Diastereoselectivity Favorskii rearrangement

Diastereoselective Favorskii ring contraction. Bicyclic a-chloro-8-lactams can undergo highly diastereoselective ring contraction to proline derivatives when treated with base, preferably barium hydroxide in aqueous solution. Thus 1 is converted into 2 and 3 in the ratio 9 1. The ring contraction of 4 to 5 is stcreospecific. in contrast, rearrangement... 

Excellent stereoselectivities can be observed in Favorskii rearrangements. In the ring contraction of cyclohexanone 17, cyclopentane 18 was observed with greater than 10 1 dr, which prompted the authors to study a series of related compounds to determine the factors that control the stereoselectivity. Of compounds 19, 20, and 21, only 19 underwent a Favorskii rearrangement, also with high diastereoselectivity, leading the authors to conclude that a 3-oxy substituent is critical to the success of the rearrangement. ... 

Favorskii ring contraction of ketone 22 followed by diastereoselective protonation led to ester 23, an intermediate in the synthesis of structures related to the guanacastepene core. In this case, an epoxide served as the leaving group. Favorskii rearrangements of various a,p-epoxyketones have been investigated. ... 

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