Pinacol type rearrangement

A cyclization of / -keto sulfone 17 in the presence of a Lewis acid gives a spirocycles via a pinacol-type rearrangement in which the sulfone group serves as a leaving group (equation 10)9. 

The reaction of crotonaldehyde and methyl vinyl ketone with thiophenol in the presence of anhydrous hydrogen chloride effects conjugate addition of thiophenol as well as acetal formation. The resulting j3-phenylthio thioacetals are converted to 1-phenylthio-and 2-phenylthio-1,3-butadiene, respectively, upon reaction with 2 equivalents of copper(I) trifluoromethanesulfonate (Table I). The copper(I)-induced heterolysis of carbon-sulfur bonds has also been used to effect pinacol-type rearrangements of bis(phenyl-thio)methyl carbinols. Thus the addition of bis(phenyl-thio)methyllithium to ketones and aldehydes followed by copper(I)-induced rearrangement results in a one-carbon ring expansion or chain-insertion transformation which gives a-phenylthio ketones. Monothioketals of 1,4-diketones are cyclized to 2,5-disubstituted furans by the action of copper(I) trifluoromethanesulfonate. ... 

Most of the carbonyls we have tested couple and undergo a pinacol-type rearrangement via a bis-siloxyalkane intermediate ... 

The presence of a quaternary atom, for instance, may be traced back to either a "pinacol" or a "Wagner-Meerwein-type" rearrangement. In the classical "pinacol rearrangement", pinacol 1 (a 1,2-diol) under strong acid conditions rearranges to pinacolone 2 which bears a quaternary atom at the a-position of the carbonyl group ... 

A new semipinacol rearrangement mediated by Sn(IV) was proposed by Bates and to explain the formation of 579 from 578 (equation 256). As stated by the authors, the mechanism of formation of 579 most likely involves an intermediate hydroxylamine 580 (equation 257). Nucleophilic addition of the hydroxylamine to the ketonic carbonyl leads to 581, which may undergo a tin-mediated pinacol-type rearrangement with preferred migration of the phenyl substituent to produce amide 582. 

Rule D. When a quarternary carbon atom is the alpha position to a secondary or tertiary double-bonded carbon atom, a pinacol type of rearrangement involving migration of a methyl group may occur. Examples of this type of rearrangement include isomerization of... 

Table 4. Cyclopentanones by Pinacol-Type Rearrangement of Cyclobutanones with oc-Lithio Selenoxides...

The acid-mediated intramolecular addition of the azide to the ketone proceeds by way of addition followed by a pinacol-type rearrangement, to give the amide 12. The regioselectivity of the bond migration is remarkable. Reduction of the amide then gave 3. 

The final, critical oxidative spirocyclization of the 2,3-disubstituted indole to the spiro oxindole was effected by treatment of 124 with tert-butyl hypochlorite in pyridine to provide the labile 125 [Fig. (34)]. The Pinacol-type rearrangement was conducted by treating compound 125 with p-toluenesulfonic acid in THF/water. It is assumed that the chlorination of 124 proceeds from the least hindered face of the indole, to give the a-chloroindolene 125. The hydration of the imine functionality must also occur from the same a-face that is syn to the relatively large chlorine atom furnishing the syn-chlorohydrin 126, that subsequently rearranges stereospecifically to the desired spiro oxindole 127. 

But now commonly associated with Tiffeneau-Demianov rearrangement The classification now covers many reactions of the "pinacol type", including ... 

Exercise 16-40 Write a sequence of reactions whereby 2-methylpropene may be converted to 2-methylpropanal by way of a pinacol-type rearrangement. Would you expect any concomitant formation of 2-butanone Explain. 

The diterpene derivative 1 was treated with BF3-OEt2 and AC2O in an attempt to induce a "pinacol-type rearrangement leading to a ring B-homo derivative. No such reaction was observed. Instead, the only product which could be isolated (33%) was shown to be 2, formed by "a profound backbone rearrangement" of 1. 

Reduction of nitro compound 269 with stannous chloride did not give the expected amino compound but rather the tricyclic 1,4-thiazepine derivative 270 after a semi-pinacol-type rearrangement (Equation 22) <2002JOC8662>. 

The synthesis of gmelanone (26) by a pinacol-type rearrangement of arboreol (25) supports its biogenesis and confirms its relative and absolute configuration.41... 

With I (X-Nalk R =H) the intermediate hydroxyphosphonate(III) is rather stable and undergoes the expected cycloelimination only poorly or not at all. However, III upon acidification undergoes a semi-pinacol type rearrangement to give aldehydes... 

A TFA-mediated pinacol-type rearrangement of arboreal 1042 leads to gmelanone 1043, confirming the biogenesis and relative absolute configuration of the latter natural product (Equation 405) <2005T8956>. 

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