Conformational Analysis of 5-Substituted 1,3-Dioxanes. 2. Phenylthio and Cyclohexylthio Groups and Their Sulfinyl and Sulfonyl Derivatives

The positions of equilibrium, established by acid catalysis, between diastereomeric cis- and trans-5-(phenylthio)-(7), 5-(phenylsulfinyl)- (8), 5-(phenylsulfonyl)- (9), 5-(cyclohexylthio)- (10), 5-(cyclohexylsulfinyl)- (11), and 5-(cyclohexylsulfonyl)-2-tert-butyl-1,3-dioxanes (12) are reported and compared with published data for the S-(methylthio)- (1), 5-(methylsulfinyl)- (2), 5-(methylsulfonyl)- (3), 5-(tert-butylthio)- (4), 5-(tert-butylsulfinyl)-(5), and 5-(tert-butylsu1fony1)-2-isopropy1-1 1,3-dioxanes (6). Although AG° values for sulfides 1, 4, and 7 are very similar, there exist significant differences in the conformational behavior of the sulfoxides and the sulfones, which result from the relative steric, electrostatic, and torsional effects arising from each substituent. The information at hand (1H and 13C NMR spectroscopic data, X-ray crystallographic analysis, etc.) shows that, while sulfone cis-6 has the S-tert-butyl group outside the ring, with both sulfonyl oxygens above the dioxane ring and eclipsing the endocyclic C-C bonds, the alkyl and aryl substituents in cis-3, cis-9, and cis-12 point inside the ring. The phenylsulfonyl- inside rotamer in cis-9 leads to steric and electron/electron repulsion that overcomes the electrostatic attractive interactions between the (negative) endocyclic oxygens and the (positive) sulfur in the sulfonyl group, so that equatorial trans-9 predominates at equilibrium, AG° = -0.44 kcal/mol. All sulfoxides place both the sulfinyl oxygen and the substituent outside the dioxane ring.