Pinacols rearrangement

The disconnection is obvious, and gives us one way of making symmetrical 1,2-diols. What makes it more than trivial is that the products undergo the pinacol rearrangement ... 

Analysis This is a t-alk ketone so a pinacol rearrangement route will be possible ... 

Synthesis Ketone A is just pinacolone, the product of the pinacol rearrangement (frames 154-5). 

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... 

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 lactone A was also used as starting material in the synthesis of the primary prostaglandins via an allylic substitution-semi-pinacolic rearrangement sequence (Ref. 2). 

Selective hydroxylation with osmium tetroxide (one equivalent in ether-pyridine at 0 ) converts (27) to a solid mixture of stereoisomeric diols (28a) which can be converted to the corresponding secondary monotoluene-sulfonate (28b) by treatment with /7-toluenesulfonyl chloride in methylene dichloride-pyridine and then by pinacol rearrangement in tetrahydrofuran-lithium perchlorate -calcium carbonate into the unconjugated cyclohepte-none (29) in 41-48 % over-all yield from (27). Mild acid-catalyzed hydrolysis of the ketal-ketone (29) removes the ketal more drastic conditions by heating at 100° in 2 hydrochloric acid for 24 hr gives the conjugated diketone (30). 

Base catalyzed pinacol rearrangement of vicinal m-glycol monotosylates is a simple and useful general method for preparing perhydroazulenes. Thus, treatment of cholestane-5a,6a-diol 6-tosylate (115a) with either one mole-equivalent of potassium t-butoxide in f-butanol at 25° or with calcium carbonate in dimethylformamide at 100° gives a quantitative yield of 10(5 6/5H)... 

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 nonphotochemical method for the preparation of D-norsteroids involves the base-catalyzed pinacol rearrangement of a pregnane-16oc,17a-diol 16a-mesylate. ... 

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

The pinacol rearrangement reaction is of limited synthetic importance although it can be a useful alternative to the standard methods for synthesis of aldehydes and ketones." Especially in the synthesis of ketones with special substitution pattern—e.g. a spiro ketone like 5—the pinacol rearrangement demonstrates its synthetic potential ... 

DimethyI-2 3-butanediol has the common name piiuicol. On heating with aqueous acid, pinacol rearranges to pinacolone, 3,3-dimethyl-2-butanone. Suggest a mechanism for this reaction. 

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... 

In a similar reaction, 2,3,6-trimethoxydibenz[6,/]oxepin gives 10,11-dihydro-2,3,6-trimeth-oxydibenz[/>,/]oxepin-cw-10,l 1-diol upon treatment with osmium(VIII) oxide in the presence of A-methylmorpholine A -oxide.262 When treated with acid the diol undergoes a pinacol rearrangement to the corresponding xanthene-9-carbaldehyde. 

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... 

A major disadvantage of this procedure is the lack of regioselectivity of the bishydroxylation which can take place on each of the four pyrrole rings of the macrotetracycle so that, in addition to the monoadduct, two bisadducts and one trisadduct are formed. The subsequent pinacol rearrangement of the diols can also occur in two directions, leading to a complex mixture of additional constitutional isomers. 

The oxidation of octaethylporphyrin 418 with hydrogen peroxide in sulfuric acid leads under pinacol rearrangement to a mixture of hydroporphyrinones, among them the expected three constitutionally isomeric isobacteriochlorins. 

The above mechanism involves a-opening of the epoxysilane, followed by a 1,2-elimination of a /3-hydroxysilanc (Peterson olefination, Chapter 10). However, it has recently been shown that aj8-dihydroxysilanes, particularly t-butyldimethylsilyl species, undergo an acid-catalysed sila-pinacol rearrangement to produce /J-aldehydo- and /i-kctosilancs (5) ... 

In order to answer the question about inherent migratory aptitudes, the obvious type of substrate to use (in the pinacol rearrangement) is R RC—CRR since the... 

---