Internal Thioaldehyde Trapping by Enes and Dienes

Intermolecular ene insertion of photochemically generated thioaldehyde MeO₂CCH=S with β-pinene occurs at room temperature to give the sulfide 2c. The internal ene insertion of thioaldehyde 6 affords a six-membered carbocycle 9 rather than the seven-membered sulfide 10 which would correspond to the regiochemistry of the intermolecular process (eq 5, 6). Likewise, internal Diels-Alder trapping occurs without a dominant role for the regiochemical preferences seen in intermolecular reactions. Desulfurization of typical internal adducts such as 25,26 or 29,30 affords 31 and 33,34, respectively.