Terpene-formaldehyde reactions. II. d-Limonene

BINOL: A Versatile Chiral Reagent

Lewis-acid catalyzed ene reactions

Update 1 of: BINOL: A Versatile Chiral Reagent

Unter „En-Reaktionen” versteht man die indirekte substituierende Addition einer Verbindung mit Doppelbindung (Enophil) an ein Olefin mit allylischem Wasserstoff (En). Lange Zeit stand die En-Reaktion im Schatten der verwandten Diels-Alder-Addition. In diesem Aufsatz wird gezeigt, das das Anwendungsgebiet der En-Reaktion von industriellen bis zu biosynthetischen Prozessen reicht. Einer Zusammenstellung der praparativen Aspekte folgte eine Diskussion des Reaktionsmechanismus.

Die En‐Reaktion

The addition of aldehydes and ketones to alkenes in the presence of Bronsted acids is usually called the Prins reaction.1,2 Addition of the protonated carbonyl compound to the alkene gives a β-hydroxy carbocation which reacts with a nucleophile such as chloride, water or acetate to give (1), adds to a second molecule of aldehyde to give (2) or loses a proton to give homoallylic alcohol (3; Scheme 1). Formaldehyde and electron-deficient carbonyl compounds undergo concerted thermal ene reactions with alkenes at temperatures ranging from 100 to 200 °C to give homoallylic alcohols (equation 1; R1 = H, CCl3, or CO2R).3–7 A wide variety of carbonyl compounds undergo Lewis acid catalyzed reactions with alkenes to give homoallylic alcohol ene adducts in excellent yield with high selectivity, although in some cases γ-chloro alcohols are formed as by-products.8,9 Recent investigations suggest that at least some of these Lewis acid catalyzed ene reactions are stepwise, rather than concerted, but that the intermediate bears more resemblance to a π-complex than to the open carbocation intermediate in the Prins reaction.

Terpene-formaldehyde reactions. II. d-Limonene

Citations

BINOL: A Versatile Chiral Reagent

Lewis-acid catalyzed ene reactions

Update 1 of: BINOL: A Versatile Chiral Reagent

Die En‐Reaktion

The Prins and Carbonyl Ene Reactions

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