Rhodium(III)-Catalyzed Intramolecular Annulations of Acrylic and Benzoic Acids to Alkynes.

Rh(III) catalysts can promote a formal (4 + 2) intramolecular oxidative annulation between acrylic or benzoic acid derivatives and alkynes. The reaction, which involves a C-H activation process, allows for a rapid assembly of appealing bicyclic pyran-2-ones and tricyclic isocoumarin derivatives in moderate to good yields. The α-pyrone moiety of the products provides for further manipulations to obtain relatively complex cyclic skeletons in a very simple manner.

Introduction

Ongoing progress in modern chemical synthesis depends on the invention of new transformations that allow a target-relevant increase in complexity without the generation of byproducts or atomic waste.[1] In this context, the functionalization of “inert” C–H bonds using transition-metal catalysis represents a particularly appealing strategy.[2] Most of the C–H activation reactions consist of couplings, cyclization, and halogenation or acetoxylation processes. However, in recent years, there has been an increasing number of reports on processes that couple the C–H activation with a formal cycloaddition, many of them using alkynes as reaction partners.[3,4] These strategies represent very powerful alternatives to classical cycloadditions of unsaturated substrates and are especially attractive to ensemble heterocyclic products.[5]

One of the landmark advances in this topic consists of the annulation between benzoic or acrylic acids and alkynes to give isocoumarin or α-pyrone products. This reaction has been performed using rhodium,[6] ruthenium,[7] and even cobalt complexes[8] and provides one of the simplest ways to ensemble these types of oxacyclic skeletons from simple precursors. Remarkably, despite the synthetic value of the reaction, we are not aware of the development of intramolecular variants. Tethering the alkyne to the acrylic or benzoic acid moiety would not only allow controlling the regioselectivity, which is an issue in the intermolecular processes, but also provide for a higher increase in skeletal complexity.

In 2013, we reported one of the first intramolecular formal annulations involving C–H activations. The reaction, shown in Scheme A, involved an acrylamide and a N-tethered alkyne and provided a practical entry to isoquinoline products.[9] In the case of the acrylic acid derivatives, the alkyne must be tethered to the carbon instead of the oxygen atom of the acid. The C–H activation would generate a rhodacycle, which after migratory insertion of the alkyne and reductive elimination should produce the bicyclic lactone adducts (Scheme B). Noteworthy, α-pyrones and related lactones fused to other rings are present in many natural products (Scheme C) and therefore the technology might find important synthetic applications in the future.

Scheme 1

Intramolecular Annulations and Natural Products with Polycyclic Pyrone Cores

Herein, we present the first examples of intramolecular, formal (4 + 2) annulations between acrylic acids and alkynes.[10] The methodology can be extended to benzoic acids and pyrrole-3-carboxylic acids, providing for the assembly of interesting polycycles in just one catalytic step. Importantly, we also present preliminary examples on the manipulation potential of the resulting pyrones.

Results and Discussion

The viability of the annulation was studied on precursor 1a, readily assembled from phenylpropynol and methyl buta-2,3-dienoate (see the Supporting Information). At the outset, we were concerned about the C–H activation step, as it might be compromised by the presence of substitution in the β-carbon of the acrylic acid moiety. However, we were pleased to find that the treatment of 1a with 2.5 mol % [Cp*RhCl2]2 in tert-amyl alcohol at 100 °C, using 2 equiv of copper diacetate as oxidant, generates the desired cycloadduct, which was isolated in 50% yield (entry 1, Table ). Using dimethylformamide (DMF) as solvent, the yield was lower (entry 2); however, switching the oxidant to silver carbonate, we obtained the expected pyrone 2a in 82% yield (entry 3).

Table 1

Optimization of the Reaction Conditionsa

entry	catalyst	solvent	T (°C)	yield (%)
1b	[Cp*RhCl2]2	tAmOH	100	50b
2b	[Cp*RhCl2]2	DMF	120	25b
3	[Cp*RhCl2]2	DMF	120	82c
4	[Cp*RhCl2]2	DCE	80	20
5	[Cp*RhCl2]2	toluene	110	36
6	[Cp*RhCl2]2	MeOH	60	71
7	[(p-cymene)RuCl2]2	DMF	120	28
8	[Cp*IrCl2]2	DMF	120
9	[Cp*Rh(CH3CN)3](SbF6)2	DMF	120	21c,d
10	[Cp*RhCl2]2	DMF	120	31e

Reaction conditions: 0.20 mmol of 1a, catalyst (2.5 mol %), 2 equiv of Ag2CO3, solvent (0.8 mL), 12 h. Yields determined by internal standard.

