CONFORMATIONAL EFFECTS ON THE 2,3-SIGMATROPIC SHIFT OF SULFUR-STABILIZED YLIDES AND ENOLATES INVOLVING BICYCLIC TRANSITION-STATES

The enolate anions of 2-(carboethoxy)-6-vinylthiane (1) or of 2-(carboethoxy)-8-vinylthiacyclooct-4-ene (2) or the corresponding sulfone 27 do not undergo an anionic 2,3-sigmatropic shift at temperatures up to 50 °C. Double-bond migration is the only observable process with 1 or 27. The ylide 29 obtained from 1 by S methylation and deprotonation does rearrange to a cycloheptene derivative 30, which can be considered as the product of a 2,3 shift (50%). However, two diastereomeric cyclopentane derivatives 31 are also formed by a Stevens rearrangement (25%). An analogous ylide 37 derived from 2 gives a Stevens product cycloheptene derivative 38 and ylide fragmentation products, the triene esters 39 and 40. No 2,3 shift is observed with 37. The reluctance of these ylides to undergo a 2,3 shift is attributed to conformational and transannular destabilization of the necessary bicyclic transition states. A similar barrier apparently prevents rearrangement of 3,6-divinyl-1,2-dithiane (15) to products of a 2,3 shift. The synthesis of 2 involves 3-carbon ring expansion of 2,5-divinyltetrahydrothiophene (8), which in turn is made by ionic cyclization of sulfide alcohol 18. Condensation of the anion of allyl tetrahydropyranyl sulfide with 5-bromopent-2-en-l-ol gives 18, the product of α alkylation, without detectable amounts of γ alkylation. © 1979, American Chemical Society. All rights reserved.