Stereochemistry pinacol rearrangements

The preparation of 620, a tricyclic intermediate suited for elaboration into quadrone, has been reported by Monti and Dean Following introduction of the proper C5, stereochemistry by alkylation of 618 under kinetically controlled conditions, diketone 619 was subjected to acid-catalyzed rearrangement. After functional group manipulation, a tandem intramolecular aldol-pinacol rearrangement gave 620. 

It appears that the stereochemistry at the migrating center has not been explicitly examined in a pinacol rearrangement but, as seen in related 1,2-migrations, retention of configuration is expected. 

The stereochemistry of 6,2-H shifts is endo, endo. Treatment of acid (760) with 50% sulfuric acid results in formation of lactone (761), contaminated by less than 3% of the 2-H species which would arise by 6,2 shift of exo-D531V The pinacolic rearrangement of labeled diol (744a) produces ketone (747a) by way of an endo, endo 6,2-D shift532). 

The pinacolic rearrangement of acyclic a-silyloxy epoxides presents an attractive procedure for the stereocontrolled preparation of aldol-type products under extremely mild conditions33 35. Treatment of epoxides with an appropriate Lewis acid catalyst results in a [1,2] shift with net inversion of epoxide stereochemistry to afford highly functionalized /J-hydroxy ketones. 

Which word or phrase best describes the stereochemistry of the product formed in the pinacol rearrangement of the diol shown ... 

Due to the symmetry of the carbocation intermediate, pinacol rearrangements of acyclic substrates are rarely stereoselective. However, conformational constraints in cyclic systems can lead to high stereoselectivities. The reactivity of the set of conformationally-locked stereoisomers 9-12 when treated with a Lewis acid is illustrative. Regardless of the stereochemistry at C-1, both C-2 (5) diastereomers (9 and 10) yield only the ketone a-(S) stereoisomer, indicating hydride migration proceeds stereoselectively from the bottom face of the ring system (pathway... 

The classic pinacol rearrangement can be used to set the stereochemistry of angular carbons in systems where an isoxazole moiety is correctly positioned to stabilize a developing carbocation. Thus, when diols 34 were exposed to a Lewis acid, ketones 35 were regio- and stereoselectively generated. 

Rearrangement of Electron-Deficient Carbon Carbocation rearrangements are well known. One named example is the pinacol rearrangement, the mechanism and stereochemistry of which have been well studied. 

Structure-activity correlations have been examined for several A-nor-thia-steroids of type (220). Several new studies have been devoted to the synthesis of A-nor-steroids. Pinacol-type rearrangement of 3a-hydroxy-2a-mesyloxy-3/S-methylcholestane (221) led to 2)8-acetyl-A-norcholestane (222). Its structure and stereochemistry were confirmed by Baeyer-Villiger oxidation, which afforded the known 2 -acetoxy-A-norcholestane (223) ... 

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