The synthesis of 4-prop-2-enyl substituted cyclopentane-1,3-dione enol esters, and an investigation of their structures by N.m.r. spectroscopy

Abstract

Alkylations of dianions derived from cyclopentane-1,3-dione and 2-methylcyclopentane-1,3-dione are shown to provide an expeditious route to the corresponding 4-alkylated derivatives (9). Unsymmetrical 4,5-disubstituted cyclopentane-1,3-diones(11) are more easily obtained by isomerisation of the corresponding 4-hydroxycyclopent-2-enones(10) in the presence of methanolic sodium methoxide.Proton and carbon-13 n.m.r. data establish that the 4-prop-2-enyl substituted cyclopentane-1,3-diones(9a–d, R = H, Me) and (11a–c) are fully enolised in solution, and that the two enolic forms [e.g.(18) and (19)] are undergoing rapid tautomerism. N.m.r. data also show that whereas the 2-methyl substituted diones (9a, b, d; R = Me) produce single enol acetates [i.e.(20), (26), and (27)] on treatment with acetic anhydride–sodium acetate, the analogues (9a, c, d; R = H) and (11a–c) instead lead to mixtures of isomeric enol acetates [e.g.(28)/(29); (35)/(36)]. In the case of the 4,5-disubstituted enol acetates (35), (36), and (41)→(44), the C-4, C-5 substituents are shown to have an anti-stereochemical relationship to one another.