Origin of Enantioselectivity: A Model Study of Sharpless …
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Fig. 2 A model structure of diastereotopic TS produced by Re-facial approach of epoxidating
oxygen
Table 2 Orientation of existing and developing chiral centres
Approaching face
Substrate: optically active allyl alcohol
Chirality of alcoholic centre
R
S
Re-face
Prochiral centre appears S
SR (98%)
SS (62%)
Si-face
Prochiral centre appears R
RR (2%)
RS (38%)
epoxidating oxygen for Si-facial attack. As a result of Re-facial approach of epoxidating oxygen on (E,R) isomer of allyl alcohol, optically active epoxide bears a
specific geometry showing (S,R) orientation for two chiral centres. Experimentally
reported data suggest this isomer is almost enantiomerically pure with ee 96%. In
case of S-configuration of starting alcohol, it projects more space-filling cyclopropyl
group towards developing epoxide ring. And these two interact with bulky isopropyl
group of chiral auxiliary which waived strict course of Re-facial approach of epoxidating oxygen and permits Si-facial attack to take place in lesser extent. As a result,