C O M M U N I C A T I O N S
Scheme 3. Synthesis of anti-1,2-Diols 9 via Hydroboration of 7 and
hydroboration of 7 was performed at 85-95 °C for 1.5 h with
(dIpc)2BH [to give the thermodynamic allylborane 12E] followed
by addition of 14 at -78 °C, anti-diol 9h was obtained with
excellent diastereoselectivity (>15:1) (entry 3). Similarly, a 5:1
mixture of anti-diols 9i and 9h was obtained in 52% yield from
12E generated by the hydroboration of 7 with (lIpc)2BH (entry 4).
The latter reaction is stereochemically mismatched.25
Thermodynamically Controlled Allylborane Isomerizationa
The data presented herein indicate that the hydroboration of 7
with (Ipc)2BH proceeds under kinetic control at 0 °C and provides
12Z with excellent selectivity. Evidently, the normally facile 1,3-
isomerization that has been documented for other allylboranes10-14
is slow in the case of 12Z owing to steric hindrance in the transition
state leading to the 1,1-diboryl species 13. However, isomerization
is readily achieved at higher temperatures, and a g12:1 mixture of
12E and 12Z is obtained at 85 °C. Thus, synthetically useful
selectivity for synthesis of either the 1,2-syn or 1,2-anti diol
diastereomers 8 and 9 can be achieved by appropriate control of
the hydroboration conditions. Applications of this method in the
synthesis of natural products will be reported in due course.
a Reactions were performed by treating 7 with (dIpc)2BH (1.0 equiv) in
toluene at 85-95 °C for 1.5 h followed by the addition of RCHO (1 equiv)
at -78 °C. The mixture was then allowed to stir at -78 °C for 4 h. The
reactions were subjected to standard oxidative workup (NaOH, H2O2) at
0 °C before product isolation. b Determined by Mosher ester analysis, unless
indicated otherwise. c See SI for % e.e. determination for 9g.
Scheme 4. Double Asymmetric Allylboration Reactions with
Aldehyde 14
Acknowledgment. Financial support provided by the National
Institutes of Health (GM038436 and GM026782) is gratefully
acknowledged.
Supporting Information Available: Experimental procedures and
spectroscopic data for all new compounds. This material is available
References
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of allenylboronate 7 was performed at 35 °C for 16 h followed by
treatment of the allylborane product with hydrocinnamaldehyde at -78
°C, a 3:1 mixture of diols 9a and 8a was obtained. Hydroboration of
7 at 65 °C for 5 h led to a 5:1 mixture of 9a and 8a. Prolonged heating
of the hydroboration reaction at 65 °C (16 h), however, only led to
decomposition. When the hydroboration of 7 was performed at 85 °C
in toluene for 1.5 h, followed by addition of hydrocinnamaldehyde at
-78 °C, anti-diol 9a was obtained with 17:1 d.r. (9a:8a) in 76% yield
and 89% e.e. (Scheme 3). The stereochemistry of anti-diol 9a (and
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1
subsequently also of 9e) was assigned by H NOE studies of the
corresponding acetonide derivatives (see SI). The hydroboration-
isomerization-allylboration sequence was then applied to a variety
of aldehydes (Scheme 3). In all cases, 1,2-anti-diols 9a-g were
obtained in good yield with g12:1 diastereoselectivity and 80-89%
e.e.
Finally, double asymmetric allylboration reactions of 12Z and
12E with chiral aldehyde 14 are summarized in Scheme 4. Kinetic
controlled hydroboration of allenylboronate 7 with either (dIpc)2BH
or (lIpc)2BH and treatment with aldehyde 14 provided syn-diols
8h or 8i with excellent diastereoselectivity (>15:1) in 72% and 65%
yield, respectively (entries 1 and 2). Alternatively, when the
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