A New Stereoselective and Convergent Approach to the Synthesis of Long- Chain Polypropionate Fragments.

Optically pure Diels-Alder adducts of 2,4-dimethylfuran and 1-cyanovinyl (1'R)- or (1'S)-camphanate are obtained readily. These 1,5-dimethyl-7-oxabicyclo[2.2.1]hept-5-en-1-yl derivatives can be converted with high stereoselectivity into polypropionate fragments such as (-)-(2R,3R,4R,5S)-5-(benzyloxy)-3-(isopropyloxy)-2,4-dimethylhept-6-enal ((-)-16) or (-)-(2R,2'S,3'S,4'S,5'S)-2-[4-(benzyloxy)tetrahydro-3,5-dimethyl-5-metho xyfur-2-yl]propanal ((-)-20). The latter underwent stereoselective cross-aldolization with the lithium enolate of (+)-(1S,4S,5S,6S)-6-exo-(benzyloxy)-1,5-endo-dimethyl-7-oxabicyclo[2.2.1 ]heptan-2-one ((+)-9) giving an aldol ((-)-23); like mode of aldolization) that can be converted into a polypropionate fragment containing nine contiguous stereogenic centers.