Stereoselective synthesis of trans-dihydronarciclasine derivatives containing a 1,4-benzodioxane moiety.

ABSTRACT: Some new trans-dihydronarciclasine derivatives containing a 1,4-benzodioxane moiety were stereoselectively synthesised using our feasible and efficient method developed recently. These new phenanthridone alkaloid analogues were obtained in both racemic and optically active forms. High enantioselectivities (up to 99% ee) were achieved by applying (8S,9S)-9-amino(9-deoxy)epiquinine as an organocatalyst. Due to a side reaction, various methoxyphenanthridine regioisomers were also prepared which afforded further synthetic trans-dihydronarciclasine analogues modified in the ring A of the phenanthridone scaffold. © Springer-Verlag GmbH Austria, part of Springer Nature 2018.

Introduction

Nowadays, hundreds of members of alkaloids are known, which have been isolated from the plants of Amaryllidaceae family. After recognising their anticancerous properties, more and more attention has been paid to their isolation and structural elucidation [1]. The alkaloids prepared from this plant family are divided into 12 groups according to their ring systems [2]. Among these diverse structures, the most potent antitumorous alkaloids can be found in the phenanthridone subgroup. Surprisingly, the detailed investigations of Amaryllidaceae alkaloids started only at the end of the nineteenth century, when Gerrard isolated lycorine (1, Fig. 1) from Narcissus pseudonarcissus in 1877 [3, 4]. Since then, several alkaloids have been obtained from various Amaryllidaceae plants including trans-dihydronarciclasine (2, Fig. 1) isolated by Pettit and co-workers from Zephyranthes candida in 1990 [5].

Fig. 1

Structures of the most important Amaryllidaceae alkaloids (1–7)

Compound 2 is the most potent natural anticancerous alkaloid among the Amaryllidaceae ones according to the data from National Cancer Institute (NCI, USA) [6]. Its 7-deoxy analogue (3, Fig. 1) was also isolated by Pettit and co-workers from Hymenocallis caribaea and Hymenocallis latifolia, in pure form, but its biological activity was found to be weaker than that of 2 [6]. This can be explained by the lack of hydroxyl group at the position A-7, similarly to the comparison of other representatives, such as pancratistatin (4) and 7-deoxypancratistatin (5) or narciclasine (6) and lycoricidine (7) (Fig. 1) [6]. Further structure–activity relationship studies have intensively been made to find more effective synthetic analogues, but the modifications rather touched the ring C of the phenanthridone skeleton [7-20] than its ring A [21-28].

Furthermore, the Amaryllidaceae alkaloids also showed significant antiviral effects. Thus, compound 1 has strong activity against herpes simplex 1 and varicella zoster DNA viruses [29] and against several RNA viruses, such as avian influenza virus (H5N1) [30] or SARS coronavirus [31]. It has also inhibitory effects against reverse transcriptase enzyme in the HIV-1 virus [32]. Besides, strong antiviral activities of some phenanthridone alkaloids including compounds 4, 6, and 7 were reported by Gabrielsen and co-workers [33, 34]. Very recently, trans-dihydronarciclasine, pancratistatin, and narciclasine have also been found to be active against Zika virus [35].

Previously, the racemic [36] and the ent-forms of 3 [37], as well as those of 2 [38, 39], were stereoselectively synthesised using our facile and efficient process developed recently. Later, we also reported the highly stereoselective synthesis of a series of analogues of 2 substituted by alkyloxy groups (ethoxy and/or methoxy) in the aromatic ring [40]. In this work, focusing further on the modification of ring A of the phenanthridone scaffold by introducing a relatively rigid substituent, the stereoselective syntheses of some new trans-dihydronarciclasine analogues containing a 1,4-benzodioxane moiety (Fig. 2), such as 2,3,4-trihydroxy-1,3,4,4a,5,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridin-6(2H)-one (8), 2,3,4,7-tetrahydroxy-1,3,4,4a,5,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridin-6(2H)-one (9), 2,3,4-trihydroxy-7-methoxy-1,3,4,4a,5,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridin-6(2H)-one (10) and 2,3,4-trihydroxy-11-methoxy-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-i]phenanthridin-6(2H)-one (11), were described in both racemic and enantiopure forms.

Fig. 2

Structures of trans-dihydronarciclasine derivatives containing a 1,4-benzodioxane moiety (8–11)

Our modifying strategy related to ring A of the phenanthridone skeleton aimed at the methylenedioxy (–OCH2O–) structural part, a rigid functional group, in trans-dihydronarciclasine, because this molecule part may also be responsible for the anticancerous activity of this alkaloid. Thus, the homologous ethylenedioxy (–OCH2CH2O–) one was introduced into the aromatic ring to obtain new analogues for further structure–activity relationship studies.

Results and discussion

Similarly to our previous synthetic works [38-40], the starting material was vanillin (12), an inexpensive and readily available substance. As seen in Scheme 1, in the initial step, compound 12 was demethylated to 3,4-dihydroxybenzaldehyde (13) in dichloromethane by AlCl3 and pyridine, using a known method [41], in good yield (92%). Hydroxyvanillin (15) was also obtained from 12, but in this case, at first, it was selectively iodinated to 5-iodovanillin (14) [42] almost quantitatively, and then compound 14 was hydrolysed with 20% aqueous NaOH solution in presence of CuSO4 [43] to give 15 in 64% yield. The dihydrobenzodioxine ring was formed with 1,2-dibromoethane in DMF, in the presence of K2CO3 [44] to afford 2,3-dihydrobenzo[b][1,4]dioxine-6-carbaldehyde (16) and 8-methoxy-2,3- dihydrobenzo[b][1,4]dioxine-6-carbaldehyde (17) in good yield (63%). In the next step, acetone was condensed with these benzaldehyde derivatives 16 and 17 in the Claisen–Schmidt reaction applying a high excess of acetone in water and basic conditions (NaOH) to give 4-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)but-3-en-2-one (18) and 4-(8-methoxy-2,3-dihydrobenzo[b][1,4]dioxin-6-yl)but-3-en-2-one (19) in good yields (83 and 62%, respectively). Since dibenzylidene acetone derivatives may also be formed during this reaction, the pure products were obtained after distillation.

The first asymmetric centre of the title molecules was formed by Michael addition of nitromethane to compounds 18 and 19 to afford (±)-4-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-5-nitropentan-2-one [(±)-20] and (±)-4-(8-methoxy-2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-5-nitropentan-2-one [(±)-21]. These racemic representatives were prepared by using Peseke and co-workers’ mild method (MeNO2, EtOH, K2CO3) [45] in very good yields (75–86%). Moreover, this reaction allows an opportunity to obtain these intermediates in enantiomerically pure form.

The results of the enantioselective Michael addition are summarised in Table 1. Using 20 mol % (8S,9S)-9-amino(9-deoxy)epiquinine (22) organocatalyst [46-53] under conditions applied previously [39, 40], the optically active nitropentanones (−)-20 and (−)-21 were prepared in good yield (67%) and excellent enantioselectivity (92–98% ee) after 7 days. The negative optical rotation values suggest, in accordance with our previous studies [39, 40], that these new intermediates enable synthesising further ent-forms of natural alkaloid analogues.

Table 1

Enantioselective Michael addition of nitromethane to 18 and 19 catalysed by (8S,9S)-9-amino(9-deoxy)epiquinine (22)

Entry	R	Product	Yield/%a	ee/%b
1	H	(−)-20	67	92
2	OMe	(−)-21	67	98

aIsolated yield

bDetermined by chiral HPLC

Henceforth, only the syntheses of the optically active compounds are shown, but the racemic derivatives can be afforded in the same way. As seen in Scheme 2, the ring C was built by the Claisen–Henry reaction using ethyl formate and dry sodium methoxide in anhydrous diethyl ether to give (−)-3-hydroxy-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-nitrocyclohexanone [(−)-23] and (−)-3-hydroxy-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-nitrocyclohexanone [(−)-24] in moderate yields (46–57%) after column chromatography or recrystallisation from ethyl acetate. Full diastereoselectivity was achieved in this cyclisation step which can be explained by the H-bond formation between the C-3 hydroxy and C-4 nitro groups (Fig. 3), as described by Walker [54] and observed by us previously [36-40]. Prior to the reduction of the nitro group into the amino one, the carbonyl group of (−)-23 and (−)-24 was protected with ethylene glycol, in the presence of oxalic acid, in anhydrous acetonitrile to afford (+)-3-hydroxy-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-nitrocyclohexanone ethylene acetal [(+)-25] and (+)-3-hydroxy-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-nitrocyclohexanone ethylene acetal [(+)-26] in good yields (75–84%). Compound (+)-25 or (+)-26 was converted to the corresponding amino derivatives, such as (+)-4-amino-3-hydroxy-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexanone ethylene acetal [(+)-27] and (+)-4-amino-3-hydroxy-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexanone ethylene acetal [(+)-28], in good yields (56–75%) using our catalytic hydrogenation method [39, 55]: 10% Pd/C (Selcat Q [56]), methanol, 60–80 °C and 12 bar. Subsequently, amino ketal (+)-27 or (+)-28 was reacted with methyl chloroformate in a biphasic solvent mixture (water and THF) to obtain (+)-3-hydroxy-4-methoxycarbonylamino-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexanone ethylene acetal [(+)-29] or (+)-3-hydroxy-4-methoxycarbonylamino-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexanone ethylene acetal [(+)-30] quantitatively. The ketal protective group of (+)-29 or (+)-30 was removed in acetone containing a catalytic amount of p-toluenesulphonic acid under reflux, but water elimination also took place to afford (−)-6-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-oxocyclohex-2-enyl)carbamic acid methyl ester [(−)-31] or (−)-6-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-oxocyclohex-2-enyl)carbamic acid methyl ester [(−)-32] in excellent yields (88–93%).

Fig. 3

Presumed structure of the 3-hydroxy-4-nitrocyclohexanone derivatives [(−)-23 or (−)-24] and its stabilisation by hydrogen bonding

Then the oxo group of (−)-31 or (−)-32 was stereoselectively reduced with NaBH4, in the presence of CaCl2, in methanol (Utimoto’s method [57]) to obtain (−)-6-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-hydroxycyclohex-2-enyl)carbamic acid methyl ester [(−)-33] or (−)-6-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-hydroxycyclohex-2-enyl)carbamic acid methyl ester [(−)-34] in good yields (67–79%) and excellent enantiopurity (> 99% ee) after recrystallisation from hexane/ethyl acetate (2:1). This very high stereoselectivity was achieved with an axial attack of the small hydride ion derived from NaBH4 enhanced by the coordination with Ca2+, resulting in an equatorial position of the hydroxy group formed newly (Scheme 3). However, the structure of the title compounds requires the inversion of this asymmetry centre. For this purpose, the Mitsunobu reaction [58] (triphenylphosphine, diethyl azodicarboxylate, anhydrous THF) seemed to be an obvious choice. Thus, compound (−)-33 or (−)-34 was converted to (+)-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-(methoxycarbonylamino)cyclohex-2-enyl benzoate [(+)-35] and (+)-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-(methoxycarbonylamino)cyclohex-2-enyl benzoate [(+)-36] using benzoic acid, under Mitsunobu conditions, in good yields (77%) after column chromatography. In the next step, the stereoselective attack of osmium tetroxide to the C=C bond of (+)-35 or (+)-36, adapting the Sharpless–Upjohn method [59] (OsO4, N-methyl-morpholine N-oxide, THF-H2O), was favoured by the steric hindrance of the bulky benzoyl group in axial position resulting in cis-diols, such as (+)-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-2,3-dihydroxy-4-(methoxycarbonylamino)cyclohexyl benzoate [(+)-37] and (+)-2,3-dihydroxy-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-(methoxycarbonylamino)cyclohexyl benzoate [(+)-38], in excellent yields (99–100%). Prior to the Bischler–Napieralski cyclisation modified by Banwell and co-workers [60], the hydroxy groups of (+)-37 or (+)-38 were protected by acetyl chloride to afford (−)-2,3-diacetoxy-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-(methoxycarbonylamino)cyclohexyl benzoate [(−)-39] and (−)-2,3-diacetoxy-5-(8′-methoxy-2′,3′-dihydrobenzo[1,4]dioxin-6′-yl)-4-(methoxycarbonylamino)cyclohexyl benzoate [(−)-40] in excellent yields (81–100%). The ring closure reaction was performed with trifluoromethanesulphonic anhydride (Tf2O) and 4-(dimethylamino)pyridine (DMAP) in dichloromethane to give (−)-2-benzoyloxy-6-methoxy-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino-[2,3-j]phenanthridin-3,4-diyl diacetate [(−)-41] in 20% yield, as well as an inseparable mixture of (−)-2-benzoyloxy-6,7-dimethoxy-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridin-3,4-diyl diacetate [(−)-42] and (−)-2-benzoyloxy-6,11-dimethoxy-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-i]phenanthridin-3,4-diyl diacetate [(−)-43] in a ratio of 1:1, in very good yield (87%). Although the 8′-deoxy derivative (−)-39 gave selectively compound (−)-41, it was converted spontaneously into the corresponding lactam one resulting in its poor yield.

As seen in Scheme 4, the acidic treatment of (−)-41 with 2 M HCl in THF afforded (−)-2-benzoyloxy-6-oxo-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-3,4-diyl diacetate [(−)-44] in moderate yield (54%), which was subsequently deacylated by the Zemplén’s method [61] (NaOMe/MeOH) in THF to give the title compound (−)-8 in good yield (71%).

