Molecular rearrangements Pinacol rearrangement

Antithetic conversion of a TGT by molecular rearrangement into a symmetrical precursor with the possibility for disconnection into two identical molecules. This case can be illustrated by the application of the Wittig rearrangement transform which converts 139 to 140 or the pinacol rearrangement transform which changes spiro ketone 141 into diol 142. 

Picric acid, synthesis of, 628 Pinacol rearrangement, 646 Pineapple, esters in, 808 Piperidine, molecular model of, 939 structure of. 918 P1TC, see Phenylisothiocyanate, 1031-1032... 

Pinacol rearrangement is a dehydration of a 1,2-diol to form a ketone. 2,3-drmethyl-2,3-butanediol has the common name pinacol (a symmetrical diol). When it is treated with strong acid, e.g. H2SO4, it gives 3,3-dimethyl-2-butanone (methyl r-butyl ketone), also commonly known as pinacolone. The product results from the loss of water and molecular rearrangement. In the rearrangement of pinacol equivalent carbocations are formed no matter which hydroxyl group is protonated and leaves. 

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 ... 

Pocker, Y. Wagner-Meerwein and pinacolic rearrangements in acyclic and cyclic systems, in Molecular Rearrangements (ed. De Mayo, P.), 1, 1-25 (Wiley, New York, 1963). 

The pinacol rearrangement proceeds over metal-substituted aluminophosphate molecular sieves (APOs) under mild conditions (batch reactor, solvent, g diol/ g catalyst = 5, 383 K, 3 h) [32]. Catalytic performance is best for APO-5 (Table 2) and surpasses that that of VPI and APO-11. Of the metals used Fe, Ni, and Cu have the highest activity and selectivity. No direct correlation was found between activity and acidity determined by pyridine TPD. For example. 

Ketones which contain a tertiary alkyl group are formed as the result of the elimination of water from pinacols. When pinacol is heated with an aqueous solution of an acid, it loses water and forms a ketone, named pinac-oliUy which is the first member of a series of substances known as pinac-olins. In the transformation a molecular rearrangement evidently takes place. The probable course of the reaction is expressed by the formulas,—... 

Hsu, B.-Y. and Cheng, S. (1998) Pinacol rearrangement over metal-substituted aluminophosphate molecular sieves. Microporous Mesoporous Mater., 21 (4-6), 505-515. 

Zimmerman, H.E. and Nuss, J.M., A Photochemical Long Range Pinacol Rearrangement Mechanistic and Exploratory Organic Photochemistry, J. Org. Chem., 51, 4604M 617, 1986. Zimmerman, H.E. and Zhu, Z., A General Predictor for Photoreactivity in Crystal Lattices Molecular Mechanics in Crystalline Media and Lock and Key Control Reaction Examples,. Am. Chem. Soc., 116, 9757-9758, 1994. 

Although molecular models showed appropriate sterochemistry and reasonable orbital overlaps, there appears to be no precedent in the literature of such pinacol-type rearrangements. At any rate, there is always a first time, and the resulting ds-ring fused system IX would be opened for double 1,2 addition at... 

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