ENYNONES IN ORGANIC-SYNTHESIS .6. SYNTHESIS OF SPIROCYCLIC METHYLENECYCLOPENTENONES AND ANALOGS OF THE METHYLENOMYCIN CLASS OF ANTIBIOTICS - MECHANISM OF PHENOL CATALYSIS

Spirocyclic methylenecyclopentenones of general structure 18 were prep ared in a single step from bis-acetylenic alcohols 29 by a process inv olving initial oxy-Cope rearrangement to afford (Z)-enynones 30-Z foll owed by electrocyclic ring closure. Mechanistic studies indicate that the initial step leading from 30-Z to 18 is a thermal 1,5-prototropic shift to afford dienols which can cyclize by a symmetry-allowed (pi(4) s + sigma(2)s + pi(2)a) process. This last step is catalyzed by certai n phenols having low oxidation potentials, most likely by a mechanism involving single electron transfer. Dramatic rate enhancements were al so observed for the cyclization of simple enynones 37 to methylenecycl opentenones 39 upon catalysis with either a-tocopherol (vitamin E, 40) or tert-butylcatechol(41). Further enhancements in both rate and yiel d were obtained under conditions of photoassisted single electron tran sfer (PET), which afforded 39 in yields of 80-98%.