Synthesis of natural products based on photochemical key transformations

Abstract- Three photochemical methods aiming primarily at syntheses of cyclopentanoid natural products are described: (A) The development and synthetic applications of four genera-tions of tricyclooctanone-type key intermediates- all products of oxadi-ir-methane rearrangements- are reviewed. Most recent achievements are the total syntheses of (-)-5-oxosilphiper-fol-6-ene and of (-)-coriolin. (B) The [2+2] photocycloaddition of 2-trimethylsiloxybutadiene to a number of standard enones proved to be quite a general re-action with appreciable yields. This finding is surprising since dienes in general are known to quench enone triplets efficiently and hence suppress chemical reactions. Both the high substrate concentrations, which can be employed in the present procedure, and the remarkable regio- and stereocontrol of the addition pro-cesses qualify this annulation as a valuable synthetic method. One photoproduct is the point of departure for an expedient as-sembly of the basic pentalenolactone skeleton. (C) Spirocyclic chiral 1,3-dioxenones, prepared in a one-step procedure, served for highly selective asymmetric [2+2] d. Mayo-type photocycloadditions with various olef ins. The value of this novel method is documented by an exploratory preparative example, a short synthesis of (+)-grandisol