In the next reactions the racemic form of (±)-42 and (±)-43 was used, but the same results could be obtained by applying their optically active ones. When the 1:1 mixture of regioisomers (±)-42 and (±)-43 was quantitatively converted into lactams, such as (±)-2-benzoyloxy-7-methoxy-6-oxo-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-3,4-diyl diacetate [(±)-45] and (±)-2-benzoyloxy-11-methoxy-6-oxo-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-i]phenanthridine-3,4-diyl diacetate [(±)-46], in the same way (Scheme 5), the methyl group of 7-methoxy derivative (±)-45 was selectively cleaved with iodotrimethylsilane (TMS-I) prepared in situ from chlorotrimethylsilane (TMS-Cl) and potassium iodide, in anhydrous acetonitrile to obtain (±)-2-benzoyloxy-7-hydroxy-6-oxo-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-3,4-diyl diacetate [(±)-47], while the methyl group in A-11 position of another regioisomer (±)-46 remained untouched allowing their separation. However, due to the poor yield (20%) of (±)-47 and the formation of further two by-products, such as (±)-3-acetamido-6-benzoyloxy-4-(8′-methoxy-7′-methoxycarbonyl-2,3-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexane-1,2-diyl diacetate [(±)-48] and (±)-3-acetamido-6-benzoyloxy-4-(8′-methoxy-5′- methoxycarbonyl-2,3-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexane-1,2-diyl diacetate [(±)-49], which proved also to be regioisomers (Fig. 4), our synthesis strategy was modified. The structures of these regioisomers were distinguished by the chemical shifts of 5′-HAr and 5′-CAr of (±)-48, as well as 7′-HAr and 7′-CAr of (±)-49, because there were significant differences between the positions of these peaks both in the 1H NMR (6.72 ppm for 5′-HAr and 6.54 ppm for 7′-HAr) and 13C NMR spectra (110.8 ppm for 5′-CAr and 102.0 ppm for 7′-CAr). These diversions were due to the anisotropic shielding effect of the adjacent methoxy group in compound (±)-49, which resulted in lower chemical shifts for those aromatic hydrogen and carbon atoms at position A-7.

Fig. 4

Structures of the isolated side products (±)-48 and (±)-49

At first, as shown in Scheme 6, the 1:1 mixture of regioisomers (−)-42 and (−)-43 was also deacylated using the Zemplén’s method resulting in (−)-6,7-dimethoxy-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-2,3,4-triol [(−)-50] and (−)-6,11-dimethoxy-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-i]phenanthridine-2,3,4-triol [(−)-51] in pure forms, in moderate yields (22–23%) after column chromatography (ethyl acetate–ethanol, 20:1). After peracetylation of (−)-50 and (−)-51, the afforded (−)-7-methoxy-6-oxo-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino-[2,3-j]phenanthridine-2,3,4-triyl triacetate [(−)-52] and (−)-11-methoxy-6-oxo-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-i]phenanthridine-2,3,4-triyl triacetate [(−)-53] were also demethylated selectively at the position A-7 using the above-mentioned method (TMS-I, acetonitrile). As a result of this cleavage, (−)-7-hydroxy-6-oxo-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-2,3,4-triyl triacetate [(−)-54] was obtained in moderate yield (55%). Then, compound (−)-52, (−)-53, or (−)-54 was also deacetylated by Zemplén’s method to afford (−)-9 in good yield (76%), and due to the regioisomer formation, two further synthetic analogues of trans-dihydronarciclasine (−)-10 and (−)-11 in moderate yields (47%).

Conclusion

In conclusion, four new synthetic trans-dihydronarciclasine analogues [(−)-8–(−)-11)] containing a relatively rigid 1,4-benzodioxane moiety in ring A were synthesised from vanillin using stereo- and enantioselective synthetic routes. These new, optically active derivatives were obtained with excellent enantiomeric purity (99% ee). According to their negative optical rotation values, these compounds appear to be new analogues of (−)-trans-dihydronarciclasine. The preparation of compounds (−)-10 and (−)-11 was due to a side reaction and regioisomer formation, and based on our modified synthesis strategy. Biological evaluations of these potentially anticancerous and antiviral molecules are in progress.

Experimental

All reagents are commercially available from Merck. Melting points were measured on a Büchi 510 apparatus using a certified mercury thermometer (ASTM 2C). Optical rotations were measured on a Perkin Elmer 241 polarimeter. IR spectra were obtained on a PerkinElmer 1600 FT-IR instrument. NMR spectra were recorded on a Bruker AV-300 instrument. HPLC analyses were carried out with a Jasco PU-1580 apparatus equipped with a Jasco UV-1575 detector (λ = 256 nm) using a Daicel Chiralpack® OD (250 × 4.6 mm × 5 μm) column (eluent: hexane/i-PrOH, 8:2; flow rate: 2.0 cm3 min−1; 20 °C). Elemental analyses were performed on a vario EL III instrument (Elementar Analysensysteme). Precoated silica gel plates (Merck 60 F254) were used for analytical TLC and Kieselgel 60 for column cromatography. Compounds 14, 15, and 22 were prepared as described previously [39], and their spectral data were found to be identical with the ones described in Ref [39].

3,4-Dihydroxybenzaldehyde (13)

A solution of 35.00 g 12 (0.23 mol) in 300 cm3 dichloromethane was cooled to 0 °C and subsequently 36.81 g AlCl3 (0.28 mol) was added. Then it was allowed to warm to rt, 81.67 cm3 pyridine (80.20 g, 1.01 mol) was added dropwise and the reaction mixture was refluxed for 24 h. After cooling, it was acidified with 20% aqueous hydrochloric acid to pH = 2. The precipitated pyridinium salt was dissolved by adding 300 cm3 water and then the aqueous phase was extracted with ethyl acetate (4 × 250 cm3). The combined organic layer was washed with brine, dried over Na2SO4, and evaporated in vacuo to give 29.22 g 13 (92%) as a pale yellow solid, which was used without further purification. M.p.: 152–153 °C (Ref. [41] 153–154 °C); its spectral data were found to be identical with the ones described in Ref. [62].

General procedure for the synthesis of 1,4-benzodioxanes 16 and 17

43.80 g potassium carbonate (0.32 mol) and 15.1 cm3 1,2-dibromoethane (32.90 g, 0.18 mol) were added to a solution of 22.00 g 13 (0.16 mol) or 26.70 g 15 (0.16 mol) in 250 cm3 DMF. The reaction mixture was stirred at 100–110 °C for 4–8 h. After cooling to rt, the precipitated inorganic salts were filtered and the reaction mixture was evaporated to 80 cm3 in vacuo. The residue was poured into 670 cm3 water. The product was isolated as specified.

2,3-Dihydrobenzo[b][1,4]dioxine-6-carbaldehyde (16)

The aqueous phase was extracted with ethyl acetate (4 × 200 cm3), and the combined organic layer was washed with brine and dried over Na2SO4. The crude product was purified by distillation in vacuo to afford 16. Yield: 63%; white solid; m.p.: 48–49 °C (Ref. [63] 49.5–50.5 °C); b.p.: 105–108 °C (0.3 mbar); 1H NMR (300 MHz, CDCl3): δ = 9.82 (s, 1H, CHO), 7.42–7.38 (m, 2H, 5-HAr and 7-HAr), 6.98 (d, J = 8.7 Hz, 1H, 8-HAr), 4.35–4.28 (m, 4H, OCH2CH2O) ppm; 13C NMR (75 MHz, CDCl3): δ = 64.0 (OCHCH2O), 64.7 (OCH2CHO), 117.8 (5-CAr or 8-CAr), 118.4 (5-CAr or 8-CAr), 124.2 (7-CAr), 130.7 (6-CAr), 143.9 (4a-CAr), 149.1 (8a-CAr), 190.7 (CHO) ppm; IR (KBr):  = 3001, 2883, 1687, 1581, 1506, 1458, 1394, 1291, 1156, 1062, 887, 777 cm−1.

8-Methoxy-2,3-dihydrobenzo[b][1,4]dioxine-6-carbaldehyde (17)

The precipitated crystals were filtered, washed with water, and dried to give 17, which was used without further purification. Yield: 63%; white crystals; m.p.: 78–79 °C. (Ref. [64] 69–72 °C); 1H NMR (300 MHz, CDCl3): δ = 9.77 (s, 1H, CHO), 7.06 (s, 2H, 5-HAr and 7-HAr), 4.40–4.28 (m, 4H, OCH2CH2O), 3.93 (s, 3H, OCH3) ppm; 13C NMR (75 MHz, CDCl3): δ = 56.3 (OCH3), 63.9 (OCHCH2O), 64.9 (OCH2CHO), 103.0 (7-CAr), 114.5 (5-CAr), 129.2 (6-CAr), 138.8 (8a-CAr), 144.0 (8-CAr), 149.6 (4a-CAr), 190.7 (CHO) ppm; IR (KBr):  = 2982, 2948, 2892, 1698, 1590, 1503, 1470, 1454, 1392, 1324, 1125, 1046, 891, 840 cm−1.

General procedure for the synthesis of 2,3-dihydrobenzodioxine butenones 18 and 19

A solution of 16.46 g 16 (0.10 mol) or 19.41 g 17 (0.10 mol) in 95 cm3 acetone (75.15 g, 1.29 mol) was added into 42 cm3 water; then the starting material was precipitated in a fine crystal form. Aqueous sodium hydroxide solution (from 1.51 g (37.76 mmol) NaOH and 6.8 cm3 H2O) and 378 cm3 water were also added and the yellow mixture was stirred for 20 h intensively at room temperature. The yellow crude product was filtered, washed with water, and dried. Finally, it was purified by distillation in vacuo to afford 18 or 19.

4-(2,3-Dihydrobenzo[b][1,4]dioxin-6-yl)but-3-en-2-one (18, C12H12O3)

Yield: 83%; white crystals; m.p.: 90–91 °C; b.p.: 153 °C (0.2 mbar); 1H NMR (300 MHz, CDCl3): δ = 7.41 (d, J = 16.2 Hz, 1H, Ar–CH = CH), 7.08–7.04 (m, 2H, 5-HAr and 7-HAr), 6.88 (d, J = 8.4 Hz, 1H, 8-HAr), 6.58 (d, J = 16.2 Hz, 1H, Ar–CH = CH), 4.31–4.27 (m, 4H, OCH2CH2O), 2.36 (s, 3H, COCH3) ppm; 13C NMR (75 MHz, CDCl3): δ = 27.4 (COCH), 64.2 (OCHCH2O), 64.5 (OCH2CHO), 116.8 (5-CAr or 8-CAr), 117.8 (5-CAr or 8-CAr), 122.3 (7-CAr), 125.5 (Ar–CH = CH), 128.0 (6-CAr), 143.1 (Ar–CH = CH), 143.7 (4a-CAr), 149.2 (8a-CAr), 192.2 (CO) ppm; IR (KBr):  = 2989, 2894, 1668, 1641, 1579, 1513, 1455, 1427, 1360, 1300, 1251, 1122, 1061, 977, 884, 806, 779 cm−1.

4-(8-Methoxy-2,3-dihydrobenzo[b][1,4]dioxin-6-yl)but-3-en-2-one (19, C13H14O4)

Yield: 62%; light yellow solid; m.p.: 96–98 °C; b.p.: 183 °C (0.2 mbar); 1H NMR (300 MHz, CDCl3): δ = 7.37 (d, J = 16.2 Hz, Ar–CH = CH), 6.75 (d, J = 1.8 Hz, 1H, 5-HAr or 7-HAr), 6.69 (d, J = 1.8 Hz, 1H, 5-HAr or 7-HAr), 6.57 (d, J = 15.9 Hz, 1H, Ar–CH = CH), 4.36–4.26 (m, 4H, OCH2CH2O), 3.90 (s, 3H, OCH3), 2.35 (s, 3H, COCH3) ppm; 13C NMR (75 MHz, CDCl3): δ = 27.4 (COCH), 56.2 (OCH3), 64.1 (OCHCH2O), 64.7 (OCH2CHO), 103.6 (7-CAr), 111.0 (5-CAr), 125.8 (Ar–CH = CH), 126.8 (6-CAr), 135.5 (8a-CAr), 143.4 (Ar–CH = CH), 144.1 (8-CAr), 149.2 (4a-CAr), 192.2 (CO) ppm; IR (KBr):  = 2996, 2940, 1666, 1638, 1589, 1508, 1453, 1321, 1274, 1126, 977, 822 cm−1.

General procedure for the synthesis of racemic nitropentanones (±)-20 and (±)-21

To a solution of 7.70 g 18 (37.74 mmol) or 8.84 g 19 (37.74 mmol) in a mixture of anhydrous 15.1 cm3 ethanol and 10.22 cm3 nitromethane (11.52 g, 0.19 mol) was added 0.11 g anhydrous potassium carbonate (0.81 mmol) and the reaction mixture was refluxed for 5–8 h. Then, it was cooled to rt and 14.3 cm3 water was added. The product was isolated as specified.

(±)-4-(2,3-Dihydrobenzo[b][1,4]dioxin-6-yl)-5-nitropentan-2-one [(±)-20, C13H15NO5]

The aqueous phase was extracted with ethyl acetate (3 × 40 cm3), and the combined organic layer was washed with brine, dried over Na2SO4, and evaporated in vacuo. Recrystallisation from ethyl acetate gave (±)-20. Yield: 75%; white crystals; m.p.: 101–102 °C; R = 0.52 (hexane/EtOAc, 1:1); 1H NMR (300 MHz, CDCl3): δ = 6.80 (d, J = 8.1 Hz, 1H, 8-HAr), 6.70 (d, J = 1.8 Hz, 1H, 5-HAr), 6.67 (dd, J = 8.4, 2.1 Hz, 1H, 7-HAr), 4.63 (dd, J = 12.3, 6.9 Hz, 1H, CH2NO2), 4.53 (dd, J = 12.3, 7.8 Hz, 1H, CH2NO2), 4.23 (s, 4H, OCH2CH2O), 3.89 (quint, J = 7.2 Hz, 1H, Ar–CH), 2.86 (d, J = 7.2 Hz, 2H, CH2CO), 2.12 (s, 3H, COCH3) ppm; 13C NMR (75 MHz, CDCl3): δ = 30.4 (COCH), 38.4 (Ar–CH), 46.2 (CHCO), 64.3 (OCHCH2O), 64.3 (OCH2CHO), 79.6 (CHNO2), 116.1 (5-CAr or 8-CAr), 117.7 (5-CAr or 8-CAr), 120.3 (7-CAr), 131.9 (6-CAr), 143.1 (8a-CAr or 4a-CAr), 143.7 (4a-CAr or 8a-CAr), 205.4 (CO) ppm; IR (KBr):  = 2878, 1717, 1592, 1508, 1433, 1384, 1313, 1220, 1161, 1130, 1071, 903, 819, 639 cm−1.

