Cyclohexanols, stereochemical effects

An interesting stereochemical profile of this cyclization is that in the five-membered product structures, substituents PhS and OH groups are placed cis to each other, whereas in six-membered products the placement is trans (Table 6, entry 7). The cis selectivity in the five-membered ring systems is not affected by a and fi-substituents of the alkoxides (entries 2, 3 and 4), indicating that the steric effect is not the dominant factor. Instead, interaction between the oxido and carbene center composes a five- (or seven-) membered transition state 28, which allows the carbene to abstract the nearest quasi axial hydrogen as a hydride to produce a carbonyl intermediate 29, leading to the cyclization products 27 and 30 (Scheme 13, Eq. 1). Similarly, the stereoselective stepwise cyclization of cis- and rra s-2-(3,3-dithiopropyl)cyclohexanol to 2-phenylthio-... 

The effect of auxiliary structure on the level of absolute stereochemical control was extensively studied, suggesting that the phenyl groups in both 5-methyl-2-(l -methyl-1-phenyl)cyclohexanol and tra .s-2-phenylcyclohexanol are important elements for practical levels of control87. Studies were also conducted to uncover the nature of the influence of the phenyl group on the competing transition states88. In all cases, those systems that provided high levels of control in ene reactions in the presence of Lewis acids afforded essentially no control when used in reactions under thermal conditions. 

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