Journal of the American Chemical Society, Vol.138, No.32, 10276-10283, 2016
Lithium Enolates Derived from Pyroglutaminol: Mechanism and Stereoselectivity of an Azaaldol Addition
A lithium enolate derived from an acetonideprotected pyroglutaminol undergoes a highly selective azaaldol addition with (E)-N-pheny1-1-[2-(trifluoroniethyl)phenyl]methanimine. The selectivity is sensitive to tetrahydrofuran (THF) concentration, temperature, and the presence of excess lithium diisopropylamide base. Rate studies show that the observable tetrasolvated dimeric enolate undergoes reversible deaggregation, with the reaction proceeding via a disolvated-monomerbased transition structure. Limited stereochemical erosion stems from the intervention of a trisolvated-monomer-based pathway, which is suppressed at low THE concentrations and elevated teniperature. Endofacial selectivity observed with excess lithium diisopropylamicle (LDA) is traced to an intermediate dianion formed by subsequent lithiation of the monomeric azaaldol adduct, which is characterized as both a dilithio form and a trilithio dianion-LDA mixed aggregate.