Pinacol rearrangement transform

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. 

The bishydroxylation of peripheral C —C double bonds of porphyrins, e.g. 6, with hydrogen peroxide under acidic conditions or with osmium(VlII) oxide yields the corresponding diols, e.g. 10, which on pinacol rearrangement are transformed into geminally dialkylated chlorins, e.g. 11.9,97... 

Isomerization of substituted styrene oxides allows the synthesis of aldehydes in high yields726 [Eq. (5.275)]. Cycloalkene oxides do not react under these conditions, whereas 2,2,3-trimethyloxirane gives isopropyl methyl ketone (85% yield). Isomerization of oxiranes to carbonyl compounds is mechanistically similar to the pinacol rearrangement involving either the formation of an intermediate carbocation or a concerted mechanism may also be operative. Glycidic esters are transformed to a-hydroxy-/3,y-unsaturated esters in the presence of Nafion-H727 [Eq. (5.276)]. 

While the hydride shift illustrated in Scheme 5.12 cannot occur as a part of the pinacol rearrangement, the intermediate carbocation is subject to alkyl migrations. As shown in Scheme 5.13, a 1,2-alkyl shift results in transfer of the cation from a tertiary center to a center adjacent to a heteroatom. As the oxygen heteroatom possesses lone electron pairs, these lone pairs serve to stabilize the cation. Thus, the illustrated 1,2-alkyl shift transforms a carbocation into a more stable carbocation. 

Analogous to the pinacole rearrangement, bicyclic triazolines are transformed on heating into aziridines, like (377), and the cyclic imines (378) or (379) depending on the nature of the substituents (6sja749). 

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. 

The thio-Prins-pinacol rearrangement was the key transformation in L.E. Overman s enantioselective total synthesis of (+)-shahamin K. Treatment of the dithioacetal substrate with DMTSF brought about the rearrangement, which gave rise to the c/s-hydroazulene core of the natural product. 

Pinacol Rearrangement.12 Certainly one of the best-known examples of a carbonium ion rearrangement is the pinacol transformation in which polysubstituted ethylene glycols are converted into substituted ketones by the action of acidic reagents such as mineral acid, acetyl chloride, or acetic acid and iodine. In accordance with Whitmore s theory of carbonium ion rearrangements,11 the mechanism of the reaction can be outlined as follows, with pinacol itself as an example ... 

It should be noted that the 2-oxazolines are connected directly with enamides, being their products of cyclization on the one hand ", as well as being the heterocyclic precursors of W-acylenamines on the other . However, it is no less important that the 2-oxazolines are cyclic derivatives (and precursors also) of the 1,2-aminoalcohols. In essence, the reactions 117 122 (equation 41) and 125- 128 (equation 43) are the transformations of ketones to 1,2-aminoalcohols, i.e. they are the reversal of one of the pinacol rearrangement variants, i.e. the Tiffeneau and MacKenzie reactions . These transformations which proceed via the iV-acyliminium intermediates 118, 126 are examples of a real retropinacol rearrangement according to structural, functional and redox features (see Section III.B.3). 

As with polystyrene sulfonic resins, Nafion-based acid catalysts are highly efficient for hydration and dehydration processes and, in general, for condensation reactions that occur with the formation of water or similar secondary products. Formation of ethers has been studied for various alcohols [109-111]. Dehydration of 1,4- and 1,5-diols at 135 °C affords the corresponding cyclic ethers such as 20 in excellent yields (Scheme 10.7), while 1,3-diols experience different transformations depending on their structure [112]. The dehydration of 1,2-diols mainly proceeds via the pinacol rearrangement. Further condensation of the initially formed carbonyl compound and unreacted diol affords 1,3-dioxolanes [113]. The catalyst could be efficiently reused following a reactivation protocol. Formation of aryl ethers is also possible, and the synthesis of dibenzofurans 21 (X = O) from 2,2 -dihydroxybiphenyls has been reported (Scheme 10.7) [114]. The related reaction... 

The pinacol rearrangement of sulfonate esters derived from a-hydroxy acetals proceeds by way of intermediate oxonium species, which upon hydrolysis are transformed to the corresponding esters. Sulfonate 24, prepared in optically pure form by classical resolution of the diastereomer-ic mixture obtained from reaction of (-)-camphorsulfonyl chloride with the racemic naph-thenyl alcohol, undergoes thermal [1,2] rearrangement to yield the corresponding ester29. 

The products obtained from pinacolic rearrangement of a-silyloxy epoxides can be further transformed in situ by allylsilane addition to the carbonyl group. Lewis acid treatment of 2,3-epoxy-l-phenyl-l-trimethylsilyloxybutane at low temperature followed by addition of 3-trimethylsilylpropene yields 1.3-diol 31 as the only product33. 

It is now known that the pinacol rearrangement is characteristic of all types of 1,2-diol, and that the process is promoted by most electrophilic catalysts. Most of the available data point to the involvement of a carbocationic intermediate, even when the hydroxyl groups are not tertiary. Evidence for a concerted process, i. e. loss of water with the synchronous migration of the substituent, has also been obtained. Other mechanistic possibilities, e. g. transformation through an epoxide or an eno-lic intermediate, can either be ruled out or regarded as playing a role in limited cases only [6]. 

Of the solid electrophilic catalysts an acidic clay [13] and alumina were applied in the early history of the pinacol rearrangement. The last two decades, with the development of a variety of new classes of solid acid has, however, brought an upsurge in research activity. Almost all types of solid acid have been tested and found to have high catalytic activity in the transformation of vicinal diols, although selectivity is not always satisfactory. 

The pinacol rearrangement of simple methyl- and phenyl-substituted diols in the presence of different montmorillonites was examined by Gutierrez and Ruiz-Hitzky [33,37-39]. The transformations were performed in dry media without solvent, with the reacting diols intercalated into the interlamellar space of the montmorillonites. High catalyst/diol ratios (> 5) and mild reaction conditions were usually used. 

Vicinal diols can be transformed to carbonyl compounds in the presence of electrophilic catalysts. This reaction, called the pinacol rearrangement, is discussed in detail in Section 5.5. 

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