Synthesis of Novel Furo-, Thieno-, and Pyrroloazepines

Abstract

The synthesis of novel furo-, thieno-, and pyrroloazepine compounds, using the oxidative radical alkylation of three five-membered heterocyclic 3-acetic acid derivatives, is described. The bicyclic systems were obtained, via a small number of steps, directly from commercially available materials.

As part of our ongoing interest in the development of novel heterocyclic compounds that inhibit the growth of cancer cells, we have employed a molecular mimicry paradigm to generate new active leads from azetopyrroloazepines 1 (Scheme  [¹] ). [¹] The mechanism of action underlying the antiproliferative activity of compound 1 is still unknown, and further exploration of the pyrroloazepinone pharmacophore is advisable. Our previous studies suggested that a pyrroloazepine group and two aromatic groups, such as those found in pyrroloazepines 2, are required for cytotoxic activity. [²]

Recently, we observed that bromine derivative 3, a conformationally constrained analogue of 2, was cytotoxic to the U-251 central nervous system cancer cell line (Figure  [¹] ). Indeed, these structures are closely related to well-known pyrroloazepine moieties that are present in a number of natural and synthetic products such as paullones 4, which have been described as potent cyclin-dependent kinase and glycogen synthase kinase-3 inhibitors [³] (Figure  [¹] ).

Kenpaullone (4a) was found to be inactive against the U-251 cancer cell line, but alsterpaullone (4b) showed a potency that was 25 times the potency of 3 (IC₅₀ = 0.68 ± 0.02 µM vs IC₅₀ = 18.14 ± 1.02 µM). One plausible explanation of the lower potency of 3, compared with 4b, is that 4b achieves a better fit at the target active site than does 3. Therefore, we hypothesize that compounds such as 5 could provide an improved fit at the active site such that they will be more cytotoxic than 3.

Here we report a novel straightforward synthetic route to the pyrrolo[2,3-d]azepinone 5 and its bioisosteric furo and thieno analogues. The retrosynthetic analysis and key strategic bond breakages are shown in Scheme  [²] . Thus, compound 5 can be synthesized by intramolecular lactamization of the amino acid 8, which could prepared by a C2 xanthate-based oxidative radical alkylation of pyrrole ­acetate 9. Compound 9 can be derived by Arndt-Eistert homologation of 1H-pyrrole-3-carboxylic acid (10).

In accordance with the synthetic strategy shown in Scheme  [³] , 1-benzyl-1H-pyrrole-3-carboxylic acid (12) was prepared in good yield from ethyl 1H-pyrrole-3-carboxylate (11) by protection with benzyl bromide in the presence of sodium hydride followed by alkaline hydrolysis; 11 was synthesized from tosylmethyl isocyanide (TosMIC) and ethyl acrylate by the van Leusen reaction. [4] Arndt-Eistert homologation [5] of 12 was accompanied by decomposition of the starting material. We assumed that acyl chloride formation with thionyl chloride was inefficient. Therefore, we required a mild method for the activation of the carboxylic acid.

Compound 12 was then converted into the corresponding diazo ketone via the mixed anhydride generated in situ with ethyl chloroformate, followed by the addition of excess ethereal diazomethane. [6] Next, the crude diazo ketone was treated with silver oxide in ethanol-1,4-dioxane at 100 ˚C to afford the homologated ethyl 1-benzyl-1H-pyrrole-3-acetate (13) in 5% yield (Scheme  [³] ). Compound 13 was also obtained, in better yields, by hydrogenolysis [7] of the pyrrole glyoxylate 15. Thus, pyrrole-3-glyoxylate 14 was prepared from pyrrole according to a procedure reported by Bray and co-workers, [8] followed by protection with benzyl bromide in the presence of sodium hydride to afford the N-benzyl derivative 15. Next, hydrogenolysis of 15 with the Pd/C/NaH₂PO₂˙H₂O system afforded 13 in 77% yield (Scheme  [³] ).

Having prepared compound 13, the next step of the synthesis of 5 involved the introduction of the C2 aminoethyl side chain in 13, as shown in Scheme  [²] . We envisaged that C2 alkylation of 13 with a cyanomethyl group by xanthate-based oxidative radical substitution, [9] followed by reduction of the resulting nitrile to an amine would provide an accessible sequence (Scheme  [4] ). Thus, alkylation of 13 with S-(cyanomethyl) O-ethyl carbonodithioate [CNCH₂SC(S)OEt] in the presence of dilauroyl peroxide (DLP) afforded a mixture 3:1 of ethyl 1-benzyl-2-(cyanomethyl)-1H-pyrrole-3-acetate (16) and ethyl 1-benzyl-2,5-bis(cyanomethyl)-1H-pyrrole-3-acetate, which could be separated by column chromatography. Reduction of the nitrile 16 with sodium borohydride/nickel(II) chloride system, [¹0] in the presence of di-tert-butyl dicarbonate, provided the N-Boc amine 17 in 79% yield; Boc protection was required to prevent dimerization and decomposition. [¹¹] N-Benzylpyrroloazepinone 19 was prepared in moderate yields by intramolecular lactamization (DMAP and EDCI) of 18, which in turn was obtained from 17 by alkaline hydrolysis and N-Boc deprotection with trifluoroacetic acid. Compound 19 can also be obtained, in 82% overall yield, from 17 by N-Boc deprotection with trifluoroacetic acid, followed by ring closure of the resulting amino ester with potassium carbonate in methanol (Scheme  [4] ). [¹²] Finally, N-benzyl deprotection of 19 using metallic sodium in liquid ammonia at -78 ˚C afforded compound 5 in 55% yield.

