Diastereoselectivity, with zinc borohydride

Nonchelate control. Diastereoselectivity for reduction is important in a synthetic context. Accessibility to defined stereoisomers by reduction of a-amino-P-hydroxy ketones is desirable. Different profiles from reduction with zinc borohydride and sodium borohydride (with slight modification of the substrates) are observed. The results are accountable in terms of chelate and nonchelate transition states."... 

Diastereoselective hydride addition is quite versatile, and it provides facile synthetic access to ( —)-pinidine (661), an alkaloid isolated from several species of Pinus, as well as its unnatural isomer (+ )-pinidine (660b). The unstable aldehyde 655, prepared in four steps from 624 [202], undergoes Grignard addition with 4-pentenylmagnesium bromide followed by Swem oxidation to afford ketone 656 in 90% yield for the two steps. Stereoselective hydride addition with L-Selectride provides the -yn-alcohol 657 (91 9), while zinc borohydride reduction provides almost exclusively the anri-alcohol 658 (>99 1) (Scheme 144). 

Diastereoselective Reductions. The diastereoselective reductions of 8-hydroxy ketones could be accomplished with NaBHa in the presence of Et2B(OMe) to form 1,3-syn diols in 68-99% yields and excellent stereochemical purities (dr>98 2) (eq 26). Et2B(OMe) could be generated in situ from Triethylborane and methanol. The method has been successfully utilized in several natural product syntheses and complements the anti-selective reductions performed with Tetramethylammonium Tri-acetoxyborohydride (see also Zinc Borohydride). 

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