Oxidations - Carbohydrates: Tools for Stereoselective Synthesis - page 328

Figure 15.11 Competing transition states for the epoxidation of 1-phenylcyclohexene with
ketones 1, 60, and 61.

thus enhancing the secondary orbital interactions between the nonbonding orbital
of the dioxirane and the * orbital of the olefin (Figure 15.4), and consequently

favoring spiro I over planar J (Figure 15.11). The results obtained from ketones
60 and 61 demonstrate that the substituents on the ketones have delicate influ-
ences on the catalytic properties of ketones.
Studies have shown that the spiro rings in the aforementioned ketones are
important for stereodifferentiation in epoxidation reactions. For a more detailed
investigation, a series of ketones ( 63-67 ) with different spiro ring substitution
patterns were subjected to epoxidation studies (Scheme 15.19) [96]. Ketones 63-67
can all be synthesized from d-fructose via the common intermediate 62. Ketone
63 is structurally similar to ketone 1. The only difference between these catalysts
is the replacement of one of the oxygens in the spiro ketal ring in 1 with a carbon
atom. Ketone 64 differs from ketone 63 by the replacement of the methyl groups
with two hydrogens. Ketones 65-67 contain spiro lactone moieties in place of the
spiro ketal in ketone 63. Epoxidation results indicated that replacement of one of
the oxygens in the spiro ring with a carbon atom leads to no significant changes
in enantioselectivity (ketone 1 versus 63 ) (Figure 15.1 and Table 15.2, entry 1).
However, the replacement of methyl groups with smaller substituents (ketone 63
versus 64 ) decreased the enantioselectivity of the epoxidation of trans -olefins,

π