These results led us to investigate the synthesis of pyrroloazepinone bioisosteres through substitution of the pyrrole nucleus by a furan 6 or a thiophene ring 7. Through application of the previously described methodology, furo- and thienoazepinones were obtained from 3-furoic acid (20) and thiophene-3-carboxylic Acid (21), respectively, as shown in Scheme  [5] .

In contrast with the pyrrole acetate 13, furan and thiophene acetates 24 and 25 were prepared in good yields by Arndt-Eistert homologation of carboxylic acids 20 and 21, respectively. In our case, carboxylic acids were prepared by oxidation, with chromic acid solution, of furan-3-carbaldehyde and thiophene-3-carbaldehyde, respectively. The carboxylic acids 20 and 21 were then converted into the corresponding α-diazo ketones 22 and 23 via acyl chlorides with thionyl chloride followed by the addition of excess ethereal diazomethane. To conclude the homologation, Wolff rearrangement [¹³] of the diazo ketones, with silver benzoate in methanol, afforded the desired methyl esters 24 and 25. Rearrangement of 22 occurred under gentle reflux, and rearrangement of 23 occurred at room temperature, respectively. C2 Oxidative radical alkylation of 24 and 25, with S-(cyanomethyl) O-ethyl carbono­dithioate in the presence of dilauroyl peroxide, provided nitriles 26 and 27 in moderate yields with recovery of a significant quantity of starting material. In these cases, the dialkylated products were not observed. Reduction of 26 and 27 with sodium borohydride and nickel(II) chloride in the presence of di-tert-butyl dicarbonate afforded N-Boc amines 28 and 29, respectively.

Finally, N-Boc deprotection of 28 and 29 with trifluoroacetic acid in dichloromethane and subsequent intramolecular lactamization of the resulting amino esters 30 and 31 with potassium carbonate in methanol afforded the azepinone derivatives 6 and 7, respectively.

Pyrroloazepinone 5 and its bioisosteric derivatives 6 and 7 were evaluated in vitro for their ability to inhibit growth of PC3 (prostate), U291 (central nervous system), K562 (leukemia), and MCF7 (breast adenocarcinoma) cancer cell lines. Unfortunately, these compounds did not inhibit the proliferation of the four cancer cell lines, and these results do not allow it to give a reasonable explanation of cytotoxic behavior of 3.

In conclusion, we have developed a novel and straightforward methodology for the synthesis of heterocyclic ring-fused pyrrolo-, furo-, and thienoazepinones via carboxylic acid homologation, oxidative radical alkylation, and intramolecular lactamization.

The starting materials and reagents were obtained from commercial suppliers and were used without further purification. Solvents were distilled before use: THF and Et₂O were dried over Na/benzophenone; pyridine, DMF, CH₂Cl₂, and DCE were distilled from CaH₂ prior to use. Ethereal solns of diazomethane (CH₂N₂) were prepared from N-methyl-N-nitroso-p-toluenesulfonamide (Diazald) as described in the literature. [6] Melting points (uncorrected) were determined in open glass capillaries using a Mel-temp II melting point apparatus. Products were purified by flash chromatography on Merck silica gel, 230-400 mesh. [¹4] IR spectra were determined on a Nicolet FT Magna-IR 750 spectrophotometer. ^¹H and ^¹³C NMR spectra were determined on a Varian Unity VXR 300 instrument relative to TMS as internal standard. Mass spectra were recorded on a Jeol JEM AX505HA EI spectrometer with a lower resolution of 70 eV. Elemental analysis (C, H, N) were performed on a CE-440 Exeter Analytical Inc.

Ethyl 1 H -Pyrrole-3-carboxylate (11)

A soln of TosMIC (4.68 g, 24 mmol) and ethyl acrylate (1.9 g, 19 mmol) in Et₂O-DMSO (1:1, 95 mL) was added dropwise at 0 ˚C to a suspension of 60% NaH dispersion in mineral oil (0.96 g, 24 mmol) in Et₂O (38 mL). The mixture was stirred for 6 h and then reaction was quenched by addition of H₂O (50 mL) and stirred for 15 min. The mixture was extracted with EtOAc (2 × 50 mL), and washed with H₂O (50 mL) and brine (50 mL). The combined organic extracts were dried (anhyd Na₂SO₄), and the solvents were evaporated in vacuo. The residue was purified by column chromatography (hexane-EtOAc, 8:2) to afford 11 (2.0 g, 75%) as a pale yellow oil. Spectroscopic data agreed well with results previously reported in the literature. [¹5]

1-Benzyl-1 H -pyrrole-3-carboxylic Acid (12)

