128. α-Alkylierung von β-Hydroxycarbonsaeuren ueber 1,3-Dioxan-4-on-Enolate. Highly Diastereoselective α-Alkylation of β-Hydroxycarboxylic Acids Through Lithium Enolates of 1,3-Dioxan-4-ones

Article abstract of DOI:10.1002/hlca.19880710527

From serine, β-hydroxyisobutyric acid ("Roche" acid) and β-hydroxybutyric acid, the dioxanones 1-6 were prepared.The generation of the enolates of type 1 with LDA at -75 deg C and alkylation gave products with trans-configuration whereas protonation of the 5-methyl-substituted enolate allowed access to the cis-configurated β-hydroxybutyric-acid derivative 12.Hydrolysis gave the free β-hydroxy acids of "syn"- and "anti"-configuration.Alkylation of the 6-unsubstituted dioxanones 1 and 3 yielded predominantly products resulting from attack in the cis-position of the t-Bu group.The "reactive" conformation of the enolates involved is tentatively derived from the product configuration.The selectivity of the alkylation is also discussed in terms of the results of an ab-initio calculation on the enolates M-P.