The site-specific Norrish IItype reaction of the gibberellin oxo ester , prepared in two steps from tetraacetyl- GA3, anhydride , to the photolactone , is reported. By potential energy calculations based on the X-ray analysis of , it is shown that the regio- and stereoselectivity of the photocyclization can be predicted involving a 9-membered cyclic transition state in the H-abstraction step. The