Pinacolin rearrangement

A rearrangement which takes place in similar compounds, and has been much studied recently, may be briefly mentioned here. This rearrangement has been named—not quite correctly—the retro-pinacoline rearrangement. 

Reference to some Russian contributions to the study of dehydration of alcohols and of the pinacoline rearrangement was made by Dolgov (67). This included work on dehydration of unsaturated alcohols. In connection with the current studies of Favorskil s school on vinyl-acetylene derivatives, the work of Zakharova on dehydration of acetyl-enylcarbinols to vinylacetylenes should be mentioned (430). 

Meerwein, H. Pinacolin rearrangement. III. Mechanism of the transformation of borneol into camphene. Ann. 1914,405,129-175. 

The preparative importance of the pinacolin rearrangement is considerable it leads not only to ketones that are hard to prepare in other ways but also from them to triply substituted acetic acids. The reagent effecting the rearrangement is always acidic, either a moderately strong organic acid or. a mineral acid. 

An analogous expansion of the quinuclidine ring during pinacoline rearrangement of the ditertiary alcohols of the type (XLV) lead to preparation of the derivatives of another novel 1-azabicyclic system—1-azabicydo (3,2,2) - nonane (XLVI) [120, 121]. 

This is clearly demonstrated in the pinacolinic deamination (cf. p. 114) of an optically active form of the amino-alcohol (50). Such reactions proceed from a conformation (antiperiplanar 50a or 50b) in which the migrating (Ph) and leaving (NH2 as N2 cf. p. 114) groups are TRANS to each other. Rearrangement via a bridged carbocation would necessarily lead to 100% inversion at the migration terminus in the product ketone (5lab), whichever initial conformation, (50a) or (50b), was involved ... 

Pinacol Rearrangement takes place when pinacol is heated with dilute acid, pinacolin formed ... 

It is suggested that the change of the a- to the /3-base takes place probably through an anhydride of the a-base by a molecular rearrangement almost identical with that of the pinacol-pinacolin type ... 

The carbon skeleton is obtained from a pre-existing one by rearrangement. Example pinacolin from pinacone. 

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