Use of chiral-pool approach into epi-thieno analogues of the scarce bioactive phenanthroquinolizidine alkaloids.

The stereoselective synthesis of epi-thieno analogues of the phenanthroquinolizidine bioactive alkaloids (-)-Cryptopleurine and (-)-(15R)-Hydroxycryptopleurine was achieved in five steps starting from easily available enantiopure (S)-2-aminoadipic acid used as chiral pool and nitrogen atom source. During these investigations, both π-cationic cyclization of chiral N-thienylmethyl-6-oxopipecolinic acids into pure (S)-keto-lactams and theirs regioselective and diastereoselective reduction, considered as key steps of this sequence, were studied. Of particular interest, the Friedel-Crafts cyclization using (CF3CO)2O/BF3·Et2O show that near the expected keto-lactams, enamides and enamidones containing trifluoromethyl residue were isolated. A mechanism leading to the latter products with high synthetic potential was discussed.

Introduction

2,3-Fused quinolizidine framework is an important motif which is found in numerous compounds among which the scarce phenanthroquinolizidines extract from three plants families. The small family of pentacyclic alkaloids (Scheme 1 ), is represented by the bioactive (−)-Cryptopleurine (R=H, 1),1a (−)-(15R)-Hydroxycryptopleurine (R=OH, 2),(a), (b) Cryptopleuridine (3),1c Boehmeriasin-A (R=OMe, 4),1d and Boehmeriasin-B (R=OH, 5).1d

Scheme 1

Structure of the phenanthroquinolizidine alkaloids, their aromatic analogues and our targeted products 10 and 11.

Based on the success of similar alkaloids with simpler skeleton, the phenanthroindolizidines, most of which demonstrate remarkable biological and pharmacological profiles, anticancer notably. In the light of these results, numerous efforts were undertaken to carry out quantitative structure–activity relationship (QSAR) studies (Scheme 1). The results of certain phenanthroquinolizidine derivatives show inhibitory activity in three human cancer cell lines with appreciable IC50 (104–130 nM), the potential to treat coronavirus infection, good to excellent in vivo antiviral activity against tobacco mosaic virus (TMV) and anti-proliferative and selective antitumor properties. While these studies were based essentially on Cryptopleurine (1) as the alkaloid model, the more recent Boehmeriasin-A (4) based compounds have shown in in vitro study anti-proliferative activity in three cancer cell lines (CEM, HeLa, and L1210) and in two endothelial cell lines (HMEC-1, BAEC) at concentration near the nanomolar range. Interestingly, during these studies, topoisomerases and SIRT2 were identified to be biological targets of these structures.

Out of such considerations, the synthesis of these natural products and derivatives has been an appealing area of research. Most of the hitherto reported approaches into these types of compounds rely on a small number of strategies; the majority of which centers on the construction of the aza-six-membered ring E from cheap educts, which used subsequently as a chiral pool and a nitrogen atom source (Scheme 1). The ring closure of the central ring D proceeded then by Friedel–Crafts cyclization as pioneered by Rapoport et al.,6, 7 the Parham-type cycloacylation and intramolecular aldol-type condensation.6, 9 Beyond other racemic experimental protocols an alternative and interesting approach based on the construction first of chiral polycyclic systems containing piperidine-2-methanol (A–D) was also developed. These key intermediates, when involved in a ring-closing metathesis afford a way to vary the nature and the size of the cycle at the end of the targets enchainment E and are used to produce the molecular diversity.

Curiously, no QSAR studies based on the modulation at the aromatic phenanthrene residue of these alkaloids have been described to this day. Along this line of reasoning, small aromatic analogues of these described phenanthroquinolizidines have been used as intermediates in the synthesis of the potent anticancer Saframycin-A (6)7b and precursors of enantiopure and non-proteogenic aromatic-δ-amino acids (7).

Other structures, exemplified by compounds 8 and 9 were also found procreative. Their preparation is based on an imine Michael reaction followed by a radical cyclization and intramolecular Schmidt reaction of acyl chlorides with alkyl azides, the last proceeding via N-acyliminium species as intermediates.

In keeping with our longstanding interest in the synthesis of indolizidinones and quinolizidinones fused to sulfur heterocyclic systems, and their use for the elaboration of alkyl(or aryl) bicyclic iminosugars and their salts, we herein report our findings on synthesis and properties of chiral thienoquinolizidindiones starting from chiral 6-oxopiperidine-carboxylic acid (6-oxopipecolinic acid). Their sequential stereoselective reduction led to the targeted thieno analogues of the bioactive phenanthroquinolizidine alkaloids 10 (R=OH) and 11 (R=H) (Scheme 1).

Results and discussion

Preparation of N-thienylmethyl-6-oxopipecolinic acids 14a,b

We firstly planned access to the requisite N-thienylmethyl-6-oxopipecolinic acids 14a,b for the Rapoport cyclization by the known protocol.7, 17 This comprises N-reductive amination by mixing commercially available (S)-2-aminoadipic acid (12) and 2(or 3)-thienaldehyde in aqueous alkaline media (NaOH 2 M), addition of sodium borohydride to the same media, followed by intramolecular N-acylation of the resulting N-thienylmethyl-aminoadipic acids 13a,b (Scheme 2 ).

Scheme 2

Synthesis scheme of oxopipecolinic acids 14a,b precursors of the π-cationic cyclization.

The cyclization of dicarboxylic acids 13a,b was performed by refluxing in ethanol for 8 h. To our surprise and contrary to the reaction profile obtained in the literature for similar sequences,7, 17 the reaction was incomplete (after cooling a small amount of unreacted (S)-2-aminoadipic acid (12) was obtained) and we observed after processing of the reaction mixture the expected carboxylic acids 14a,b (in 79% and 76% isolated yields, respectively) in addition to N-substituted amino acids 13a,b in 9/1 ratio. Complete cyclization of 13a,b to 14a,b was carried out by heating in water at reflux for 4 h.

Interestingly, the structure of both carboxylic acids 14a and 14b was secured by an X-ray crystallographic analysis (see also the ORTEP drawing of 14a,b in the ESI part). Hence the possible epimerisation at the α-position of the cyclic lactam-acid under alkaline and thermal conditions can be excluded.

π-Cyclization of N-thienylmethyl-6-oxopipecolinic acids 14a,b

In the first set of Friedel–Crafts cyclization attempts of N-thienylmethyl-6-oxopipecolinic acids 14a,b, standard Rapoport conditions were used.7a In stereospecific syntheses of the bioactive alkaloids (+)-Tylophorine and the piperidine analogue (−)-Cryptopleurine the reaction conditions were optimized. To our surprise, no reaction occurred, rather a complete polymerization of the reactants was observed under all reaction conditions tried.

Standard Rapoport conditions having failed to produce the expected tricyclic keto-lactams 15a,b, other cyclization conditions of 14a,b were then investigated in the hope to gain some insights in this cyclization and optimize the process for further general applications.

Thus under similar conditions (Method A) starting from carboxylic acid 14a, but with AlCl3 instead of SnCl4 as catalyst at 10–40 °C then 0–40 °C for 2 h after the addition of the catalyst, the cyclization reaction yields the expected tricyclic product 15a in 32% yield. Similarly, the carboxylic acid 14b led to the keto-lactam 15b in a yield up to 42%. The change of oxalyl chloride for thionyl chloride resulted in the erosion of the reaction yield both in the case of 15a (29%) and 15b (31%). It is worth mentioning that these cyclization conditions are effective in producing thienoindolizidindiones17, (a), (c) and benzothienoindolizidindiones(b), 19 in high yields (of ≈75%), which is absolutely not the case in the thienoquinolizidindiones series.

For the above reason, another method B based on the use of the combination of (CF3CO)2O and BF3·Et2O to promote the Friedel–Crafts cyclization was attempted. Interestingly, under these cyclization conditions (rt, 12 days), two compounds 15a,b (55.5 and 59.4%, respectively) and 16a,b (7.3 and 9.6%, respectively) were detected in a better result than method A (Scheme 3 ). Similar yields of 16a,b and 15a,b were obtained when the reaction was carried out without solvent in clean TFAA and BF3-etherate. In fact, when the reaction was carried out at reflux, except for keto-lactams of 15a (53.0%), 15b (57.3%) and enamides 16a (5.2%) and 16b (5.8%), unexpected trifluoroenamidones 17a,b were also isolated in very low yields (Scheme 3). Under various reaction conditions the acids 14a,b led in addition to ketones 15a,b also to enamides 16a,b with yields culminated at 9.6% in both cases; the yields of trifluoroenamidones 17a,b were not affected. Apparently, 17a,b are formed from 16a,b at higher temperature by an independent reactions with TFAA (Scheme 4 ), as demonstrated experiments at 60 °C in a pressurized tube, 16a,b both giving a near quantitative yield of 17a,b.

Scheme 3

π-Cationic cyclization of N-thienylmethyl-6-oxopipecolinic acids 14a,b.

Scheme 4

A plausible mechanism of the formation of enamides 16a,b and enamidones 17a,b from carboxylic acid 14a,b.