To a soln of 11 (2.6 g, 19 mmol) in DMF (40 mL), 60% NaH dispersion in mineral oil (1.6 g, 38 mmol) was added portionwise at 0 ˚C. The mixture was stirred for 30 min and then BnBr (3.42 g, 20 mmol) was added dropwise. The mixture was allowed to reach r.t. and was stirred for 3 h. H₂O (80 mL) was added and the mixture was extracted with EtOAc (2 × 80 mL). The combined extracts were washed with H₂O (2 × 40 mL) and brine (1 × 40 mL) and dried (anhyd Na₂SO₄). The solvents were evaporated in vacuo to give a residue that was purified by column chromatography (hexane-EtOAc, 8:2) to afford ethyl 1-benzyl-1H-pyrrole-3-carboxylate (3.0 g, 70%) as a yellow oil. Spectroscopic data agreed well with results previously reported in the literature. [¹6] To a soln of the N-benzyl derivative (1.7 g, 7.4 mmol) in EtOH-H₂O (1:1, 148 mL), a soln of 10% KOH (37 mL) was added and the mixture was stirred at reflux for 12 h. After removal of EtOH in vacuo, the mixture was acidified to pH ˜1 with 2 M HCl and extracted with Et₂O (90 mL). The combined organic extracts were dried (anhyd Na₂SO₄), and the solvents were evaporated in vacuo to afford 12 (1.24 g, 47%) as a white solid; mp 88-92 ˚C.

IR (KBr): 3031 (br), 1655, 1538, 1445, 1388, 1350, 1244, 958, 743, 726 cm^-¹.

^¹H NMR (300 MHz, CDCl₃-DMSO-d ₆): δ = 5.05 (s, 2 H), 6.57 (dd, J = 2.7, 1.5 Hz, 1 H), 6.64 (t, J = 2.7 Hz, 1 H), 7.13-7.17 (m, 2 H), 7.28-7.36 (m, 4 H).

^¹³C NMR (75 MHz, CDCl₃-DMSO-d ₆): δ = 53.1, 109.9, 116.0, 121.5, 124.1, 125.8, 126.7, 126.4, 128.2, 128.9, 136.2, 166.0.

MS (EI, 70 eV): m/z (%) = 201 (84) [M⁺], 91 (100).

Ethyl 1-Benzyl-1 H -pyrrole-3-acetate (13)

Et₃N (3.3 mL) and ClCO₂Et (2.6 g) were added at 0 ˚C to a soln of 12 (4 g, 20 mmol) in THF (100 mL). The mixture was stirred for 30 min and an excess of CH₂N₂ (3-5 equiv), as an ethereal soln, was added at 0 ˚C. The mixture was allowed to reach r.t. and was stirred for 16 h. The solvent and excess CH₂N₂ were evaporated in vacuo. An excess of Ag₂O (10 g, 50 mmol, 5 equiv) was added to a soln of the crude residue in a mixture of EtOH-1,4-dioxane (2:1, 100 mL). The mixture was refluxed for 2 h, it was allowed to reach r.t. and filtered, and the solvents were evaporated in vacuo. The residue was purified by column chromatography (hexane-EtOAc, 9:1) to afford the desired product (0.26 g, 5%) as a colorless oil.

IR (film): 2981, 2932, 1733, 1499, 1451, 1346, 1302 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 1.24 (t, J = 7.2 Hz, 3 H), 3.46 (s, 2 H), 4.14 (q, J = 7.2 Hz, 2 H), 4.99 (s, 2 H), 6.10-6.11 (m, 1 H), 6.58-6.61 (m, 2 H), 7.09-7.13 (m, 2 H), 7.23-7.34 (m, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 14.1, 33.1, 53.3, 60.4, 109.3, 115.9, 119.9, 121.1, 127.0, 127.6, 128.6, 137.9, 172.4.

MS (EI, 70 eV): m/z (%) = 243 (53) [M⁺], 170 (100), 91 (63.9).

Ethyl 2-Oxo-2-(1 H -pyrrol-3-yl)acetate (14)

To a soln of pyrrole (4.82 g, 72 mmol) in DMF (100 mL), 60% NaH dispersion in mineral oil (3.17 g, 79 mmol) was added portionwise at 0 ˚C. The mixture was stirred for 45 min and then TIPSCl (13.9 g, 72 mmol) was added dropwise. The mixture was allowed to reach r.t. and stirred for 2 h. The mixture was diluted with H₂O (100 mL) and extracted with Et₂O (2 × 100 mL). The combined organic extracts were dried (anhyd Na₂SO₄), and the solvent was evaporated in vacuo. The residue was purified by column chromatography (hexane-EtOAc, 98:2) to afford 1-(triisopropylsilyl)pyrrole (15.6 g, 97%) as a brown oil. A soln of the N-TIPS-pyrrole (13.0 g, 58 mmol), ethyl oxalyl chloride (23.9 g, 175 mmol), and pyridine (13.8 g, 175 mmol) in DCE (100 mL) was gently refluxed for 16 h. The mixture was allowed to reach r.t. and the solvent was evaporated in vacuo. The residue was purified by column chromatography (hexane-EtOAc, 7:3) to afford 14 (6.0 g, 62%) as a brown solid. Spectroscopic data agreed well with the results previously reported in the literature. [7]

Ethyl 2-(1-Benzyl-1 H -pyrrol-3-yl)-2-oxoacetate (15)

To a soln of 14 (2.8 g, 17 mmol) in DMF (35 mL), 60% NaH dispersion in mineral oil (0.816 g, 20.4 mmol) was added portionwise at 0 ˚C. The mixture was stirred for 45 min and BnBr (3.77 g, 22.1 mmol) was added dropwise. The mixture was allowed to warm to r.t. and was stirred for 3 h. After addition of H₂O (50 mL), the mixture was extracted with EtOAc (2 × 50 mL). The combined organic extracts were washed with H₂O (2 × 50 mL) and dried (anhyd Na₂SO₄). The solvent was evaporated in vacuo, and the residue was purified by column chromatography (hexane-EtOAc, 8:2) to afford 15 (4.0 g, 91%) as a solid.

