INTRAMOLECULAR [4-CYCLOADDITION OF A PHOTOGENERATED O-QUINONE METHIDEIN AQUEOUS-SOLUTION(2])

Using an appropriately substituted o-hydroxybenzyl alcohol precursor 5 , it is shown that a photogenerated o-quinone methide can undergo effi cient intramolecular [4 + 2] (Diels-Alder) cycloaddition in aqueous CH 3CN solution at 15 degrees C, to generate the hexahydrocannabinol ring system 11. The dienophile tether must be sufficiently electron rich, having at least three alkyl groups. Laser flash photolysis studies sho w the intermediacy of an o-quinone methide (h(max) approximate to 400 nm) which has a lifetime > 2 ms. Quantum yields for reaction and fluor escence parameters are also reported which depend strongly on the prop ortion of water in the H2O-CH3CN solvent mixture. (C) 1997 Elsevier Sc ience S.A.