Synthesis of Highly Functionalized 1,3-Oxathioles via an Unusual [4+1] Annulation of α,α'-Dioxothione with 1,2-Diaza-1,3-dienes

Abstract

The present method provides highly functionalized 1,3-oxathioles through an unusual base-promoted [4+1] annulation between α,α'-dioxothione, generated in situ from a phthalimide precursor, and 1,2-diaza-1,3-dienes. An intriguing scaffold comprised of spiro-fused oxathiole and pyrazolone components was also obtained in moderate yield.

Key words

oxathioles - Michael addition - [4+1] annulation - spiro compounds

• References and Notes

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• 20 To a solution of oxathiole 4e (0.194 g, 0.5 mmol) in CH₂Cl₂ (4 mL) was added TFA (0.057 g, 0.5 mmol) at 25 °C. The solution was stirred for 2 h at r.t., the solvent was removed in vacuo and the residue was purified by column chromatography (silica gel; cyclohexane–EtOAc, 80:20) to give 5 as a pale-yellow waxy solid (0.048 g, 38%). Ethyl 2,6-Dimethyl-9-oxo-1-oxa-4-thia-7,8-diazaspiro-[4.4]nona-2,6-diene-3-carboxylate (5): ¹H NMR (400 MHz, CDCl₃): δ = 1.29 (t, J = 7.2 Hz, 3 H), 2.20 (s, 3 H), 2.31 (s, 3 H), 4.22 and 4.23 (q, J = 7.2 Hz, 2 H), 8.57 (s, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 12.7 (q), 14.2 (q), 14.3 (q), 61.4 (t), 87.8 (s), 101.7 (s), 156.9 (s), 159.0 (s), 161.8 (s), 171.7 (s). IR (Nujol): 3349, 3200, 1736, 1712, 1641, 1284, 1087 cm^–1. MS: m/z (%) = 256 (35) [M]⁺, 213 (100), 185 (66), 141 (36). Anal. Calcd for C₁₀H₁₂N₂O₄S (256.28): C, 46.87; H, 4.72; N, 10.93. Found: C, 46.74; H, 4.77; N, 11.01.
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