Furthermore, with polyphosphoric acid (PPA) at 100–105 °C under conditions C,15, 17 carboxylic acids 14a,b provided the thienoquinolizidindiones 15a,b as sole reaction products in yields of 41% and 52%, respectively. In addition to the best yields observed in the cyclization reaction, the latter is easy to implement, the reaction time being very short, but continuation of the reaction resulted in drastic decrease of reaction yields. Ultimately, the use of Eaton's reagent (P2O5/MeSO3H: 1/10 w/w) at 90 °C for 14a and 75 °C for 14b according to the conditions D used by Rigo et al., afforded the cyclized products 15a and 15b after 1 or 1.5 h of the reaction in good yields of 71% and 78%, respectively.

The formation of the N-substituted lactams 16a,b can be explained by the formation of the acylium cations Ia,b, followed by the elimination of CO and the deprotonation of the stable N-acyliminium species IIa,b. Trifluoroacetylen-amidones 17a,b were then formed by the nucleophilic substitution reaction of the trifluoroacetic anhydride with enamides 16a,b (Scheme 4).

Interestingly, when the carboxylic acid 14a was subjected to trifluoroacetic anhydride alone at 60 °C for 16 h, only keto-lactam 15a and enamidone 17a were isolated in 55.4% and 18.6%, respectively. Finally, the chiral integrity of the stereogenic centre during the Friedel–Crafts cyclization was secured by checking the 1H and 13C NMR spectra of the enantiopure (S)-15a and corresponding racemate (±)-15a in the presence of the chiral shift reagent Eu(hfc)3.

Diastereoselective reduction of thienoquinolizidindiones 15a,b

According to previous reports on the stereoselective reduction of indolizidindiones containing thiophene17, (a), (c) or benzothiophene ring into corresponding enantiopure alcohols, reduction of ketones 15a and 15b was examined by using a variety of hydride reagents (Scheme 5 ).

Scheme 5

Diastereoselective reduction of tricyclic ketones (S)-15a,b. Hydride reagents used are NaBH4 with or without additive (NiCl2, CeCl3), LiBH4, Red-Al, DIBAL, l-Selectride or k-Selectride at different reaction temperatures.

Thus, we assume that in each case the simple borohydride and aluminohydride reductions of the ketones 15a and 15b afforded diastereomeric mixture of alcohols trans-10Aa,b and cis-10Aa,b although trans-alcohols 10A were largely favored.

It is clear that the selective reduction of the ketones 15a,b with borohydride reagents reflects a preference for an axial hydride attack from the more hindered endo face of the tricyclic system. For instance, the NaBH4 reduction at 0 °C in methanol gave a mixture of trans-10Aa,b and cis-10Aa,b diastereoisomers in the ratio of 90/10 with 73% yield for 10Aa and 81% yield for 10Ab. The same reaction but conducted at −80 °C, afforded a 95/5 mixture of trans/cis, while an addition of complexing agents such as CeCl3 (Luche reagent) resulted in the same 95/5 ratio, thus demonstrating the ineffectiveness of the Lewis acid such as CeCl3 used herein. A high diastereospecificity (trans/cis=99/1) in favor of the trans-10Aa diastereoisomer was reached when NaBH4 was combined with NiCl2 at −80 °C.

Conversely, the use of Red-Al or DIBAL at the same temperature of −75 °C gave a mixture of trans/cis diastereoisomers in the ratio of 90/10 in both cases, but in lower yields not exceeding 51% in the best case. While the stereochemistry distributions was in accord with our earlier investigations in indolizidindiones containing thiophene and benzothiophene nucleus,15, 17, 19 it was significantly different with that reported by Rapoport during the synthesis of (−)-Cryptopleurine (1), with only one exception.7a The stereochemical assignments of these alcohols are based on analysis of the 1H NMR spectra which show the interaction constants of J=9.3 Hz for the trans-isomer 10Aa and J=9.5 for the trans-isomer 10Ab.

The stereoselectivity can be reversed by using a bulkier hydride reagent that can block axial approach of the hydride to the benefit of the equatorial one, thus being more in line with the results obtained during the investigations of Rapoport.7a Thus, use of bulkier hydride reagents such as l-Selectride and k-Selectride solution in THF afforded alcohols cis-10Aa and cis-10Ab as major products. In both cases, the reaction conducted at −80 °C is more stereoselective (18/82 and 10/90) compared to the reaction taking place at −40 °C (29/71 and 19/81).

Ultimately, pure diastereomers cis-10Aa and cis-10Ab were isolated by simple crystallization from EtOAc in the yields of 62% and 60%, respectively. Further, the stereochemical assignments of these cis-alcohols are based on analysis of their 1H NMR spectra which show the interaction constants for cis-10Aa isomer J=1.1 Hz and J=1.8 Hz for cis-10Ab isomer. Elsewhere, attempts to convert alcohol trans-10Aa into the expected alcohol cis-10Aa by alternative Mitsunobu inversion were unsuccessful.

Accessing the targeted thienoquinolizidinols 10 and thienoquinolizidines 11

With the ultimate objective to prepare enantiopure thieno analogues 11Ba,b 15a of the naturally occurring (−)-Cryptopleurine (1) and (−)-(15R)-Hydroxycryptopleurine (2), the formal reduction of lactam carbonyl group and/or OH function into corresponding alkane was envisioned as highlighted in Scheme 6 .

Scheme 6

Synthesis of quinolizidinols trans-10Ba,b and cis-10Ba,b. Reagents and conditions: (i) Ac2O, Et3N, DAMP, CH2Cl2, rt, TLC. (ii) LAH, THF, reflux, 2.5 h. (iii) ET3SiH, TFA.

In this perspective, the amido-alcohols trans-10Aa,b were acetylated using standard conditions (Scheme 6). Thus, reaction with acetic anhydride in the presence of dry triethylamine and catalytic amounts of DMAP led to the acetoxy derivatives trans-18Aa,b in yields of 74% ((4R,4aS)-18Aa) and 72% ((9aS,10S)-18Ab), respectively. The structure of trans-18Ab was learned from crystallographic analysis thus confirming the structure integrity during this transformation (See Fig. 1 for the ORTEP drawing). Ultimately, these trans-derivatives were efficiently converted into the expected quinolizidinols trans-10Ba,b in good yields (71% and 74%) by using LAH reduction in refluxing THF for 1 h according to Green's protocol. The latter was developed for stereoselective synthesis of the alkaloids (−)-2-Epilentiginosine and (+)-Lentiginosine. With this sequence in hand, alcohols cis-10Aa,b were then converted to acetoxy derivatives cis-18Aa,b in yields of 72% ((4S,4aS)-18Aa) and 71% ((9aS,10R)-18Ab), respectively, which finally provided quinolizidinols cis-10Ba,b in 71.7% and 71.5% yield, respectively.

Fig. 1

ORTEP drawing of trans-18Ab.

The first attempts to remove the OH function from enantiopure cis- and trans-10Ba,b substrates with triethylsilane in TFA have failed to provide the targeted quinolizidines (S)-11Ba,b (Scheme 6). Taking into account these unsatisfactory results, we turned then our attention first to reduce the OH function into corresponding alkane residue in the presence of the lactam function followed by its reduction in the ultimate stage (Scheme 7 ).

Scheme 7

Synthesis of the targeted quinolizidines (S)-11Ba,b. Reagents and conditions: (i) TFA, Et3SiH, rt, 12 h. (ii) LAH, THF, reflux, 2.5 h.

Thus, treating of alcohols trans-10Aa,b with TFA in the presence of triethylsilane hydride led to the lactams 11Aa and 11Ab in 72% and 87% yield, respectively, after flash chromatography purification and recrystallization. Finally, the synthesis of the thieno analogues of the naturally occurring cryptopleurines 1 and 2 was achieved with LAH reduction of the lactam function in refluxing THF. The reduction reaction occurred cleanly within only 1 h to provide the optically pure title compounds (S)-4a,5,6,7,8,10-hexahydro-4H-thieno[3,2-b]quinolizine (11Ba) in 65% yield and its regioisomer (S)-6,7,8,9,9a,10-hexahydro-4H-thieno[2,3-b]-quinolizine (11Bb) in 71% yield after recrystallization from dry i-hexane.

The formation of racemic thieno analogues (±)-11Ba and (±)-11Bb of the bioactive alkaloids Cryptopleurine (1) and Boehmeriasin-A (4), were reported from our laboratory in high yilelds. The sequence used consisted of dehydration of thienoquinolizidinols (±)-10Ba,b in refluxing mixture of acetic acid/perchloric acid, followed by borohydride reduction of the resulting and non-isolated thienoquinol-izidinium salts.

Conclusions

We carried out stereoselective synthesis of epi-thieno analogues of the phenanthroquinolizidine bioactive alkaloids (−)-Cryptopleurine and (−)-(15R)-Hydroxycryptopleurine in five steps starting from available enantiopure (S)-2-aminoadipic acid used as chiral pool and nitrogen atom source. During these investigations, we envisaged both π-cationic cyclization of chiral N-thienylmethyl-6-oxopipecolinic acids into pure (S)-keto-lactams and their regioselective and diastereoselective reduction. The latter reductions, considered as key steps of this sequence, were investigated in order to get some insights necessary for further applications in the syntheses of these types of compounds. The overall yield of these transformations of approximately 19% in the series a and 27% in the series b outlines the higher reactivity at the C 2-position of the thiophene ring (series a) compared to its C 3-position (series b) during the important cyclization step.