Cu(OAc)2·H2O (2 equiv) instead of Ag2CO3.

Isolated yield.

5 mol % of catalyst.

1 equiv of Ag2CO3.

With other solvents, such as 1,2-dichloroethane, toluene, or MeOH, we observed poorer conversions. Interestingly, using [(p-cymene)RuCl2]2 as catalyst, which had been previously used in intermolecular annulations, the yield was modest (entry 7), whereas with [Cp*IrCl2]2, the starting material was fully recovered (entry 8). The cationic catalyst [Cp*Rh(CH3CN)3](SbF6)2 also resulted in a less effective process (entry 9). The yield of the product was also lower when only 1 equiv of silver carbonate was used (entry 10).

With the optimized conditions at hand, we studied the scope with other precursors (see Table ). The reaction is also efficient with acrylic acid derivatives featuring longer connecting tethers and therefore products with 6- and 7-membered rings containing either a fully carbon chain or an ether were obtained in good yields (2b–e, 50–73%). Further enlarging the connection slows down the reaction and thus the 8-membered product 2f was isolated in low yield (12%). The presence of an aliphatic substituent (n-Bu) instead of the phenyl ring in the alkyne is tolerated, albeit the yield was slightly lower (2g, 49%). A methyl group in the α-position of the acrylic acids does not compromise the reaction, and the expected adducts were obtained in good yields (2h and 2i, 52–80%). We also found that the reaction is efficient with substrates containing other aromatic substituents at the alkyne, with either electron-rich (2j and 2k, 75 and 90% yield) or electron-deficient groups (2l, 63% yield), as well as for heteroaromatic substituents (2m and 2n, 56 and 73% yield). We have also tested the reaction in a precursor bearing a nitrogen atom in the connecting tether. The reaction worked, but the product 2o was isolated in lower yield .

Table 2

Substrate Scope for Substituted Acrylic Acidsa

Conditions: 50 mg of 1, 2 equiv of Ag2CO3, 2.5 mol % of [Cp*RhCl2]2, DMF (0.3 M), 120 °C, 12 h. Isolated yields.

At this point, we also assayed the reaction with benzoic instead of acrylic acid derivatives. These precursors were readily synthetized from commercially available products in one or two steps (see the Supporting Information). Using the same conditions as above, we obtained the expected products 2p and 2q, featuring 6- and 7-membered rings in 47 and 77% yields, respectively (Table ). Interestingly, we also demonstrated that N-alkyl 3-carboxy pyrroles 1r and 1s participate in the process to give the corresponding adducts 2r and 2s featuring seven- and eight-membered rings in excellent yields (80 and 73% respectively). Pyrrole-fused polycyclic products are present in many natural compounds and therefore this methodology might provide an easy way to generate these types of structures.

Table 3

Scope for Aromatic Substratesa

Conditions: 50 mg of 1, 2 equiv of Ag2CO3, 2.5 mol % of [Cp*RhCl2]2, DMF (0.3 M), 120 °C, 12 h. Isolated yields.

Although the above cycloadducts are of intrinsic interest, we envisioned that the pyrone structure might allow designed manipulations to obtain other relevant scaffolds. Therefore, the treatment of product 2e with potassium hydroxide in ethanol at room temperature promoted a quantitative conversion to the keto-carboxylic acid 3. Formally, the transformation of 1e to 3 can be visualized as a cyclization/hydration process, something that does not seem to be easy to achieve using alternative technologies (Scheme ).[11]

Scheme 2

Hydrolysis of the α-Pyrone

Moreover, the presence of a cis-diene in pyrone adducts suggested the possibility of performing Diels–Alder cycloadditions. Remarkably, the treatment of pyrone 2d, prepared in a 2 mmol scale from 1d, with 2 equiv of N-phenylmaleimide at 140 °C produced bis-adduct 4 in good yield and complete stereoselectivity. The structure of this pentacyclic adduct was confirmed by X-ray diffraction crystallography (see Scheme and Figure ).

Scheme 3

Cascade Diels–Alder Processes

Conditions: (a) 2.05 mmol of 1d, 1.5 equiv of Ag2CO3, 2.5 mol % of [Cp*RhCl2]2, DMF (10.2 mL, 0.2 M), 120 °C, 24 h, 77% yield.

Figure 1

X-ray structure of cycloadduct 4. Hydrogens omitted for clarity.

This surprising transformation can be explained in terms of a cascade process involving several Diels–Alder reactions.[12] An initial (4 + 2) Diels–Alder cycloaddition, even between electron-deficient partners, is followed by a retro-cycloaddition with concomitant extrusion of carbon dioxide. Finally, the resulting intermediate reacts with another equivalent of maleimide through a second Diels–Alder to form the polycyclic structure. Overall, the transformation of 1d into 4 (two steps) conveys the formation of five carbon–carbon bonds and five cycles, therefore providing for an impressive increase in complexity with total atom economy.

