Boron rearrangements, review

There are many applications of boron rearrangements in organic synthesis and the subject has been comprehensively reviewed. " This methodology offers considerable scope in the formation of carbon-carbon bonds. 

The arylsulphonate of an alcohol cyclizes under the influence of boron trifluoride at ambient temperature a substituent in the para position of the substrate may appear in an unexpected position in the product as a result of a rearrangement which does not occur with a rneta- or or//ro-placed substituent [2170]. Chloroacetaidehyde reacts with the uracil-6-thiol (99J) under mildly basic conditions to form a fused thiophene ring (review of recent work on such compounds [3981]) unsubstituted at the 2 and 3 positions [3682]. A fused 2-chlorothiophene ring is formed when the t-alcohol (993) is heated with thionyl chloride [3914]. 

More recently, several variations of the Claisen rearrangement have been reported that also possess some of the features of the Ireland variant, including the amide Claisen, S,S- and N,S-ketene acetal Claisen and the zwitterionic Qaisen rearrangements [2]. This review will focus on the Ireland variant of the Qaisen rearrangement in which a sUyl ketene acetal is an intermediate, although mention will be made of boron ketene acetals where appropriate. 

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