In particular, during Friedel–Crafts reaction, we have shown that the protocol using Eaton's reagent is superior in terms of yields. On the other hand, the approach using the combination of (CF3CO)2O/BF3·Et2O is also very interesting since apart from expected keto-lactams, enamides and enamidones containing trifluoromethyl residue were isolated. A mechanism leading to the latter products, which have high synthetic potential, was discussed.

After screening of different reducing agents, the diastereoselective reduction of the ketone function of the pure tricyclic keto-lactams 15 was achieved to provide both cis-10Aa,b and trans-10Aa,b alcohol-lactams in good yields. Thienoquinolizidinols cis-10Ba,b and trans-10Ba,b were reached by lactamic carbonyl reduction in tandem with the acetate deprotection, but their transformation into the targeted epi-thienoquinolizidine analogues (S)-11Ba,b using various reducing agents failed in all attempts. However, this operation could be conducted efficaciously starting from alcohol-lactams cis-10Aa,b and trans-10Aa,b by first reducing the alcohol into enantiopure tricyclic lactams (S)-11Aa,b followed by their reduction at the lactamic ketone.

Experimental section

General remarks

Melting points were obtained using a Boetius apparatus and are corrected. Commercial reagents were used without further purification. All solvents were distilled before use. Flash column liquid chromatography (FLC) was performed on silica gel Kieselgel 60 (40–63 μm, 230–400 mesh) and analytical thin-layer chromatography (TLC) was performed on aluminum plates pre-coated with either 0.2 mm (DC-Alufolien, Merck) or 0.25 mm silica gel 60 F254 (ALUGRAM-SIL G/UV254, Macherey-Nagel). The compounds were visualized by UV fluorescence and by dipping the plates in an aqueous H2SO4 solution of cerium sulfate/ammonium molybdate followed by charring with a heat gun. HPLC analyses were performed on Varian system 9012 with diode array Varian 9065 polychrom UV detector: column CC 250/3 Nucleosil 120-5 C18, 250×3 mm (Macherey Nagel). Mobile phase: solvent A: water/acetonitrile/methanesulfonic acid (1000/25/1), solvent B: water/aceto-nitrile/methanesulfonic acid (25/1000/1), elution mode: gradient with 5–50% solvent B, flow rate: 0.65 mL/min, UV detection: 210 nm (DAD), 35 °C, 20 min. GC–MS analyses were performed on GC–MS Varian Saturn 2100 T, ion trap MS detector, 70 eV. Column: Varian, FactorFour capillary column VF -5 ms 30mx0.25 mm ID, DF=0.25. Optical rotations were measured with a POLAR L-lP polarimeter (IBZ Messtechnik) with a water-jacketed 10.000 cm cell at the wavelength of sodium line D (λ=589 nm). Specific rotations are given in units of 10−1 deg cm2 g−1 and concentrations are given in g/100 mL. Infrared spectra were recorded on a Nicolet 5700 FTIR spectrometer as KBr discs (KBr) or as thin films on KBr plates (film). NMR spectra were recorded on an Inova 600 Varian spectrometer in CDCl3. Chemical shifts (δ) are quoted in ppm and are referenced to the tetramethylsilane (TMS) as internal standard. The qCOSY, NOESY and DIFFNOE techniques were used in assignment of 1H–1H relationships and the determination of relative configuration. The qHSQC and qHMBC techniques were used throughout for the assignment of the 1H–13C relationships. The elemental analyses were carried out by the microanalysis laboratory of INSA, F-76130 Mt. St. Aignan, France. High-resolution spectrometry was performed on Micromass Q-Tof Micro MS system with ESI+ ionization (measured mass represents M+H+).

(S)-4a,5,6,7-Tetrahydro-4H-thieno[3,2-b]quinolizine-4,8(10H)-dione (15a)

Method A

Oxalyl chloride (1.74 mL, 2.1 mmol) was added at 10 °C to a solution of a freshly crystallized piperidine-carboxylic acid 14a (2.39 g, 10 mmol) in dry dichloromethane (50 mL). The mixture was stirred at 40 °C for 30 min, cooled with an ice bath, and then AlCl3 (3.33 g, 2.5 mmol) was added slowly by keeping temperature below 0 °C. After stirring at 20 °C for 2 h dichloromethane was added (20 mL), and then ice (20 g) and water (20 mL) were added to quench AlCl3. The two phases were separated and the aqueous layer was extracted with dichloromethane (2×20 mL), then was dried with MgSO4 and concentrated. The resulting crude yellow product (0.98 g, 44.3%) was purified by flash column chromatography (dichloromethane) to yield keto-lactam 15a. Recrystallization from cyclohexane gave pure 15a (0.71 g, 32%) as a light yellow crystal; mp 120.2–120.7 °C; [α]D 22=+2.73 (c 1.06, MeOH); R =0.53 (CH2Cl2/Acetone, 3/1); IR (ν, cm−1, KBr): 3078, 2952, 2873, 1677, 1628, 1523, 1459, 1402, 1328, 1249, 1180, 1153, 1095, 1042, 964, 910, 889, 866, 833, 814, 739, 725, 655, 626, 612, 496, 491, 427. 1H NMR (600 MHz, CD3OD): δ 7.36 (s, 2H), 5.96 (d, 1H, J=17.4 Hz), 4.31 (t, 1H, J=5.9 Hz), 4.28 (d, 1H, J=17.5 Hz), 2.17 (dddd, 2H, J=13.7, 9.7, 6.0 and 3.2 Hz), 1.80 (ddt, 2H, J=10.5, 6.0 and 3.6 Hz), 1.76–1.69 (m, 2H). 13C NMR (150 MHz, CD3OD): δ 189.84, 171.79, 153.67, 136.62, 126.26, 125.30, 63.64, 42.13, 33.21, 23.34, 19.75. HRMS calcd for C11H11NO2S (221.28) [M+1]+: 222.0510, found 222.0504.

(S)-7,8,9,9a-Tetrahydro-4H-thieno[2,3-b]quinolizine-6,10-dione (15b)

This product was obtained from 14b (3.58 g, 15 mmol), oxalyl chloride (2.61 mL, 3.15 mmol) and AlCl3 (5.0 g, 3.75 mmol) in the same way as for 15a, yield 1.39 g, 42%; mp 120.5–121.4 °C, [α]D 22=−3.97 (c 1.0, MeOH); R =0.42 (CH2Cl2/Acetone, 3/1); IR (ν, cm−1, KBr): 3124, 2962, 2860, 1670, 1636, 1537, 1447, 1427, 1414, 1326, 1314, 1237, 1213, 1172, 1095, 1020, 922, 840, 741, 629, 524, 490, 415. 1H NMR (600 MHz, CD3OD): δ 7.93 (d, 1H, J=5.0 Hz), 7.17 (d, 1H, J=5.0 Hz), 5.84 (d, 1H, J=17.3 Hz), 4.35 (t, 1H, J=5.8 Hz), 4.12 (d, 1H, J=17.3 Hz), 2.41 (tdq, 3H, J=17.5, 11.9 and 5.8 Hz), 2.18 (dddd, 1H, J=13.7, 10.0, 6.1 and 3.3 Hz), 1.80 (ddt, 1H, J=10.5, 6.2 and 3.7 Hz), 1.77–1.72 (m, 1H). 13C NMR (150 MHz, CD3OD): δ 188.86, 172.01, 151.07, 137.15, 135.21, 127.51, 63.86, 43.08, 33.24, 23.47, 19.74. HRMS calcd for C11H11NO2S (221.28) [M+1]+: 222.0510, found 222.0506.

1-(Thien-2-ylmethyl)-3,4-dihydropyridin-2(1H)-one (16a) and 1-(thien-3-ylmethyl)-3,4-dihydropyridin-2(1H)-one (16b)

Method B (i, room temperature)

A stirred mixture of a freshly crystallized acid 14a (1.0 g, 4.18 mmol) and trifluoroacetic anhydride (1.2 mL, 8.35 mmol) in dry 1,2-dichloroethane (20 mL) was stirred for 45 min, then cooled with an ice bath and boron trifluoride etherate (4.75 mL, 38.4 mmol) was added. After stirring at 20 °C for 12 days under nitrogen atmosphere solvents were evaporated, then a saturated solution of K2CO3 in water (150 mL) was added and the mixture was stirred at room temperature for 2 h. The aqueous phase was extracted with dichloromethane; the organic phase was washed with water, dried (Na2SO4). Evaporation of the solution afforded a dark oil as a mixture of 15a and 16a, which was purified by chromatography (20 mm×30 cm, 80 g, CH2Cl2/acetone 20/1) on silica gel column to provide finally 16a 59 mg (7.3%) as a light yellow oil and 15a (510 mg, 55.5%) as a pale powder.

1-(Thien-2-ylmethyl)-3,4-dihydropyridin-2(1H)-one (16a)

IR (ν, cm−1, KBr): 3305, 2951, 1616, 1472, 1441, 1412, 1363, 1329, 1272, 1187, 1078, 1039, 1010, 956, 849, 701, 553. 1H NMR (600 MHz, CD3OD): δ 7.38–7.13 (m, 1H), 6.96 (ddd, 1H, J=7.7, 3.6 and 1.7 Hz), 6.10 (dd, 1H, J=7.8 and 1.4 Hz), 5.16 (dt, 1H, J=8.1 and 4.2 Hz), 2.55 (t, 2 H, J=8.0 Hz), 2.32 (qt, 2H, J=6.9 and 1.9 Hz). 13C NMR (150 MHz, CD3OD): δ 169.10, 139.69, 128.89, 126.72, 126.41, 125.46, 106.74, 43.82, 31.30, 20.31. HRMS calcd for C10H11NOS (193.27) [M+1]+: 194.0561, found 194.0539.