Conclusions

In summary, we have developed the first examples of intramolecular (4 + 2) annulations between acrylic acids and alkynes. The oxidative annulation is promoted by Rh(III) catalysts and allows for the straightforward assembly of bicyclic and tricyclic pyrone-containing adducts from simple precursors. We also presented preliminary demonstrations of relevant manipulations of the cycloadducts, including a remarkable thermal cycloaddition cascade in the presence of an electron-poor dienophile.

Experimental Section

General Comments

Dry solvents were freshly distilled under argon from an appropriate drying agent before use. Dried N,N-dimethylformamide was purchased from Sigma-Aldrich and used without further purification. [RhCp*Cl2]2 (99%) [12354-85-7] and Ag2CO3 (99%) [534-16-7] were purchased from Sigma-Aldrich. All other chemicals were purchased from Sigma-Aldrich, TCI, Alfa Aesar, and Strem-Chemicals and used without further purification. All reactions dealing with air and moisture-sensitive compounds were carried out in an oven-dried reaction flask under an argon atmosphere with dry solvents. The abbreviation “rt” refers to reactions carried out approximately at 23 °C. Reaction mixtures were stirred using Teflon-coated magnetic stirring bars. Reaction temperatures were maintained using Thermowatch-controlled silicone oil baths. Thin-layer chromatography (TLC) was performed on silica gel plates, and components were visualized by observation under UV light and/or by treating the plates with p-anisaldehyde or phosphomolybdic acid solutions, followed by heating. Flash chromatography was carried out on silica gel unless otherwise stated. Dryings were performed with anhydrous Na2SO4 or MgSO4. Concentration refers to the removal of volatile solvents via distillation using a Büchi rotary evaporator followed by residual solvent removal under high vacuum. Melting points (mp) were determined with an M-560 BÜCHI apparatus. NMR spectra were recorded in CDCl3 at 300 MHz (Varian), unless other solvents are specified. Carbon types and structural assignments were determined from distortionless enhancement by polarization transfer-NMR. NMR spectra were analyzed using MestreNova NMR data processing software (www.mestrelab.com). 1,3,5-Trimethoxybenzene was used as internal standard. The following abbreviations are used to indicate signal multiplicity: s, singlet; d, doublet; t, triplet; q, quartet; dd, double doublet; ddd, doublet of doublet of doublets; td, triple doublet; dt, doublet of triplets; dq, doublet of quartet; m, multiplet; brs, broad singlet. Electrospray ionization mass spectra (ESI-MS) were acquired using an IT-MS Bruker AmaZon SL at CIQUS and also using chemical ionization (CI) electron impact (EI), electrospray ionization (ESI), or atmospheric-pressure chemical ionization (APCI) at the CACTUS facility of the University of Santiago de Compostela. The reactions were monitored by TLC.

General Procedure for the Annulation Reaction

An oven-dried Schlenk tube equipped with a septum and magnetic stirring bar was charged with acrylic or benzoic acid (0.23 mmol), bis(dichloro(pentamethylcyclopentadienyl)rhodium) (2.5 mol %), silver carbonate (0.46 mmol), and DMF (0.8 mL). The reaction mixture was then stirred at 120 °C. After 24 h, the reaction mixture was concentrated in vacuum. Purification of the crude product was done by flash chromatography on silica gel (1:10–1:3 EtOAc/hexane).

4-Phenyl-3H-furo[3,4-c]pyran-6(1H)-one (2a)

Dark-yellow solid, mp 117–118 °C (41 mg, 82%). 1H NMR (300 MHz, CDCl3): δ 7.65–7.57 (m, 2H), 7.53–7.45 (m, 3H), 6.19–6.12 (m, 1H), 5.09 (s, 2H), 4.87 (d, J = 1.6 Hz, 2H) 13C NMR (75 MHz, CDCl3): δ 162.7 (C), 161.1 (C), 152.4 (C), 131.4 (C), 130.8 (CH), 129.1 (CH), 126.6 (CH), 116.1 (C), 104.4 (CH), 70.9 (CH2), 70.1 (CH2). High-resolution mass spectroscopy (HRMS) (EI-FIA-TOF): m/z calculated for C13H10O3 ([M]+) 214.0630, found 214.0636.

