Читать книгу Catalytic Asymmetric Synthesis - Группа авторов - Страница 22

Enamine chemistry was extended to dienamine chemistry: diarylprolinol silyl ether reacts with an α,β‐unsaturated aldehyde to generate an iminium ion, which is further converted into a dienamine. As dienamines are electron‐rich, they act as reactive dienes in the Diels‐Alder reaction. The 1,4‐position of the original aldehyde reacts with a dienophile. Jørgensen reported the reaction of a dienamine with azodicarboxylate, in which only the s‐cis dienamine from a mixture of s‐cis and s‐trans isomers reacts in a concerted fashion (Diels‐Alder reaction) to afford the product with excellent enantioselectivity (Eq. 1.15) [34].

(1.15)

A dienamine is successfully utilized in the enantioselective synthesis of a key intermediate of 14β‐steroids by the reaction of an enal with a cyclic dienophile in the presence of diphenylprolinol silyl ether (Eq. 1.16). This reaction allows easy access to an optically active steroid core with a variety of substituents in the A ring in high yields and up to more than 99% ee. (+)‐Estrone was efficiently synthesized by using this reaction as a key step [35].

(1.16)

This dienamine system was further extended to trienamine chemistry. Chen and Jørgensen reported the generation of an electron‐rich trienamine from a polyconjugated 2,4‐dienal. By a reaction with a dienophile, a cycloaddition reaction proceeded to afford a spirocyclic oxindole possessing a cyclohexene moiety with an exclusive β,ε‐regioselectivity and excellent stereoselectivity (Eq. 1.17) [36].

(1.17)