Page 832 - Advanced Organic Chemistry Part B - Reactions & Synthesis

808 Scheme 9.3. (Continued)

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5302 (1993).

demonstrate the high diastereoselectivity of the allylboration reaction. Entry 3 examines
the facial selectivity of glyceraldehyde acetonide toward the achiral reagents derived
from butenyl pinacol borane. It was found that the reaction with the Z-2-butenyl
derivative is highly enantioselective, the E-isomer was much less so. It was suggested
that steric interaction of the E-methyl group with the dioxolane in the expected TS
ring led to involvement of a second transition structure.

H O
O O O
O H O H CH 3 B
B O B O O
O O O O O
H CH 3
O
CH 3 H
two competing transition
strongly favored
for Z- boronate structures for E - boronate
Entry 4 shows the reaction of 9-(E-2-butenyl)-9BBN with methyl pyruvate. This
reaction is not very stereoselective, which is presumably due to a modest preference
for the orientation of the methyl and methoxycarbonyl groups in the TS. Only use of
an extremely sterically demanding pyruvic ester achieved high diastereoselectivity.

CH 3 CO R
2
CH 3 B CH 3 B
RO C O CH 3 O
2
R product ratio
CH 3 73 27
Ph 80 20
2,6-diMePh 75 25
100
2,4,6-tri-t-BuPh 0