1-Phenyl-7,8-dihydropyrano[4,3-c]pyran-3(5H)-one (2b)

White solid, mp 128–129 °C (29.5 mg, 59%). 1H NMR (300 MHz, CDCl3): δ 7.65–7.59 (m, 2H), 7.49–7.45 (m, 2H), 7.34–7.30 (m, 1H), 6.01 (s, 1H), 4.67 (d, J = 1.4 Hz, 2H), 3.83 (t, J = 5.7 Hz, 2H), 2.77 (t, J = 5.6 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ 161.8 (CO), 154.6 (C), 130.4 (CH), 128.7 (CH), 128.6 (CH), 125.6 (C), 124.3 (C), 109.9 (C), 107.8 (CH), 67.3 (CH2), 65.5 (CH2), 25.3 (CH2). HRMS (ESI-FIA-TOF): m/z calculated for C14H13O3 ([M + H]+) 229.0857, found 229.0857.

1-Phenyl-5,6,7,8-tetrahydro-3H-isochromen-3-one (2c)

Light-yellow solid, mp 113–114 °C (25 mg, 50%). 1H NMR (300 MHz, CDCl3): δ 7.60–7.51 (m, 2H), 7.47–7.38 (m, 3H), 6.08 (s, 1H), 2.70 (t, J = 5.9 Hz, 2H), 2.60 (t, J = 6.2 Hz, 2H), 1.82–1.61 (m, 4H). 13C NMR (75 MHz, CDCl3): δ 162.5 (C), 158.5 (C), 156.8 (C), 132.6 (C), 129.8 (CH), 128.9 (CH), 128.3 (CH), 114.0 (C), 111.4 (CH), 29.7 (CH2), 25.3 (CH2), 22.7 (CH2), 21.6 (CH2). Low-resolution mass spectrometry (LRMS) (m/z, CI) 227.2 ([M+H]+, 100), 213.2 (54), 199.2 (23), 105.1 (52). HRMS (EI-FIA-TOF): m/z calculated for C15H14O2 ([M]+) 226.0994, found 226.0980.

1-Phenyl-8,9-dihydro-7H-pyrano[4,3-c]oxepin-3(5H)-one (2d)

White solid, mp 91–92 °C (35.5 mg, 71%). 1H NMR (300 MHz, CDCl3): δ 7.47–7.35 (m, 5H), 6.11 (s, 1H), 4.40 (s, 2H), 3.88 (t, J = 5.1 Hz, 2H), 2.78–2.69 (m, 2H), 1.78 (p, J = 5.2 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ 161.9 (C), 158.0 (C), 157.6 (C), 132.4 (C), 129. 9 (CH), 128.9 (CH), 128.4 (CH), 116.8 (C), 112.5 (CH), 74.9 (CH2), 73.5 (CH2), 30.9 (CH2), 27.6 (CH2). HRMS (ESI-FIA-TOF): m/z calculated for C15H13O3 ([M + H]+) 243.1016, found 243.1016.

1-Phenyl-6,7,8,9-tetrahydrocyclohepta[c]pyran-3(5H)-one (2e)

White solid, mp 74–77 °C (36.5 mg, 73%). 1H NMR (300 MHz, CDCl3): δ 7.50–7.39 (m, 5H), 6.11 (s, 1H), 2.68–2.62 (m, 2H), 2.60–2.54 (m, 2H), 1.80–1.63 (m, 6H). 13C NMR (75 MHz, CDCl3): δ 163.4 (C), 162.7 (C), 157.0 (C), 133.0 (C), 129.7 (CH), 129.1 (CH), 128.4 (CH), 118.9 (C), 111.9 (CH), 36.5 (CH2), 31.3 (CH2), 29.3 (CH2), 28.3 (CH2), 28.3 (CH2). HRMS (APCI-FIA-TOF): m/z calculated for C16H17O2 ([M + H]+) 241.1223, found 241.1223.

1-Phenyl-7,8,9,10-tetrahydropyrano[4,3-c]oxocin-3(5H)-one (2f)

Yellowish solid, mp 118–120 °C (6 mg, 12%). 1H NMR (300 MHz, CDCl3): δ 7.53–7.40 (m, 5H), 6.14–6.07 (m, 1H), 4.69–4.62 (m, 2H), 3.86 (t, J = 4.9 Hz, 2H), 2.88 (t, J = 6.3 Hz, 2H), 1.81–1.64 (m, 4H). 13C NMR (75 MHz, CDCl3): δ 162.1 (C), 160.4 (C), 159.4 (C), 133.141 (C), 130.1 (CH), 129.1 (CH), 128.6 (CH), 115.8 (C), 110.4 (CH), 72.6 (CH2), 71.4 (CH2), 27.9 (CH2), 25.5 (CH2), 25.3 (CH2). LRMS (m/z, CI) 257.1 ([M+H]+, 100), 229.1 (6). HRMS (EI-FIA-TOF): m/z calculated for C16H16O3 ([M]+) 256.1099, found 256.1094.

