Free radical alkylation of tri-O-benzoyl-6-exo-bromo-levoglucosan: approaches to the synthesis of okadaic acid

Synthetic studies toward (-)-FR901483 using a conjugate allylation to install the C-1 quaternary carbon

Two approaches to the aza-tricyclo dodecane skeleton of (-)-FR901483 are reported. Both routes utilized a Grignard addition to an N-acylpyridinium salt to establish the absolute stereochemistry at C-6 and a highly diastereoselective conjugate allylation reaction to form the quaternary center at C-1 of the natural product in an excellent yield. Although the desired polysubstituted piperidine intermediates were prepared regio- and stereoselectively, the construction of the C-8/C-9 bond connectivity could not be achieved. All attempts at a pinacol cyclization or an intramolecular 6-exo-tet epoxide opening were unsuccessful because of an unfavorable A(1,3) strain inherent in the molecule.