Polyketides, spiroacetal

Analogous treatment of the unsaturated ester 207 using 1,1-dibromoethane in this case cleanly produced the dihydropyran 208 in high yield on a gram scale (Scheme 2.73) [41bj. Cydic enol ether 208 was then efficiently advanced to a spiroacetal, which corresponds to the core structure of the bioactive polyketide spirofungin A. 

The products of such processes have been manipulated in a variety of ways. Thus, the derived 0-TMS ethers undergo TMSNTf2-promoted conversion into cw-2,6-disubstituted tetrahydropyrans, acetals, or spiroacetals. In certain cases, 2,6-bridged tetrahydropyrans can be formed in the presence of Sn(OTf)2. Reactions analogous to those shown in eq 10 have been used during the development of total syntheses of the polyketide (+)-dactylolide as well as the alkaloids (—)-adaline and (—)-euphococcinine. ... 

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