Reduction norephedrine-borane

In this model the sense of asymmetric induction is controlled by two principle factors (i) coordination of borane on the least-hindered face of the bicyclic ring system and (ii) coordination of the Lewis acid syn to the small group (R ). The latter point is in good agreement with the structural data that has been presented in this chapter, and is further supported by results from the asymmetric reduction of oxime ethers (Figure 51). As predicted by the model (75 and 78), (E) and (Z)-oxime ethers afford enantiomeric amines upon reduction by the reagent derived from (-)-norephedrine and borane (2 equiv.). Here, Lewis acid coordination is dictated by the ( )/(Z) stereochemistry of the oxime ether rather than by the rule of coordination syn to the small group. 

The reduction of ketoxime ethers by borane in the presence of (—)-norephedrine yields (S)-amines from a/ i-kctoximcs and (/ )-isomers from. s i/ -ketoximes, e.g. from phenyl-4-tolyl-methyl ketoxime methyl ether40. 

The one-step transformation of 2- or 3-(l-hydroxyalkyl)-2,3-dihydrobenzofurans to 2- or 3-acylbenzofurans with A -bromosuccinimide was performed with good yields <97H(45)1657>. The asymmetric reduction of dihydrobenzofuran ketoxime ethers to enantiomerically enriched chiral hydroxylamines with reagents prepared from borane and norephedrine has been investigated <97TA497>. An enzymatic resolution of a 3-hydroxymethylbenzofuran using Candida rugosa lipase provides an enantioselective synthesis of vitamin E related antioxidants <97TA45>. 

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