Regioselective Synthesis of 2-Arylthio-4-methoxybenzoates by Formal [3+3] Cyclocondensations of 3-Arylthio-1-trimethylsilyloxybuta-1,3-dienes with 3-Oxo-orthoesters

Abstract

A variety of 2-arylthio-4-methoxybenzoates are regio­selectively prepared by the first formal [3+3] cyclocondensations of 3-arylthio-1-trimethylsilyloxybuta-1,3-dienes with 3-oxo-ortho­esters.

Key Words

arenes - cyclizations - diaryl sulfides - regioselectivity - silyl enol ethers

References and Notes

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18

General Experimental Procedure
To a CH₂Cl₂ soln (3 mL/mmol of 1) of 2 (1.5 mmol) and 1 (2.25 mmol) was added TiCl₄ (2.25 mmol) at -78 ˚C. The solution was allowed to warm to 20 ˚C within 20 h. To the solution was added HCl (10%, 25 mL). The organic and the aqueous layer were separated and the latter was extracted with CH₂Cl₂ (3 × 20 mL). The combined organic layers were dried (Na₂SO₄), filtered, and the filtrate was concentrated in vacuo, and the residue was purified by chromatography (SiO₂, EtOAc-n-heptane = 1:9).
4-Methoxy-6-methyl-2-(3-tolylsulfanyl)benzoic Acid Methyl Ester (3b)
Starting with 1a (2.25 mmol), 2b (1.5 mmol), and TiCl₄ (2.25 mmol), 3b was isolated as a yellow oil (226 mg, 50%). ^¹H NMR (250 MHz, CDCl₃): δ = 2.25 (s, 3 H, CH₃), 2.30 (s, 3 H, CH₃), 3.69 (s, 3 H, OCH₃), 3.87 (s, 3 H, OCH₃), 6.57-7.39 (m, 6 H, Ar). ^¹³C NMR (63 MHz, CDCl₃): δ = 20.4 (CH₃), 21.2 (CH₃), 51.9 (OCH₃), 55.2 (CH₃O), 114.1, 114.5, (CH_Ar), 126.9, 128.3 (C), 129.4, 131.5, 132.1, 133.0 (CH_Ar), 138.3, 139.1, 140.1, 146.9, 168.8 (C). IR (KBr): ν = 2998 (w), 2947 (w), 2834 (w), 2735 (w), 2570 (w), 1724 (s), 1589 (s), 1564 (m), 15560 (m), 1471 (m), 1426 (m), 1380 (w), 1302 (m), 1264 (s), 1217 (s), 1187 (m), 1136 (s), 1090 (s), 1047 (s), 998 (m), 960 (m), 776 (s), 691 (s), 608 (m). MS (EI): m/z (%) = 302 (100) [M⁺], 272 (11), 271 (61), 270 (21), 269 (86), 256 (18), 255 (49),228 (17), 227 (13), 184 (15), 121 (3), 91 (4), 65 (5). HRMS (EI): m/z calcd for C₁₇H₁₈O₂O₃S [M⁺]: 302.09712; found: 302.097152.