(±)-4-(8-Methoxy-2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-5-nitropentan-2-one [(±)-21, C14H17NO6]

After crystallisation at 0 °C, the precipitated crystals were filtered, washed with water and dried to afford (±)-21. Yield: 86%; white solid; m.p.: 99–101 °C; R = 0.33 (hexane/EtOAc, 1:1); 1H NMR (300 MHz, CDCl3): δ = 6.35 (d, J = 1.8 Hz, 1H, 5-HAr or 7-HAr), 6.34 (d, J = 1.8 Hz, 1H, 5-HAr or 7-HAr), 4.63 (dd, J = 12.3, 6.9 Hz, 1H, CH2NO2), 4.53 (dd, J = 12.3, 7.8 Hz, 1H, CH2NO2), 4.29–4.22 (m, 4H, OCH2CH2O), 3.88 (quint, J = 7.2 Hz, 1H, Ar–CH), 3.86 (s, 3H, OCH3), 2.86 (dd, J = 6.9, 1.5 Hz, 2H, CH2CO), 2.12 (s, 3H, COCH3) ppm; 13C NMR (75 MHz, CDCl3): δ = 30.4 (COCH), 38.9 (Ar–CH), 46.2 (CHCO), 56.2 (OCH3), 64.2 (OCHCH2O), 64.4 (OCH2CHO), 79.5 (CH2NO2), 103.8 (7-CAr), 108.3 (5-CAr), 131.0 (8a-CAr), 132.6 (6-CAr), 144.2 (8-CAr), 149.4 (4a-CAr), 205.4 (CO) ppm; IR (KBr):  = 2962, 1716, 1599, 1513, 1438, 1386, 1326, 1221, 1130, 1046, 951, 844, 669 cm−1.

General procedure for the enantioselective Michael addition using organocatalyst 22

A solution of 8.17 g 18 (40.00 mmol) or 9.37 g 19 (40.00 mmol) and 20 mol % (8S,9S)-9-amino(9-deoxy)epiquinine (22) in 54 cm3 anhydrous nitromethane was stirred at rt for 7 d, and then the solvent was evaporated in vacuo. The residue was purified by column chromatography (hexane/EtOAc, 1:1) to obtain optically active (−)-20 or recrystallised from MeOH to give (−)-21. Spectroscopic data and elemental analysis for these compounds matched those for the racemates as given above.

(−)-4-(2,3-Dihydrobenzo[b][1,4]dioxin-6-yl)-5-nitropentan-2-one [(−)-20, C13H15NO5]

Yield: 67%; white solid; m.p.: 75–77 °C; R = 0.52 (hexane/EtOAc, 1:1); HPLC: Chiralpack® OD (hexane/i-PrOH = 8:2, flow rate 2.0 cm3 min−1, 256 nm, 20 °C), t(−): 25 min, t(+): 18 min; [α]D22 = − 16.1° (c = 0.94, acetone); ee 92%.

(−)-4-(8-Methoxy-2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-5-nitropentan-2-one [(−)-21, C14H17NO6]

Yield: 67%; white crystals; m.p.: 111–115 °C; R = 0.33 (hexane/EtOAc, 1:1); HPLC: Chiralpack® OD (hexane/i-PrOH = 8:2, flow rate 2.0 cm3 min−1, 256 nm, 20 °C), t(−): 32 min, t(+): 28 min; [α]D22 = − 15.0° (c = 1.5, acetone); ee 98%.

General procedure for the Claisen–Henry reaction

Dry and freshly prepared sodium methoxide powder (from 2.90 g sodium (0.13 mol) and 70 cm3 anhydrous methanol) was suspended in 100 cm3 anhydrous diethyl ether. Subsequently, 14.50 cm3 ethyl formate (13.36 g, 0.18 mol) and 8.07 g (−)-20 (30.48 mmol) or 9.00 g (−)-21 (30.48 mmol) were added, and the reaction mixture was stirred at rt for 20 h. It was cooled to 0 °C and 51 cm3 water was added dropwise. After separating, the aqueous phase was acidified with acetic acid to pH = 4 at 0 °C. The precipitated crystals were filtered, washed with water and dried. The crude product (−)-23 or (−)-24 was purified as specified.

(−)-3-Hydroxy-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-nitrocyclohexanone [(−)-23, C14H15NO6]

Recrystallisation from ethyl acetate gave light yellow crystals. Yield: 46%; m.p.: 206 °C; 1H NMR (300 MHz, DMSO-d): δ = 6.96 (d, J = 1.8 Hz, 1H, 5-HAr), 6.86 (dd, J = 8.4, 1.8 Hz, 1H, 7-HAr), 6.78 (d, J = 8.1 Hz, 1H, 8-HAr), 5.98 (d, J = 4.5 Hz, 1H, OH), 5.67 (dd, J = 11.7, 2.1 Hz, 1H, 4-H), 4.69–4.63 (m, 1H, 3-HCy), 4.21 (s, 4H, OCH2CH2O), 3.83 (td, J = 12.9, 4.5 Hz, 1H, 5-H), 3.04 (dd, J = 14.4, 2.7 Hz, 1H, 2-Hβ,Cy), 2.65 (t, J = 14.1 Hz, 1H, 6-Hβ), 2.39 (dt, J = 14.4, 3.0 Hz, 1H, 2-Hα,Cy), 2.31 (ddd, J = 14.7, 5.1, 2.1 Hz, 1H, 6-Hα) ppm; 13C NMR (75 MHz, DMSO-d): δ = 40.4 (5-C), 46.4 (2-CCy), 47.3 (6-C), 63.9 (OCHCH2O), 64.0 (OCH2CHO), 69.6 (3-CCy), 89.4 (4-C), 115.7 (8-CAr), 117.0 (5-CAr), 120.1 (7-CAr), 133.6 (6-CAr), 142.3 (8a-CAr), 143.2 (4a-CAr), 206.0 (CO) ppm; IR (KBr):  = 3318, 2984, 2919, 1714, 1591, 1554, 1511, 1461, 1373, 1309, 1248, 1127, 1067, 889, 814 cm−1; [α]D22 = − 18.4° (c = 1, CHCl3).

(−)-3-Hydroxy-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-nitrocyclohexanone [(−)-24, C15H17NO7]

It was purified by column chromatography (hexane/EtOAc, 1:2) to afford a light yellow solid. Yield: 57%; m.p.: 200–201 °C; R = 0.19 (hexane/EtOAc, 1:1); 1H NMR (300 MHz, DMSO-d): δ = 6.66 (d, J = 1.2 Hz, 1H, 5-HAr), 6.56 (d, J = 1.5 Hz, 1H, 7-HAr), 5.99 (d, J = 4.5 Hz, 1H, OH), 5.70 (dd, J = 11.7, 1.8 Hz, 1H, 4-H), 4.70–4.64 (m, 1H, 3-HCy), 4.18 (s, 4H, OCH2CH2O), 3.81 (td, J = 12.9, 4.2 Hz, 1H, 5-H), 3.74 (s, 3H, OCH3), 3.02 (dd, J = 14.1, 2.1 Hz, 1H, 2-Hβ,Cy), 2.67 (t, J = 14.1 Hz, 1H, 6-Hβ), 2.40 (dt, J = 14.7, 2.7 Hz, 1H, 2-Hα,Cy), 2.33 (ddd, J = 14.7, 4.5, 2.1 Hz, 1H, 6-Hα) ppm; 13C NMR (75 MHz, DMSO-d): δ = 39.3 (5-C), 46.4 (2-CCy), 47.3 (6-C), 55.8 (OCH3), 63.6 (OCHCH2O), 63.9 (OCH2CHO), 69.6 (3-CCy), 89.2 (4-C), 104.0 (7-CAr), 108.1 (5-CAr), 132.0 (8a-CAr), 132.6 (6-CAr), 143.5 (4a-CAr), 148.6 (8-CAr), 206.1 (CO) ppm; IR (KBr):  = 3337, 2962, 2867, 1709, 1599, 1554, 1514, 1465, 1376, 1253, 1130, 1050, 888, 825 cm−1; [α]D22 = − 11.4° (c = 0.5, THF).

General procedure for the synthesis of ethylene acetals (+)-25 and (+)-26

36 cm3 ethylene glycol (39.89 g, 0.64 mol) and 4.40 g (−)-23 (15.00 mmol) or 4.85 g (−)-24 (15.00 mmol) were added to a solution of 12.62 g anhydrous oxalic acid (0.14 mol) in 213 cm3 anhydrous acetonitrile. The mixture was stirred at rt for 3–4 d. Then, it was poured into a cooled and saturated 594 cm3 NaHCO3 solution. The precipitated solid was collected by filtration, washed with water, and dried to give (+)-25 or (+)-26.

(+)-3-Hydroxy-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-nitrocyclohexanone ethylene acetal [(+)-25, C16H19NO7]

Yield: 84%; white solid; m.p.: 224–228 °C; 1H NMR (300 MHz, CDCl3): δ = 6.79 (d, J = 8.1 Hz, 1H, 8-HAr), 6.75–6.70 (m, 2H, 5-HAr and 7-HAr), 4.70 (dd, J = 12.0, 3.0 Hz, 1H, 4-H), 4.63 (dq, J = 10.2, 3.0 Hz, 1H, 3-HCy), 4.22 (s, 4H, OCH2CH2Obenzodioxane), 4.09–3.91 (m, 5H, OH and OCH2CH2Oacetal), 3.75 (td, J = 12.6, 3.9 Hz, 1H, 5-H), 2.19 (dt, J = 14.4, 3.0 Hz, 1H, 2-Hα,Cy), 2.04 (dd, J = 14.4, 2.7 Hz, 1H, 2-Hβ,Cy), 2.00 (dt, J = 12.9, 3.6 Hz, 2H, 6-Hα), 1.79 (t, J = 13.5 Hz, 1H, 6-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 37.7 (5-C), 38.5 (2-CCy), 41.5 (6-C), 64.3 (OCHCH2Oacetal, 2′-C or 3′-C), 64.4 (2′-C or 3′-C), 65.1 (OCH2CHOacetal), 69.7 (3-CCy), 91.1 (4-CCy), 107.6 (1-C), 116.1 (5-CAr), 117.6 (8-CAr), 120.2 (7-CAr), 132.6 (6-CAr), 140.3 (8a-CAr), 141.9 (4a-CAr) ppm; IR (KBr):  = 3502, 2980, 2943, 2887, 1592, 1551, 1507, 1458, 1386, 1362, 1243, 1125, 1050, 953, 890, 820, 640 cm−1; [α]D22 = + 47.0° (c = 0.5, DMF).

(+)-3-Hydroxy-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-nitrocyclohexanone ethylene acetal [(+)-26, C17H21NO8]

Yield: 75%; white solid; m.p.: 178–180 °C; 1H NMR (300 MHz, CDCl3): δ = 6.40 (s, 2H, 5-HAr and7-HAr), 4.71 (dd, J = 12.0, 2.7 Hz, 1H, 4-H), 4.63 (dq, J = 9.9, 3.0 Hz, 1H, 3-HCy), 4.28–4.22 (m, 4H, OCH2CH2Obenzodioxane), 4.09–3.92 (m, 5H, OH and OCH2CH2Oacetal), 3.86 (s, 3H, OCH3), 3.74 (td, J = 12.6, 4.2 Hz, 1H, 5-H), 2.19 (dt, J = 14.7, 3.0 Hz, 1H, 2-Hα,Cy), 2.07–1.98 (m, 2H, 2-Hβ,Cy and 6-Hα), 1.80 (t, J = 13.5 Hz, 1H, 6-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 38.2 (5-C), 38.4 (2-CCy), 41.5 (6-C), 56.2 (OCH3), 64.2 (OCHCH2Oacetal), 64.3 (2′-C and 3′-C), 65.1 (OCH2CHOacetal), 69.6 (3-CCy), 91.0 (4-C), 104.0 (7-CAr), 107.6 (1-C), 108.3 (5-CAr), 131.8 (8a-CAr), 132.4 (6-CAr), 144.1 (4a-CAr), 148.9 (8-CAr) ppm; IR (KBr):  = 3514, 2974, 2939, 2895, 1599, 1544, 1513, 1460, 1385, 1352, 1249, 1127, 1051, 950, 840 cm−1; [α]D22 = + 13.8° (c = 1, CHCl3).

General procedure for the synthesis of amines (+)-27 and (+)-28

Over a 10% Pd/C catalyst (Selcat Q, 0.75 g), 2.53 g (+)-25 (7.50 mmol) or 2.75 g (+)-26 (7.50 mmol) was hydrogenated in 50 cm3 MeOH, in a 250 cm3 stainless steel autoclave equipped with a magnetic stirrer (stirring speed: 1100 rpm). The reduction was carried out at 12 bar and 60–80 °C for 6 h. After the hydrogen uptake was finished, the catalyst was removed by filtration and the filtrate was concentrated in vacuo to afford (+)-27 or (+)-28.

(+)-4-Amino-3-hydroxy-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexanone ethylene acetal [(+)-27, C16H21NO5]

Yield: 56%; light brown oil; 1H NMR (300 MHz, CDCl3): δ = 6.81 (d, J = 8.4 Hz, 1H, 8-HAr), 6.71 (d, J = 1.2 Hz, 1H, 5-HAr), 6.67 (dd, J = 8.4, 1.8 Hz, 1H, 7-HAr), 4.23 (s, 4H, OCH2CH2Obenzodioxane), 4.06–3.85 (m, 5H, 3-HCy and OCH2CH2Oacetal), 2.77–2.71 (m, 2H, 4-H, 5-H), 2.20 (bs, 2H, NH2), 2.11 (dt, J = 14.1, 2.7 Hz, 1H, 2-Hα,Cy), 1.80 (dd, J = 14.1, 3.0 Hz, 1H, 2-Hβ,Cy), 1.88–1.77 (m, 2H, 6-Hα and 6-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 39.2 (2-CCy), 41.6 (6-C), 43.7 (5-C), 57.3 (4-C), 64.0 (OCHCH2Oacetal), 64.3 (2′-C and 3′-C), 64.8 (OCH2CHOacetal), 70.4 (3-CCy), 108.8 (1-C), 116.4 (8-CAr), 117.4 (5-CAr), 120.7 (7-CAr), 135.4 (6-CAr), 142.4 (8a-CAr), 143.6 (4a-CAr) ppm; IR (KBr):  = 3503, 2929, 2880, 1589, 1508, 1433, 1371, 1289, 1245, 1110, 1067, 1002, 948, 888, 815, 747 cm−1; [α]D22 = + 15.5° (c = 1, CHCl3).

