Epoxide ring, conformation

A detailed examination of OSO4 reactions with A -steroids has been reported." The A-ring conformation of the reactant or derived complex is important in determining the stereoselectivity of these reactions, and the major role of the proximate substituents is to anchor the appropriate conformation favouring a- or /3-attack. Studies on the stereochemistry of electrophilic attack on cholest-5-en-3-one continue." As with bromine chloride," appreciable /3-attack occurs and the 5/3,6j8-epoxide was isolated along with the previously reported 5a,6a-epoxide and the Baeyer-Villiger product, the A-homo-enol lactone (58). Base-catalysed... 

A number of studies have now been made, notably by Buchanan and coworkers, with the object of determining the position of the equilibrium between the various pairs of interconvertible epoxides the findings are summarized in Chart I. For the dianhydro compounds, the epoxides that contain the free hydroxyl group quasi-axial, and that also have the possibility of a polar interaction between the epoxide ring and the 1,6-anhydro bridge, are clearly less stable than those in which it is quasi-equatorial, although the relative importance of these two factors is uncertain. The situation is less clear for the monocyclic epoxides. The half-chair conformations indicated are considered to be favored on the basis that the alkyl... 

Intramolecular opening of 2-azabicyclo[3.3.0]oct-7-ene-3 endo-carboxamide epoxide displays a dependence on the substituent at the nuclear nitrogen [46], The conformational preference for a proximal attack by. the amide function is counterbalanced by an accentuated polarity alternating sequence which enforces the acceptor role of the distal carbon of the epoxide ring when the nuclear nitrogen is part of a carbamate group. 

It is impossible for the CO bonds of the product epoxide ring to adopt perfectly axial and equatorial positions. If you make a model of cyclohexene oxide you will see that the ring is a slightly deformed chair—it is more of a half-chair conformation in which four of the carbon atoms are in the same plane (you met this on p. 461). 

A ten-membered lactone 19 with quite similar conformative prerequisites, where the ideally puckered ring conformation is stabilized by three equatorial methyl groups, is also reported to be epoxidized with complete diastereoselectivity156 the two additional methyl groups enhance the diastereoselectivity considerably compared to 3-methylcyclodecene (vide supra). 

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