Computer-assisted site-specific functionalization in the gibberellin series; norrish type II photocyclization via a 9-membered transition state

The site-specific Norrish II type 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 structure of the photolactone was determined by spectroscopic data, especially NOE measurements.