(+)-4-Amino-3-hydroxy-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexanone ethylene acetal [(+)-28, C17H23NO6]

Yield: 75%; grey semi-solid; 1H NMR (300 MHz, CDCl3): δ = 6.40 (d, J = 1.8 Hz, 1H, 5-HAr or 7-HAr), 6.35 (d, J = 1.2 Hz, 5-HAr or 7-HAr), 4.30–4.24 (m, 4H, OCH2CH2Obenzodioxane), 4.07–3.87 (m, 5H, 3-HCy and OCH2CH2Oacetal), 3.86 (s, 3H, OCH3), 2.76–2.73 (m, 2H, 4-H and 5-H), 2.12 (dt, J = 14.1, 2.7 Hz, 1H, 2-Hα,Cy), 2.07 (bs, 2H, NH2), 1.94 (dd, J = 14.1, 3.0 Hz, 1H, 2-Hβ,Cy), 1.90–1.78 (m, 2H, 6-Hα and 6-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 39.2 (2-CCy), 41.6 (6-C), 44.3 (5-C), 56.1 (OCH3), 57.3 (4-C), 64.1 (OCHCH2Oacetal), 64.3 (2′-C or 3′-C), 64.5 (2′-C or 3′-C), 64.8 (OCH2CHOacetal), 70.5 (3-CCy), 103.7 (7-CAr), 108.9 (1-C), 109.1 (5-CAr), 131.9 (6-CAr), 134.5 (8a-CAr), 144.1 (8-CAr), 149.0 (4a-CAr) ppm; IR (KBr): = 3503, 2929, 1598, 1558, 1508, 1458, 1371, 1341, 1216, 1126, 1069, 952, 887 cm−1; [α]D22 = + 6.3° (c = 1, methanol).

General procedure for the synthesis of carbamates (+)-29 and (+)-30

Half of the required methyl chloroformate (0.64 cm3, 0.80 g, 8.47 mmol), 3% aqueous NaOH solution (20 cm3), and subsequently the other half of methyl chloroformate (0.64 cm3, 0.80 g, 8.47 mmol) were added to a solution of 2.54 g (+)-27 (8.27 mmol) or 2.79 g (+)-28 (8.27 mmol) in 51 cm3 tetrahydrofuran. The reaction mixture was stirred rigorously at rt for 2 h, then it was poured into 113 cm3 water and extracted with ethyl acetate (4 × 105 cm3). The combined organic layer was washed with brine, dried over Na2SO4, and the solvent was evaporated in vacuo to give (+)-29 or (+)-30.

(+)-3-Hydroxy-4-methoxycarbonylamino-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexanone ethylene acetal [(+)-29, C18H23NO7]

Yield: 100%; brown oil; 1H NMR (300 MHz, CDCl3): δ = 6.78 (d, J = 8.1 Hz, 1H, 8-HAr), 6.72–6.68 (m, 2H, 5-HAr and 7-HAr), 4.98 (d, J = 9.0 Hz, 1H, NH), 4.22 (s, 4H, OCH2CH2Obenzodioxane), 4.15–4.08 (m, 1H, 3-HCy), 4.04–3.88 (m, 4H, OCH2CH2Oacetal), 3.81 (m, 1H, 4-H), 3.56 (d, J = 9.6 Hz, 1H, OH), 3.50 (s, 3H, NHCOOCH), 2.91 (td, J = 11.7, 3.6 Hz, 1H, 5-H), 2.08 (dt, J = 14.4, 2.7 Hz, 1H, 2-Hα,Cy), 2.00 (dd, J = 14.4, 2.7 Hz, 1H, 2-Hβ,Cy), 1.91 (dt, J = 13.2, 3.3 Hz, 1H, 6-Hα), 1.84 (t, J = 12.6 Hz, 1H, 6-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 38.6 (2-CCy), 40.8 (5-C), 42.5 (6-C), 51.9 (NHCOOCH), 56.3 (4-C), 64.2 (OCHCH2Oacetal), 64.3 (2′-C and 3′-C), 64.9 (OCH2CHOacetal), 69.6 (3-CCy), 108.5 (1-C), 116.6 (5-CAr), 117.2 (8-CAr), 120.4 (7-CAr), 134.5 (6-CAr), 142.3 (8a-CAr), 143.3 (4a-CAr), 156.4 (NHCOOCH3) ppm; IR (KBr):  = 3471, 3318, 2950, 2890, 1712, 1589, 1543, 1506, 1447, 1341, 1289, 1226, 1132, 1065, 993, 950, 919, 882, 817, 777, 723 cm−1; [α]D22 = + 17.8° (c = 1, methanol).

(+)-3-Hydroxy-4-methoxycarbonylamino-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexanone ethylene acetal [(+)-30, C19H25NO8]

Yield: 99%; white solid (fluffy); m.p.: 84–90 °C; 1H NMR (300 MHz, CDCl3): δ = 6.39 (s, 2H, 5-HAr and 7-HAr), 4.96 (d, J = 9.3 Hz, 1H, NH), 4.30–4.22 (m, 4H, OCH2CH2Obenzodioxane), 4.15–4.08 (m, 1H, 3-HCy), 4.05–3.91 (m, 5H, 4-H, OCH2CH2Oacetal), 3.86 (s, 3H, OCH), 3.57 (d, J = 9.6 Hz, 1H, OH), 3.51 (s, 3H, NHCOOCH), 2.90 (td, J = 11.4, 3.0 Hz, 1H, 5-H), 2.09 (dt, J = 14.4, 3.0 Hz, 1H, 2-Hα,Cy), 2.01 (dd, J = 14.1, 3.3 Hz, 1H, 2-Hβ,Cy), 1.92 (dt, J = 13.2, 3.3 Hz, 1H, 6-Hα), 1.83 (t, J = 13.2 Hz, 1H, 6-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 38.7 (2-CCy), 41.2 (5-C), 42.8 (6-C), 51.9 (NHCOOCH), 56.0 (4-C), 56.1 (OCH3), 64.2 (OCHCH2Oacetal, 2′-C or 3′-C), 64.5 (2′-C or 3′-C), 64.9 (OCH2CHOacetal), 69.7 (3-CCy), 103.2 (7-CAr), 108.5 (1-C), 109.4 (5-CAr), 131.8 (8a-CAr), 133.6 (6-CAr), 143.8 (8-CAr), 148.7 (4a-CAr), 156.4 (NHCOOCH3) ppm; IR (KBr):  = 3447, 3340, 2944, 1714, 1596, 1536, 1511, 1457, 1435, 1340, 1259, 1225, 1123, 1080, 1052, 887 cm−1; [α]D22 = + 10.0° (c = 1, CHCl3).

General procedure for the synthesis of enones (−)-31 and (−)-32

A solution of 3.62 g (+)-29 (9.91 mmol) or 3.92 g (+)-30 (9.91 mmol) and 3.37 g p-TsOH (17.72 mmol) in 235 cm3 acetone was heated to reflux and stirred for 1 h. After cooling to rt, it was poured into 461 cm3 saturated NaHCO3 solution and extracted with ethyl acetate (4 × 138 cm3). The combined organic layer was washed with brine, dried over Na2SO4, and the solvent was evaporated in vacuo to afford (−)-31 or (−)-32.

(−)-6-(2′,3′-Dihydrobenzo[b][1,4]dioxin-6′-yl)-4-oxocyclohex-2-enyl)carbamic acid methyl ester [(−)-31, C16H17NO5]

Yield: 93%; white solid; m.p.: 153–155 °C; 1H NMR (300 MHz, CDCl3): δ = 6.94 (d, J = 10.2 Hz, 1H, 2-HCy), 6.83 (d, J = 8.1 Hz, 1H, 8-HAr), 6.74 (d, J = 1.8 Hz, 1H, 5-HAr), 6.70 (dd, J = 8.1, 1.8 Hz, 1H, 7-HAr), 6.07 (dd, J = 10.2, 2.4 Hz, 1H, 3-HCy), 4.82–4.71 (m, 1H, NH), 4.61–4.56 (m, 1H, 1-H), 4.25 (s, 4H, OCH2CH2O), 3.60 (s, 3H, NHCOOCH), 3.20–3.11 (m, 1H, 6-H), 2.68–2.65 (m, 2H, 5-Hα and 5-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 44.9 (5-C), 47.1 (6-C) 52.3 (C-1), 53.3 (NHCOOCH), 64.3 (OCHCHO), 116.0 (5-CAr), 117.6 (8-CAr), 120.2 (7-CAr), 129.2 (3-CCy), 132.9 (6-CAr), 143.0 (8a-CAr), 143.7 (4a-CAr), 151.6 (2-CCy), 156.6 (NHCOOCH3), 197.5 (4-C) ppm; IR (KBr):  = 3330, 2984, 2884, 1698, 1683, 1591, 1541, 1509, 1458, 1385, 1244, 1130, 1053, 927, 890, 817, 773 cm−1; [α]D22 = − 162.0° (c = 1, acetone).

(−)-6-(8′-Methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-oxocyclohex-2-enyl)carbamic acid methyl ester [(−)-32, C17H19NO6]

Yield: 88%; light brown oil; 1H NMR (300 MHz, CDCl3): δ = 6.94 (d, J = 9.9, 1H, 2-HCy), 6.40 (d, J = 1.8 Hz, 1H, 5-HAr or 7-HAr), 6.36 (d, J = 1.8 Hz, 1H, 5- HAr or 7-HAr), 6.07 (dd, J = 10.2, 2.4 Hz, 1H, 3-HCy), 4.82 (d, J = 7.5 Hz, 1H, NH), 4.65–4.59 (m, 1H, 1-H), 4.30–4.24 (m, 4H, OCH2CH2O), 3.86 (s, 3H, OCH3) 3.61 (s, 3H, NHCOOCH), 3.20–3.11 (m, 1H, 6-H), 2.68–2.64 (m, 2H, 5-Hα and 5-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 45.0 (5-C), 47.6 (6-C), 52.3 (NHCOOCH), 53.1 (1-C), 56.1 (OCH3), 64.2 (OCHCH2O), 64.4 (OCH2CHO), 103.0 (7-CAr), 108.8 (5-CAr), 129.2 (3-CCy), 132.0 (8a-CAr), 132.4 (6-CAr), 144.1 (8a-CAr), 149.1 (4a-CAr), 151.7 (2-CCy), 156.3 (NHCOOCH3), 197.5 (4-C) ppm; IR (KBr):  = 3341, 2946, 1699, 1680, 1598, 1512, 1462, 1385, 1342, 1242, 1127, 1050, 887, 652 cm−1; [α]D22 = − 116.2° (c = 1, CHCl3).

General procedure for the synthesis of enols (−)-33 and (−)-34

A solution of 2.63 g (−)-31 (8.67 mmol) or 2.89 g (−)-32 (8.67 mmol) and 1.97 g CaCl2 (17.75 mmol) in 264 cm3 methanol was stirred for 30 min at rt. Then, it was cooled to 0 °C and 0.49 g NaBH4 (12.95 mmol) was added in one portion. It was further stirred at 0 °C for 30 min, then poured into 372 cm3 water and extracted with ethyl acetate (4 × 188 cm3). The combined organic layer was washed with brine, dried over Na2SO4, and the solvent was evaporated in vacuo. The residue was recrystallised from hexane/EtOAc (2:1) to give (−)-33 or (−)-34.

(−)-6-(2′,3′-Dihydrobenzo[b][1,4]dioxin-6′-yl)-4-hydroxycyclohex-2-enyl)carbamic acid methyl ester [(−)-33, C16H19NO5]

Yield: 67%; white solid; m.p.: 166–168 °C; 1H NMR (300 MHz, CDCl3): δ = 6.80 (d, J = 8.1 Hz, 1H, 8-HAr), 6.72 (d, J = 1.8 Hz, 1H, 5-HAr), 6.68 (dd, J = 8.1, 1.8 Hz, 1H, 7-HAr), 5.81 (dd, J = 10.2, 1.2 Hz, 1H, 3-HCy), 5.75 (d, J = 10.2 Hz, 1H, 2-HCy), 4.60–4.50 (m, 1H, NH), 4.50–4.38 (m, 1H, 4-H), 4.34–4.27 (m, 1H, 1-H), 4.24 (s, 4H, OCH2CH2O), 3.55 (s, 3H, NHCOOCH), 2.65–2.57 (m, 1H, 6-H), 2.24 (dd, J = 12.0, 5.4 Hz, 1H, 5-Hβ), 1.80 (td, J = 12.9, 9.9 Hz, 1H, 5-Hα) ppm; 13C NMR (75 MHz, CDCl3): δ = 40.6 (5-C), 45.7 (6-C), 52.0 (NHCOOCH), 53.5 (1-C), 64.3 (OCH2CH2O), 67.8 (4-C), 116.0 (5-CAr), 117.3 (8-CAr), 120.2 (7-CAr), 131.1 (2-CCy), 132.4 (3-CCy), 135.1 (6-CAr), 142.5 (8a-CAr), 143.5 (4a-CAr), 156.5 (NHCOOCH3) ppm; IR (KBr):  = 3329, 2926, 2876, 1687, 1592, 1533, 1509, 1457, 1435, 1315, 1290, 1126, 1102, 1049, 928, 885, 759, 634 cm−1; HPLC: Chiralpack® OD (hexane/i-PrOH = 8:2, flow rate 2.0 cm3 min−1, 256 nm, 20 °C), t(−): 23 min, t(+): 30 min; [α]D22 = − 129.0° (c = 1, CHCl3); ee > 99%.

(−)-6-(8′-Methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-hydroxycyclohex-2-enyl)carbamic acid methyl ester [(−)-34, C17H21NO6]

Yield: 79%; white solid; m.p.: 166 °C; 1H NMR (300 MHz, CDCl3): δ = 6.39 (d, J = 1.8 Hz, 1H, 5-HA or 7-HAr), 6.36 (d, J = 1.8 Hz, 1H, 5-HA or 7-HAr), 5.82 (dd, J = 10.2, 1.2 Hz, 1H, 3-HCy), 5.76 (d, J = 10.8 Hz, 1H, 2-HCy), 4.60–4.50 (m, 1H, NH), 4.50–4.40 (m, 1H, 4-H), 4.24–4.30 (m, 5H, 1-H and OCH2CH2O), 3.86 (s, 3H, OCH3) 3.56 (s, 3H, NHCOOCH), 2.66–2.56 (m, 1H, 6-H), 2.26 (dd, J = 12.3, 5.4 Hz, 1H, 5-Hβ), 1.80 (td, J = 12.9, 9.9 Hz, 1H, 5-Hα) ppm; 13C NMR (75 MHz, CDCl3): δ = 40.0 (5-C), 46.1 (6-C), 52.1 (NHCOOCH), 53.7 (1-C), 56.1 (OCH3), 64.3 (OCHCH2O), 64.5 (OCH2CHO), 67.7 (4-C), 103.1 (7-CAr), 108.9 (5-CAr), 131.1 (2-CCy), 132.4 (3-CCy), 133.7 (6-CAr), 134.1 (8a-CAr), 143.9 (8-CAr), 148.9 (4a-CAr), 156.4 (NHCOOCH3) ppm; IR (KBr):  = 3333, 2930, 1693, 1599, 1539, 1512, 1457, 1341, 1239, 1129, 1049, 887, 655 cm−1; HPLC: Chiralpack® OD (hexane/i-PrOH = 8:2, flow rate 2.0 cm3 min−1, 256 nm, 20 °C), t(−): 30 min, t(+): 25 min; [α]D22 = − 107.8° (c = 1, CHCl3); ee > 99%.

