Enantioselective Isothiourea-Catalysed Michael–Michael–Lactonisation­ Cascade Reaction for the Synthesis of δ-Lactones and 1,2,3,4-Substituted Cyclopentanes

Dedicated to Professor Dieter Enders on the occasion of his 70^th birthday

Abstract

This manuscript describes the application of α,β-unsaturated acyl ammonium intermediates in a Michael–Michael–lactonisation cascade process to furnish δ-lactones. Generation of α,β-unsaturated acyl ammonium intermediates was achieved upon addition of isothiourea catalyst HyperBTM into α,β-unsaturated acid chlorides. Subsequent reaction with enone-malonates gave access to δ-lactones in 20–64% yield, 72.5:27.5 to 95:5 er and 81:19 to >95:5 dr. Additionally, application of a ring-opening protocol yielded 1,2,3,4-substituted cyclopentanes in 28–77% yield, 76:24 to 98:2 er and 86:14 to >95:5 dr. Interestingly, the highest er was observed at high reaction temperatures, with 70 °C proving optimal. This effect was investigated by conducting an Eyring analysis, which indicated that differential activation entropy rather than differential activation enthalpy is responsible for enantiodiscrimination in this process.

Key words

Lewis base catalysis - α,β-unsaturated acyl ammonium intermediates - isothiourea catalysis - cascades - enantioselective catalysis

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