SIMPLE DESIGNS FOR THE CONSTRUCTION OF COMPLEX TRANS-FUSED POLYETHER TOXIN FRAMEWORKS - A LINEAR STRATEGY BASED ON ENTROPICALLY FAVORED OXIRANE RING ENLARGEMENT IN EPOXYCYCLOALKENES FOLLOWED BY CARBON-CARBON OR CARBON-OXYGEN BOND-FORMING CYCLIZATIONS

A successful design for the construction of trans-fused medium-size cyclic ethers is described. The key features of the synthesis are as follows: (i) intramolecular oxirane ring expansion in cycloalkenes to give bridged oxabicyclic systems and (ii) linear, one- or two-directional synthetic operations which generate external oxocycles in single reaction steps. The general approach involves the intramolecular addition of a stable gamma-alkoxy-substituted allylstannane to an aldehyde carbonyl group, and the entire reaction is conducted in a one-pot process which includes the following: (i) vic-diol fragmentation from the bridged oxabicyclic precursor and (ii) Lewis acid-induced cyclization of the resulting aldehyde-allylic tin system. While the present strategy was mostly developed around racemic models, the potential for adoption of;enantioselective features is immediate. The versatility, scope, limitations, and potential applications of the present technology are discussed in detail.