1-(Thien-3-ylmethyl)-3,4-dihydropyridin-2(1H)-one (16b)

This product was obtained from 14b (1.0 g, 4.18 mmol), trifluoroacetic anhydride (1.2 mL, 8.35 mmol) and boron trifluoride etherate (4.75 mL, 38.4 mmol) in dry 1,2-dichloroethane (20 mL) at room temperature for 12 days in the same way as for 16a. Two compounds were isolated, 15b (550 mg, 59.4%) and compound 16b as light yellow oil (77.5 mg, 9.6%). IR (ν, cm−1, KBr): 3305, 3097, 2949, 1613, 1471, 1441, 1410, 1329, 1268, 1229, 1170, 1077, 1013, 961, 891, 847, 830, 793, 759, 723, 694, 628, 573, 552, 481. 1H NMR (600 MHz, CD3OD): δ 7.29 (ddt, 1H, J=4.9, 3.0 and 0.4 Hz), 7.13 (1H, ddt, J=3.0, 1.3 and 0.8 Hz), 7.00 (ddt, 1H, J=4.9, 1.3 and 0.4 Hz), 6.06 (dt, 1H, J=7.7 and 1.6 Hz), 5.15 (dtt, 1H, J=7.7, 4.4 and 0.5 Hz), 4.67 (dd, 2H, J=0.9 and 0.4 Hz), 2.63–2.32 (m, 2H), 2.32 (dtdd, 2H, J=7.9, 4.4, 1.6 and 0.9 Hz). 13C NMR (150 MHz, CD3OD): δ 168.71, 137.73, 128.67, 127.14, 126.01, 122.03, 105.99, 43.83, 30.91, 19.91. HRMS calcd for C10H11NOS (193.27) [M+1]+: 194.0561, found 194.0541.

1-(Thiophen-2-ylmethyl)-5-(2,2,2-trifluoroacetyl)-3,4-dihydropyridin-2(1H)-one (17a) and 1-(thiophen-3-ylmethyl)-5-(2,2,2-trifluoroacetyl)-3,4-dihydropyridin-2(1H)-one (17b)

Method B (ii, reflux)

This reaction was carried out starting from carboxylic acid 14a (1.0 g, 4.18 mmol), trifluoroacetic anhydride (1.2 mL, 8.35 mmol) and boron trifluoride etherate (4.75 mL, 38.4 mmol) in dry 1,2-dichloroethane (35 mL) at reflux for 6 h. Three compounds 17a (27 mg, 2.3%), 16a (42 mg, 5.2%) and 15a (490 mg, 53.0%) were isolated from the reaction mixture via silica gel chromatography eluting with a gradient of 0–20 percent of acetone in DCM.

1-(Thiophen-2-ylmethyl)-5-(2,2,2-trifluoroacetyl)-3,4-dihydropyridin-2(1H)-one (17a)

This compound was isolated as colorless oil. 1H NMR (600 MHz, CDCl3): δ 7.55 (q, 1H, J=1.0 Hz), 7.30 (dd, 1H, J=5.1 and 1.3 Hz), 7.05 (ddt, 1H, J=3.5, 1.4 and 0.7 Hz), 6.99 (dd, 1H, J=5.1 and 3.5 Hz), 4.97 (d, 2H, J=0.8 Hz), 2.77–2.59 (m, 4H). 19F NMR (600 MHz, CDCl3): δ −69.89 (s); 13C NMR (150 MHz, CDCl3): δ 178.22 (q, 2 J C,F=34.3 Hz, CF3–CO), 168.87, 144.92 (q, 3 J C,F=4.6 Hz), 137.32, 127.71, 127.22, 126.67, 116.66 (q, 1 J C,F=291.1 Hz, CF3–CO), 111.98, 45.35, 30.21, 18.71. HRMS calcd for C12H10F3NO2S (289.27) [M+1]+: 290.0384, found 290.0359.

1-(Thiophen-3-ylmethyl)-5-(2,2,2-trifluoroacetyl)-3,4-dihydropyridin-2(1H)-one (17b)

This product was obtained from 14b (1.0 g, 4.18 mmol), trifluoroacetic anhydride (1.2 mL, 8.35 mmol) and boron trifluoride etherate (4.75 mL, 38.4 mmol) in dry 1,2-dichloroethane (35 mL) at reflux for 6 h. Three compounds 17b (31 mg, 2.6%), 16b (47 mg, 5.8%) and 15b (530 mg, 57.3%) were isolated via silica gel chromatography eluting with a gradient of 0–20 percent of acetone in CH2Cl2. 1-(Thiophen-3-ylmethyl)-5-(2,2,2-trifluoro-acetyl)-3,4-dihydropyridin-2(1H)-one ( ). This compound was isolated as pale yellow oil. 1H NMR (600 MHz, CDCl3): δ 7.50 (d, 1H, J=1.5 Hz), 7.30 (dd, 1H, J=5.0 and 2.9 Hz), 7.20 (td, 1H, J=5.0, 2.9 and 0.8 Hz), 6.96 (dd, 1H, J=5.0 and 1.4 Hz), 4.78 (s, 2H), 3.10–2.27 (m, 4H). 19F NMR (600 MHz, CDCl3): δ −69.92 (s); 13C NMR (150 MHz, CDCl3): δ 177.43 (q, 2 J C,F=34.3 Hz, CF3–CO), 169.12, 145.42 (q, 3 J C,F=4.4 Hz), 136.06, 127.27, 126.96, 123.95, 116.66 (q, 1 J C,F=291.1 Hz, CF3–CO), 111.66, 45.82, 30.19, 18.68. HRMS calcd for C12H10F3NO2S (289.27) [M+1]+: 290.0384, found 290.0348.

Method B (iii, without solvent)

To a stirred mixture of a freshly crystallized acid 14a or 14b (0.5 g, 2.09 mmol) and trifluoroacetic anhydride (3.0 mL, 20.9 mmol, 10 equiv) was added in one portion boron trifluoride etherate (2.84 mL, 23 mmol). After stirring at 20 °C for 20 h under nitrogen atmosphere the solid was filtered (1H NMR indicated formation of pure ketone 15a, 260 mg, 56.1% or 15b, 260 mg, 56.2%), the remaining solvents were evaporated and saturated solution of K2CO3 in water (60 mL) added and the mixture stirred at room temperature for 10 min. The aqueous phase was extracted with dichloromethane (3×35 mL); the organic phase was washed with water, dried (Na2SO4). Evaporation of the solution afforded a dark oil as a mixture of 15a and 16a or 15b and 16b which was purified by chromatography (15 mm×30 cm, 80 g, CH2Cl2, CH2Cl2/acetone 20/1) on silica gel column to provide finally 16a (39 mg, 9.7%) as a light yellow oil and 15a (67 mg, 14.5%, total yield of 15a was 297 mg, 70.6%) or 16b (39 mg, 9.7%) as a colorless oil and 15b (84 mg, 18.2%, total yield of 15b 344 mg, 74.4%).

Method B (16a,b to 17a,b)

A stirred mixture of a freshly chromatographed 16a or 16b (250 mg, 1.29 mmol) and trifluoroacetic anhydride (2 mL, 15.18 mmol) was heated in a sealed tube at 60 °C. After stirring for 20 h sealed tube was cooled to room temperature, the solvent was evaporated, saturated solution of K2CO3 in water (10 mL) was added and the mixture stirred at room temperature for 10 min. The aqueous phase was extracted with dichloromethane (3×10 mL); the organic phase was washed with water, dried (Na2SO4). Evaporation of the solution afforded a yellow oil (310 mg), which was purified by chromatography (10 mm×30 cm, 50 g, CH2Cl2) on silica gel column to provide finally 17a (299 mg, 80%) as a pale yellow oil or 17b (310 mg, 83%) as colorless oil.

Method C

A mixture of freshly crystallized carboxylic acid 14a (1.5 g, 6.27 mmol) and freshly prepared PPA (30 g) was heated at 105 °C for 45 min. The reaction mixture was cooled, neutralized with satd NaHCO3 solution and the aqueous layer was extracted with CH2Cl2 (3×30 mL). The combined extracts were washed with H2O (3×30 mL), brine (30 mL), dried (Na2SO4) and evaporated under reduced pressure to give crude products 15a (0.86 g, 62%) which were purified on a small silica gel column using CH2Cl2 as eluent (0.69 g, 50%). Recrystallization from cyclohexane gave pure 15a (0.56 g, 40.4%); R =0.52 (CH2Cl2/Acetone, 3/1).

Thieno[2,3-b]quinolizine-6,10-dione (15b)

This product was obtained from 14b (1.5 g, 6.27 mmol) and PPA (30 g) at 110 °C for 1.5 h in the same way as for 14a, yield 0.73 g, 52.6% (cyclohexane); R =0.42 (CH2Cl2/Acetone, 3/1).

