DEDICATED TO PROFESSOR HC BROWN ON HIS ... - Sigma-Aldrich

tested, including heterocyclic 36 and
fluorinated aldehydes 37 (Scheme 6). With
chiral aldehydes, the reagent controlled the
diastereoselectivity leading to high de’s and
ee’s. 38 High selectivities were also achieved
in the allylboration of a series of dialdehydes
(Figure 4). 39 Recently, it has been shown
that this reagent provides high ee’s of
homoallylic amines produced by the
allylboration of N-silyl imines (eq 18). 40
Several remarkably successful “allyl”borating
agents have been synthesized using
pinane as the chiral auxiliary (Figure 5).
Discussion of these reagents follows.
28 VOL. 35, NO. 1 • 2002
Figure 4. High Enantioselectivities in the Allylboration of Dialdehydes.
Figure 5. Pinane-Based “Allyl”borating Reagents.
Scheme 7. Methallylboration of Aldehydes.
5.1. B-Methallyldiisopinocampheylborane
This reagent, readily prepared from
Ipc2BOMe and methallyllithium, converts
aldehydes into methallyl alcohols in very
high ee’s (Scheme 7). 41 The alcohols are
valuable intermediates, which can be
elaborated into more complex acyclic
compounds via diastereoselective epoxidation
and iodocyclization.
5.2. 3,3-Dimethylallyldiisopinocampheylborane
This reagent is synthesized by
the hydroboration of 1,1-dimethylallene. Its
eq 18
allylboration reaction typically provides
products with predictable configurations in
89–96% ee (Scheme 8). 42
5.3. [(Z)-3-Alkoxyallyl]diisopinocampheylborane
Metal salts of allyl alkyl ethers remain
in the Z form due to chelation. 43
Transmetalation with boron retains the Z
stereochemistry. 44 The synthesis of [(Z)-3alkoxyallyl]diisopinocampheylborane
was
achieved by reaction of the lithium salt of
allyl alkyl ether with B-methoxydiisopinocampheylborane,
followed by treatment
with BF3:Et2O. 45 The reaction of the title