4-Butyl-3H-furo[3,4-c]pyran-6(1H)-one (2g)

Viscous yellow oil (24.5 mg, 49%). 1H NMR (300 MHz, CDCl3): δ 6.01–6.00 (m, 1H), 4.81–4.79 (m, 1H), 4.79–4.77 (m, 1H), 2.42 (t, J = 7.5 Hz, 2H), 1.75–1.61 (m, 2H), 1.46–1.16 (m, 2H), 0.93 (t, J = 7.3 Hz, 3H). 13C NMR (75 MHz, CDCl3): δ 163.3 (C), 160.5 (C), 157.7 (C), 116.3 (C), 103.5 (CH), 71.3 (CH2), 68.63 (CH2), 32.38 (CH2), 28.72 (CH2), 22.30 (CH2), 13.7 (CH3). HRMS (QI): m/z calculated for C11H15O3 ([M + H]+) 195.1021, found 195.1026.

4-Methyl-1-phenyl-6,7-dihydrocyclopenta[c]pyran-3(5H)-one (2h)

Reddish solid (26 mg, 52%). 1H NMR (300 MHz, CDCl3): δ 7.77–7.66 (m, 2H), 7.49–7.33 (m, 3H), 2.95 (t, J = 7.3 Hz, 2H), 2.74 (t, J = 7.5 Hz, 2H), 2.14–2.00 (m, 5H). 13C NMR (75 MHz, CDCl3): δ 164.2 (C), 160.7 (C), 150.4 (C), 132.7 (C), 129.4 (CH), 128.6 (CH), 127.0 (CH), 120.1 (C), 116.8 (C), 31.5 (CH2), 30.7 (CH2), 25.7 (CH2), 13.3 (CH3). LRMS (m/z, CI) 227.0 ([M+H]+, 100), 199.0 (10). HRMS (EI-FIA-TOF): m/z calculated for C15H14O2 ([M]+) 226.0994, found 226.0995.

4-Methyl-1-phenyl-5,6,7,8-tetrahydro-3H-isochromen-3-one (2i)

Light-yellow solid, mp 84–87 °C (40 mg, 80%). 1H NMR (300 MHz, CDCl3): δ 7.57–7.51 (m, 2H), 7.45–7.38 (m, 3H), 2.68–2.55 (m, 4H), 2.07 (s, 3H), 1.84–1.74 (m, 2H), 1.66–1.58 (m, 2H). 13C NMR (75 MHz, CDCl3): δ 163.7 (C), 153.8 (C), 151.9 (C), 133.0 (C), 129.6 (CH), 129.1 (CH), 128.4 (CH), 119.8 (C), 114.0 (C), 27.7 (CH2), 26.2 (CH2), 22.4 (CH2), 22.0 (CH2), 12.1 (CH3). LRMS (m/z, CI) 241.1 ([M+H]+,100), 213.1 (14). HRMS calculated for C16H16O2 240.1150, found 240.1147. HRMS (EI-FIA-TOF): m/z calculated for C16H16O2 ([M]+) 240.1150, found 240.1147.

1-(3,5-Dimethylphenyl)-4-methyl-5,6,7,8-tetrahydro-3H-isochromen-3-one (2j)

Reddish solid (37.5 mg, 75%). 1H NMR (300 MHz, CDCl3): δ 7.08 (s, 2H), 6.95 (s, 1H), 2.66–2.40 (m, 4H), 2.26 (s, 6H), 1.99 (s, 3H), 1.79–1.63 (m, 2H), 1.62–1.44 (m, 2H). 13C NMR (75 MHz, CDCl3): δ 163.7 (C), 154.0 (C), 152.0 (C), 137.8 (C), 132.6 (C), 131.1 (CH), 126.7 (CH), 119.2 (C), 113.7 (C), 27.8 (CH2), 26.3 (CH2), 22.4 (CH2), 22.1 (CH2), 21.4 (CH3), 12.1 (CH3). HRMS (EI-FIA-TOF): m/z calculated for C18H20O2 ([M]+) 268.1463, found 268.1470.