Method D

A mixture of freshly prepared carboxylic acid 14a (5.0 g, 20.9 mmol) and Eaton's reagent (P2O5/CH3SO3H/1/10 w/w) (15 mL) was heated at 90 °C for 1 h. The reaction mixture was cooled and ice (20 g) and water (80 mL) were added carefully. The aqueous phase was extracted with CH2Cl2 (3×60 mL). The combined organic extracts were washed with saturated NaHCO3 (50 mL), H2O (30 mL), brine (20 mL), dried (Na2SO4) and evaporated under reduced pressure to give crude 15a (4.11 g, 89%) as a yellow semi-crystalline solid which was purified on a small silica gel (120 g) column using CH2Cl2, CH2Cl2/acetone 25/1 as eluent gave pure 15a (3.6 g, 78%). The analytically pure compound 15a was obtained by crystallization from cyclohexane; R =0.52 (CH2Cl2/Acetone, 3/1).

This product was obtained from acid 14b (5.0 g, 20.9 mmol) and Eaton's reagent (P2O5/CH3SO3H: 1/10 w/w) (30 mL) at 75 °C for 90 min in the same way as for 15a, yield 3.28 g (71%), R =0.42 (CH2Cl2/Acetone, 3/1).

(4R,4aS)-4-Hydroxy-4a,5,6,7-tetrahydro-4H-thieno[3,2-b] quinolizin-8(10H)-one (trans-10Aa)

To a solution of a freshly crystallized ketone 15a (2.21 g, 10 mmol) in methanol (80 mL) was added in a small portions sodium borohydride (0.45 g, 12 mmol) at −45 °C. The mixture was then stirred at −45 °C during 4 h, until total disappearance of starting materials was observed (TLC). The solution was carefully neutralized with concentrated HCl and the solvent was removed under vacuum. The obtained solution was then extracted with dichloromethane. The organic layer was dried over MgSO4 and concentrated to afford a solid as a mixture of two diastereomers trans-10Aa and cis-10Aa (2.13 g, 95.5%) in 9/1 ratio (from 1H NMR spectra). Recrystallization from toluene gave single alcohol trans-10Aa (1.64 g, 73.5%) as colorless crystals; mp 171.6–172.4 °C (decomposition); [α]D 21=+75.2 (c 1.04, MeOH); R =0.32 (CH2Cl2/Acetone, 3/1); IR (ν, cm−1, KBr): 3338, 3282, 3093, 2958, 2893, 1605, 1468, 1434, 1410, 1329, 1304, 1235, 1181, 1145, 1059, 1022, 951, 835, 806, 714, 672, 660, 618, 598, 581, 534, 504, 453, 414. 1H NMR (600 MHz, CD3OD): δ 7.26 (d, 1H, J=5.2 Hz), 7.06 (d, 1H, J=5.2 Hz), 5.50 (d, 1H, J=16.7 Hz), 4.57 (d, 1H, J=9.3 Hz), 4.07 (d, 1H, J=16.8 Hz), 3.43 (tt, 1H, J=8.6 and 3.7 Hz), 2.41 (t, 2H, J=6.3 Hz), 2.23 (ddt, 1H, J=13.5, 6.7 and 3.5 Hz), 2.07 (dtt, 1H, J=14.1, 9.9 and 4.5 Hz), 1.92 (ttd, 1H, J=11.2, 7.6 and 3.3 Hz), 1.82–1.72 (m, 1H). 13C NMR (150 MHz, CD3OD): δ 172.06, 140.18, 133.38, 126.87, 124.87, 68.81, 60.86, 42.86, 33.29, 24.46, 18.13. HRMS calcd for C11H13NO2S (223.29) [M+1]+: 224.0667, found 224.0661.

(9aS,10S)-10-Hydroxy-8,9,9a,10-tetrahydro-4H-thieno [2,3-b]quinolizin-6(7H)-one (trans-10Ab)

This product was obtained from freshly crystallized keto-lactam 15b (2.21 g, 10 mmol), methanol (80 mL) and sodium borohydride (0.45 g, 12 mmol) in the same way as for 5a as a mixture of two diastereomers trans-10Ab and cis-10Ab (2.05 g, 94.4%) in 9/1 ratio (from 1H NMR spectra). Yield 1.81 g (81.2%), colorless crystals (toluene); mp 190.5–191.4 °C (decomposition); [α]D 22=+ 94.2 (c 1.0, MeOH); R =0.27 (CH2Cl2/Acetone, 3/1); IR (ν, cm−1, KBr): 3514, 3316, 3066, 2881, 2667, 1616, 1471, 1457, 1437, 1410, 1344, 1328, 1308, 1230, 1174, 1100, 1074, 1036, 1022, 987, 870, 856, 839, 800, 738, 690, 656, 632, 596, 568, 532, 504, 482. 1H NMR (600 MHz, CD3OD): δ 7.33 (dd, 1H, J=5.2 and 0.9 Hz), 6.83 (d, 1H, J=5.1 Hz), 5.37 (dd, 1H, J=16.7 and 1.5 Hz), 4.71 (dd, 1H, J=9.5 and 2.2 Hz), 3.94 (dd, 1H, J=16.8 and 2.3 Hz), 3.44 (ddd, 1H, J=9.4, 5.8 and 3.7 Hz), 2.45–2.38 (m, 2H), 2.23 (ddd, 1H, J=13.2, 6.5 and 3.2 Hz), 2.08 (dddd, 1H, J=13.8, 10.9, 5.8 and 3.4 Hz), 1.92 (ddt, 1H, J=14.1, 7.0 and 3.2 Hz), 1.84–1.72 (m, 1H). 13C NMR (150 MHz, CD3OD): δ 172.31, 140.34, 133.84, 126.54, 125.40, 68.68, 61.39, 43.79, 33.29, 24.42, 18.09. HRMS calcd for C11H13NO2S (223.29) [M+1]+: 224.0667, found 224.0658.

(4S,4aS)-4-Hydroxy-4a,5,6,7-tetrahydro-4H-thieno[3,2-b]quinolizin-8(10H)-one (cis-10Aa)

The freshly crystallized ketone 15a (884 mg, 4 mmol) was dissolved in dry THF (80 mL) and cooled to −85 °C with stirring. 1 M solution of l-Selectride in THF (12 mL, 12 mmol) was added (90 min) dropwise via a syringe and the reaction mixture was stirred for 12 h at −80 °C, then was quenched with sodium hydroxide aqueous solution (1 M, 5 mL) and hydrogen peroxide (5 mL, 30% in water) at −40 °C. The reaction mixture was then stirred 1 h at 0 °C, concentrated under vacuum and extracted with CH2Cl2 (3×50 mL). The combined organic layers were washed with water, brine (2×20 mL) and dried over anhydrous MgSO4. After filtration, the filtrate was concentrated in vacuo to afford a solid as a mixture of two diastereomers trans-10Aa and cis-10Aa (632 mg, 71%) in 18/82 ratio (from 1H NMR spectra). Recrystallization of the solid from AcOEt (85 mL, to the 65 °C hot solution of AcOEt was added a few crystals of pure cis-diastereomer and during 1 h, cooled slowly to 45 °C, allowed for an additional hour at the same temperature and then cooled to 0 °C for 24 h) gave cis-10Aa (552 mg, 61%) as a colorless crystals; mp 177.5–179.1 °C; [α]D 23=+7.8 (c 1.06, MeOH); R =0.21 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3311, 3217, 3006, 2872, 2652, 1602, 1458, 1448, 1426, 1402, 1352, 1336, 1312, 1225, 1172, 1098, 1071, 1029, 1021, 983, 8762, 852, 834, 798, 732, 686, 605, 591, 562, 522, 498, 471. 1H NMR (600 MHz, CD3OD): δ 7.29 (dd, 1H, J=5.1 and 0.5 Hz), 7.01 (d, 1H, J=5.1 Hz), 5.53 (d, 1H, J=17.3 Hz), 4.54 (d, 1H, J=1.1 Hz), 4.14 (d, 1H, J=17.3 Hz), 3.72 (td, 1H, J=6.7 and 1.9 Hz), 2.43 (t, 2H, J=6.4 Hz), 2.21 (dddd, 1H, J=12.4, 9.3, 6.4 and 2.9 Hz), 2.18–2.12 (m, 1H), 2.11–1.78 (m, 1H), 1.76 (ddt, 1H, J=16.6, 9.7 and 3.6 Hz). 13C NMR (150 MHz, CD3OD): δ 173.67, 137.89, 135.42, 127.91, 124.89, 67.65, 58.88, 43.10, 33.61, 26.14, 19.83. HRMS calcd for C11H13NO2S (223.29) [M+1]+: 224.0667, found 224.0659.

