Neighbouring-group derivatives

B. Solvolysis of Phosphoric Acid Derivatives.—Interest continues in neighbouring-group participation in the solvolysis of phosphate esters. As a potential model compound for investigating the mechanism of ribo-nuclease action, the phenyl hydrogen phosphate ester of c/j-3,4-tetrahydro-furandiol (24) has been the subject of a detailed study. Above (and probably also below) pH 4 hydrolysis gives solely the cyclic phosphate (25)... 

The mechanism of the novel transformation of a-nitro- to a-hydroxy-ketones has been probed. The reaction, which proceeds under basic aqueous conditions, requires that the Q -nitro substrate be CH-acidic in the a -position, and that it be readily depro-tonated under the conditions employed. NO2 -OH exchange occurs with retention of configuration, with the hydroxyl oxygen being predominantly derived from the solvent. A mechanism involving neighbouring-group participation, via a Favorskii-like cyclopropanone intermediate, is proposed. 

Volume 10 is devoted to formation and solvolysis of esters and related reactions, with discussion of the effect of neighbouring groups and biological implications, e.g. enzyme action, where appropriate. The first chapter deals mainly with esters of the inorganic acids of phosphorus and sulphur, Chapter 2 with the formation and solvolysis of esters of organic acids and the final chapter with the solvolysis of related derivatives of carboxylic acids, e.g. halides, amides, anhydrides, cyanides, carbamic acid derivatives. 

The initial suggestion that a remote double bond could be involved in a displacement reaction was made by Shoppee in 1946 in order to account for the stereochemical results of the transformations of 3-cholesteryl derivatives Building on the considerable work then underway on neighbouring group participation in ionization reactions Shoppee suggested that the double bond in the cholesteryl system, 1, could function in a similar manner to Lewis base containing groups such as acetate or alkoxy, etc. 

Neighbouring group participation by a 5a-azido group is indicated in the conversion of the azido alcohol (26) into the 6ji -chloro derivative (27) by thionyl chloride, again with retention of configuration [i2y]. 

Stereoselective hydroxylation at C(5) has been achieved from the corresponding 3-amino-derivatives by neighbouring-group participation. A possible mechanism involving a dihydro-oxazinium cyclic intermediate has been proposed. 

The effect of a heteroatom 0 to the oxirane ring has been studied in the reaction of monosubstituted oxiranes 131 and benzylamine. If X = 0, the reaction occurs via the transition state 132 with neighbouring-group participation. Neighboring-group participation has similarly been observed in the reactions of mono- and tri-substituted oxiranes and )3-substituted primary amines. The product ratio of the aminoalcohols formed from the reactions of a-vinyloxiranes and primary amines depends on the geometry of the oxirane.Base-catalyzed addition of oxiranes to oximes leads not only to 0-alkyl but to A -alkyl derivatives (Eq, 319). ... 

---