IR (film): 3120, 2983, 1731, 1656, 1526, 1271, 1239, 1159, 1032, 713 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 1.38 (t, J = 7.2 Hz, 3 H), 4.35 (q, J = 7.2 Hz, 2 H), 5.08 (s, 2 H), 6.65 (d, J = 3.0 Hz, 1 H), 6.80 (d, J = 3.0 Hz, 1 H), 7.14-7.18 (m, 2 H), 7.31-7.39 (m, 3 H), 7.74 (t, J = 1.8 Hz, 1 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 14.0, 54.0, 61.9, 111.1, 121.6, 123.3, 127.3, 128.3, 129.0, 130.6, 135.9, 163.0, 178.3.

MS (EI, 70 eV): m/z (%) = 257 (2.5) [M⁺], 184 (100), 91 (77.9).

Ethyl 1-Benzyl-1 H -pyrrole-3-acetate (13)

A soln of NaH₂PO₂˙H₂O (8.4 g, 78 mmol) in H₂O (7.5 mL) was added to a suspension of 15 (4.0 g, 15.5 mmol) in the presence of 10% Pd/C (1.6 g, 10 wt %) in 1,4-dioxane (46 mL). The mixture was refluxed for 4 h and then allowed to cool to r.t. Another portion of Pd/C (1.6 g, 10 wt %) and a soln of NaH₂PO₂˙H₂O (8.4 g, 78 mmol) in H₂O (7.5 mL) were added, and refluxing was continued for an additional 4 h. The mixture was allowed to reach r.t., was filtered through Celite, and the precipitate was washed with several portions of EtOAc. The combined organic extracts were evaporated in vacuo. The residue was purified by column chromatography (hexane-EtOAc, 9:1) to give 13 (2.92 g, 77%).

Ethyl 1-Benzyl-2-(cyanomethyl)-1 H -pyrrole-3-acetate (16), 3:1 Mixture with Ethyl 1-Benzyl-2,5-bis(cyanomethyl)-1 H -pyrrole-3-acetate; Typical Procedure

To a soln of 13 (2.92 g, 12.0 mmol) and CNCH₂SC(S)OEt (2.9 g, 18.0 mmol) in DCE (25 mL) under reflux was added DLP (5-10 mol%) every 90 min for 12 h. The mixture was allowed to reach r.t., and the solvent was evaporated in vacuo. The mixture was separated by column chromatography (hexane-EtOAc, 9:1).

Ethyl 1-Benzyl-2-(cyanomethyl)-1 H -pyrrole-3-acetate (16)

Pale yellow semisolid; yield: 1.5 g (44%).

IR (KBr): 2923, 2852, 2249, 1730, 1495, 1446, 1302, 1269, 1210, 1113, 1034, 726 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 1.26 (t, J = 7.2 Hz, 3 H), 3.45 (s, 2 H), 3.55 (s, 2 H), 4.15 (q, J = 7.2 Hz, 2 H), 5.14 (s, 2 H), 6.12 (d, J = 2.7 Hz, 1 H), 6.68 (d, J = 2.7 Hz, 1 H), 7.01-7.06 (m, 2 H), 7.28-7.36 (m, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 13.7, 14.1, 32.6, 51.2, 60.9, 109.3, 115.9, 116.2, 117.5, 122.7, 126.4, 127.9, 129.0, 136.8, 171.5.

MS (EI, 70 eV): m/z (%) = 282 (40.8) [M⁺], 208 (57.8), 91 (100).

Ethyl 1-Benzyl-2,5-bis(cyanomethyl)-1 H -pyrrole-3-acetate

Pale yellow viscous oil; yield: 0.57 g (14.5%).

^¹H NMR (200 MHz, CDCl₃): δ = 1.27 (t, J = 7.2 Hz, 3 H), 3.46 (s, 2 H), 3.53 (s, 2 H), 3.60 (s, 2 H), 4.16 (q, J = 7.2 Hz, 2 H), 5.18 (s, 2 H), 6.21 (s, 1 H), 6.68-6.92 (m, 2 H), 7.26-7.35 (m, 3 H).

