Semipinacol rearrangements rearrangement reaction

Pan, C.-A., Wang, B.-M., Tu, Y.-Q., Song, Z.-L. Samarium-catalyzed tandem semipinacol rearrangement/Tishchenko reaction of a-hydroxy epoxides a novel approach to highly stereoselective construction of 2-quaternary 1,3-diol units. Angew. Chem., Int. Ed. Engl. 2001,40, 3877-3880. 

Scheme 1.20 A tandem semipinacol rearrangement/Schmidt reaction in the total synthesis of r+Vstemonamine ri31V ...

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. 

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.

Fig. 14.20. Second semipinacol rearrangement of a glycol monotosylate. The reaction involves two steps in basic media, since the [l,2]-rear-rangement and the dissociation of the tosylate occur at the same time. Under reaction conditions this rearrangement is followed by a base-catalyzed epimerization. This is why instead of the initially cis-annulated bicyclic ketone D its trans-isomer C is isolated.

Semipinacol rearrangement of a-haloalkyl aryl acetates has produced good yields of 2-aryl alkanoic esters. The acids formed on hydrolysis are important anti-inflammatory agents (such as Ibuprofen) and the reaction has been reviewed in some detail1010. In a similar process, oc-bromoalkyl aryl acetals thermally rearrange in protic solvents (under neutral or slightly basic conditions) via a 1,2-aryl shift (equation 193)1010,1011. 

Semipinacol rearrangements are pinacol reactions with no choice about which way to go... 

One of the further reactions this carbocation can undergo is rearrangement. If the starting amine is a 2-amino alcohol, the cation can be stabilized by a semipinacol rearrangement. 

The mechanism of this benzilic acid rearrangement starts with attack of hydroxide on one of the carbonyl groups. The tetrahedral intermediate can collapse in a reaction reminiscent of a semipinacol rearrangement, carbonyl group is formed here... 

Selectivity in rearrangement reactions is affected by the electronic nature of both the group that migrates and the group that is left behind. But there is more Stereochemistry is important too. The outcome of diazotization and semipinacol rearrangement (Tiffeneau-Demjanov rearrangement) of this amino-alcohol depends entirely on the diaster eoisomer you start with. There are four diastereoisomers, and we have drawn each one in the only conformation it can reasonably adopt, with the f-butyl group equatorial. 

A simple procedure for the synthesis of 3-ethoxycarbonylbenzofurans from salicylaldehydes and ethyl diazoacetate has been developed. The reaction is believed to occur via a semipinacol rearrangement and tautomerization to a /3-hydroxy acrylate which is trapped by the adjacent phenoxy group (Equation 118) <2006S1711>. 

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