Intermolecular ene insertion of photochemically generated thioaldehyde Me02CCH= S with 8-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 ...