An efficient synthesis of (Z)-gamma-fluoroallylphosphonates using a base-promoted deconjugation of (E)-gamma-fluorovinylphosphonates, and its utility as fluoroolefin-containing building block

A three-step synthesis of gamma-fluoroallylphosphonates starting with alpha,beta-unsaturated aldehydes is described. Treatment with diethyl phosphite in the presence of KF gives alpha-hydroxyallyl phosphonate in excellent yield; DAST deoxofluorination produces the corresponding gamma-fluorovinylphosphonate through a S(N)2' mechanism, and finally, a base-promoted double bond migration leads to the desired gamma-fluoroallylphosphonate. gamma-Fluoroallylphosphonate is a useful building block in the synthesis of fluoroolefins. An exploratory study yielded excellent yields of (Z)-diethyl 1-benzyl-3-fluoro-2-butenylphosphonate and (Z)-diethyl 1,3-difluoro-2-hexenylphosphonate. Treatment of (E)-diethyl 3-fluoro-2-hexenylphosphonate with LiN(TMS)(2) and benzaldehyde in THF led to the preferential formation of syn-(Z)-diethyl 3-fluoro-1-(hydroxybenzyl)-2-butenylphosphonate. (C) 2000 Elsevier Science S.A. All rights reserved.