General procedure for the Mitsunobu reaction

0.74 cm3 diethyl azodicarboxylate (0.82 g, 4.71 mmol) in 2.6 cm3 anhydrous THF at 0 °C was added dropwise to a solution of 1.09 g (−)-33 (3.58 mmol) or 1.20 g (−)-34 (3.58 mmol) and 1.15 g triphenylphosphine (4.38 mmol) in 56 cm3 anhydrous THF, and the mixture was stirred for 10 min. Then 0.47 g benzoic acid (3.85 mmol) was also added, and the reaction mixture was stirred at 0 °C for 2 h, then heated to 45–50 °C and further stirred for 5 h. The solvent was evaporated in vacuo and the residue was purified as specified.

(+)-5-(2′,3′-Dihydrobenzo[b][1,4]dioxin-6′-yl)-4-(methoxycarbonylamino)cyclohex-2-enyl benzoate [(+)-35, C23H23NO6]

It was purified by column chromatography (CH2Cl2/acetone, 20:1) to afford a pale yellow solid. Yield: 77%; m.p.: 58–63 °C; R = 0.51 (CHCl3/methanol, 100:1); 1H NMR (300 MHz, CDCl3): δ = 8.05 (d, J = 7.2 Hz, 2H, 2-HBz and 6-HBz), 7.57 (t, J = 7.2 Hz, 1H, 4-HBz), 7.45 (t, J = 7.2 Hz, 2H, 3-HBz and 5-HBz), 6.81 (d, J = 8.4 Hz, 1H, 8-HAr), 6.76 (d, J = 1.8 Hz, 1H, 5-HAr), 6.72 (dd, J = 8.4 Hz, 7-HAr), 6.08–5.99 (m, 2H, 2-HCy and 3-HCy), 5.53–5.48 (m, 1H, 1-H), 4.67–4.54 (m, 1H, NH), 4.41–4.29 (m, 1H, 4-H), 4.24 (s, 4H, OCH2CH2O), 3.58 (s, 3H, NHCOOCH), 2.93–2.83 (m, 1H, 5-H), 2.19–2.15 (m, 2H, 6-Hα and 6-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 38.0 (6-C), 42.9 (5-C), 52.1 (NHCOOCH), 53.2 (4-C), 64.3 (OCHCH2O), 64.4 (OCH2CHO), 66.8 (1-C), 116.3 (5-CAr), 117.4 (8-CAr), 120.3 (7-CAr), 125.5 (2-CCy), 128.4 (3-CBz and 5-CBz), 129.6 (2-CBz and 6-CBz), 130.3 (1-CBz), 133.0 (4-CBz), 135.1 (6-CAr), 135.8 (3-CCy), 141.2 (8a-CAr), 143.5 (4a-CAr), 156.4 (NHCOOCH3), 165.9 (PhCO) ppm; IR (KBr):  = 3356, 3036, 2947, 1716, 1591, 1510, 1452, 1315, 1271, 1109, 1053, 1025, 953, 896, 810, 713 cm−1; [α]D22 = + 82.5° (c = 1, CHCl3).

(+)-5-(8′-Methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-(methoxycarbonylamino)cyclohex-2-enyl benzoate [(+)-36, C24H25NO7]

It was isolated by column chromatography (CHCl3/acetone, 20:1) to give a white solid. Yield: 77%; m.p.: 66–71 °C; R = 0.35 (CHCl3/methanol, 100:1); 1H NMR (300 MHz, CDCl3): δ = 8.05 (d, J = 6.9 Hz, 2H, 2-HBz and 6-HBz), 7.58 (tt, J = 7.2, 1.2 Hz, 1H, 4-HBz), 7.45 (t, J = 7.5 Hz, 2H, 3-HB and 5-HBz), 6.43 (d, J = 1.8 Hz, 1H, 5- HAr or 7-HAr), 6.40 (d, J = 1.8 Hz, 1H, 5- HAr or 7-HAr), 6.08–5.99 (m, 2H, 2-HCy and 3-HCy), 5.54–5.49 (m, 1H, 1-H), 4.68–4.65 (m, 1H, NH), 4.43–4.33 (m, 1H, 4-H), 4.30–4.24 (m, 4H, OCH2CH2O), 3.87 (s, 3H, OCH3), 3.58 (s, 3H, NHCOOCH), 2.92–2.84 (m, 1H, 5-H), 2.20–2.16 (m, 2H, 6-Hα and 6-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 36.0 (6-C), 42.0 (5-C), 52.1 (NHCOOCH), 53.4 (4-C), 56.2 (OCH3), 64.3 (OCHCH2O), 64.5 (OCH2CHO), 66.7 (1-C), 103.2 (7-CAr), 109.2 (5-CAr), 125.6 (2-CCy), 128.4 (3-CBz and 5-CBz), 129.6 (2-CBz and 6-CBz), 130.3 (1-CBz), 132.0 (6-CAr), 133.0 (4-CBz), 134.1 (8a-CAr), 135.8 (3-CCy), 143.9 (8-CAr), 148.9 (4a-CAr), 156.4 (NHCOOCH3), 165.9 (PhCO) ppm; IR (KBr):  = 3362, 2931, 1716, 1599, 1511, 1453, 1362, 1340, 1271, 1129, 1053, 1025, 887, 649 cm−1; [α]D22 = + 90.3° (c = 1, CHCl3).

General procedure for synthesis of cis-diols (+)-37 and (+)-38

To a solution of 1.15 g (+)-35 (2.80 mmol) or 1.23 g (+)-36 (2.80 mmol) in a mixture of 17 cm3 tetrahydrofuran and 2.8 cm3 water, 0.71 g N-methylmorpholine-N-oxide (6.06 mmol) and subsequently 1.22 cm3 4% aqueous OsO4 solution (48.6 mg, 0.19 mmol) under argon were added. The mixture was stirred at rt for 24 h, then it was poured into 112 cm3 saturated Na2S2O3 solution and extracted with ethyl acetate (4 × 60 cm3). The combined organic layer was washed with brine, dried over Na2SO4, and the solvent was evaporated in vacuo to afford (+)-37 or (+)-38.

(+)-5-(2′,3′-Dihydrobenzo[b][1,4]dioxin-6′-yl)-2,3-dihydroxy-4-(methoxycarbonylamino)cyclohexyl benzoate [(+)-37, C23H25NO8]

Yield: 99%; white solid (fluffy); m.p.: 92 °C; 1H NMR (300 MHz, CDCl3): δ = 8.03 (d, J = 7.5 Hz, 2H, 2-HBz and 6-HBz), 7.60 (t, J = 7.5 Hz, 1H, 4-HBz), 7.47 (t, J = 7.5 Hz, 2H, 3-HBz and 5-HBz), 6.80 (d, J = 8.1 Hz, 1H, 8-HAr), 6.73 (d, J = 1.8 Hz, 1H, 5-HAr), 6.68 (dd, J = 8.1, 1.8 Hz, 1H, 5-HAr), 5.41 (q, J = 2.7 Hz, 1H, 1-H), 4.71–4.62 (m, 1H, NH), 4.25–4.18 (m, 5H, OCH2CH2O and 2-HCy), 4.06–3.95 (m, 2H, 3-HCy and 4-H), 3.58 (s, 3H, NHCOOCH), 2.82 (ddd, J = 13.5, 11.1, 3.0 Hz, 1H, 5-H), 2.30 (ddd, J = 15.9, 11.4, 2.4 Hz, 1H, 6-Hβ), 2.02 (dt, J = 14.7, 3.3 Hz, 1H, 6-Hα) ppm; 13C NMR (75 MHz, CDCl3): δ = 32.9 (6-C), 41.9 (5-C), 52.6 (NHCOOCH), 55.7 (4-C), 64.3 (OCH2CH2O), 70.1 (2-CCy), 71.3 (1-C), 74.4 (3-CCy), 116.3 (5-CAr), 117.6 (8-CAr), 120.4 (7-CAr), 128.5 (3-CBz and 5-CBz), 129.6 (2-CBz and 6-CBz), 130.0 (1-CBz), 133.3 (4-CBz), 133.8 (6-CAr), 142.7 (8a-CAr), 143.7 (4a-CAr), 158.9 (NHCOOCH3), 165.1 (PhCO) ppm; IR (KBr):  = 3421, 2928, 2877, 1716, 1591, 1541, 1509, 1456, 1374, 1338, 1273, 1113, 1070, 1045, 887, 714 cm−1; [α]D22 = + 65.2° (c = 1, CHCl3).

(+)-2,3-Dihydroxy-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-(methoxycarbonylamino)cyclohexyl benzoate [(+)-38, C24H27NO9]

Yield: 100%; white solid (fluffy); m.p.: 112–120 °C; 1H NMR (300 MHz, CDCl3): δ = 8.04 (dd, J = 7.2, 1.5 Hz, 2H, 2-HBz and 6-HBz), 7.61 (t, J = 7.2 Hz, 1H, 4-HBz), 7.48 (t, J = 7.5 Hz, 2H, 3-HBz and 5-HBz), 6.39 (d, J = 1.8 Hz, 1H, 5-HAr or 7-HAr), 6.35 (d, J = 1.8 Hz, 1H, 5-HAr or 7-HAr), 5.43 (q, J = 3.0 Hz, 1H, 1-H), 4.65–4.64 (m, 1H, NH), 4.56 (bs, 1H, OH), 4.30–4.23 (m, 4H, OCH2CH2O), 4.21–4.18 (m, 1H, 2-HCy), 4.06–3.95 (m, 2H, 3-HCy and 4-H), 3.86 (s, 3H, OCH3), 3.59 (s, 3H, NHCOOCH), 3.18 (bs, 1H, OH), 2.80 (ddd, J = 12.9, 10.8, 3.0 Hz, 1H, 5-H), 2.31 (ddd, J = 14.1, 12.0, 2.4 Hz, 1H, 6-Hβ), 2.04 (dt, J = 14.4, 3.0 Hz, 1H, 6-Hα) ppm; 13C NMR (75 MHz, CDCl3): δ = 32.9 (6-C), 42.6 (5-C), 52.6 (NHCOOCH), 55.5 (4-C), 64.2 (OCHCH2O), 64.4 (OCHCH2O), 70.2 (2-CCy), 71.4 (1-C), 74.1 (3-CCy), 103.0 (7-CAr), 109.3 (5-CAr), 128.5 (3-CBz and 5-CBz), 129.6 (2-CBz and 6-CBz), 129.9 (1-CBz), 132.1 (6-CAr), 132.9 (8a-CAr), 133.3 (4-CBz), 144.0 (8-CAr), 149.2 (4a-CAr), 158.9 (NHCOOCH3), 165.1 (PhCO) ppm; IR (KBr):  = 3364, 2930, 1716, 1599, 1541, 1511, 1455, 1370, 1339, 1274, 1127, 1071, 1048, 887, 715 cm−1; [α]D22 = + 63.2° (c = 1, CHCl3).

General procedure for acetylation

A solution of 0.99 g (+)-37 (2.22 mmol) or 1.05 g (+)-38 (2.22 mmol) in 8.28 cm3 acetyl chloride (9.14 g, 0.12 mol) was stirred at rt for 20–24 h. Then, it was poured into 618 cm3 saturated NaHCO3 solution at 0 °C and extracted with ethyl acetate (4 × 110 cm3). The combined organic layer was washed with brine, dried over Na2SO4 and concentrated in vacuo to give (−)-39 or (−)-40.

(−)-2,3-Diacetoxy-5-(2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-(methoxycarbonylamino)cyclohexyl benzoate [(−)-39, C27H29NO10]

Yield: 81%; colourless semi-solid; 1H NMR (300 MHz, CDCl3): δ = 8.07 (dd, J = 7.2, 1.2 Hz, 2H, 2-HBz and 6-HBz), 7.62 (t, J = 7.2 Hz, 1H, 4-HBz), 7.49 (t, J = 7.5 Hz, 2H, 3-HBz and 5-HBz), 6.79 (d, J = 8.1 Hz, 1H, 8-HAr), 6.73 (d, J = 1.8 Hz, 1H, 5-HAr), 6.70 (dd, J = 8.4, 1.8 Hz, 1H, 7-HAr), 5.49 (t, J = 3.0 Hz, 1H, 1-HCy or 2-HCy), 5.37 (dd, J = 10.8, 3.0 Hz, 1H, 3-HCy), 5.28 (q, J = 3.3 Hz, 1H, 1-HCy or 2-HCy), 4.44 (d, J = 9.0 Hz, 1H, NH), 4.27–4.16 (m, 5H, OCH2CH2O and 4-H), 3.50 (s, 3H, NHCOOCH), 2.95 (td, J = 10.8, 7.2 Hz, 1H, 5-H), 2.23 (s, 3H, COCH3), 2.19–2.09 (m, 2H, 6-Hα and 6-Hβ), 2.02 (s, 3H, COCH3) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.8 (COCH), 21.0 (COCH), 33.6 (6-C), 42.9 (5-C), 52.1 (NHCOOCH), 53.4 (4-C), 64.3 (OCH2CH2O), 69.3 (2-CCy), 69.4 (1-C), 71.4 (3-CCy), 116.5 (5-CAr), 117.3 (8-CAr), 120.6 (7-CAr), 128.6 (3-CBz and 5-CBz), 129.4 (1-CBz), 129.8 (2-CBz and 6-CBz), 133.5 (4-CBz), 133.6 (6-CAr), 142.6 (8a-CAr), 143.4 (4a-CAr), 156.4 (NHCOOCH3), 164.9 (PhCO), 169.4 (COCH3), 170.7 (COCH3) ppm; IR (KBr):  = 3397, 2926, 1749, 1721, 1591, 1522, 1509, 1456, 1373, 1271, 1249, 1157, 1070, 1052, 933, 887, 808, 776, 717 cm−1; [α]D22 = − 22.4° (c = 1, CHCl3).

