Lithium Enolates Derived from Pyroglutaminol: Mechanism and Stereoselectivity of an Azaaldol Addition

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• 作者：Michael J. Houghton ; Christopher J. Huck ; Stephen W. Wright ; David B. Collum
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：August 17, 2016
• 年：2016
• 卷：138
• 期：32
• 页码：10276-10283
• 全文大小：542K
• 年卷期：0
• ISSN：1520-5126

文摘

A lithium enolate derived from an acetonide-protected pyroglutaminol undergoes a highly selective azaaldol addition with (E)-N-phenyl-1-[2-(trifluoromethyl)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-monomer-based transition structure. Limited stereochemical erosion stems from the intervention of a trisolvated-monomer-based pathway, which is suppressed at low THF concentrations and elevated temperature. Endofacial selectivity observed with excess lithium diisopropylamide (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.