Oxidations - Carbohydrates: Tools for Stereoselective Synthesis - page 327

Figure 15.9 Examples of epoxidation of cis - and terminal olefins with d-glucose-derived

ketone 60.

Figure 15.10

Spiro transition states for the epoxidation with ketone 60.

Determination of the absolute configurations of several epoxide products
revealed that the olefin substituents with -system prefer to be proximal to the

oxazolidinone moiety of the catalyst. There seems to be an attractive interaction
between the R π substitution on the olefin and the oxazolidinone of the ketone
catalyst (spiro C favored over spiro D, Figure 15.10). The epoxidation of 1-
phenylcyclohexene with ketone 1 and 60 also validate this hypothesis. While the
(R,R) enantiomer was obtained in 98% ee with ketone 1 via favored spiro E transi-
tion state, the opposite absolute configuration was obtained in 39% ee with ketone
60 (Figure 15.11) [93, 95]. This result indicated that planar H became the major
transition state for the epoxidation with ketone 60, presumably due to the attractive
interaction between the oxazolidinone moiety of the catalyst and the phenyl group
of the substrate [93]. Interestingly, the epoxidation of 1-phenylcyclohexene with
carbocyclic analog of ketone 60 ( 61 ) (Figure 15.11) gave the (R,R) epoxide in 40%
ee, suggesting that spiro I became the major transition state again in this case
(Figure 15.11) [95]. X-ray studies showed that ketones 60 and 61 have very similar
conformations in the solid state. It is likely that the pyranose oxygen in ketone 60
influences the epoxidation transition state via electronic effects rather than con-
formational effects. The replacement of the pyranose oxygen with a carbon atom
causes an increase in energy of the nonbonding orbital of the dioxirane oxygen,

π