TANDEM ALDOL-CYCLIZATION SEQUENCE FOR THE CONSTRUCTION OF CYCLE ETHERS - THE FORMATION OF SUBSTITUTED TETRAHYDROFURANS

The application of a tandem deconjugative aldol-cyclization Sequence f or the construction of substituted tetrahydrofurans was examined. The aldol condensation of alkenoates proceeded with alkylation at the alph a-position to generate homoallylic alcohol moieties. These compounds c ould be induced to cyclize under the influence of iodine via an endo m ode. The stereoselectivity for the cyclization occurred in good to exc ellent fashion. X-ray crystal structure analysis of three of the tetra hydrofurans established unambiguously the product stereochemistry. Thi s was: used to propose a transition structure for the cyclization whic h correctly predicts the observed product stereochemistry. By this met hod, virtually all the possible stereoisomers for the substituted tetr ahydrofurans can be constructed by judicious choice of aldol product a nd/or olefin geometry.