4-Methyl-1-(naphthalen-1-yl)-5,6,7,8-tetrahydro-3H-isochromen-3-one (2k)

Yellow solid, mp 94–96 °C (45 mg, 90%). 1H NMR (300 MHz, CDCl3): δ 7.97–7.84 (m, 2H), 7.75–7.64 (m, 1H), 7.57–7.43 (m, 4H), 2.69 (t, J = 7.7 Hz, 2H), 2.21 (t, J = 6.4 Hz, 2H), 2.13 (s, 3H), 1.83–1.72 (m, 2H), 1.62–1.50 (m, 2H). 13C NMR (75 MHz, CDCl3): δ 163.9 (C), 153.5 (C), 151.7 (C), 133.8 (C), 131.3 (C), 130.5 (C), 130.2 (CH), 128.7 (CH), 128.2 (CH), 127.1 (CH), 126.4 (CH), 125.3 (CH), 125.2 (CH), 120.1 (C), 115.5 (C), 27.7 (CH2), 25.1 (CH2), 22.1 (CH2), 22.1 (CH2), 12.1 (CH3). HRMS (EI-FIA-TOF): m/z calculated for C20H18O2 ([M]+) 290.1307, found 290.1310.

4-Methyl-1-(4-(trifluoromethyl)phenyl)-5,6,7,8-tetrahydro-3H-isochromen-3-one (2l)

Yellow solid, mp 134–135 °C (31.5 mg, 63%). 1H NMR (300 MHz, CDCl3): δ 7.69–7.66 (m, 4H), 2.69–2.55 (m, 4H), 2.12–2.06 (m, 3H), 1.87–1.72 (m, 2H), 1.71–1.57 (m, 2H). 13C NMR (75 MHz, CDCl3): δ 163.2 (C), 151.9 (C), 151.6 (C), 136.4 (C), 131.4 (C, q, J = 32.3 Hz), 129.5 (CH), 125.4 (CH, q, J = 3.7 Hz), 124.0 (CF3, q, J = 272.6 Hz), 120.9 (C), 115.0 (C), 27.7 (CH2), 26.1 (CH2), 22.3 (CH2), 21.9 (CH2), 12.2 (CH3). HRMS (EI-FIA-TOF): m/z calculated for C17H15F3O2 ([M]+) 308.1024, found 308.1006.

1-(Thiophen-3-yl)-7,8-dihydropyrano[4,3-c]pyran-3(5H)-one (2m)

Yellow solid (39.2 mg, 56%), mp 116–117 °C. 1H NMR (300 MHz, CDCl3): δ 7.75 (dd, J = 3.0, 1.3 Hz, 1H), 7.49 (dd, J = 5.2, 1.3 Hz, 1H), 7.40 (dd, J = 5.1, 2.9 Hz, 1H), 5.97–5.90 (m, 1H), 4.62 (d, J = 1.6 Hz, 2H), 3.88 (t, J = 5.8 Hz, 2H), 2.81 (t, J = 5.8 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ 161.4 (C), 154.6 (C), 153.7 (C), 133.3 (C), 127.7 (CH), 127.1 (CH), 126.2 (CH), 109.0 (C), 107.2 (CH), 67.4 (CH2), 65.3 (CH2), 25.2 (CH2). HRMS (APCI-FIA-TOF): m/z calculated for C12H11O3S ([M + H ]+) 235.0424, found 235.0423.

1-(Thiophen-2-yl)-7,8-dihydropyrano[4,3-c]pyran-3(5H)-one (2n)

Yellow solid (51.1 mg, 73%), mp 142–143 °C. 1H NMR (300 MHz, CDCl3): δ 7.63 (dd, J = 3.9, 1.0 Hz, 1H), 7.55 (dd, J = 5.1, 1.1 Hz, 1H), 7.16 (dd, J = 5.1, 3.8 Hz, 1H), 5.91 (s, 1H), 4.61 (d, J = 1.5 Hz, 2H), 3.94 (t, J = 5.9 Hz, 2H), 2.83 (t, J = 5.9 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ 160.8 (C), 154.5 (C), 152.9 (C), 134.3 (C), 129.9 (CH), 129.6 (CH), 128.0 (CH), 108.3 (C), 107.0 (CH), 67.6 (CH2), 65.2 (CH2), 25.0 (CH2). HRMS (APCI-FIA-TOF): m/z calculated for C12H11O3S ([M + H]+) 235.0422, found 235.0423.

2-Benzyl-4-phenyl-2,3-dihydropyrano[3,4-c]pyrrol-6(1H)-one (2o)

Yellow amorphous solid (17.5 mg, 35%). 1H NMR (300 MHz, CDCl3): δ 7.67–7.58 (m, 2H), 7.50–7.39 (m, 3H), 7.38–7.28 (m, 5H), 6.12 (s, 1H), 3.94 (s, 2H), 3.88 (s, 2H), 3.73 (s, 2H). 13C NMR (75 MHz, CDCl3): δ 162.63 (C), 161.5 (C), 137.9 (C), 131.8 (C), 130.5 (CH), 128.9 (CH), 128.8 (CH), 128.7 (C), 128.7 (CH), 127.7 (CH), 127.0 (CH), 116.9 (C), 106.1 (CH), 59.9 (CH2), 57.3 (CH2), 55.8 (CH2). HRMS (APCI-FIA-TOF): m/z calculated for C20H18NO2 ([M + H]+) 304.1329, found 304.1332.