(9aS,10R)-10-Hydroxy-8,9,9a,10-tetrahydro-4H-thieno [2,3-b]quinolizin-6(7H)-one (cis-10Ab)

This product was obtained from freshly crystallized keto-lactam 15b (1.33 g, 6 mmol), dry THF (100 mL) and l-Selectride (18 mL of a 1.0 mol dm−3 solution in THF) in the same way as for cis-10Aa as a mixture of two diastereomers and trans-10Ab and cis-10Ab (1.22 g, 91%) in 14/86 ratio (from 1H NMR spectra). Recrystallization of the solid from AcOEt (120 mL) gave optically pure cis-10Ab (797 mg, 59.4%) as a colorless crystals; mp 196.8–198.5 °C (decomposition); [α]D 21=+ 27.3 (c 1.13, MeOH); R =0.24 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3313, 3086, 3066, 2887, 2841, 1615, 1472, 1457, 1437, 1410, 1344, 1329, 1308, 1262, 1230, 1174, 1101, 1074, 1035, 1022, 987, 970, 912, 901, 872, 856, 839, 800, 738, 690, 659, 631, 596, 568, 532, 504, 482, 460. 1H NMR (600 MHz, CD3OD): δ 7.38 (d, 1H, J=5.1 Hz), 6.87 (dd, 1H, J=5.2 and 0.5 Hz), 5.38 (d, 1H, J=17.3 Hz), 4.63 (d, 1H, J=1.8 Hz), 4.03 (d, 1H, J=17.3 Hz), 3.76 (td, 1H, J=6.6 and 2.2 Hz), 2.42 (t, 2H, J=6.4 Hz), 2.28–2.04 (m, 3H), 1.85–1.65 (m, 1H). 13C NMR (150 MHz, CD3OD): δ 173.89, 136.55, 134.73, 127.13, 125.75, 66.98, 59.34, 44.19, 33.64, 26.33, 19.78. HRMS calcd for C11H13NO2S (223.29) [M+1]+: 224.0667, found 224.0661.

(4R,4aS)-8-Oxo-4a,5,6,7,8,10-hexahydro-4H-thieno[3,2-b]quinolizin-4-yl acetate (trans-18Aa)

To a solution of a freshly crystallized trans-10Aa (1.56 g, 7.0 mmol) in 35 mL of dry CH2Cl2 was added acetic anhydride (1.43 g, 1.31 mL, 14 mmol), 4-(dimethylamino)pyridine (DMAP, 85 mg, 0.7 mmol), and triethylamine (1.42 g, 1.95 mL). The reaction mixture was stirred until disappearance of the starting material (monitored by TLC). The mixture was quenched with a saturated aqueous NaHCO3 solution. The aqueous layer was extracted with diethyl ether and the organic layers were washed with a saturated aqueous CuSO4 solution and water, dried over MgSO4, and concentrated under vacuum. The yellow oil was purified by flash chromatography on silica gel column (20 mm×25 cm, 80 g, cyclohexane/CH2Cl2). An oil, which quickly crystallized on standing in a fridge, was obtained in the yield of 1.63 g (88%). Recrystallization from a mixture of cyclohexane/i-hexane (1/50) gave 1.33 g (71.6%) an analytical sample of trans-18Aa; mp 95.3–96.5 °C; [α]D 21=−22.6 (c 1.31, MeOH); R =0.53 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3101, 2947, 2877, 1731, 1635, 1454, 1413, 1373, 1333, 1233, 1178, 1141, 1022, 961, 942, 833, 787, 704, 667, 613, 521, 465, 449. 1H NMR (600 MHz, CDCl3): δ 7.28 (d, 1H, J=5.2 Hz), 6.81 (t, 1H, J=5.1 Hz), 5.96 (dt, 1H, J=9.6 and 1.6 Hz), 5.55 (d, 1H, J=16.7 Hz), 4.10 (d, 1H, J=16.7 Hz), 3.73 (ddd, 1H, J=9.3, 5.8 and 3.3 Hz), 2.46–2.39 (m, 2H), 2.16 (s, CH3), 2.09–2.00 (m, 1H), 1.98–1.88 (m, 2H), 1.77 (tdd, 1H, J=10.2, 7.5 and 5.3 Hz). 13C NMR (150 MHz, CDCl3): δ 171.09, 170.54, 134.34, 133.98, 125.01, 124.02, 68.56, 56.69, 41.26, 31.71, 23.09, 19.45, 16.67. HRMS calcd for C13H15NO3S (265.33) [M+1]+: 266.0773, found 266.0768.

(9aS,10S)-6-Oxo-6,7,8,9,9a,10-hexahydro-4H-thieno-[2,3-b]quinolizin-10-yl acetate (trans-18Ab)

This product was obtained from freshly crystallized trans-10Ab (1.56 g, 7.0 mmol), CH2Cl2 (40 mL), acetic anhydride (1.43 g, 1.31 mL, 14 mmol), 4-dimethylaminopyridine (DMAP, 85 mg, 0.7 mmol) and triethylamine (1.42 g, 1.95 mL) in the same way as for trans-18Aa, yield 1.37 g, 73.7% (DME/i-hexane 1:60), colorless crystals; R =0.49 (CH2Cl2/acetone, 3/1); mp 100.6–102.4 °C; [α]D=−39.6 (c 1.05, MeOH); IR (ν, cm−1, KBr): 2941, 2844, 1734, 1645, 1458, 1434, 1414, 1366, 1339, 1245, 1222, 1177, 1101, 1017, 967, 953, 899, 839, 791, 710, 686, 617, 598, 528, 417. 1H NMR (600 MHz, CD3OD): δ 7.26 (d, 1H, J=5.2 Hz), 6.81 (d, 1H, J=5.1 Hz), 5.99 (d, 1H, J=9.3 Hz), 5.55 (d, 1H, J=16.4 Hz), 3.91 (d, 1H, J=16.6 Hz), 3.78–3.64 (m, 1H), 2.50 (ddd, 1H, J=10.8, 6.9 and 2.9 Hz), 2.47–2.40 (m, 1H), 2.18 (s, CH3), 2.08–2.00 (m, 1H), 1.98–1.91 (m, 2H), 1.84–1.77 (m, 1H). 13C NMR (150 MHz, CD3OD): δ 170.74, 169.50, 135.21, 133.36, 126.41, 124.49, 69.07, 57.07, 42.55, 32.47, 24.13, 20.97, 17.40. HRMS calcd for C13H15NO3S (265.33) [M+1]+: 266.0773, found 266.0766.

(4S,4aS)-8-Oxo-4,4a,5,7,8,10-hexahydro-6H-thieno[3,2-b]quinolizin-4-yl acetate (cis-18Aa)

To a solution of a freshly crystallized cis-10Aa (1.12 g, 5.0 mmol) in 35 mL of dry CH2Cl2 was added acetic anhydride (1.02 g, 0.94 mL, 10 mmol), 4-dimethylaminopyridine (DMAP, 61 mg, 0.5 mmol), and triethylamine (1.02 g, 1.41 mL). The reaction mixture was stirred until disappearance of the starting material (the reaction was monitored by TLC). The mixture was diluted with CH2Cl2 (50 mL) and quenched with a saturated aqueous NaHCO3 solution. The aqueous layer was extracted with CH2Cl2 (50 mL) and the organic layers were washed with a saturated aqueous CuSO4 solution, HCl (10%, 25 mL), Na2CO3 (25%, 30 mL) and water, dried over MgSO4, and concentrated under vacuum. An oil, which quickly crystallized on standing, was obtained in the yield of (1.27 g, 96%). Recrystallization from a mixture of cyclohexane/i-hexane (1/50) gave 0.95 g (71.7% calculated on starting cis-10Aa) of an analytical sample of cis-18Aa as colorless crystals; R =0.47 (CH2Cl2/acetone, 3/1); mp 102.3–103.8 °C; [α]D 23=−2.65 (c 1.08, MeOH); IR (ν, cm−1, KBr): 3446, 3246, 2952, 1732, 1635, 1624, 1436, 1416, 1372, 1365, 1304, 1224, 1167, 1142, 1064, 1036, 1017, 942, 912, 833, 738, 714, 662, 647, 615, 602, 519, 463, 433. 1H NMR (600 MHz, CDCl3): 7.33 (d, 1H, J=5.1 Hz), 6.97 (d, 1H, J=5.1 Hz), 5.82 (d, 1H, J=2.4 Hz), 5.55 (d, 1H, J=17.6 Hz), 4.11 (d, 1H, J=17.6 Hz), 3.95 (td, 1H, J=7.0 and 2.2 Hz), 2.83 (d, 1H, J=19.2 Hz), 2.35 (t, 1H, J=6.4 Hz), 2.11 (dtd, 1H, J=13.8, 6.9 and 3.4 Hz), 2.04 (s, 3H, OCH3), 2.00 (qd, 1H, J=7.0 and 4.3 Hz), 1.92 (dddd, 1H, J=13.4, 10.3, 7.0 and 3.5 Hz), 1.74 (dtq, 1H, J=13.6, 6.9 and 3.7 Hz). 13C NMR (150 MHz, CDCl3): 170.87, 170.25, 138.00, 133.94, 128.06, 124.65, 68.42, 56.28, 42.20, 33.53, 25.87, 20.95, 19.73. HRMS calcd for C13H15NO3S (265.33) [M+1]+: 266.0773, found 266.0768.

