Pinacol rearrangement examples

The pinacol rearrangement is frequently observed when geminal diols react with acid. The stmcture of the products from unsymmetrical diols can be predicted on the basis of ease of carbocation formation. For example, l,l-diphenyl-2-metltyl-l,2-propanediol rearranges to... 

A simple and direct approach to 10(5 4j H)<2Z)eo-5-lceto derivatives lacking functionality in ring A is the controlled pinacol rearrangement of vicinal cw-diols analogous to the process described in the previous section. An example is the reaction of cholestane-4a,5a-diol 4-tosylate (136) with 1 mole-equivalent of potassium t-butoxide or with dimethylforraamide-calcium carbonate at reflux which gives a quantitative yield of Q(5ApH)abeo-cholestan-5-one (137). ... 

A reaction that follows this pattern is the acid-catalyzed conversion of diols to ketones, which is known as the pinacol rearrangement.60 The classic example of this reaction is the conversion of 2,3-dimethylbutane-2,3-diol(pinacol) to methyl /-butyl ketone (pinacolone).61... 

These reactions involve addition of the diazoester to an adduct of the carbonyl compound and the Lewis acid. Elimination of nitrogen then triggers migration. Sections B and C of Scheme 10.2 give some additional examples of pinacol rearrangements involving diazo and diazonium intermediates. 

The only example of formation of a monocyclic cyclobutane derivative, pinonic acid (3-acetyl-2,2-dimethylcyclobutaneacetic acid, 31) by pinacol rearrangement from dihydroxy-a-cam-pholenic acid by dry distillation, was reported in the early literature.97... 

Some selected examples of such a pinacol rearrangement applied in the preparation of cy-clopentanones are shown in Table 2 further examples can be found in refs 35-39. A number of cyclopentenones, 2,2-disubstituted cyclopentane-1,3-diones and 2,2-disubstituted 3-methylenecyclopentanones were synthesized by this method. 

We have already mentioned that migratory aptitudes are dependent on the reaction and on the conditions under which the reaction is carried out. An example of the latter type of variation is that in the pinacol rearrangement of triphenylethylene glycol, the phenyl/hydrogen migration ratio may vary by a factor of 180 (from 7.33 to 0.41) when the catalyst is changed from concentrated sulfuric acid to HC1 in water/dioxane.48... 

The most general method for the synthesis of tetrahydrofurans based upon the IMSC methodology was developed by Overman et al. [53, 54, 94—96] For example, condensation of alcohol 221 with an aldehyde or a ketone in the presence of a Lewis acid leads to the formation of the carbocations 222a,b. The tertiary carboca-tion 222a undergoes a pinacol rearrangement and forms the desired heterocycle 224 (Scheme 13.82). Overman et al. used this approach during the synthesis of the various cladiellin diterpenes, which possess the core skeleton 224 [53]. 

The mechanism for this reaction, shown in Figure 22.5, involves a carbocation rearrangement that occurs by an allowed [1,2] sigmatropic shift. The product of this rearrangement, a protonated ketone, is considerably more stable than the initial carbocation, so the migration is quite favorable. Another example of the pinacol rearrangement is provided in the following equation ... 

Among the most famous examples of a reaction involving a 1,2-alkyl shift is the pinacol rearrangement. This reaction, shown in Scheme 5.10, results in the conversion of a vicinal diol to a ketone. 

Using our knowledge of alcohol reactions, we can explain results that seem strange at first glance. The following dehydration is an example of the pinacol rearrangement ... 

Acid Form - Pseudoliquid Phase Behavior. Owing to a high affinity for polar molecules, large quantities of molecules such as alcohols and ether are absorbed within the bulk phase of crystalline heteropolyacids. The amounts of pyridine, methanol, and 2-propanol absorbed correspond to 50-100 times that which can be adsorbed on the surface, while nonpolar molecules like ethylene and benzene are adsorbed at the surface only. Catalytic reactions of polar molecules occiu both on the surface and in the bulk, so that the solid heteropolyacid behaves as a highly concentrated solution, called a pseudoliquid phase . The dehydration of alcohols, various conversions of methanol and dimethyl ether to hydrocarbons in gas-solid systems, and the alkylation of phenol and pinacol rearrangements can all occur in the pseudoliquid. The transient response using isotopically labeled 2-propanol provides evidence for the pseudoliquid phase behavior of H3PW12O40. This behavior influences the selectivity, for example, the aUcene/aUcane ratio, in the conversion of dimethyl ether. 

Many common rearrangement reactions are related to the rearrangement of 1,2-dihydrojqr compounds to carbonyl compounds. Often these reactions are called pinacol rearrangements, because one of the first examples was the transformation of pinacol to pinacolone ... 

The second example looks at first to be a similar pinacol rearrangement. But the resulting ketone cannot easily be transformed into the product. 

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