4-Methyl-2,3-dihydro-6H-pyrano[3,4,5-de]chromen-6-one (2p)

White solid, mp 161–162 °C (23.5 mg, 47%). 1H NMR (300 MHz, CDCl3): δ 7.7 (d, J = 7.8 Hz, 1H), 7.2 (t, J = 7.9 Hz, 1H), 7.1 (d, J = 8.0 Hz, 1H), 4.2 (t, J = 5.7 Hz, 2H), 2.7 (t, J = 5.8 Hz, 2H), 2.1 (s, 3H). 13C NMR (75 MHz, CDCl3): δ 162.2 (C), 152.0 (C), 147.5 (C), 128.1 (CH), 124.1 (C), 121.6 (CH), 120.9 (CH), 120.4 (C), 104.0 (C), 65.8 (CH2), 23.9 (CH2), 15.9 (CH3). HRMS (APCI-FIA-TOF): m/z calculated for C12H11O3 ([M + H]+) 202.0700, found 202.0703.

5-Phenyl-3,4-dihydro-2H,7H-oxepino[4,3,2-de]isochromen-7-one (2q)

White solid, mp 155–156 °C (38.5 mg, 77%). 1H NMR (300 MHz, CDCl3): δ 7.96 (dd, J = 7.5, 1.7 Hz, 1H), 7.53–7.47 (m, 2H), 7.40–7.33 (m, 3H), 7.32–7.22 (m, 2H), 4.27 (t, J = 6.8 Hz, 2H), 2.86 (t, J = 6.4 Hz, 2H), 2.12 (p, J = 6.7 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ 162.1 (C), 158.6 (C), 150.2 (C), 133.0 (C), 131.1 (C), 129.7 (CH), 129.2 (CH), 128.8 (CH), 128.5 (CH), 126.4 (CH), 124.3 (CH), 122.9 (C), 111.9 (C), 73.5 (CH2), 29.2 (CH2), 25.9 (CH2). HRMS (APCI-FIA-TOF): m/z calculated for C18H15O3 ([M + H]+) 279.1016, found 279.1016.

2-Methyl-5-phenyl-6,7,8,9-tetrahydro-3H-4-oxa-9a-azabenzo[cd]azulen-3-one (2r)

Yellowish solid, mp 193–194 °C, (40 mg, 80%). 1H NMR (300 MHz, CDCl3): δ 7.52–7.46 (m, 2H), 7.43–7.34 (m, 3H), 6.50 (s, 1H), 4.16–4.10 (m, 2H), 2.89–2.81 (m, 2H), 2.36 (s, 3H), 2.11 (p, J = 6.6 Hz, 2H), 1.93 (p, J = 6.8 Hz, 2H). 13C NMR (75 MHz, CDCl3): δ 160.6 (C), 151.0 (C), 140.8 (C), 133.3 (C), 129.2 (CH), 128.9 (CH), 128.2 (CH), 124.8 (CH), 118.0 (C), 109.0 (C), 108.6 (C), 47.2 (CH2), 27.6 (CH2), 25.3 (CH2), 25.1 (CH2), 10.7 (CH3). HRMS (APCI-FIA-TOF): m/z calculated for C18H18NO2 ([M + H]+) 280.1340, found 280.1332.

2-Methyl-5-phenyl-7,8,9,10-tetrahydro-3H,6H-4-oxa-10a-azacycloocta[cd]inden-3-one (2s)

Light orange solid, mp 216–217 °C (36.5 mg, 73%). 1H NMR (300 MHz, CDCl3): δ 7.48–7.41 (m, 2H), 7.41–7.34 (m, 3H), 6.46 (s, 1H), 4.33–4.21 (m, 2H), 2.84 (t, J = 7.1 Hz, 2H), 2.38 (s, 3H), 1.95–1.76 (m, 4H), 1.58–1.46 (m, 2H). 13C NMR (75 MHz, CDCl3): δ 160.6 (C), 152.2 (C), 140.5 (C), 133.6 (C), 129.0 (CH), 129.0 (CH), 128.2 (CH), 125.2 (CH), 117.9 (C), 108.2 (C), 107.6 (C), 47.9 (CH2), 30.1 (CH2), 28.4 (CH2), 25.4 (CH2), 21.3 (CH2), 10.8 (CH3). HRMS (APCI-FIA-TOF): m/z calculated for C19H20NO2 ([M + H]+) 294.1488, found 294.1489.