Ethyl 1-Benzyl-2-{2-[( tert -butoxycarbonyl)amino]ethyl}-1 H -pyrrole-3-acetate (17); Typical Procedure

To a mixture of 16 (0.14 g, 0.5 mmol), Boc₂O (0.21 g, 1.0 mmol), and NiCl₂˙6 H₂O (0.117 g, 0.5 mmol) in MeOH (5 mL) was added at 0 ˚C NaBH₄ (0.132 g, 3.5 mmol) in small portions over 15 min; the reaction was exothermic after each addition. The mixture was allowed to reach r.t. and stirred overnight. The solvent was evaporated in vacuo, and the residue was dissolved in EtOAc (30 mL) and filtered through Celite and the precipitate was washed with several portions of EtOAc. The combined organic extracts were evaporated in vacuo. The residue was purified by flash chromatography (hexane-EtOAc, 8:2) to afford 17 (0.154 g, 79%) as a colorless viscous oil.

IR (film): 3379, 2977, 2932, 1709, 1499, 1453, 1365, 1251, 1170, 1031 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 1.25 (t, J = 7.2 Hz, 3 H), 1.42 (s, 9 H), 2.69 (t, J = 6.9 Hz, 2 H), 3.13-3.11 (m, 2 H), 3.42 (s, 2 H), 4.15 (q, J = 7.2 Hz, 2 H), 4.82 (br s, 1 H), 5.05 (s, 2 H), 6.10 (d, J = 2.4 Hz, 1 H), 6.58 (d, J = 2.4 Hz, 1 H), 6.98-7.00 (m, 2 H), 7.23-7.32 (m, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 14.1, 24.8, 28.3, 32.6, 40.3, 50.6, 60.6, 108.9, 114.0, 121.3, 126.4, 127.4, 128.7, 138.3, 155.9, 172.6.

MS (EI, 70 eV): m/z (%) = 386 (9.5) [M⁺], 256 (44), 59 (96.5), 43 (100).

1-Benzyl-4,6,7,8-tetrahydropyrrolo[3,2- d ]azepin-5(1 H )-one (19); Typical Procedure

To a soln of 17 (1.23 g, 3.18 mmol) in CH₂Cl₂ (32 mL) at 0 ˚C was added TFA (9.5 mL). The mixture was allowed to reach r.t. and was stirred for 1 h. The solvent and the excess TFA were evaporated in vacuo. The residue was dissolved in anhyd MeOH (32 mL) and K₂CO₃ (3.82 g) was added. The mixture was stirred for 16 h and then the solvent was evaporated in vacuo. The residue was dissolved with EtOAc (100 mL), washed with H₂O (60 mL), and dried (anhyd Na₂SO₄). The solvent was evaporated in vacuo and the residue was purified by column chromatography (hexane-EtOAc, 8:2) to afford 19 (0.63 g, 82%) as a white solid; mp 199-200 ˚C.

IR (KBr): 3209, 3087, 2905, 1667, 1490, 1444, 1400, 1356, 799, 721 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 2.61 (t, J = 5.7 Hz, 2 H), 3.51 (m, 2 H), 3.59 (s, 2 H), 4.95 (s, 2 H), 5.98 (d, J = 3.0 Hz, 1 H), 6.06 (s, 1 H), 6.60 (d, J = 3.0 Hz, 1 H), 6.95-6.98 (m, 2 H), 7.22-7.33 (m, 3 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 27.0, 34.7, 39.4, 50.5, 108.9, 111.5, 120.3, 125.8, 126.3, 127.4, 128.8, 137.8, 176.2.

MS (EI, 70 eV): m/z (%) = 240 (78.2) [M⁺], 183 (61.2), 91 (100).

Anal. Calcd for C₁₅H₁₆N₂O: C, 74.97; H, 6.71; N, 11.66. Found: C, 73.79; H, 6.80; N, 10.92.

4,6,7,8-Tetrahydropyrrolo[3,2- d ]azepin-5(1 H )-one (5)

To a soln of 19 (0.20 g, 0.84 mmol) in THF (15 mL) at -78 ˚C was added liquid NH₃ (∼15 mL) , followed by Na (100 mg, 4.1 mmol) in small portions. The mixture was stirred at -78 ˚C for 30 min, then quenched with solid NH₄Cl (400 mg). The mixture was allowed to reach r.t. and NH₃ was evaporated. Brine (20 mL) was added, and the mixture was extracted with CH₂Cl₂ (3 × 40 mL). The combined organic extracts were dried (anhyd Na₂SO₄) and the solvent evaporated in vacuo. The residue was purified by flash chromatography (CH₂Cl₂-MeOH, 97:3) to afford 5 (70 mg, 55%) as a white solid; mp >210 ˚C.

IR (KBr): 3241, 2886, 1669, 1479, 1428, 1353, 1139, 721 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 2.76 (t, J = 5.4 Hz, 2 H), 3.46 (s, 2 H), 3.47-3.53 (m, 2 H), 5.81 (t, J = 2.4 Hz, 1 H), 6.54 (t, J = 2.4 Hz, 1 H), 7.17 (br s, 1 H), 10.01 (br s, 1 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 27.3, 34.0, 38.8, 108.1, 109.1, 115.2, 124.6, 175.3.

MS (EI, 70 eV): m/z (%) = 150 (76.8) [M⁺], 93 (100).

Anal. Calcd for C₈H₁₀N₂O: C, 63.98; H, 6.71; N, 18.65. Found: C, 63.61; H, 6.47; N, 18.88.