(9aS,10R)-6-Oxo-6,7,8,9,9a,10-hexahydro-4H-thieno-[2,3-b]quinolizin-10-yl acetate (cis-18Ab)

This product was obtained from freshly crystallized cis-11Ab (1.56 g, 7.0 mmol), CH2Cl2 (45 mL), acetic anhydride (1.43 g, 1.31 mL, 14 mmol), 4-dimethylaminopyridine (DMAP, 85 mg, 0.7 mmol) and triethylamine (1.42 g, 1.95 mL) in the same way as for cis-18Aa, yield 1.33 g, (71.5%), colorless crystals; R =0.42 (CH2Cl2/acetone, 4/1); mp 162.2–163.1 °C (cyclohexane/i-hexane, 1/40); [α]D 22=+ 0.58 (c 1.12, MeOH); R =0.39 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3431, 3255, 3116, 3072, 2951, 1732, 1639, 1445, 1417, 1371, 1334, 1293, 1227, 1186, 1161, 1089, 1084, 1063, 1020, 954, 910, 887, 862, 794, 716, 692, 674, 644, 604, 528, 512, 457, 432. 1H NMR (600 MHz, CD3OD): δ 7.42 (d, 1H, J=5.2 Hz), 6.89 (d, 1H, J=5.2 Hz), 5.84 (d, 1H, J=2.2 Hz), 5.37 (d, 1H, J=17.4 Hz), 4.08 (d, 1H, J=17.4 Hz), 3.97 (td, 1H, J=6.8 and 2.2 Hz), 2.44 (t, 2H, J=6.4 Hz), 2.12 (ddt, 1H, J=13.8, 6.9 and 3.7 Hz), 2.08–2.01 (m, 1H), 2.06 (s, CH3), 1.98 (ddd, 1H, J=9.8, 8.0 and 4.9 Hz), 1.77 (dtq, 1H, J=13.3, 6.7 and 3.5 Hz). 13C NMR (150 MHz, CD3OD): δ 173.55, 172.23, 136.69, 131.87, 128.84, 125.55, 68.90, 57.33, 44.09, 33.55, 26.23, 20.88, 19.67. HRMS calcd for C13H15NO3S (265.33) [M+1]+: 266.0773, found 266.0766.

(4R,4aS)-4a,5,6,7,8,10-Hexahydro-4H-thieno[3,2-b]-quinolizin-4-ol (trans-10Ba)

Lithium aluminum hydride (0.38 g, 10 mmol) was added to a solution of a freshly crystallized acetyl thienoderivative trans-18Aa (530 mg, 2 mmol) in dry THF (20 mL) at room temperature and the mixture then heated under reflux for 2.5 h. The slurry was then warmed to ambient temperature and after an additional 40 min was carefully quenched with 2/1 w/w NaSO4·10H2O/Celite (10 g). Gas Evolution! A dry diethyl ether (20 mL) was then added to a solution and after 30 min, the suspension was dried over MgSO4 (3 g), filtered and concentrated in vacuo to give a residue (393 mg, 94%). Recrystallization of the solid twice from anhydrous n-hexane gave pure thienoquindolizinol trans-10Ba as a pale cream crystals (301 mg, 72%); mp 127.6–128.4 °C; [α]D 23=+31.7 (c 1.08, MeOH); R =0.16 (CH2Cl2:acetone, 3:1); IR (ν, cm−1, KBr): 3072, 2933, 2854, 2781, 1454, 1435, 1408, 1325, 1293, 1271, 1205, 1190, 1173, 1136, 1114, 1098, 1076, 1045, 1028, 1009, 947, 847, 792, 772, 720, 692, 604, 531, 486, 436. 1H NMR (600 MHz, CD3OD): δ 7.15 (d, 1H, J=5.1 Hz), 6.96 (d, 1H, J=5.1 Hz), 4.34 (dd, 1H, J=8.1 and 1.5 Hz), 3.90 (d, 1H, J=8.1 Hz), 3.43 (d, 1H, J=15.0 Hz), 3.04 (d, 1H, J=11.5 Hz), 2.32–2.26 (m, 2H), 2.15–2.07 (m, 1H), 1.90–1.81 (m, 1H), 1.77–1.69 (m, 1H), 1.66–1.56 (m, 1H), 1.42–1.26 (m, 2H). 13C NMR (150 MHz, CD3OD): δ 137.39, 132.79, 125.51, 122.78, 70.33, 65.68, 55.46, 53.27, 29.67, 25.08, 23.50. HRMS calcd for C11H15NOS (209.31) [M+1]+: 210.0874, found 210.0868.

(9aS,10S)-6,7,8,9,9a,10-Hexahydro-4H-thieno[2,3-b]-quinolizin-10-ol (trans-10Bb)

This product was obtained from freshly crystallized acetyl derivative trans-18Ab (796 mg, 3.0 mmol) and lithium aluminum hydride (570 mg, 1.5 mmol) in dry THF (35 mL) in the same way as for trans-10Aa, yield 477 mg, 76% (1,2-dimethoxyethane), colorless crystals; mp 191.2–193.8 °C (decomposition); [α]D 21=+5.1 (c 1.02, MeOH); R =0.17 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3072, 2932, 2781, 1435, 1408, 1327, 1293, 1204, 1189, 1173, 1114, 1090, 1077, 1044, 1029, 1009, 989, 947, 847, 791, 772, 720, 692, 605, 532, 437. 1H NMR (600 MHz, CD3OD): δ 7.29 (d, 1H, J=5.1 Hz), 6.76 (d, 1H, J=5.1 Hz), 4.47 (d, 1H, J=8.1 Hz), 3.03 (dd, 1H, J=11.3 and 4.1 Hz), 3.02 (dt, 1H, J=14.3 and 5.3 Hz), 2.34 (dt, 1H, J=12.9 and 2.7 Hz), 2.29 (td, 1H, J=12.2 and 2.9 Hz), 2.12 (ddd, 1H, J=10.9, 8.2 and 3.1 Hz), 1.89–1.81 (m, 1H), 1.72 (dq, 1H, J=13.2 and 3.0 Hz), 1.60 (qt, 1H, J=12.8 and 3.7 Hz), 1.42–1.25 (m, 2H). 13C NMR (150 MHz, CD3OD): δ 139.31, 135.06, 126.04, 125.36, 71.83, 67.05, 56.83, 55.92, 30.87, 26.23, 24.70. HRMS calcd for C11H15NOS (209.31) [M+1]+: 210.0874, found 210.0866.

(4S,4aS)-4,4a,5,7,8,10-Hexahydro-6H-thieno[3,2-b]-quinolizin-4-ol (cis-10Ba)

Lithium aluminum hydride (0.323 g, 8.5 mmol) was added to a solution of a freshly crystallized acetyl derivative cis-18Aa (450 mg, 1.7 mmol) in dry THF (20 mL) at room temperature and the mixture then heated under reflux for 1.5 h. The resulting mixture was cooled and saturated NH4Cl added cautiously until the lithium complex was destroyed. The mixture was then diluted with water (20 mL) and dichloromethane (50 mL) and stirred 2 h at 30 °C. The dichloromethane layer was separated and the aqueous layer extracted with dichloromethane (2×20 mL). The combined extracts were washed with water, brine, dried over MgSO4 and concentrated in vacuo to give a residue (341 mg). Recrystallization of the solid from i-hexane gave pure quindolizinol cis-10Ba as a white crystals (252 mg, 71%); mp 154.2–156.1 °C; [α]D 23=−5.86 (c 1.06, MeOH); R =0.19 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3219, 2939, 2895, 2848, 2763, 1446, 1414, 1392, 1348, 1295, 1277, 1240, 1138, 1109, 1086, 1076, 1038, 987, 974, 879, 845, 831, 775, 758, 689, 632, 578, 480, 416. 1H NMR (600 MHz, CD3OD): δ 7.21 (dd, 1H, J=5.1 and 0.9 Hz), 6.98 (d, 1H, J=5.1 Hz), 4.35 (d, 1H, J=2.1 Hz), 3.96 (d, 1H, J=15.1 Hz), 3.29 (d, 1H, J=15.0 Hz), 3.11 (dt, 1H, J=11.3, 2.7 Hz), 2.29 (dt, 1H, J=11.5, 3.1 Hz), 2.23 (ddd, 1H, J=12.6, 11.4, 2.9 Hz), 1.96–1.82 (m, 2H), 1.71–1.62 (m, 2H), 1.59 (ddd, 1H, J=12.9, 9.4 and 3.5 Hz), 1.42 (qt, 1H, J=13.0 and 3.6 Hz). 13C NMR (150 MHz, CD3OD): δ 137.91, 135.87, 128.10, 124.05, 67.14, 64.37, 57.46, 55.34, 28.65, 26.53, 25.33. HRMS calcd for C11H15NOS (209.31) [M+1]+: 210.0874, found 210.0868.

(9aS,10R)-6,7,8,9,9a,10-Hexahydro-4H-thieno[2,3-b]-quinolizin-10-ol (cis-10Bb)

This product was obtained from freshly crystallized acetyl derivative cis-18Ab (795 mg, 3.0 mmol) and lithium aluminum hydride (570 mg, 1.5 mmol) in dry THF (35 mL) in the same way as for cis-18Ba, yield 426 mg, 68% (i-hexane), colorless crystals; mp 161.8–163.6 °C; [α]D 21=−12.9 (c 1.02, MeOH); R =0.16 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3205, 2937, 2882, 2781, 1454, 1398, 1330, 1285, 1274, 1238, 1160, 1110, 1092, 1078, 1034, 1012, 987, 894, 847, 830, 798, 747, 723, 694, 635, 592, 582, 517, 459, 405. 1H NMR (600 MHz, CD3OD): δ 7.32 (d, 1H, J=5.1 Hz), 6.75 (d, 1H, J=5.1 Hz), 4.45 (d, 1H, J=2.9 Hz, OH), 3.80 (d, 1H, J=15.0 Hz), 3.14 (d, 1H, J=15.1 Hz), 3.08 (dq, 1H, J=11.6, 2.0 Hz), 2.34 (dt, 1H, J=11.5 and 3.0 Hz), 2.21 (ddd, 1H, J=14.3, 11.7 and 2.9 Hz), 1.96–1.79 (m, 2H), 1.69 (dq, 2H, J=14.1, 3.3 and 2.9 Hz), 1.61 (qt, 1H, J=12.9 and 3.5 Hz), 1.41 (qt, 1H, J=13.1 and 3.7 Hz). 13C NMR (150 MHz, CD3OD): δ 135.96, 134.53, 125.12, 124.03, 65.25, 63.36, 55.94, 55.06, 27.33, 25.02, 23.86. HRMS calcd for C11H15NOS (209.31) [M+1]+: 210.0874, found 210.0866.

