PHOTOCHEMICAL-REACTIONS OF SULFIDE-CONTAINING ALKYL PHENYLGLYOXYLATES

Ethylthioalkyl phenylglyoxylates (2) underwent regioselective photocyc lization to produce up to 13-membered ring thiacyclols (3) in good yie lds. Cyclization occurred between the excited carbonyl group and the c arbon a to the sulfur on the remote side. The more methylenes placed b etween the carbonyl group and the sulfide, the lower the yield of 3 wh ile intermolecular hydrogen abstraction induced reductive dimerization and intramolecular Norrish Type II photolysis become competitive. The remote cyclization can be rationalized by a photoinduced electron tra nsfer from the sulfur atom to the excited carbonyl group followed by p roton transfer and subsequent cyclization of the resulting biradical, Rate constants for electron transfer increase as the chain connecting the electron donor and acceptor becomes longer and more flexible. (C) 1997 Elsevier Science Ltd.