STEREOCHEMISTRY OF RING ENLARGEMENT BY [2,3] SIGMATROPIC REARRANGEMENT OF CYCLIC SULFONIUM YLIDES - SYNTHESIS OF 5-METHYLTHIACYCLOALK-4-ENES

Sulfonium ylides generated in situ from r-l, t-2-dimethyl-2-vinylthiolanium (lb), r-l, t-2-dimethyl-2-vinylthianium (2b), and r-1, c-2-dimethyl-2-vinylthianium (2c) hexafluorophosphates undergo highly stereoselective but nonstereospecific [2, 3] sigmatropic ring enlargement. The six-membered methylide from 2b, where the carbanionic and vinyl moieties are on the same side of the ring, reacts faster than the ylide from 2c; however, both give (E)-5-methylthiacyclonon-4-ene as the exclusive product. The five-membered methylide from lb ring expands, giving a ca. 17:1 mixture of (Z)- and (E)-5-methylthiacyclooct-4-ene. The isomeric methylide from lc, where the carbanionic and vinyl moieties are on opposite sides of the ring, does not appreciably react in the [2, 3] sigmatropic fashion but undergoes βelimination, probably by the α, β mechanism. The results are rationalized in terms of ground-state conformational effects, in accord with an early transition state for the [2, 3] sigmatropic rearrangement of allylic sulfonium ylides. Copyright © 1979, American Chemical Society. All rights reserved.