Abstract
Practical and stereocontrolled new methods for the synthesis of highly potent synthetic pyrethroids, 2, 2-dimethyl-3- (2-chloro- 3, 3, 3- trifluoro- 2- propenyl) cyclopropanecarboxylates are described. The key of the methods is the transformation of RCHO to RCH=C (Cl) CF3. This was achieved (1) by aldehyde addition of a zinc carbenoid CF3CCl2ZnCl prepared from CCl3CF3 and Zn, followed by acetylation and reduction; (2) by one-pot procedure using CF3CCl3/Zn (> 2mol) /Ac2O (1mol); or (3) by conversion to RCH (OH) CX = CF2 with CF3CCl3/Zn (> 2mol) /AlCl3 (cat.) followed by regio- and stereoselective fluorination. Application of these to 3- formyl- 2, 2- dimethycyclopropanecarboxylates afforded the trans isomer of the target compounds. A series of transformation involving addition of CF3CCl2ZnCl to 3- methy- 2- butenal, conversion into Me2C = CHCH (COCHN2) CCl2CF3, intramolecular carbene addition, and reduction gave the cis acid of the aimed pyrethroids.
