Claisen rearrangements based on vinyl fluorides

Abstract

2-Fluoroalk-1-en-3-ols ( 4), available from terminal alkenes ( 1) by bromofluorination, subsequent dehydrobromination of the 1-bromo-2-fluoroalkanes ( 2) to form 2-fluoroalkenes ( 3) and selenium dioxide mediated allylic oxidation with tert-butylhydroperoxide, undergo Johnson-Claisen rearrangement on treatment with trimethyl orthoacetate to give methyl 4-fluoroalk-4-enoates ( 7) in high yields. In contrast Ireland-Claisen rearrangement of 3-acetoxy-2-fluorodec-1-ene ( 9b) with triethylamine and TMSOTf in ether failed. Instead of the expected formation of a carboxylic acid, selective C-silylation of the α-position to the carboxyl group to form 14 occurred. However, Ireland-Claisen rearrangement was successful with corresponding chloroacetates 10 and propionates 11 of four 2-fluoroalk-1-en-3-ols ( 4) to give 2-chloro-4-fluoroalk-4-enecarboxylic acids ( 15) or its 2-methyl derivatives 16, respectively, in moderate yields. These [3,3]-sigmatropic rearrangements are diastereoselective giving trans-configured double bonds, exclusively. Corresponding esters derived from ( Z)-2-fluorocyclododec-2-enol ( 22), did rearrange to yield mixtures of diastereomers much less selectively. Also 2-fluorodec-2-enol ( 6), which was prepared by rearrangement of 2-fluoro-2-octyloxirane ( 5) with TMSOTf and triethylamine, was successfully applied as a starting material for [3,3]-sigmatropic rearrangements. The corresponding 3-(1-fluoroethenyl)alkanoic acid derivatives 17 and 18 were formed in moderate yield.