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

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.