(−)-2,3-Diacetoxy-5-(8′-methoxy-2′,3′-dihydrobenzo[b][1,4]dioxin-6′-yl)-4-(methoxycarbonylamino)cyclohexyl benzoate [(−)-40, C28H31NO11]

Yield: 100%; white solid (fluffy); m.p.: 114–116 °C; 1H NMR (300 MHz, CDCl3): δ = 8.08 (dd, J = 7.2, 1.2 Hz, 2H, 2-HBz and 6-HBz), 7.62 (t, J = 7.5 Hz, 1H, 4-HBz), 7.49 (t, J = 7.5 Hz, 2H, 3-HBz and 5-HBz), 6.40 (d, J = 1.8 Hz, 1H, 5-HAr or 7-HAr), 6.28 (s, 1H, 5-HAr or 7-HAr), 5.49 (t, J = 3.3 Hz, 1H, 1-HCy or 2-HCy), 5.38 (dd, J = 10.5, 3.0 Hz, 1H, 3-HCy), 5.28 (q, J = 3.3 Hz, 1H, 1-HCy or 2-HCy), 4.49 (d, J = 9.9 Hz, 1H, NH), 4.29–4.21 (m, 5H, OCH2CH2O, 4-H), 3.86 (s, 3H, OCH3), 3.51 (s, 3H, NHCOOCH), 2.96 (td, J = 11.1, 7.2 Hz, 1H, 5-H), 2.23 (s, 3H, COCH), 2.19 (dd, J = 8.4, 2.4 Hz, 1H, 6-Hβ), 2.12 (dt, J = 15.3, 2.7 Hz, 1H, 6-Hα), 2.02 (s, 3H, COCH) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.8 (COCH), 21.0 (COCH), 33.9 (6-C), 43.2 (5-C), 52.1 (NHCOOCH), 53.5 (4-C), 64.2 (OCHCH2O), 64.5 (OCH2CHO), 69.3 (2-CCy), 69.4 (1-C), 71.4 (3-CCy), 103.7 (7-CAr), 109.1 (5-CAr), 128.6 (3-CBz and 5-CBz), 129.4 (1-CBz), 129.8 (2-CBz and 6-CBz), 133.6 (6-CAr), 120.6 (8a-CAr), 133.5 (4-CBz), 143.9 (8-CAr), 148.8 (4a-CAr), 156.5 (NHCOOCH3), 164.9 (PhCO), 169.4 (COCH3), 170.6 (COCH3) ppm; IR (KBr):  = 3365, 2930, 1750, 1726, 1599, 1540, 1512, 1455, 1370, 1273, 1240, 1128, 1051, 887, 715 cm−1; [α]D22 = − 6.0° (c = 1, CHCl3).

General procedure for the modified Bischler–Napieralski cyclisation

A solution of 1.39 g (−)-39 (2.64 mmol) or 1.47 g (−)-40 (2.64 mmol) and 0.97 g 4-(dimethylamino)pyridine (7.94 mmol) in 73 cm3 anhydrous dichloromethane was cooled to 0 °C. A solution of 2.34 cm3 trifluoromethanesulphonic anhydride (3.92 g, 13.91 mmol) in 12 cm3 anhydrous dichloromethane was added dropwise. The reaction mixture was stirred for 20–24 h while being allowed to warm to rt. Then, it was diluted with 46 cm3 dichloromethane, subsequently washed with 656 cm3 saturated NaHCO3 solution, 656 cm3 20% aqueous AcOH, and 656 cm3 saturated NaHCO3 solution. The organic layer was dried over Na2SO4 and evaporated in vacuo. The crude product was purified as specified.

(−)-2-Benzoyloxy-6-methoxy-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridin-3,4-diyl diacetate [(−)-41, C27H27NO9]

This product was converted spontaneously into the corresponding lactam due to traces of acid. It was separated from the lactam derivative by column chromatography (CHCl3/acetone, 20:1) to afford a pale yellow oil. Yield: 20%; R = 0.69 (CH2Cl2/methanol, 100:1); 1H NMR (300 MHz, CDCl3): δ = 8.05 (dd, J = 8.1, 1.2 Hz, 2H, 2-HBz and 6-HBz), 7.58 (t, J = 7.5 Hz, 1H, 4-HBz), 7.44 (t, J = 7.5 Hz, 2H, 3-HBz and 5-HBz), 7.21 (s, 1H, 7-HAr), 6.74 (s, 1H, 12-HAr), 5.57–5.53 (m, 1H, 3-H), 5.52 (dd, J = 10.8, 3.0 Hz, 1H, 4-H), 5.43 (q, J = 2.4 Hz, 1H, 2-H), 4.30–4.20 (m, 4H, OCH2CH2O), 3.80 (s, 3H, OCH3), 3.51 (dd, J = 13.8, 10.8 Hz, 1H, 4a-H), 2.90 (td, J = 12.9, 3.6 Hz, 1H, 12b-H), 2.60 (dt, J = 14.4, 3.0 Hz, 1H, 1-Hα), 2.13 (s, 3H, COCH), 2.09 (s, 3H, COCH), 2.07–1.97 (m, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.8 (COCH), 21.0 (COCH), 27.4 (1-C), 32.8 (12b-C), 52.5 (NCOCH), 57.5 (4a-C), 64.2 (OCHCH2O), 64.6 (OCH2CHO), 69.4 (3-C), 69.6 (2-C), 72.4 (4-C), 112.5 (12-CAr), 114.5 (7-CAr), 119.1 (6a-CAr), 128.5 (3-CBz and 5-CBz), 129.4 (1-CBz), 129.8 (3-CBz and 5-CBz), 133.5 (4-CBz), 134.4 (12a-CAr), 142.1 (8a-CAr), 146.1 (4a-CAr), 160.5 (6-C), 165.1 (PhCO), 169.4 (COCH3), 170.5 (COCH3) ppm; [α]D22 = − 63.9° (c = 1, CHCl3).

(−)-2-Benzoyloxy-6,7-dimethoxy-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridin-3,4-diyl diacetate [(−)-42, C28H29NO10]/(−)-2-Benzoyloxy-6,11-dimethoxy-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-i]phenanthridin-3,4-diyl diacetate [(−)-43, C28H29NO10]

It was purified by column chromatography (CHCl3/acetone, 20:1) to give a light brown solid (fluffy). Yield: 87%; mixture of regioisomers (1:1); R = 0.53 (CH2Cl2/methanol, 20:1); 1H NMR (300 MHz, CDCl3): δ = 8.07 (dd, J = 6.9, 1.5 Hz, 2H, 2-HBz and 6-HBz), 8.05 (dd, J = 6.9, 1.5 Hz, 2H, 2-HBz and 6-HBz), 7.58 (tt, J = 7.5, 1.5 Hz, 1H, 4-HBz), 7.46 (t, J = 7.5 Hz, 2H, 3-HBz and 5-HBz), 7.45 (t, J = 7.5 Hz, 2H, 3-HBz and 5-HBz), 6.55 (s, 1H, HAr), 6.40 (s, 1H, HAr), 5.57–5.53 (m, 2H, 2 × 3-H), 5.49 (dd, J = 10.8, 3.0 Hz, 2H, 2 × 4-H), 5.46–5.40 (m, 2H, 2 × 2-H), 4.42–4.16 (m, 8H, 2 × OCH2CH2O), 3.91 (s, 3H, Ar-OCH3), 3.84–3.83 (3 × s, 9H, Ar-OCH3, 2 ×CNOCH3), 3.40 (2 × dd, J = 13.5, 10.5 Hz, 2H, 2 × 4a-H), 2.82–2.55 (m, 4H, 2 × 12b-H, 2 × 1-Hα), 2.12 (s, 6H, 2 × COCH3), 2.09 (s, 3H, COCH3), 2.08 (s, 3H, COCH3), 1.90 (2 × dt, J = 12.3, 2.7 Hz, 2H, 1-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.8 (COCH), 21.0 (COCH), 27.5 (1-C), 27.6 (1-C), 33.4 (10b-C), 33.6 (10b-C), 52.6 (2 × CNOCH), 56.9 (OCH3), 57.0 (2 × 4a-C), 61.7 (OCH3), 63.9 (OCHCH2O), 64.2 (OCHCH2O), 64.3 (OCH2CHO), 64.5 (OCHCH2O), 69.3 (2 × 3-CCy), 69.6 (2-CCy), 69.7 (2-CCy), 72.4 (4-C), 72.6 (4-C), 99.4 (12-C), 108.1 (12-C), 128.5 (3-CBz and 5-CBz), 128.6 (3-CBz and 5-CBz), 129.3 (1-CBz), 129.4 (1-CBz), 129.8 (2-CBz and 6-CBz), 132.1 (2 × 6a-C), 133.4 (4-CBz), 133.5 (4-CBz), 134.9 (2 × 12a-C), 135.3 (7a-C and 10a-C), 142.5 (6b-C), 146.5 (7-C), 150.6 (11-C and 11a-C), 160.4 (6-C), 165.1 (PhCO), 165.2 (PhCO), 169.4 (2 × COCH3), 170.4 (2 × COCH3) ppm; IR (KBr):  = 2944, 1752, 1637, 1600, 1500, 1437, 1371, 1334, 1269, 1239, 1096, 1070, 714 cm−1; [α]D22 = − 114.1° (c = 1, CHCl3).

General procedure for the synthesis of lactams (−)-44 and (±)-45/(±)-46

To a solution of 0.61 g (−)-41 (1.13 mmol) or 0.70 g (±)-42/(±)-43 (1.13 mmol) in 55 cm3 tetrahydrofuran, 2.80 cm3 2 M aqueous HCl was added, and it was stirred at rt for 22 h. Then, it was poured into 120 cm3 saturated NaHCO3 solution and extracted with ethyl acetate (4 × 30 cm3). The combined organic layer was washed with brine, dried over Na2SO4 and the solvent was evaporated in vacuo. The crude product was dissolved in 5.30 cm3 acetyl chloride (4.78 g, 0.061 mol) and stirred at rt for 20 h. Then, it was poured into 380 cm3 saturated NaHCO3 solution and extracted with ethyl acetate (4 × 80 cm3). The combined organic layer was washed with brine, dried over Na2SO4, and the solvent was evaporated in vacuo to afford (−)-44 or (±)-45/(±)-46.

(−)-2-Benzoyloxy-6-oxo-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-3,4-diyl diacetate [(−)-44, C26H25NO9]

Yield: 54%; yellow oil; 1H NMR (300 MHz, CDCl3): δ = 8.05 (dd, J = 7.2, 1.2 Hz, 2H, 2-HBz and 6-HBz), 7.59 (tt, J = 7.5, 1.2 Hz, 1H, 4-HBz), 7.46 (t, J = 7.5 Hz, 2H, 3-HBz and 5-HBz), 6.74 (s, 1H, 7-HAr), 6.57 (s, 1H, 12-HAr), 6.08 (s, 1H, NH), 5.60 (t, J = 3.0 Hz, 1H, 3-H), 5.44 (q, J = 3.0 Hz, 1H, 2-H), 5.36 (dd, J = 10.8, 3.0 Hz, 1H, 4-H), 4.31–4.22 (m, 4H, OCH2CH2O), 3.87 (dd, J = 12.0, 11.1 Hz, 1H, 4a-H), 3.26 (td, J = 12.6, 3.6 Hz, 1H, 12b-H), 2.62 (dt, J = 14.4, 3.0 Hz, 1H, 1-Hα), 2.11 (s, 3H, COCH3), 2.10 (s, 3H, COCH3), 2.08–1.08 (m, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.8 (COCH), 20.9 (COCH), 26.8 (1-C), 34.7 (12b-C), 53.0 (4a-C), 64.1 (OCHCH2O), 64.8 (OCH2CHO), 67.7 (2-C), 69.3 (4-C), 71.9 (3-C), 112.6 (7-C), 117.8 (12-C), 122.3 (6a-C), 128.7 (3-CBz, 5-CBz), 129.1 (1-CBz), 129.8 (2-CBz, 6-CBz), 133.7 (4-CBz), 137.0 (12a-C), 142.8 (7a-C), 147.4 (11a-C), 165.0 (6-C), 165.7 (PhCO), 169.2 (COCH3), 170.3 (COCH3) ppm; IR (KBr):  = 2928, 1754, 1726, 1669, 1498, 1455, 1368, 1368, 1317, 1266, 1234, 1097, 1065, 925, 803, 712 cm−1; [α]D22 = − 78.7° (c = 1, CHCl3).

(±)-2-Benzoyloxy-7-methoxy-6-oxo-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-3,4-diyl diacetate [(±)-45, C27H27NO10]/(±)-2-Benzoyloxy-11-methoxy-6-oxo-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-i]phenanthridine-3,4-diyl diacetate [(±)-46, C27H27NO10]

The mixture of these regioisomers (1:1) proved to be inseparable in this step. Their isolation was achieved after selective demethylation, as described below.

General procedure for the Zemplén deacylation

77 cm3 0.56 M methanolic solution of sodium methoxide was added dropwise at rt to a solution of 1.15 g (−)-42/(−)-43 (2.13 mmol) in 153 cm3 anhydrous tetrahydrofuran, and the reaction mixture was stirred for 2 h. Then it was poured into 500 cm3 water and extracted with ethyl acetate (4 × 120 cm3). The combined organic layer was washed with brine, dried over Na2SO4, and the solvent was evaporated in vacuo. The regioisomers were separated by column chromatography (EtOAc/ethanol, 20:1) to give (−)-50 and (−)-51.