(S)-4a,5,6,7-Tetrahydro-4H-thieno[3,2-b]quinolizin-8(10H)-one ((S)-11Aa)

Triethylsilane (1.3 mL, 8 mmol) was added dropwise to a stirred solution of alcohol trans-10Aa (1.17 g, 5.2 mmol) in trifluoroacetic acid (10 mL) at 0 °C. The resulting yellow solution was stirred at room temperature for 12 h. The reaction mixture was concentrated in vacuo, diluted with water (20 mL), made alkaline carefully with 10% Na2CO3, and extracted with dichloromethane (3×25 mL). The combined extracts were washed with water (2×15 mL), dried over anhydrous MgSO4, and concentrated in vacuo. The residue (1.01 g, 93%) was purified by flash chromatography on a silica gel column eluting with dichloromethane gave a white solid (783 mg, 72%), which after recrystallization from i-hexane gave (S)-11Aa as a colorless crystals (680 mg, 62%); mp 101.4–102.3 °C; [α]D 21=+98.5 (c 1.0, MeOH); R =0.44 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3060, 2947, 2829, 1623, 1463, 1440, 1417, 1355, 1342, 1331, 1308, 1235, 1175, 1138, 1097, 1009, 899, 831, 750, 698, 651, 608, 651, 608, 511, 489, 472, 437. 1H NMR (600 MHz, CD3OD): δ 7.22 (d, 1H, J=5.0 Hz), 6.79 (d, 1H, J=5.0 Hz), 5.34 (d, 1H, J=17.8 Hz), 4.00 (d, 1H, J=16.7 Hz), 3.78 (dd, 1H, J=10.2 and 5.1 Hz), 2.98–2.85 (m, 2H), 2.43 (t, 2H, J=6.4 Hz), 2.18 (dddd, 1H, J=15.2, 9.0, 5.9 and 2.8 Hz), 1.93 (dtd, 1H, J=13.3, 10.0, 9.3 and 6.5 Hz), 1.89–1.76 (m, 2H). 13C NMR (150 MHz, CD3OD): δ 172.2, 134.9, 131.8, 127.8, 124.4, 54.9, 49.4, 43.1, 33.7, 33.6, 29.4, 18.9. HRMS calcd for C11H13NOS (207.29) [M+1]+: 208.0718, found 208.0712.

(S)-8,9,9a,10-Tetrahydro-4H-thieno[2,3-b]quinolizin-6(7H)-one ((S)-11Ab)

This product was obtained from freshly crystallized alcohol trans-10Ab (1.50 g, 6.7 mmol), triethylsilane (2.2 mL, 13.5 mmol) and trifluoroacetic acid (25 mL) in the same way as for (S)-11Aa, yield 1.21 g, 87% (n-hexane), colorless crystals; mp 82.4–84.2 °C; [α]D 21=+29.2 (c 1.01, MeOH); R =0.38 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3255, 3086, 2945, 2850, 1633, 1574, 1461, 1445, 1434, 1411, 1334, 1303, 1285, 1364, 1234, 1175, 1152, 1092, 1082, 1020, 980, 889, 836, 751, 741, 707, 671, 654, 586, 537, 493, 461, 438. 1H NMR (600 MHz, CD3OD): δ 7.21 (d, 1H, J=5.1 Hz), 6.82 (d, 1H, J=5.1 Hz), 5.34 (d, 1H, J=16.4 Hz), 4.00 (dd, 1H, J=16.8 and 2.6 Hz), 3.78 (dq, 1H, J=10.3 and 5.2 Hz), 2.98–2.84 (m, 2H), 2.43 (t, 2H, J=6.4 Hz), 2.18 (dddd, 1H, J=15.3, 9.0, 5.9 and 2.8 Hz), 1.94 (ddt, 1H, J=12.9, 6.3 and 2.8 Hz), 1.88–1.76 (m, 2H). 13C NMR (150 MHz, CD3OD): δ 171.1, 132.2, 131.3, 124.5, 123.2, 53.9, 42.7, 32.2, 31.5, 28.1, 17.5; HRMS calcd for C11H13NOS (207.29) [M+1]+: 208.0718, found 208.0710.

(S)-4a,5,6,7,8,10-Hexahydro-4H-thieno[3,2-b]quinol-izine ((S)-11Ba)

Lithium aluminum hydride (0.75 g, 2 mmol) was added to a solution of the lactam (S)-11Aa (628 mg, 3 mmol) in dry THF (30 mL) at room temperature and the mixture then heated under reflux for 1 h. The resulting mixture was cooled, NH4Cl and water added cautiously until the lithium complex was destroyed. The mixture was then diluted with water (20 mL) and dichloromethane (50 mL). The dichloromethane layer was separated and the aqueous layer extracted with dichloromethane (2×20 mL). The combined extracts were washed with water, brine, dried over MgSO4, and concentrated in vacuo to give a residue (553 mg, 89%). Recrystallization of the solid from n-hexane gave pure tricyclic amine (S)-11Ba as a colorless crystals (404 mg, 65%); mp 51–54 °C; [α]D 22=+110.4 (c 1.08, MeOH); R =0.23 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3377, 3113, 3084, 2928, 2850, 2762, 1566, 1444, 1402, 1342, 1289, 1242, 1185, 1132, 1110, 1075, 1031, 1017, 982, 915, 868, 833, 746, 699, 609, 521, 474, 455. 1H NMR (600 MHz, CD3OD): δ 7.16 (dd, 1H, J=5.1, 1.0 Hz), 6.75 (d, 1H, J=5.0 Hz), 3.93 (dd, 1H, J=14.9 and 1.2 Hz), 3.39 (ddd, 1H, J=15.0, 3.2 and 1.7 Hz), 3.07 (dtd, 1H, J=11.7, 3.4 and 1.7 Hz), 2.76 (ddd, 1H, J=16.2, 4.4 and 1.7 Hz), 2.49 (dddd, 1H, J=16.4, 10.6, 2.7 and 1.3 Hz), 2.34 (tt, 1H, J=10.3 and 3.6 Hz), 2.25 (td, 1H, J=12.1 and 3.0 Hz), 1.92–1.84 (m, 1H), 1.82–1.77 (m, 1H), 1.76–1.70 (m, 1H), 1.63 (qt, 1H, J=14.1 and 4.0 Hz), 1.46–1.33 (m, 2H). 13C NMR (150 MHz, CD3OD): δ 134.38, 132.25, 127.53, 123.62, 59.49, 57.00, 55.06, 34.29, 34.15, 26.59, 25.09. HRMS calcd for C11H15NS (193.31) [M+1]+: 194.0925, found 194.0920.

(S)-6,7,8,9,9a,10-Hexahydro-4H-thieno[2,3-b]quinol-izine ((S)-11Bb)

This product was obtained from freshly crystallized lactam (S)-11Ab (829 mg, 4 mmol) and lithium aluminum hydride (1.13 g, 3 mmol) in dry THF (40 mL) in the same way as for (S)-11Ba, yield 548 mg, 71% (n-hexane), colorless crystals; mp 55.6–57.8 °C; [α]D 23=+26.8 (c=1.05, MeOH); R =0.20 (CH2Cl2/acetone, 3/1); IR (ν, cm−1, KBr): 3109, 3068, 2955, 2780, 1556, 1441, 1373, 1319, 1286, 1213, 1148, 1131, 1083, 1036, 999, 939, 838, 762, 693, 671, 622, 596, 557, 505, 469. 1H NMR (600 MHz, CD3OD): δ 7.14 (d, 1H, J=5.1 Hz), 6.74 (d, 1H, J=5.1 Hz), 3.85 (d, 1H, J=14.7 Hz), 3.24 (dd, 1H, J=14.9 and 2.4 Hz), 3.09–3.02 (m, 1H), 2.87 (dd, 1H, J=16.6 and 4.2 Hz), 2.68–2.59 (m, 1H), 2.37 (tt, 1H, J=7.4 and 4.0 Hz), 2.24 (td, 1H, J=11.9, 11.5 and 2.4 Hz), 1.90 (dt, 1H, J=9.6 and 3.1 Hz), 1.79 (dd, 1H, J=9.7 and 3.5 Hz), 1.76–1.69 (m, 1H), 1.68–1.56 (m, 1H), 1.39 (qd, 2H, J=12.0, 11.0 and 3.4 Hz). 13C NMR (150 MHz, CD3OD): δ 133.81, 133.51, 125.76, 123.84, 59.76, 56.92, 56.16, 34.19, 33.38, 26.52, 25.06. HRMS calcd for C11H15NS (193.31) [M+1]+: 194.0925, found 194.0919.