Intramolecular [4+2] cycloaddition of a photogenerated o-quinone methide in aqueous solution

Using an appropriately substituted o-hydroxybenzyl alcohol precursor 5, it is shown that a photogenerated o-quinone methide can undergo efficient intramolecular [4 + 2] (Diels-Alder) cycloaddition in aqueous CH3CN 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 show the intermediacy of an o-quinone methide (h(max) approximate to 400 nm) which has a lifetime > 2 ms. Quantum yields for reaction and fluorescence parameters are also reported which depend strongly on the proportion of water in the H2O-CH3CN solvent mixture. (C) 1997 Elsevier Science S.A.