Diols rearrangements with acid

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... 

The MNDO method has been employed405 to study the reaction pathway and to optimize the structures of reactant, product, and transition state of the acid-catalysed rearrangement of 1,2-propylene glycol, and the unimolecular dehydration of protonated a,co-diols in the gas phase has been examined406 by tandem mass spectrometric experiments. It has been shown that the reaction of l,2-diarylcyclopropane-l,2-diols (342) with acids yields primarily the a,//-unsaturated ketones (343) in which the aryl... 

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. 

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

When the 1,2-diol pinacol is treated with acid, a rearrangement takes place. 

Allylic alcohols undergo symmetry-allowed orthoester Claisen rearrangement, when treated with trialkyl orthoacetate in the presence of an acid catalyst. When this reaction is applied to 2-butyne-l,4-diols, one of the products formed is a 1,3-diene. 1,3-Dienes also result when this reaction is performed on bisally lie alcohols (equation 91). Regiospe-cific conversion of allylic alcohols to two-carbon extended dienoate esters, by performing an orthoester Claisen rearrangement with phenylsulhnyl orthoacetate, has been described (equation 92). ... 

Earlier, we saw that vicinal diols perform a rearrangement when treated with acid. If an a-diketone is treated with a base, it will undergo a rearrangement. 

In the pinacol-pinacolone rearrangement a 1,2-diol rearranges to give a carbonyl group that is adjacent to a quaternary carbon. There are many variations of this reaction for example, the semipinacol rearrangement, in which a (P-amino alcohol is treated with nitrous acid. This reaction may be modified so that a ring expansion takes place, in which case it is called the Tiffeneu-Demyanov ring expansion. 

Other catalytic effects [1, 801-802, at end]. Treatment of a steroid 5a,6a-diol (1) with a trace of perchloric acid in refluxing dioxane results in rearrangement to the 5-epimeric ketones (2) and (3). On treatment with sodium ethoxide in ethanol. (3) is converted into (2), the point of equilibrium being 93% of the 5a-ketone and 7% of the 50-ketone. ... 

The first synthesis of this exquisite molecule in racemic modification was reported by Underwood and Ramamoorthy (152) as early as 1970. In our preparation of trishomocubane (D3) in an optically active modification (148), we followed Barborak s elegant procedure (153) and started from the easily accessible diene adduct 185. A [n2 + n2 ] photocyclization followed by hydride reduction converted 185 into the glycol 186, whose acidic rearrangement gave the pentacyclic diol 187 with the trishomocubane D3 framework. Removal of one of the functional groups from diol 187 provided 188, and optical resolu-... 

Pinacolic rearrangement of the 20)8-ethynylpregnane-17a,20a-diol (548) with either thionyl chloride or formic acid gave the D-homo-ketone (549) a similar reaction is known in the 20-methyl analogue. ... 

The derived 2,6a-diols (667) rearrange on treatment with acid, apparently through the delocalised homo-allylic cation (668), to give the A -compounds (669) and (670). 

Diol 307, prepared from 282 in a sequence similar to the one described above, forms acetonide 308 when treated with acetone under acidic conditions. Exposure of 308 to SnCU produces highly functionalized tetrahydrofuran 309 via a pinacol-type rearrangement with complete preservation of enantiomeric purity [106]. 

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