Epoxidation torsional effect

Stereoselectivity in the epoxidation of monocyclic or fused cyclohexene derivatives in a halfchair conformation is governed by steric repulsions torsional effects are mostly negligible. One exception is the epoxidation of sterpurene (1), which forms a single epoxide 2 structural assignment of this epoxide was not attempted171. 

Further work on the conformation of certain germacrane sesquiterpenoids has been performed using both X-ray analyses and the intramolecular nuclear Overhauser effect (n.o.e.). Complete details of the X-ray analysis of the silver nitrate adduct of germacratriene have been published. In addition to demonstrating its conformation as (222), this analysis correlates the known reactivity (to epoxidation) of the three double bonds (4=5 > 1=10 > 7=11) with their torsional strains. The X-ray analysis of a heavy-atom derivative of pyrethrosin (223) has revealed that some stereochemical adjustments are required with respect... 

The torsional rigidity of an epoxide (or episulfide or ethyleneimine) group and of a carbon-carbon double bond must have a definite effect on the shape of any ring to which the former are attached or in which the latter is incorporated. When these groupings are associated with a pyranoid ring, the latter must adopt a half-chair, a boat, or a skew conformation, and it has been calculated that, for cyclohexene, the half-chair is energetically more favored than the boat by 2.7 Kcal./mole. The conformation of... 

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