Beckmann fragmentation, ring

Fragmentation of the intermediate or concerted formation of nitriles from the activated oxime (Scheme 9, pathway 3) this is the Beckmann fragmentation. In some circumstances this pathway becomes dominant, particularly when there are quaternary carbons adjacent to the oxime. This transformation has found particular utility in ring-cleavage processes (sometimes called abnormal or second-order Beckmann rearrangements). 

Beckmann fragmentation is frequently applied to cyclic oximes resulting in a ring-cleavage reaction. Normally, a nitrile-alkene compound is obtained from the oxime and further transformation is usual. 

The D-ring of steroids has been cleaved frequently by a Beckmann fragmentation. Typical strategy uses enolate chemistry to introduce an oximino group at the a-carbonyl carbon. Reduction of the carbonyl (or addition of a carbanion) produces an a-hydroxy oxime which serves as good substrate for the fragmentation. Two examples of this strategy are shown in equations 213 and 214. 

Shimizu used this version of the Beckmann fragmentation to construct the characteristic A ring of the steroidal alkaloids of salamanders from an androstane-... 

Shimizu used this version of the Beckmann fragmentation to construct the characteristic A ring of the steroidal alkaloids of salamanders from an androstane-3-one. Thus treatment of (12), prepared as described above from S androstane-17i3-ol-3-one, with tosyl chloride followed by desulfurization gave (14). This intermediate was converted to (15) by epoxidation followed by reaction with... 

The 2)5-hydroxy-3-oximino-compound (379) suffers Beckmann fragmentation with thionyl chloride to give the cyanoaldehyde (380), offering a controlled method for unsymmetrical cleavage of the ring. ... 

Another classical example is Stork s 1972 total synthesis of racemic byssochlamic acid (51), which constitutes the first application of a Beckmann fragmentation in the total synthesis of a complex natural product 1 Scheme S.131. Bicyclic oxime 49 underwent Beckmann fragmentation to generate the nine-membered intermediate 50, which was converted to ( )-byssochlamic acid (51) after ethyl substituent formation and conversion of the benzene rings to the two maleic anhydrides. 

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