SCHEDULED MAINTENANCE
Silyl enol ethers as new protecting groups for alkyl 4-halo-3-oxobutanoates; the preparation of pure (3-alkoxycarbonyl-2-oxopropyl)triphenylphosphonium salts
Abstract
A new method has been developed for the preparation of pure
(3-alkoxycarbonyl-2-oxopropyl)triphenylphosphonium salts 8. Alkyl
4-bromo-3-oxobutanoates and alkyl 4-chloro-3-oxobutanoates 7 are
protected as the trimethylsilyl enol ethers prior to treatment of the
resulting bromo(trimethylsilyl enol ether) esters 20c and 21c with
triphenylphosphine in toluene and then addition of a little water to
give pure (3-isopropoxycarbonyl-2-oxopropyl)triphenylphosphonium bromide
8c. Bromo(silyl enol ether) esters react more efficiently with
triphenylphosphine than the chloro(silyl enol ether) esters.
tert-Butyldimethylsilyl enol ethers of alkyl
4-bromo-3-oxobutanoates and alkyl 4-chloro-3-oxobutanoates 7 also react
with triphenylphosphine. Protection of isopropyl 4-bromo-3-oxobutanoate
7c as the enol acetate followed by subsequent reaction with
triphenylphosphine gives
(Z)-(2-acetoxy-3-isopropoxycarbonylbut-2-enyl)
triphenylphosphonium bromide 17.
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