(−)-6,7-Dimethoxy-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-2,3,4-triol [(−)-50, C17H21NO7]

Yield: 22%; white solid; m.p.: 133–136 °C; R = 0.42 (EtOAc/methanol, 20:1); 1H NMR (300 MHz, DMSO-d): δ = 6.55 (s, 1H, 12-H), 4.83 (d, J = 3.3 Hz, 1H, OH), 4.53 (d, J = 3.6 Hz, 1H, OH), 4.31–4.18 (m, 5H, OCH2CH2O and OH), 3.87 (q, J = 2.7 Hz, 1H, 2-H), 3.79–3.74 (m, 4H, 3-H and OCH3), 3.71 (s, 3H, CNOCH3), 3.68 (dd, J = 10.2, 3.0 Hz, 1H, 4-H), 2.87 (dd, J = 13.2, 9.9 Hz, 1-H, 4a-H), 2.39 (td, J = 12.9, 3.3 Hz, 1H, 12b-H), 2.04 (dt, J = 13.8, 3.3 Hz, 1H, 1-Hα), 1.62 (ddd, J = 14.4, 11.1, 1.8 Hz, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, DMSO-d): δ = 28.9 (1-C), 32.2 (12b-C), 52.1 (CNOCH), 58.9 (4a-C), 60.8 (OCH3), 63.7 (OCHCH2O), 64.2 (OCH2CHO), 68.6 (2-C), 71.2 (4-C), 71.8 (3-C), 107.5 (12-C), 111.9 (6a-C), 135.8 (7a-C), 136.7 (12a-C), 146.1 (7-C), 146.6 (11a-C), 159.1 (6-C) ppm; IR (KBr):  = 3408, 2926, 1717, 1637, 1608, 1574, 1483, 1437, 1333, 1226, 1122, 1040, 812 cm−1; [α]D22 = − 49.0° (c = 0.3, methanol).

(−)-6,11-Dimethoxy-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-i]phenanthridine-2,3,4-triol [(−)-51, C17H21NO7]

Yield: 23%; white solid; m.p.: 132–134 °C; R = 0.26 (EtOAc/methanol, 20:1); 1H NMR (300 MHz, DMSO-d): δ = 6.49 (s, 1H, 12-H), 4.84 (d, J = 3.3 Hz, 1H, OH), 4.54 (d, J = 3.3 Hz, 1H, OH), 4.34–4.14 (m, 5H, OCH2CH2O and OH), 3.89 (q, J = 2.4 Hz, 1H, 2-H), 3.80 (s, 3H, OCH3), 3.77 (t, J = 3.0 Hz, 1H, 3-H), 3.71 (s, 3H, CNOCH3), 3.69 (overlapped dd, 1H, 4-H), 2.87 (dd, J = 13.8, 10.2 Hz, 1-H, 4a-H), 2.42 (td, J = 12.9, 3.3 Hz, 1H, 12b-H), 2.14 (dt, J = 12.9, 3.0 Hz, 1H, 1-Hα), 1.62 (ddd, J = 14.4, 12.0, 2.4 Hz, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, DMSO-d): δ = 28.9 (1-C), 32.5 (12b-C), 51.9 (CNOCH), 55.6 (OCH3), 58.9 (4a-C), 63.0 (OCHCH2O), 63.8 (OCH2CHO), 68.6 (2-C), 71.3 (4-C), 71.8 (3-C), 99.6 (12-C), 107.2 (6a-C), 131.4 (10a-C), 136.9 (12a-C), 141.9 (6b-C), 150.5 (11-C), 159.4 (6-C) ppm; IR (KBr):  = 3420, 2924, 1716, 1699, 1635, 1602, 1558, 1457, 1384, 1334, 1132, 1065 cm−1; [α]D22 = − 63.8° (c = 0.3, methanol).

General procedure for the synthesis of triacetoxy lactams (−)-52 and (−)-53

To a solution of 0.23 g (−)-50 (0.65 mmol) or 0.23 g (−)-51 (0.65 mmol) in 32 cm3 tetrahydrofuran, 1.60 cm3 2 M aqueous HCl was added and the reaction mixture was stirred at rt for 22 h. After evaporation of the solvent in vacuo, the residue was dissolved in 2.44 cm3 acetyl chloride (2.20 g, 28.03 mmol) and stirred at rt for 20 h. Then, it was poured into 186 cm3 saturated NaHCO3 solution at 0 °C and extracted with ethyl acetate (4 × 40 cm3). The combined organic layer was washed with brine, dried over Na2SO4, and the solvent was evaporated in vacuo. The crude product was purified by preparative TLC (CHCl3/acetone, 3:1) to afford (−)-52 or (−)-53.

(−)-7-Methoxy-6-oxo-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-2,3,4-triyl triacetate [(−)-52, C22H25NO10]

Yield: 87%; white solid; m.p.: 122–125 °C; R = 0.39 (CHCl3/acetone, 3:1); 1H NMR (300 MHz, CDCl3): δ = 6.52 (s, 1H, 12-H), 6.03 (s, 1H, NH), 5.42 (t, J = 3.0 Hz, 1H, 3-H), 5.18 (q, J = 3.0 Hz 1H, 2-H), 5.16 (dd, J = 11.1, 2.7 Hz, 1H, 4-H), 4.36–4.26 (m, 4H, OCH2CH2O), 3.94 (s, 3H, OCH3), 3.71–3.63 (m, 1H, 4a-H), 3.06 (td, J = 12.6, 3.6 Hz, 1H, 12b-H), 2.40 (dt, J = 14.7, 3.0 Hz, 1H, 1-Hα), 2.13 (s, 3H, COCH3), 2.07 (s, 3H, COCH3), 2.06 (s, 3H, COCH3), 1.87 (ddd, J = 14.7, 12.9, 2.4 Hz, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.7 (COCH), 20.8 (COCH), 21.0 (COCH), 26.7 (1-C), 35.3 (12b-C), 52.2 (4a-C), 61.9 (OCH), 64.1 (OCHCH2O), 64.6 (OCH2CHO), 67.4 (3-C), 68.6 (2-C), 71.6 (4-C), 107.9 (12-C), 115.5 (6a-C), 133.8 (7a-C), 137.1 (12a-C), 147.4 (7-C), 151.0 (11a-C), 163.6 (6-C), 169.1 (COCH3), 169.4 (COCH3), 170.4 (COCH3) ppm; IR (KBr):  = 3197, 3089, 2931, 2874, 1751, 1670, 1606, 1476, 1372, 1332, 1244, 1224, 1118, 1062, 1041, 859 cm−1; [α]D22 = − 77.2° (c = 0.5, CHCl3).

(−)-11-Methoxy-6-oxo-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-i]phenanthridine-2,3,4-triyl triacetate [(−)-53, C22H25NO10]

Yield: 47%; white solid; m.p.: 208–211 °C; R = 0.17 (CHCl3/acetone, 3:1); 1H NMR (300 MHz, DMSO-d): δ = 7.98 (s, 1H, NH), 6.48 (s, 1H, 12-H), 5.23 (t, J = 3.0 Hz, 1H, 3-H), 5.06 (q, J = 2.7 Hz, 1H, 2-H), 4.91 (dd, J = 10.8, 2.7 Hz, 2H, 4-H), 4.37–4.12 (m, 4H, OCH2CH2O), 3.83 (s, 3H, OCH3), 3.53–3.45 (m, 1H, 4a-H), 2.95 (td, J = 12.3, 3.3 Hz, 1H, 12b-H), 2.11 (s, 3H, COCH3), 2.04 (s, 3H, COCH3), 1.95 (s, 3H, COCH3), 1.78 (ddd, J = 14.4, 12.9, 2.4 Hz, 1H, 1-Hβ) ppm (the sign of 1-Hα is covered by that of DMSO-d); 13C NMR (75 MHz, DMSO-d): δ = 20.4 (COCH), 20.7 (COCH), 20.9 (COCH), 26.0 (1-C), 35.3 (12b-C), 51.6 (4a-C), 55.7 (OCH3), 63.0 (OCHCH2O), 63.1 (OCH2CHO), 66.6 (3-C), 68.1 (2-C), 70.5 (4-C), 99.5 (12-C), 111.0 (6a-C), 131.9 (7a-C), 134.1 (12a-C), 144.6 (7-C), 150.6 (11a-C), 162.5 (6-C), 169.1 (COCH3), 169.3 (COCH3), 169.9 (COCH3) ppm; IR (KBr):  = 3195, 3091, 2939, 1752, 1667, 1597, 1494, 1451, 1370, 1330, 1251, 1157, 1128, 1060, 1028, 799 cm−1; [α]D22 = − 135.5° (c = 0.5, CHCl3).

General procedure for the selective demethylation

To a solution of 0.18 g (±)-45/(±)-46 (0.35 mmol) or 0.16 g (−)-52 (0.35 mmol) and 57.3 mg potassium iodide (0.35 mmol) in 16 cm3 anhydrous acetonitrile, 48.7 mg chlorotrimethylsilane (0.45 mmol) in 3.1 cm3 anhydrous acetonitrile was added. The reaction mixture was heated to 60 °C and stirred for 4 h. Then, it was cooled to 0 °C and 26 cm3 water was added dropwise to quench the reaction. After extraction with ethyl acetate (4 × 26 cm3), the combined organic layer was washed with brine, dried over Na2SO4, and the solvent was evaporated in vacuo. The residue was purified by preparative TLC (EtOAc/heptane, 1:1) to give (±)-47 or (−)-54. Compound (±)-46, as well as the by-products (±)-48 and (±)-49, was isolated from the crude product obtained by the conversion of (±)-45/(±)-46.

(±)-2-Benzoyloxy-7-hydroxy-6-oxo-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-3,4-diyl diacetate [(±)-47, C26H25NO10]

Yield: 20%; white solid; m.p.: 201–204 °C; 1H NMR (300 MHz, CDCl3): δ = 12.52 (s, 1H, OH), 8.03 (dd, J = 7.8, 1.2 Hz, 2H, 2-HBz and 6-HBz), 7.59 (tt, J = 7.5, 1.2 Hz, 1H, 4-HBz), 7.46 (t, J = 7.5 Hz, 2H, 3-HBz and 5-HBz), 6.27 (s, 1H, 12-H), 6.10 (s, 1H, NH), 5.60 (t, J = 3.0 Hz, 1H, 3-H), 5.43 (q, J = 3.0 Hz, 1H, 2-H), 5.33 (dd, J = 10.8, 2.7 Hz, 1H, 4-H), 4.31 (s, 4H, OCH2CH2O), 3.85 (dd, J = 12.3, 11.1 Hz, 1H, 4a-H), 3.20 (td, J = 12.9, 3.3 Hz, 1H, 12b-H), 2.60 (dt, J = 14.7, 2.7 Hz, 1H, 1-Hα), 2.12 (s, 3H, COCH3), 2.11 (s, 3H, COCH3), 2.01 (ddd, J = 14.7, 12.6, 2.4 Hz, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.7 (COCH), 20.8 (COCH), 26.5 (1-C), 34.3 (12b-C), 52.8 (4a-C), 64.1 (OCHCH2O), 64.8 (OCH2CHO), 67.5 (3-C), 69.1 (2-C), 71.9 (4-C), 103.7 (12-C), 104.5 (6a-C), 128.6 (3-CBz and 5-CBz), 129.1 (1-CBz), 129.8 (2-CBz and 6-CBz), 131.1 (7a-C), 132.2 (12a-C), 133.7 (4-CBz), 148.2 (7-C), 152.6 (11a-C), 164.9 (6-C), 169.2 (COPh), 170.1 (COCH3), 170.3 (COCH3) ppm; IR (KBr):  = 3446, 2930, 1753, 1731, 1683, 1652, 1448, 1362, 1270, 1239, 1157, 1069, 1028, 803, 711 cm−1.

(−)-7-Hydroxy-6-oxo-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridine-2,3,4-triyl triacetate [(−)-54, C21H23NO10]

Yield: 55%; white solid; m.p.: 154–156 °C; R = 0.85 (hexane/EtOAc, 1:2); 1H NMR (300 MHz, CDCl3): δ = 12.54 (s, 1H, OH), 6.25 (s, 1H, 12-H), 5.95 (s, 1H, NH), 5.44 (t, J = 3.0 Hz, 1H, 3-H), 5.20–5.16 (m, 2H, 2-H and 4-H), 4.32 (s, 4H, OCH2CH2O), 3.76 (dd, J = 12.6, 11.1 Hz, 1H, 4a-H), 3.10 (td, J = 13.2, 3.3 Hz, 1H, 12b-H), 2.43 (dt, J = 14.4, 2.7 Hz, 1H, 1-Hα), 2.13 (s, 3H, COCH3), 2.09 (s, 3H, COCH3), 2.08 (s, 3H, COCH3), 1.90 (ddd, J = 14.4, 12.6, 2.7 Hz, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.7 (COCH), 20.8 (COCH), 21.0 (COCH), 26.4 (1-C), 34.1 (12b-C), 52.8 (4a-C), 64.1 (OCHCH2O), 64.8 (OCH2CHO), 67.3 (3-C), 68.5 (2-C), 71.8 (4-C), 103.7 (12-C), 104.5 (6a-C), 131.1 (7a-C), 132.2 (12a-C), 148.2 (7-C), 152.6 (11a-C), 169.1 (6-C), 169.3 (COCH3), 170.1 (COCH), 170.3 (COCH) ppm; IR (KBr):  = 3337, 2937, 1752, 1652, 1586, 1447, 1370, 1246, 1225, 1125, 1056, 1035, 858 cm−1; [α]D22 = − 58.0° (c = 0.5, CHCl3).

(±)-2-Benzoyloxy-11-methoxy-6-oxo-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-i]phenanthridine-3,4-diyl diacetate [(±)-46, C27H27NO10]

Yield: 18%; white solid (fluffy); m.p.: 237–238 °C; R = 0.51 (hexane/EtOAc, 1:2); 1H NMR (300 MHz, CDCl3): δ = 8.06 (dd, J = 8.1, 0.9 Hz, 2H, 2-HBz and 6-HBz), 7.60 (tt, J = 7.5, 1.2 Hz, 1H, 4-HBz), 7.47 (t, J = 7.5 Hz, 2H, 3-HBz and 5-HBz), 7.08 (s, 1H, 12-HAr), 6.37 (s, 1H, NH), 5.59 (t, J = 3.0 Hz, 1H, 3-H), 5.45 (q, J = 3.0 Hz, 1H, 2-H), 5.36 (dd, J = 10.8, 3.0 Hz, 1H, 4-H), 4.54–4.50 (m, 1H, OHCHCH2O), 4.39–4.23 (m, 3H, OHCHCHO), 3.92 (s, 3H, OCH3), 3.77 (dd, J = 12.0, 11.4 Hz, 1H, 4a-H), 3.19 (td, J = 12.0, 3.0 Hz, 1H, 12b-H), 2.64 (dt, J = 12.6, 3.0 Hz, 1H, 1-Hα), 2.11 (s, 6H, 2 × COCH3), 2.07–1.97 (m, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.7 (COCH), 20.8 (COCH), 27.0 (1-C), 35.5 (12b-C), 52.1 (4a-C), 56.2 (OCH3), 63.9 (OCHCH2O), 64.6 (OCH2CHO), 67.7 (2-C), 69.2 (4-C), 71.5 (3-C), 99.2 (6a-C), 110.6 (12-C), 128.7 (3-CBz and 5-CBz), 129.0 (1-CBz), 129.8 (2-CBz and 6-CBz), 132.5 (10a-C), 133.7 (4-CBz), 134.2 (12a-C), 145.5 (6b-C), 151.7 (11-C), 165.0 (6-C), 165.1 (COPh), 169.2 (COCH3), 170.4 (COCH3) ppm; IR (KBr):  = 3179, 3078, 2935, 1753, 1723, 1668, 1598, 1495, 1451, 1369, 1330, 1269, 1238, 1137, 1097, 1060, 715 cm−1.

