Beckmann rearrangements seven-membered rings

Six-membered rings can also serve as precursors to seven-membered rings by ring-transformation processes. Thus, Beckmann rearrangement of 308 selectively afforded the azepinones 309 (e.g., R=R = R2 = H, R3 = Me 50%) (Equation 45) <1998JGM198>, while reaction of 310 with azide gave the azido azepine 311 (Equation 46) <1998H(48)427>. 

The Beckmann rearrangement of cyclic oximes results in lactams. This is exemplified in Figure 11.38 with the generation of e-caprolactam, the monomer of nylon-6. The nitrilium ion intermediate cannot adopt the preferred linear structure because it is embedded in a seven-membered ring. Therefore, in this case the intermediate might better be described as the resonance hybride of the resonance forms A (C=N+ triple bond) and B (C+=N double bond). The C,N multiple bond in this intermediate resembles the bond between the two C atoms in benzyne that do not carry H atoms. 

The sulfur-containing ring, and the stereochemistry, of biotin are already defined and, in the seven steps that follow, the most important is the breaking open of the seven-membered ring by a Beckmann rearrangement, which you will meet in Chapter 37. 

Novel syntheses of a-aminophosphonic acid derivatives (453 and 454) were disclosed, using the Beckmann rearrangement as a key synthetic step. The ring size was crucial and only seven-membered cycles could be formed in fair yield (51-62%) (equations 193 and 194). 

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