Regioselectivity of alkylations

The regioselectivity of alkylations of silyl dienol ethers has been studied87,88. These reactions favor y-alkylation products. In contrast, alkylations of the corresponding lithium enolates mainly occur in the a-position. Substituents on the silyl diene unit, as well as the substituents at the silicon, strongly influence the regioselectivity of the reaction87 91,... 

TABLE 2. Regioselectivity of alkylation of the pyrrolidine enamine of 2-methylcyclohexanone... 

A survey of work since 1975 on the derivatization of ascorbic acid is reviewed from the perspective of the organic chemistry of ascorbic acid. Recent advances in the control of regioselectivity of alkylative derivatization of ascorbic acid have been made possible by the utilization of di- and trianions of ascorbic acid. Their use has allowed the facile synthesis of inorganic esters of ascorbic acid. New synthesis of acetal and ketal, side-chain oxidized, and deoxy derivatives are reviewed. The total synthesis of a new side-chain oxidized ascorbic acid derivative, 5-ketoascorbic acid, is reported. 

It is well established since the work by Kornblum [92] that the regioselectivity of alkylation of an ambident anion as -naphthol anion is essentially dependent on the reaction medium. Whereas 0-alkylation is selectively obtained under anionic activation conditions (dipolar aprotic solvents or PTC), selective C-alkyl-ation remains an unsolved problem. 

Scheme 21). Deprotonation of keten thioacetals gives the allylic anion. The regioselectivity of alkylation of such species has continued to attract attention. ... 

Factors influencing the regioselectivity of alkylation of allylic anions continue to be studied, a-Alkylation of the keten thioacetalide anion (13) is favoured by hard leaving groups/ whereas the a-aminonitrile anion (14) reacts with ketones in the a-position at low temperature (-78 °C), but behaves as a homoenolate equivalent (y-attack) at At the higher temperature, competing aldol... 

The alkylation of p-methoxyphenylpyridine is not inhibited by water as solvent but water slightly modifies the regioselectivity of alkylation as a small amount of alkylation at mefa-position is observed beside the major alkylation at ortho-position [109]. 

SCHEME 7.33. Regioselectivity of alkylation of Li-enolates derived from Al-acyl sultams. 

The regioselectivity of alkylation and carbonyl addition of various thioallyl anions has been examined, particularly with regard to chelation and ion-pairing effects. Thus, Biellmann and co-workers have found that thioallyl-lithium (50), in the presence of [2,2,2]cryptate, adds to acetone exclusively from the... 

The regioselectivity of alkylation has also been studied with cyclic thioallyl anions, with thioallyl-copper systems which involve 5n2 reactions, and with the dianion of prop-2-enethiol (5 ). > The products of addition of aldehyde at the a-position of (50) have been converted into cyclopropane... 

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