3-Furoic Acid (20); Typical Procedure

Aq 1.32 M chromic acid soln [¹7] (50 mL) was added dropwise at 0-5 ˚C to a soln of furan-3-carbaldehyde (5.0 g, 52 mmol) in Et₂O (100 mL). The mixture was stirred for 3 h and then diluted with H₂O (50 mL). The organic layer was separated, dried (anhyd Na₂SO₄), and the solvent was evaporated in vacuo to give 20 (3.2 g, 55%) as a white solid; mp 120 ˚C (Lit. [¹8] 120-122 ˚C).

1-Diazo-2-(3-furoyl)ethanone (22); Typical Procedure

SOCl₂ (6 mL) was added to 3-furoic acid 20 (2.3 g, 20 mmol). The mixture was stirred at r.t. overnight and the excess SOCl₂ was evaporated in vacuo. The crude acyl chloride was used in the next step without further purification. To the soln of acyl chloride and Et₃N (3.0 mL) in THF (100 mL), a freshly prepared ethereal soln of diazomethane (60 mL) was added at 0 ˚C. The mixture was stirred for 2 h and the excess diazomethane, if present, was decomposed by the addition of 1 M AcOH. The solvent was evaporated in vacuo, and the residue was diluted with CH₂Cl₂ (100 mL). The mixture was sequentially washed with H₂O (50 mL), sat. NaHCO₃ soln (50 mL), and brine (50 mL), then dried (anhyd Na₂SO₄). The solvent was evaporated in vacuo and the residue was purified by flash chromatography (EtOAc-hexane, 2:8) to afford 22 (1.9 g, 70%) as a yellow solid; mp 37-38 ˚C.

IR (KBr): 3090, 3054, 2124, 1609, 1512, 1405, 1357, 1158, 733 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 5.58 (s, 1 H), 6.67 (dd, J = 4.5, 0.9 Hz, 1 H), 7.44-7.45 (m, 1 H), 7.91 (dd, J = 1.5, 0.9 Hz, 1 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 54.5, 108.2, 126.2, 144.1, 144.6, 180.2.

MS (EI, 70 eV): m/z (%) = 136 (100) [M⁺], 95 (81).

Methyl Furan-3-acetate (24); Typical Procedure

A soln of silver benzoate (0.46 g, 2.6 mmol) in Et₃N (4.2 mL) was added to a soln of 22 (1.4 g, 10.2 mmol) in MeOH (50 mL). The mixture was refluxed gently 2 h and then the solvent was evaporated under reduced pressure. The residue was dissolved in Et₂O (30 mL), washed with sat. NaHCO₃ soln (30 mL) and H₂O (30 mL), dried (anhyd Na₂SO₄), and concentrated in vacuo. The crude product was purified by flash chromatography (silica gel, EtOAc-hexane, 2:98) to give 24 (1.2 g, 85%) as a colorless oil.

IR (film): 2955, 1741, 1437, 1276, 1245, 1166, 1021, 874 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 3.46 (s, 2 H), 3.71 (s, 3 H), 6.37-6.38 (m, 1 H), 7.37-7.38 (m, 2 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 30.1, 51.9, 111.2, 117.2, 140.3, 142.9, 171.5.

MS (EI, 70 eV): m/z (%) = 140 (46.2) [M⁺], 112 (27.8), 81 (100).

Methyl 2-(Cyanomethyl)furan-3-acetate (26)

Following the typical procedure for 16 using 24 (1.0 g, 7.1 mmol) gave 26 (0.76 g, 60%) as the sole product as a yellow pale semisolid.

IR (film): 2955, 2922, 2257, 1738, 1437, 1275, 1202, 1171 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 3.45 (s, 2 H), 3.71 (s, 3 H), 3.77 (s, 2 H), 6.35 (d, J = 2.1 Hz, 1 H), 7.35 (d, J = 1.8 Hz, 1 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 15.8, 30.4, 52.2, 110.3, 112.9, 115.0, 139.7, 142.3, 170.7.

MS (EI, 70 eV): m/z (%) = 179 (35) [M⁺], 120 (95), 119 (100).

Anal. Calcd for C₉H₉NO₃: C, 60.33; H, 5.06; N, 7.82. Found: C, 60.83; H, 5.01; N, 7.25.

Methyl {2-[( tert -Butoxycarbonyl)amino]ethyl}furan-3-acetate (28)

Following the typical procedure for 17 using 26 (0.5 g, 2.8 mmol) gave 28 (0.57 g, 72%) as a colorless viscous oil.

IR (film): 3372, 2975, 1738, 1713, 1517, 1252, 1170 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 1.43 (s, 9 H), 2.79 (t, J = 6.6 Hz, 2 H), 3.38 (s, 2 H), 3.34-3.42 (m, 2 H), 3.70 (s, 3 H), 4.85 (s wide, 1 H), 6.30 (d, J = 2.1 Hz, 1 H), 7.29 (d, J = 1.8 Hz, 1 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 26.5, 28.3, 30.2, 38.0, 52.0, 79.2, 111.9, 113.3, 141.0, 150.0, 155.8, 171.7.

MS (EI, 70 eV): m/z (%) = 283 (2) [M⁺], 166 (100), 57 (98.7).

