Beckmann reaction stereochemistry

The oxime nitrogen can also participate in cycloamination reactions to give pyrroles. Thus, treatment of oxime esters such as 185 with Pd(Ph3P)4 readily affords 186 [129]. The pentafluorophenyl group is necessary for good results otherwise a Beckmann rearrangement can unfavorably enter the picture. The oxime stereochemistry makes no difference on the outcome of the reaction. In addition to 186, pyrroles 187 and 188 were also prepared in this study (among others) [129]. 

Both oximes (10) and their ester (11) or ether derivatives can be used in the classical Beckmann rearrangement and the reaction usually proceeds under acidic or neutral conditions (although basic conditions may also be used). In sharp contrast, only 0-oxime esters can be used as starting materials for the Neber rearrangement and basic conditions are always necessary. The Neber rearrangement is not stereospecific, as the stereochemistry of the starting material E or Z) does not influence the outcome of the reaction. In... 

However, the stereochemistry of the oxime cannot be easily controlled and this may be a drawback for the synthetic utility of the Beckmann rearrangement. When a mixture of oximes is obtained from the ketone and when the isomerization of the oxime cannot be prevented during the rearrangement reaction, a mixture of amides is obtained. In other less favourable cases, the intended oxime cannot be obtained and the wrong amide will result from the rearrangement reaction. 

Unlike the similar Beckmann rearrangement, the stereochemistry of the oxime is not critical to die outcome of the reaction. 

The experiments also provide stereochemical evidence that a carbocation is an intermediate in both reactions. Both starting materials are ns-decalins but the product is a frans-decalin. The carbocation intermediate has no stereochemistry and can react with the nitrile from either face. Axial attack is preferred and it gives the stable frcws-decal in. The formation of the carbocation is shown only by the Beckmann fragmentation formation from the alcohol by the S l mechanism is obvious. 

The rearrangement of ketone nitrones with tosyl chloride in pyridine has been described in full. The reaction has resemblances to the Beckmann rearrangement of oximes, but does not depend upon nitrone stereochemistry, and gives N-alkyl-(usually A-methyl) lactams. 

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