Semipinacol rearrangements

A new semipinacol rearrangement mediated by Sn(IV) was proposed by Bates and to explain the formation of 579 from 578 (equation 256). As stated by the authors, the mechanism of formation of 579 most likely involves an intermediate hydroxylamine 580 (equation 257). Nucleophilic addition of the hydroxylamine to the ketonic carbonyl leads to 581, which may undergo a tin-mediated pinacol-type rearrangement with preferred migration of the phenyl substituent to produce amide 582. 

Extension of the reaction to propargylic cyclobutenols results in a tandem carbene inser-tion-semipinacol rearrangement of the cyclobutenol ring, leaving the cyclopropyl ring intact (equation 81)151. 

The semipinacol rearrangement is the rearrangement that ensues when a /1-amino alcohol is deaminated as in the following equation. See Problem 6.4. 

Scheme 36 Chloronium-induced semipinacol rearrangement for the installation of the quaternary stereocenter of welwitindolinone A. Model studies...

Fig. 14.8. Tandem rearrangement comprising a Wagner-Meerwein rearrangement and a semipinacol rearrangement.

Fig. 14.17. "Accidental" dia-stereoselectivity in the semipinacol rearrangement of an epoxide. The more substituted carbenium ion is formed exclusively during ring-opening because of product development control. Only two H atoms are available for possible migrations, and no alkyl groups. In general, diastereos-electivity may or may not occur, depending on which one of the diastereotopic H atoms migrates in which one of the diastereotopic conformers. The present case exhibits diastereo-selectivity.

Fig. 14.18. Mechanism-based diastereoselectivity in the semipinacol rearrangement of an epoxide. This rearrangement is stereoselective, since there is only one H atom in the position next to the sextet center and the H atom undergoes the [l,2]-migration on the same face of the five-mem bered ring.

Fig. 14.19. First semipinacol rearrangement of a glycol monotosylate. The reaction involves three steps in neutral media formation of a carbe-nium ion, rearrangement to a carboxonium ion, and deprotonation to the ketone.

Other leaving groups in other glycol derivatives facilitate other semipinacol rearrangements. Molecular nitrogen, for example, is the leaving group in... 

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