4,6,7,8-Tetrahydro-5 H -furo[3,2- d ]azepin-5-one (6)

Following the typical procedure for 19 using 28 (0.53 g, 1.9 mmol) gave 6 (0.21 g, 75%) as a white solid; mp 113-114 ˚C.

IR (KBr): 3221, 3089, 2953, 1662, 1478, 1425, 1409, 1179, 1126 cm^-¹.

^¹H NMR (500 MHz, CDCl₃): δ = 2.83-2.85 (m, 2 H), 3.48 (s, 2 H), 3.54-3.57 (m, 2 H), 6.17 (d, J = 1.5 Hz, 1 H), 6.70 (br s, 1 H), 7.26 (d, J = 2.0 Hz, 1 H).

^¹³C NMR (125 MHz, CDCl₃): δ = 28.5, 32.5, 38.8, 111.1, 112.1, 140.6, 148.6, 175.5.

MS (EI, 70 eV): m/z (%) = 151 (100) [M⁺], 94 (78).

Anal. Calcd for C₈H₉NO₂: C, 63.56; H, 6.00; N, 9.27. Found: C, 63.66; H, 6.42; N, 9.78.

Thiophene-3-carboxylic Acid (21)

Following the typical procedure for 20 using thiophene-3-carbaldehyde (4.0 g, 33 mmol) gave 21 (3.34 g, 80%) as a pale yellow solid; mp 139-140 ˚C (Lit. [¹9] 136-141 ˚C).

1-Diazo-2-(thiophen-3-yl)ethanone (23)

Following the typical procedure for 22 using 21 (2.5 g, 20 mmol) gave 23 (2.7 g, 90%) as a yellow solid; mp 57-58 ˚C.

IR (KBr): 3081, 3051, 2108, 1597, 1427, 1356, 1236, 829 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 5.74 (s, 1 H), 7.33 (dd, J = 5.1, 3.0 Hz, 1 H), 7.41 (dd, J = 5.1, 1.2 Hz, 1 H), 7.88 (dd, J = 3.1, 1.2 Hz, 1 H).

^¹³C NMR (75 MHZ, CDCl₃): δ = 54.4, 125.8, 126.6, 129.0, 140.7, 180.5.

MS (EI, 70 eV): m/z (%) = 152 (100) [M⁺], 111 (64.3), 96 (88.7).

Methyl Thiophene-3-acetate (25)

Following the typical procedure for 24 using 23 (1.2 g, 7.6 mmol), with the reaction carried out at r.t., gave 25 (0.94 g, 80%) as a colorless oil.

IR (film): 2954, 1741, 1438, 1337, 1268, 1155, 1014 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 3.66 (s, 2 H), 3.70 (s, 3 H), 7.04 (dd, J = 5.1, 1.2 Hz, 1 H), 7.13-7.15 (m, 1 H), 7.29 (dd, J = 5.1, 3.3 Hz, 1 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 35.6, 51.9, 122.8, 125.7, 128.4, 133.5, 171.5.

MS (EI, 70 eV): m/z (%) = 156 (28.7) [M⁺], 97 (100).

Methyl 2-(Cyanomethyl)thiophene-3-acetate (27)

Following the typical procedure for 16 using 25 (0.78 g, 5 mmol) gave 27 (0.44 g, 46%) as the sole product as a yellow pale semisolid.

IR (film): 2926, 2254, 1736, 1436, 1266, 1174 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 3.63 (s, 2 H), 3.71 (s, 3 H), 3.90 (s, 2 H), 6.94 (d, J = 5.4 Hz, 1 H), 7.21 (d, J = 5.1 Hz, 1 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 16.6, 29.5, 52.2, 123.3, 124.4, 127.1, 129.9, 132.0, 170.4.

MS (EI, 70 eV): m/z (%) = 195 (39) [M⁺], 136 (100).

Anal. Calcd for C₉H₉NO₂S: C, 55.37; H, 4.65; N, 7.17. Found: C, 55.14; H, 4.11; N, 7.45.

Methyl 2-{2-[( tert -Butoxycarbonyl)amino]ethyl}thiophene-3-acetate (29)

Following the typical procedure for 17 using 27 (0.44 g, 2.25 mmol) gave 29 (0.52 g, 78%) as a colorless viscous oil.

IR (film): 3374, 2975, 1738, 1713, 1517, 1252, 1170 cm^-¹.

^¹H NMR (300 MHz, CDCl₃): δ = 1.44 (s, 9 H), 2.97 (t, J = 6.6 Hz, 2 H), 3.33-3.40 (m, 2 H), 3.57 (s, 2 H), 3.69 (s, 3 H), 4.84 (br s, 1 H), 6.90 (d, J = 5.1 Hz, 1 H), 7.12 (d, J = 5.1 Hz, 1 H).

^¹³C NMR (75 MHz, CDCl₃): δ = 28.3, 30.4, 33.7, 41.5, 52.0, 79.3, 122.8, 126.7, 129.1, 130.1, 155.8, 171.5.

MS (EI, 70 eV): m/z (%) = 299 (82) [M⁺], 243 (11.2), 182 (66.8), 57 (100).

