One-Pot Stereoselective Synthesis of Pyrano[3,2-c]benzothiopyrans: A New Generation and [4+2] Cycloaddition of ortho-Thioquinonemethides

Abstract

Pyrano[3,2-c]benzothiopyrans are synthesized from thiosalicylaldehyde derivatives and unsaturated alcohols in the presence of trimethyl orthoformate and a catalytic amount of p-toluenesulfonic acid with complete trans-stereoselectivity via intramolecular [4+2] cycloaddition of ortho-thioquinonemethides. A more efficient generation and cycloaddition of ortho-thioquinone­methides with Lewis acid is also described.

Key words

stereoselective synthesis - [4+2] cycloaddition - Lewis acid - sulfur - heterocycles

References

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9

Compound 5a: ¹H NMR (400 MHz, CDCl₃): δ = 1.21 (d, J = 6.1 Hz, 3 H), 1.45 (m, 1 H), 1.50 (s, 3 H), 1.71-1.80 (m, 2 H), 1.92 (m, 1 H), 2.47 (ddd, J = 2.9, 10.0, 11.5 Hz, 1 H), 3.42 (s, 3 H), 3.67 (dqd, J = 2.0, 6.1, 11.1 Hz, 1 H), 4.60 (m, 2 H), 4.85 (d, J = 11.2 Hz, 1 H), 4.93 (d, J = 11.2 Hz, 1 H), 4.95 (d, J = 10.0 Hz, 1 H), 7.21 (ddd, J = 1.7, 7.3, 7.6 Hz, 1 H), 7.23 (ddd, J = 1.5, 7.3, 7.6 Hz, 1 H), 7.43 (dd, J = 1.7, 7.6 Hz, 1 H), 7.56 (dd, J = 1.5, 7.6 Hz, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 21.4, 22.2, 30.7, 33.7, 48.5, 56.1, 74.4, 78.6, 79.5, 112.3, 127.2, 127.6, 128.0, 131.6, 135.5, 141.4, 145.8.

10

Compound 6 was purified by silica gel column chromatography, and was characterized by NMR.