A Copper(I)-Catalyzed Photobicyclization Route to exo-1,2-Polymethylene- and 7-Hydroxynorbornanes. Nonclassical 2-Bicycloheptyl and 7-Norbornyl Carbenium Ion Intermediates

Article abstract of DOI:10.1021/jo00363a029

A synthesis of 7-hydroxynorbornanes from 3-hydroxy-1,6-heptadienes is achieved by copper(I) trifluoromethanesulfonate catalyzed 2? + 2? photobicyclization and solvolytic rearrangement of intermediate cyclobutylcarbinyl alcohols.The rearrangements are remarkably stereoselective and stereospecific. exo-1,2-Polymethylene-7-hydroxynorbornanes are obtained stereoselectively from both cis- and trans-2-allyl-1-vinylcycloalkanols owing to rapid cis-trans epimerization at the 3-position of intermediate 2,3-polymethylenebicyclo<3.2.0>hept-2-yl carbenium ions.The importance of bridged (nonclassical) carbenium ion intermediates in the 2-bicyclo<3.2.0>heptyl to 7-norbornyl rearrangements is poignantly evidenced by stereospecific inversion at the migration origin which delivers 1,2,2-trimethylnorbornan-syn-7-ol in the face of a severe countervailing steric impediment.