Reductive Cleavage of Allyllic Ketals by an Arene-Catalysed Lithiation: A Simple and Direct Route to Masked Lithium Homoenolates

Abstract

The reaction of acrolein diethyl acetal 1 with an excess of lithium powder and a catalytic amount of 4,4′-di- tert-butylbiphenyl (DTBB; 2.5 mol %) in the presence of a carbonyl compound 2 in THF at 0°C leads, after hydrolysis with water, to the corresponding γ-coupling products 3 as a Z/E mixture, the Z-isomer being mainly the major one. When the final hydrolysis was carried out in acidic conditions (3 N hydrochloric acid) followed by treatment with silicon compounds of the type R 3SiNu in the presence of boron trifluoride etherate in dichloromethane at temperatures ranging between −78 and 20°C, the corresponding substituted tetrahydrofurans 4 were obtained. The same Barbier-type reaction applied to 2-cyclopentenone ethylene ketal 5 yields, after acid hydrolysis, the expected 3-substituted cyclopentanones 6. In all cases overall yields are moderates.