Carvones borohydride

In order to study the factors determining the regioselectivity of sodium borohydride reduction of a, -unsaturated ketones, reactions with 3-methylcyclohexenone, carvone and cholestenone were carried out in 2-propanol, diglyme, triglyme or pyridine. Mixtures of 1,2- and 1,4-reduction products were obtained in the alcoholic and ether solvents, whereas pure 1,4-reduction was observed in pyridine. Addition of triethylamine to NaBH4 in diglyme led to formation of triethylamine borine, EtsN BHs. Similarly, with pyridine, pyridine borine could be isolated, leading to exclusive 1,4-reductions. 

The predominance of axial attack also extends to monocyclic 2-cyclohcxcnoncs. for example the reductions of (/ )-2-methyl-5-(1-meLhylethenyl)-2-cyclohexenone (carvone)119, (S)-3-methyl-6-( l-methylethyl)-2-cyclohexenone (piperitonc)99, and (/ )-4-( l-methylethyl)-2-cvclo-hexenone (cryptone)120. However, the selectivities are often rather modest, probably due to conformational flexibility in the starting materials11 The use of sodium borohydride/ceri-um(III) chloride does not necessarily improve selectivity in fact, Ihis reagent gives preferentially the cfs-product with d.r. 65 35 on reduction of piperitone". 

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