Pinacol dehydration

An unexpected rearrangement of compound (48), obtainable by oxidation of lycoctonine followed by pinacolic dehydration and Curtius or Hofmann degradation has been observed. Treatment of this primary amine with nitrous acid affords two products (49) and (50) pathways for their formation have... 

Transformation Products from Lycoctonine. - Edwards has reported an unexpected rearrangement during deamination of 4-amino-4-des(oxy-methylene)-anhydrolycoctonam (117). In studies of the chemistry of lycoctonine (3), treatment of its derivative lycoctonamic acid (118) with very dilute sulfuric acid afforded the pinacolic dehydration product (119). This acid was converted to (117) by Curtius or Hofmann degradation. When (117) was treated with nitrous acid, the hydroxyketolactam (120) and the aldehydolactam acid (121) were obtained as the main products in 50 and 20% yields, respectively. 

On the basis of extensive chemical and spectral studies, the authors suggested that the pinacolic dehydration product (185) and compound 187 are the most likely intermediates in the transformation to lycoctamone (186). They postulated that the repulsive interactions between the C-18 substituent and the C-6 hydrogen and between the C-9 hydrogen and the C-6 methoxyl group are the main driving forces in this transformation. 

Pinacol upon dehydration with acid catalysts e.g., by distillation from 6A sulphuric acid or upon refluxing for 3—4 hours with 50 per cent, phosphoric acid or hydrated oxalic acid) is transformed into methyl ter<.-butyr ketone or plnacolone ... 

Pinacol (tetramethylethyleneglycol). Pinacol hydrate may be dehydrated in the following manner (compare Section 11,39). Mix 100 g. of pinacol hydrate with 200 ml. of benzene and distil a mixture of water and benzene passes over. Separate the lower layer and return the upper layer... 

The pinacol rearrangement is a dehydration reaction that converts a 1,2-diol into a ketone. The reaction involves two carbocation intermediates. 

Self-condensation of the substituted propiophenone, 15, by the pinacol reaction proceeds to give the glycol, 16, as the meso isomer. (If it is assumed that the transition state for this reaction resembles product, this stereoselectivity can be rationalized on the grounds of steric interaction compare A, which leads to the observed product, with B.) Dehydration under very specialized conditions (acetyl chloride, acetic anhydride) affords the bisstyrene-type diene (17). Removal of the acyl groups by means of base affords the synthetic estrogen, dien-... 

Although acetone is used widely as an industrial solvent, nevertheless it has become the by-product of the acetone-butanol fermentation and there is always the fear of overproduction. There is thus a need for an extension of the industrial utilization of acetone. A possibility in this direction may be in its conversion into pinacol, the preparation of which has recently been improved by McHenry, Drum and O Connor. It is obtained together with isopropyl alcohol by electrolytic reduction of acetone under controlled conditions. Pinacol (LXVI) may be dehydrated to 2,3-dimethylbutadiene which can be converted into a synthetic rubber, or converted through pinacolone (LXVII) into neohexane... 

Dehydration of diols pinacol rearrangement. Preparation of pinacolone... 

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. 

Superacid dehydrative cyclization of pinacols such as (93) gives condensed aromatic compounds as shown, presumably via dicationic species like (94).137... 

The dehydration of ditertiary alcohols in the presence of hydrobromic acid may lead to dienes (e.g. pinacol to 2,3-dimethylbuta- 1,3-diene, cognate preparation in Expt 5.12), although in this case some concomitant rearrangement to t-butyl methyl ketone (pinacolone, Expt 5.98) occurs under the acidic conditions employed. 

Pinacol. Pinacol hydrate may be dehydrated in the following manner (compare Section 2.23, p. 168, Drying by distillation). Mix 100 g of pinacol hydrate with 200 ml of benzene (CAUTION) and distil a mixture of water and benzene passes over. Separate the lower layer and return the upper layer of benzene to the distilling flask. Repeat the process until the benzene distillate is clear. Finally distil the anhydrous pinacol and collect the fraction boiling at 169-173 °C (50 g). The pure pinacol has m.p. 43 °C, but on exposure to moist air the m.p. gradually falls to 29-30 °C and then rises to 45-46 °C when hydration to the hexahydrate is complete. 

In the 1-electron reduction of A1 4-3-ketosteroids (164), various stereoisomers of pinacols are formed according to the pH. The protonized form of the ketosteroid, reduced in acidic solution, gives rise to a pinacol with hydroxyl groups in the a-position. In alkaline media, the unprotonized ketosteroid is reduced with the formation of the isomer with the hydroxyls in the p-position. The structure of the products prepared by controlled potential electrolysis, are supported by the rates of dehydration and periodic acid oxidations. For A4-3-ketosteroids, the difference in the composition of products obtained in acidic and alkaline media is less pronounced. 

The pinacol rearrangement is a dehydration of an alcohol that results in an unexpected product. When hot sulfuric acid is added to an alcohol, the expected product of dehydration is an alkene. However, if the alcohol is a vicinal diol, the product will be a ketone or aldehyde. The reaction follows the mechanism shown, below. The first hydroxyl group is protonated and removed by the acid to form a carboca-tion in an expected dehydration step. Now, a methyl group may move to fonn an even more stable carbocation. This new carbocation exhibits resonance as shown. Resonance Structure 2 is favored because all tire atoms have an octet of electrons. The water deprotonates Resonance Structure 2, forming pinacolone and regenerating the acid catalyst. 

Pinacol and other highly substituted 1,2-diols tend to undergo dehydration with rearrangement under acid-catalysis. 

The basic principles illustrated above are applied in explaining the mechanism of many common organic reactions (11). Whitmore, in explaining the 1, 2 shift in dehydration and the rearrangements of glycols (pinacols), applied this mechanism with success (31, 32). 

The synthetic oestrogen dienoestrol 43 might be made10 by dehydration of the symmetrical diol 44 and hence by pinacol dimerisation of the ketone 45. Successful pinacol with magnesium metal gave 44 that could be dehydrated with AcCl in AC2O. 

Though restricted by the need for symmetry, this is a useful approach to t-alkyl ketones which are otherwise difficult to make.7 The crowded alkenes 40 can be made by dehydration of alcohols 41 and hence from the ketone 42 and RLi or RMgX. As 42 has a t -alkyl substituent it is a candidate for the pinacol approach. 

This reagent is both a Lewis acid and a dehydrating agent. Thus it effects Pummerer rearrangement of sulfoxides and rearrangement of pinacol to pinacolone via an epoxide.2... 

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 ... 

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