R-Alpine borane

B-3-Pinanyl-9-borab1cyclot3.3.1]nonane, 1, 1s also available from Aldrich Chemical Company under the trade-name, R-Alpine-Borane. 

Scheme 4.6 Reagents and conditions a) NiCl2, CrCl2, THE, 0°C b) Dess-Martin periodinane, CH2CI2, 95% c) (R)-alpine borane, THE, 86% d) NSNHNH2, DEAD, PhaP, THE, -15°C to rt, 77% e) PTSA, MeOH/CHaCla, 50°C, 85% f) Pd(PPh3)4, K2CO3, DME, 100°C, 76%...

The mildness and selectivity of the reagent are illustrated in the reduction of vinylacetylenic ketone. This ketone is extremely sensitive to acid- or base-catalyzed isomerization of the cis double bond. (R)-Alpine-Borane (from 100% ee (-i-)-a-pinene) provides the (S)-enantiomer of alcohol in >98% ee, while the reagent from (-)-a-pinene (90% ee) gives 87% ee of (R)-alcohol (Eq. 26.8) [14]. 

An efficient stereoselective reduction of 22-keto-23-acetylenic steroid to anti-Cram product 22-(R)-hydroxy-23-acetylenic steroid and Cram product 22-(S)- hydroxy-23-acetylenic steroid has been achieved using (R)-Alpine-Borane [(-i-)-a-pinene, 92% ee] (125 1, R-.S) and L-selectride (lithium tri-sec-butylboro-hydride) (1 11, R S), respectively [19] (Chart 26.7). (S)-Alpine-Borane (2 M in THE) prepared from (-)-a-pinene (92% ee) provides unexpectedly low 1 2.7, R S ratio due to the influence of the a-chiral center at C-20 of the steroid, and also the reduction is much slower than with (R)-Alpine-Borane. 

Using neat (S)-Alpine-Borane the reaction is 67% complete in 92 h, and 22-(S)- is obtained in a 7 1 ratio. It is thus apparent that for (R)-Alpine-Borane the double asymmetric inductions are working together, whereas in (S)-Alpine-Bo-rane they are working in opposite direction. A similar change in selectivities is observed in the asymmetric reduction of these acetylenic ketones using (+)-and (-)-A-methylephedrine/LAH [18]. The faster rate of reduction with (R)-... 

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