Synthesis of Acid 3

To a solution of 1-phenyl-6,7,8,9-tetrahydrocyclohepta[c]pyran-3(5H)-one (2e) (50 mg, 0.21 mmol) in EtOH (1 mL) was added a saturated aqueous solution of KOH (93.4 mg, 1.66 mmol, 10 M), and the reaction mixture was stirred for 4 h at room temperature. The mixture was poured into an ice bath, and then the solution was acidified by the addition of HCl (0.5 M). EtOAc (5 mL) was added and the layers were separated. The aqueous layer was extracted twice with EtOAc (2 × 5 mL), and the combined organic extracts were washed with brine, dried, and concentrated under reduced pressure. Purification of the crude product by flash chromatography on silica gel (1:5–3:1 EtOAc/hexane) afforded the desired product 2-(2-benzoylcyclohept-1-en-1-yl)acetic acid (3) (53 mg, 99% yield). 1H NMR (300 MHz, CDCl3): δ 7.84 (d, J = 7.0 Hz, 2H), 7.60 (t, J = 7.4 Hz, 1H), 7.47 (t, J = 7.5 Hz, 2H), 3.10 (s, 2H), 2.51–2.43 (m, 4H), 1.86–1.76 (m, 2H), 1.67–1.52 (m, 4H). 13C NMR (75 MHz, CDCl3): δ 202.1 (C), 172.6 (C), 142.4 (C), 135.9 (C), 134.0 (CH), 129.9 (CH), 128.9 (CH), 44.0 (CH2), 35.7 (CH2), 32.2 (CH2), 32.0 (CH2), 26.5 (CH2), 25.8 (CH2). HRMS (APCI-FIA-TOF): m/z calculated for C16H19O3 ([M + H]+) 259.1324, found 259.1329.

Procedure for a Large-Scale Catalytic Annulation of 1d

An oven-dried Schlenk tube equipped with a septum and magnetic stirring bar was charged with (E)-4-((5-phenylpent-4-yn-1-yl)oxy)but-2-enoic acid (500.0 mg, 2.05 mmol), bis(dichloro(pentamethylcyclopentadienyl)rhodium) (32 mg, 0.05 mmol), silver carbonate (850 mg, 3.08 mmol), and DMF (7 mL). The reaction mixture was then stirred at 120 °C. After 24 h, the reaction mixture was concentrated in vacuum. Purification of the crude product by flash chromatography on silica gel (1:10–1:3 EtOAc/hexane) afforded 384 mg of 4-phenyl-3H-furo[3,4-c]pyran-6(1H)-one (2d) (77% yield).

Procedure for the Cascade Diels–Alder Reaction

An oven-dried sealed tube equipped with a magnetic stirring bar was charged with N-phenylmaleimide (75.2 mg, 0.43 mmol), 1-phenyl-8,9-dihydro-7H-pyrano[4,3-c]oxepin-3(5H)-one (2d) (30 mg, 0.12 mmol), and dioxane (1 mL). The reaction mixture was then stirred at 140 °C. After 36 h, the reaction mixture was concentrated in vacuo. Purification of the crude product by flash chromatography on silica gel (1:10–2:1 EtOAc/hexane) afforded the desired product as a white solid (65.5 mg, 97% yield). White solid, mp 249–250 °C. 1H NMR (300 MHz, CDCl3): δ 7.51–7.33 (m, 11H), 7.29–7.22 (m, 4H), 4.27 (s, 2H), 3.76 (t, J = 6.2 Hz, 2H), 3.69 (t, J = 3.0 Hz, 1H), 3.44 (d, J = 8.3 Hz, 2H), 3.16 (dd, J = 8.3, 3.0 Hz, 2H), 2.54–2.45 (m, 2H), 1.69–1.57 (m, 2H). 13C NMR (75 MHz, CDCl3): δ 175.3 (C), 173.9 (C), 139.1 (C), 139.0 (C), 134.6 (C), 131.6 (C), 131.0 (CH), 129.3 (CH), 128. 9 (CH), 127.8 (CH), 126.8 (CH), 126.3 (CH), 71.1 (CH2), 70.9 (CH2), 53.8 (C), 48.1 (CH), 44.0 (CH), 37.5 (CH), 30.4 (CH2), 29.3 (CH2). HRMS (APCI-FIA-TOF): m/z calculated for C35H25N6O2 ([M + H]+) 545.2071, found 545.2084.