4,6,7,8-Tetrahydro-5 H -thieno[3,2- d ]azepin-5-one (7)

Following the typical procedure for 19 using 27 (0.5 g, 1.7 mmol) gave 7 (0.2 g, 70%) as a solid; mp 149-150 ˚C.

IR (KBr): 3190, 3095, 2916, 1692, 1630, 1478, 1431, 1403 cm^-¹.

^¹H NMR (500 MHz, CDCl₃): δ = 3.05 (t, J = 5.5 Hz, 2 H), 3.63-3.66 (m, 2 H), 3.74 (s, 2 H), 6.54 (br s, 1 H), 6.72 (d, J = 5.5 Hz, 1 H), 7.09 (d, J = 5.0 Hz, 1 H).

^¹³C NMR (125 MHz, CDCl₃): δ = 29.7, 36.5, 122.5, 127.6, 129.9, 135.1, 174.8.

MS (EI, 70 eV): m/z (%) = 167 (64.3) [M⁺], 110 (100).

Anal. Calcd for C₈H₉NOS: C, 57.46; H, 5.42; N, 8.38. Found: C, 57.83; H, 5.74; N, 8.67.

Acknowledgment

We wish to thank DGAPA, UNAM (Proyect PAPIIT 1N-204910) for financial support. We also thank R. Patiño, J. Perez, L. Velazco, H. Rios, N. Zavala, E. Huerta, and A. Peña for technical support. We also thank M.Sc. Vilchis, M. and M.Sc. Solano, J. for cytotoxicity tests. Carlos Villarreal is CONACyT Graduate Scholarship holder.

References

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• 17 Furniss BS. Hannaford AJ. Smith PWG. Tatchell AR. Vogel’s Textbook of Practical Organic Chemistry   5th ed.:  Longman; Essex: 1989.  p.609 
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References

• 1 Martínez R. Clara-Sosa A. Ramírez Apan Ma T. Bioorg. Med. Chem.  2007,  15:  3912 
  CrossrefSearch in Google Scholar
• 2 Martínez R. Ávila JG. Ramírez MaT. Pérez A. Martínez A. Bioorg. Med. Chem.  2006,  14:  4007 
  CrossrefSearch in Google Scholar
• 3a Lahusen T. De Siervi A. Kunick C. Senderowicz A. Mol. Carcinog.  2003,  36:  183 
  Search in Google Scholar
• 3b Kunick C. Zeng Z. Gussio R. Zaharevitz D. Leost M. Totzke F. Schaechtele C. Kubbutat MHG. Meijer L. Lemcke T. ChemBioChem  2005,  6:  541 
  Search in Google Scholar
• 4 van Leusen AM. Siderius H. Hoogenboom H. Van Leusen D. Tetrahedron Lett.  1972,  13:  5337 
  CrossrefSearch in Google Scholar
• 5 Bachman WE. Stuve WS. Org. React.  1942,  1:  38 
  Search in Google Scholar
• 6 Furniss BS. Hannaford AJ. Smith PWG. Tatchell AR. Vogel’s Textbook of Practical Organic Chemistry   5th ed.:  Longman; Essex: 1989.  p.430 
  Search in Google Scholar
• 7 Demopoulos VJ. Synth. Commun.  1989,  19:  2585 
  CrossrefSearch in Google Scholar
• 8 Bray BL. Mathies PH. Naef R. Solas DR. Tidwell TT. Artis DR. Muchowski JM. J. Org. Chem.  1990,  55:  637 
  Search in Google Scholar
• 9 Osornio YM. Cruz-Almanza R. Jiménez-Montaño V. Miranda LD. Chem. Commun.  2003,  2316 
  Crossref
• 10 Caddick S. Judd DB. Lewis AK.de K.. Reich MT. Williams MRV. Tetrahedron  2003,  59:  5417 
  CrossrefSearch in Google Scholar
• 11 Caddick S. Haynes AK.de K. Judd DB. Williams MRV. Tetrahedron Lett.  2000,  41:  3513 
  CrossrefSearch in Google Scholar
• 12 Reyes-Gutierrez PE. Torres-Ochoa RO. Martínez R. Miranda LD. Org. Biomol. Chem.  2009,  7:  1388 
  CrossrefSearch in Google Scholar
• 13 Newman MS. Beal PF. J. Am. Chem. Soc.  1950,  72:  5163 
  CrossrefSearch in Google Scholar
• 14 Still WC. Khan M. Mitra A. J. Org. Chem.  1978,  43:  2923 
  CrossrefSearch in Google Scholar
• 15 Flitsch W. Pandl K. Ruβkamp P. Liebigs Ann. Chem.  1983,  529 
  Crossref
• 16 Shim YK. Youn JI. Chun JS. Park TH. Kim MH. Kim WJ. Synthesis  1990,  753 
  Thieme Connect
• 17 Furniss BS. Hannaford AJ. Smith PWG. Tatchell AR. Vogel’s Textbook of Practical Organic Chemistry   5th ed.:  Longman; Essex: 1989.  p.609 
  Search in Google Scholar
• 18 Ghosh A. Raha CR. Indian Chem. Soc.  1954,  31:  461 
  Search in Google Scholar
• 19 Friedmanh L. Fishel DL. Shechter H. J. Org. Chem.  1965,  30:  1457 
  CrossrefSearch in Google Scholar