Oxidations - Carbohydrates: Tools for Stereoselective Synthesis - page 336

Figure 15.23 Ketones 73-76 by Shing et al .

Figure 15.24

Examples of epoxidation with ketone 76b.

63% yield and 65% ee using this ketone [108]. Later, Shing and coworkers also
reported l-arabinose-derived uloses 74 and 75 [109]. Ulose 74 gave 90% ee for the
epoxidation of trans -stilbene; however, the yield was unsatisfactory due to the
decomposition of the catalyst during epoxidation. To overcome this problem, more
sterically demanding

-pivalate ulose 75 was studied (Figure 15.23). The yield of
the epoxidation was dramatically improved and up to 68% ee was obtained for
phenylstilbene oxide. Another series of l-arabinose-derived uloses containing
tunable steric sensors ( 76 ) were also reported by Shing and coworkers (Figure
15.23) [110, 111]. Enantioselectivity increased with increasing R group size. Good
yield and enantioselectivities were achieved with various trans - and trisubstituted
olefins with ketone 76b (Figure 15.24). Epoxide 78, synthesized from olefin 77
using ketone 76a in 68% ee, was transformed into taxol side-chain 79 in five
steps(Scheme 15.23) [112].

α

Scheme 15.23 Synthesis of taxol side chain 79.

In 2003, Zhao and coworkers described compounds 80-82 as epoxidation cata-
lysts for several unfunctionalized olefins (Figure 15.25) [113]. Up to 94% ee was
obtained for the epoxidation of trans -stilbene with aldehyde 80. Also in 2003, Wong
and coworkers described a

-cyclodextrin-derived ketoester 83 for the epoxidation
of several unfunctionalized olefins (Figure 15.26) [114]. Up to 40% ee was obtained
for p -chlorostyrene with ketoester 83.

β