(±)-3-Acetamido-6-benzoyloxy-4-(8′-methoxy-7′-methoxycarbonyl-2,3-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexane-1,2-diyl diacetate [(±)-48, C30H33NO12]

Yield: 8%; white solid; m.p.: 244–245 °C; R = 0.48 (CHCl3/acetone, 20:1); 1H NMR (300 MHz, CDCl3): δ = 8.05 (dd, J = 7.8, 1.2 Hz, 2H, 2-HBz and 6-HBz), 7.61 (tt, J = 7.5, 1.2 Hz, 1H, 4-HBz), 7.49 (t, J = 7.2 Hz, 2H, 3-HBz and 5-HBz), 6.72 (s, 1H, 5-HAr), 6.17 (d, J = 9.6 Hz, 1H, NH), 5.46 (t, J = 2.7 Hz, 1H, 2-HCy), 5.29 (q, J = 3.0 Hz, 1H, 1-H), 5.22 (dd, J = 10.8, 3.0 Hz, 1H, 3-HCy), 4.49 (q, J = 10.5 Hz, 1H, 4-H), 4.34–4.23 (m, 4H, OCH2CH2O), 3.81 (s, 3H, Ar-OCH3), 3.60 (s, 3H, COOCH3), 2.97 (td, J = 12.0, 4.2 Hz, 1H, 5-H), 2.25 (s, 3H, 2-CHOCOCH), 2.25–2.21 (m, 1H, 6-Hα), 2.17–2.13 (m, 1H, 6-Hβ), 2.01 (s, 3H, 3-CHOCOCH), 1.75 (s, 3H, NHCOCH) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.8 (3-CHOCOCH), 21.1 (2-CHOCOCH), 23.1 (NHCOCH), 32.4 (6-C), 38.8 (5-C), 51.5 (4-C), 52.2 (COOCH), 61.5 (Ar-OCH3), 64.1 (OCHCH2O), 64.3 (OCH2CHO), 69.0 (2-CCy), 69.4 (1-C), 72.0 (3-CCy), 110.8 (5-CAr), 121.3 (7-CAr), 128.6 (3-CBz and 5-CBz), 129.4 (1-CBz), 129.7 (2-CBz and 6-CBz), 130.3 (6-CAr), 133.6 (4-CBz), 135.9 (8a-CAr), 146.0 (8-CAr) 146.2 (4a-CAr), 164.6 (OCOPh), 169.1 (COOCH3), 169.6 (2-CHOCOCH3), 170.0 (NHCOCH3), 170.8 (3-CHOCOCH3) ppm; IR (KBr):  = 3368, 2951, 1755, 1731, 1675, 1608, 1541, 1509, 1442, 1374, 1337, 1274, 1221, 1168, 1128, 1110, 1069, 716 cm−1.

(±)-3-Acetamido-6-benzoyloxy-4-(8′-methoxy-5′-methoxycarbonyl-2,3-dihydrobenzo[b][1,4]dioxin-6′-yl)cyclohexane-1,2-diyl diacetate [(±)-49, C30H33NO12]

Yield: 13%; white solid (fluffy); m.p.: 109–112 °C; R = 0.40 (CHCl3/acetone, 20:1); 1H NMR (300 MHz, CDCl3): δ = 8.07 (dd, J = 7.8, 1.2 Hz, 2H, 2-HBz and 6-HBz), 7.62 (tt, J = 7.5, 1.2 Hz, 1H, 4-HBz), 7.50 (t, J = 7.2 Hz, 2H, 3-HBz and 5-HBz), 6.54 (s, 1H, 7-HAr), 5.83 (d, J = 9.9 Hz, 1H, NH), 5.48 (t, J = 3.0 Hz, 1H, 2-HCy), 5.27 (q, J = 3.0 Hz, 1H, 1-H), 5.25 (dd, J = overlapped and 3.3 Hz, 1H, 3-HCy), 4.66 (q, J = 10.8 Hz, 1H, 4-H), 4.32–4.22 (m, 4H, OCH2CH2O), 3.92 (s, 3H, Ar-OCH3), 3.53 (s, 3H, COOCH3), 3.09 (td, J = 12.0, 4.2 Hz, 1H, 5-H), 2.26 (s, 3H, 2-CHOCOCH), 2.26–2.22 (m, 1H, 6-Hα), 2.17–2.13 (m, 1H, 6-Hβ), 2.02 (s, 3H, 3-CHOCOCH), 1.74 (s, 3H, NHCOCH) ppm; 13C NMR (75 MHz, CDCl3): δ = 20.8 (3-CHOCOCH), 21.2 (2-CHOCOCH), 23.2 (NHCOCH), 33.2 (6-C), 39.2 (5-C), 50.6 (4-C), 52.2 (COOCH), 56.4 (Ar-OCH3), 64.2 (OCHCH2O), 64.4 (OCH2CHO), 69.1 (2-CCy), 69.5 (1-C), 71.8 (3-CCy), 102.0 (7-CAr), 116.2 (5-CAr), 128.6 (3-CBz and 5-CBz), 129.4 (1-CBz), 129.7 (2-CBz and 6-CBz), 130.3 (6-CAr), 131.8 (8a-CAr), 133.6 (4-CBz), 141.2 (4a-CAr), 150.2 (8-CAr) 164.6 (OCOPh), 168.5 (COOCH3), 169.6 (2-CHOCOCH3), 169.9 (NHCOCH3), 170.7 (3-CHOCOCH3) ppm; IR (KBr):  = 2948, 1750, 1729, 1670, 1541, 1493, 1456, 1371, 1332, 1276, 1239, 1160, 1112, 1069, 716 cm−1.

General procedure for the modified Zemplén’s deacetylation

To a solution of 84.2 mg (−)-44 (0.17 mmol), 78.7 mg (−)-52 (0.17 mmol), 78.7 mg (−)-53 (0.17 mmol), or 76.3 mg (−)-54 (0.17 mmol) in 12.5 cm3 anhydrous tetrahydrofuran, 6.50 cm3 0.53 M methanolic solution of sodium methoxide was added dropwise at rt and the reaction mixture was stirred also at rt for 2 h. Then Amberlyte IR-120 (strongly acidic resin) was added until the pH became 6. The solid resin was filtered and washed with 10 cm3 methanol, and then the filtrate was concentrated in vacuo. The crude product was purified by preparative TLC (EtOAc/ethanol, 6:1) to afford (−)-8, (−)-9, (−)-10, or (−)-11.

(−)-2,3,4-Trihydroxy-1,3,4,4a,5,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridin-6(2H)-one [(−)-8, C15H17NO6]

Yield: 71%; white solid (fluffy); m.p.: 162 °C; R = 0.52 (EtOAc/methanol, 20:1); 1H NMR (300 MHz, DMSO-d): δ = 7.30 (s, 1H, NH), 6.90 (s, 1H, 7-H), 6.77 (s, 1H, 12-H), 5.17–4.93 (m, 2H, 2 × OH), 4.93–4.79 (m, 1H, OH), 4.29–4.24 (m, 4H, OCH2CH2O), 3.91–3.84 (m, 1H, 2-H), 3.72–3.68 (m, 2H, 3-H and 4-H), 2.86 (td, J = 11.7, 3.0 Hz, 1H, 12b-H), 2.09 (dt, J = 13.5, 3.0 Hz, 1H, 1-Hα), 1.63 (ddd, J = 14.7, 11.7, 2.1 Hz, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, DMSO-d): δ = 28.7 (1-C), 34.2 (12b-C), 55.7 (4a-C), 64.4 (OCHCH2O), 64.9 (OCH2CHO), 69.1 (2-C), 70.2 (4-C), 72.2 (3-C), 112.8 (7-C), 116.5 (12-C), 123.1 (6a-C), 136.4 (12a-C), 142.3 (7a-C), 147.1 (11a-C), 164.7 (6-C) ppm; IR (KBr):  = 3446, 2910, 1716, 1683, 1580, 1509, 1473, 1374, 1315, 1225, 1148, 1076, 1034, 912, 889, 790 cm−1; [α]D22 = − 26.7° (c = 1, methanol); ee > 99%.

(−)-2,3,4,7-Tetrahydroxy-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridin-6(2H)-one [(−)-9, C15H17NO7]

Yield: 76%; white solid; m.p.: 174–176 °C; R = 0.76 (EtOAc/methanol, 4:1); 1H NMR (300 MHz, DMSO-d): δ = 13.0 (s, 1H, OH), 7.45 (s, 1H, NH), 6.25 (s, 1H, 12-H), 5.01–4.92 (m, 2H, 2 × OH), 4.89–4.82 (m, 1H, OH), 4.28–4.22 (m, 4H, OCH2CH2O), 3.91–3.83 (m, 1H, 2-H or 3-H or 4-H), 3.77–3.65 (m, 2H, 2 × 2-H or 3-H or 4-H), 2.81 (td, J = 12.3, 3.3 Hz, 1H, 12b-H), 2.10–1.98 (m, 1H, 1-Hα), 1.66–1.56 (m, 1H, 1-Hβ) ppm (the sign of 4a-H is covered by that of water in DMSO-d); 13C NMR (75 MHz, DMSO-d): δ = 27.8 (1-C), 33.1 (12b-C), 55.1 (4a-C), 63.4 (OCHCH2O), 64.4 (OCH2CHO), 68.4 (2-C), 69.5 (4-C), 71.6 (3-C), 102.8 (12-C), 104.4 (6a-C), 130.1 (8a-C), 134.4 (12a-C), 147.5 (7-C), 151.5 (11a-C), 169.7 (6-C) ppm; IR (KBr):  = 3421, 2926, 1647, 1626, 1587, 1448, 1400, 1362, 1281, 1230, 1126, 1064, 1030, 910, 811 cm−1; [α]D22 = − 18.5° (c = 0.13, ethanol); ee > 99%.

(−)-2,3,4-Trihydroxy-7-methoxy-1,2,3,4,4a,9,10,12b-octahydro[1,4]dioxino[2,3-j]phenanthridin-6(2H)-one [(−)-10, C16H19NO7]

Yield: 47%; white solid; m.p.: 163–168 °C; R = 0.39 (EtOAc/methanol, 4:1); 1H NMR (300 MHz, DMSO-d): δ = 6.91 (s, 1H, NH), 6.53 (s, 1H, 12-H), 5.10–4.91 (m, 1H, OH), 4.91–4.67 (m, 2H, 2 × OH), 4.31–4.21 (m, 4H, OCH2CH2O), 3.89–3.84 (m, 1H, 2-H or 3-OH or 4-H), 3.74 (s, 3H, OCH3), 3.72–3.65 (m, 2H, 2 × 2-H or 3-OH or 4-H), 3.17 (dd, J = 11.1, 10.5 Hz, 1H, 4a-H), 2.74 (td, J = 12.6, 3.6 Hz, 1H, 12b-H), 2.03 (dt, J = 12.9, 2.7 Hz, 1H, 1-Hα), 1.63–1.53 (m, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, DMSO-d): δ = 28.3 (1-C), 34.8 (12b-C), 54.5 (4a-C), 60.9 (OCH3), 63.6 (OCHCH2O), 64.2 (OCH2CHO), 68.5 (2-C), 69.4 (4-C), 71.5 (3-C), 107.3 (12-C), 116.1 (6a-C), 135.9 (10a-C), 136.1 (12a-C), 146.5 (6b-C), 149.4 (11-C), 162.6 (6-C) ppm; IR (KBr):  = 3392, 2927, 1717, 1652, 1475, 1331, 1226, 1122, 1068, 1039, 910 cm−1; [α]D22 = − 34.5° (c = 0.63, ethanol); ee > 99%.

(−)-2,3,4-Trihydroxy-11-methoxy-1,2,3,4,4a,8,9,12b-octahydro[1,4]dioxino[2,3-i]phenanthridin-6(2H)-one [(−)-11, C16H19NO7]

Yield: 47%; white solid; m.p.: 204–207 °C; R = 0.20 (EtOAc/methanol, 4:1); 1H NMR (300 MHz, DMSO-d): δ = 6.76 (s, 1H, NH), 6.45 (s, 1H, 12-H), 4.96 (d, J = 3.3 Hz, 1H, OH), 4.89 (d, J = 6.3 Hz, 1H, OH), 4.78 (d, J = 2.4 Hz, 1H, OH), 4.31–4.11 (m, 4H, OCH2CH2O), 3.91–3.86 (m, 1H, 2-H or 3-H or 4-H), 3.81 (s, 3H, OCH3), 3.73–3.65 (m, 2H, 2 × 2H or 3-H or 4-H), 3.19 (dd, J = 11.4, 10.2 Hz, 1H, 4a-H), 2.77 (td, J = 12.6, 3.3 Hz, 1H, 12b-H), 2.13 (dt, J = 13.2, 3.3 Hz, 1H, 1-Hα), 1.69–1.60 (m, 1H, 1-Hβ) ppm; 13C NMR (75 MHz, DMSO-d): δ = 28.3 (1-C), 35.0 (12b-C), 54.5 (4a-C), 55.6 (OCH3), 63.0 (OCHCH2O), 63.7 (OCH2CHO), 68.6 (2-C), 69.6 (4-C), 71.6 (3-C), 99.3 (12-C), 111.0 (6a-C), 131.5 (10a-C), 136.1 (12a-C), 144.5 (6b-C), 150.5 (11-C), 162.7 (6-C) ppm; IR (KBr):  = 3399, 2923, 1648, 1600, 1495, 1455, 1363 1329, 1131, 1068, 900 cm−1; [α]D22 = − 66.5° (c = 0.25, ethanol); ee > 99%.

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Supplementary material 1 (PDF 11859 kb)