Thieme Chemistry Journal Awardees - Where Are They Now? Microwave-Assisted Rhodium-Catalyzed Decarbonylation of Functionalized 3-Formyl-2H-chromenes: A Sequence for Functionalized Chromenes like Deoxycordiachromene

Abstract

3-Formyl-2H-chromenes which are readily accessible through an oxa-Michael reaction of salicylaldehydes and α,β-unsaturated aldehydes undergo a smooth decarbonylation reaction upon treatment with rhodium catalysts. With our method, a great variety of functionalized chromenes is accessible in a two-step sequence from salicylaldehydes.

Key words

domino oxa-Michael-aldol reaction - decarbonylation - benzopyran - rhodium-catalysis - natural products

References and Notes

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Typical Procedure for the Rh-Catalyzed Deformylation of 3-Formyl-2 H -chromenes
Formylchromene 3a (390 µmol), RhCl₃˙xH₂O (19.5 µmol, 5 mol%), and dppp (39.0 µmol, 10 mol%) in diglyme (2 mL) were refluxed under argon for 16 h. After cooling, pentane (10 mL) was added, and the mixture was washed with H₂O (5 × 5 mL). The organic layer was dried over Na₂SO₄ and evaporated. The crude product was purified by column chromatography on SiO₂.

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Selected Data
Compound 4a: ^¹H NMR (400 MHz, CDCl₃): δ = 1.40 (s, 3 H), 1.58 (s, 3 H), 1.60-1.79 (m, 2 H), 1.67 (s, 3 H), 2.12 (m_c, 2 H), 5.11 (tt, J = 7.2, 1.4 Hz, 1 H), 5.56 (d, J _cis = 9.8 Hz, 1 H), 6.36 (d, J _cis = 9.8 Hz, 1 H), 6.77 (d, J = 8.0 Hz, 1 H), 6.82 (ddd, J = 7.6, 7.4, 1.2 Hz, 1 H), 6.96 (dd, J = 7.6, 1.6 Hz, 1 H), 7.09 (ddd, J = 8.0, 7.4, 1.6 Hz, 1 H). ^¹³C NMR (100 MHz, CDCl₃): δ = 17.6, 22.7, 25.7, 26.5, 41.3, 78.4, 116.1, 120.5, 121.1, 122.8, 124.1, 126.3, 129.0, 129.6, 131.7, 153.2. MS-FAB: m/z (%) = 229.2 (11) [M⁺ + H], 228.2 (18) [M⁺], 145.1 (100) [M⁺ - C₆H₁₁], 136.1 (10). HRMS: m/z calcd for C₁₆H₂₀O: 228.1514. Found: 228.1510. Anal. Calcd for C₁₆H₂₀O: C, 84.16; H, 8.83. Found: C, 84.14; H, 8.69.