ESTER HOMOLOGATION REVISITED - A RELIABLE, HIGHER YIELDING AND BETTER UNDERSTOOD PROCEDURE

Enolate anions 3 and 6, prepared via enolization of a-bromo and dibromo ketones 4 and 5 were converted m high yield to ynolate anions 10 by respective addition of lithium tetramethylpiperidide (to effect deprotonation, 3 --> 7) or butyllithium (to effect metal-halogen exchange, 6 --> 7). Mixtures of such enolates were abo obtainable from esters 1 m a large-scale (25 mmol) via in situ formation and addition of lithiodibromomethane (from methylene bromide and lithium tetramethylpiperidide), followed by treatment of the resulting adducts with lithium hexamethyldisilazide to ensure complete enolization. Addition of sec-butyllithium and n-butyllithium to effect ynolate anion formation, followed by quenching of the reaction mixtures into acidic ethanol, reproducibly afforded homologated esters 8 in 67-90% yield. Demonstrated for ethyl esters 1 having the carbethoxy moiety attached to primary, secondary, tertiary, aryl, and alkenyl groups, this general procedure provides a convenient, large-scale alternative to the classical Arndt-Eistert sequence.