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A technical scale preparation of optically active (1 R , cis ,α S )-cypermethrine 4 from racemic m -phenoxybenzaldehyde cyanohydrin acetate ( RS )- 1 and (1 R , cis )-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylic acid chloride (1 R , cis )- 3 is described. Key steps of the new procedure are a lipase catalyzed enantioselective transesterification of ( RS )- 1 with n -butanol and direct acylation of the mixture of ( R )- 1 and ( S )-cyanohydrin ( S )- 2 with (1 R , cis )- 3 to give enantiomerically pure (1 R , cis ,α S )- 4 . The unchanged ( R )- 1 is removed from (1 R , cis ,α S )- 4 by distillation, and is racemized with triethylamine to give ( RS )- 1 which is returned to the process. The total yield of (1 R , cis ,α S )- 4 referred to ( RS )- 1 is 80%. |
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