Formal Homo-Nazarov and Other Cyclization Reactions of Activated Cyclopropanes

• Published in: Chemistry : a European journal Vol. 17; no. 51; pp. 14527 - 14538
• Main Authors: De Simone, Filippo, Saget, Tanguy, Benfatti, Fides, Almeida, Sofia, Waser, Jérôme
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 16.12.2011; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: alkaloids; Alkaloids - chemical synthesis; Alkaloids - chemistry; Asymmetry; Carbonyls; Catalysis; Catalysts; Chemistry; Chemistry, Multidisciplinary; Cyclization; Cyclopentanes - chemical synthesis; Cyclopentanes - chemistry; cyclopropane; Cyclopropanes - chemistry; Esters; heterocyclic compounds; Indoles; Indoles - chemical synthesis; Indoles - chemistry; Ketones; Ketones - chemistry; Molecular Structure; Organic compounds; Physical Sciences; regioselectivity; Science & Technology; Stereoisomerism; regioselectivity; CARBONYLS; SUBSTITUTION; cyclopropane; ACID; alkaloids; catalysis; KETONES; ALKENES; OLEFIN PARTICIPATION; EFFICIENT CATALYSIS; CYCLOADDITIONS; EBURNAMONINE; heterocyclic compounds; CYCLOPENTENONES
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201102583

The Nazarov cyclization of divinyl ketones gives access to cyclopentenones. Replacing one of the vinyl groups by a cyclopropane leads to a formal homo‐Nazarov process for the synthesis of cyclohexenones. In contrast to the Nazarov reaction, the cyclization of vinyl‐cyclopropyl ketones is a stepwise process, often requiring harsh conditions. Herein, we describe two different approaches for further polarization of the three‐membered ring of vinyl‐cyclopropyl ketones to allow the formal homo‐Nazarov reaction under mild catalytic conditions. In the first approach, the introduction of an ester group α to the carbonyl on the cyclopropane gave a more than tenfold increase in reaction rate, allowing us to extend the scope of the reaction to non‐electron‐rich aryl donor substituents in the β position to the carbonyl on the cyclopropane. In this case, a proof of principle for asymmetric induction could be achieved using chiral Lewis acid catalysts. In the second approach, heteroatoms, especially nitrogen, were introduced β to the carbonyl on the cyclopropane. In this case, the reaction was especially successful when the vinyl group was replaced by an indole heterocycle. With a free indole, the formal homo‐Nazarov cyclization on the C3 position of indole was observed using a copper catalyst. In contrast, a new cyclization reaction on the N1 position was observed with Brønsted acid catalysts. Both reactions were applied to the synthesis of natural alkaloids. Preliminary investigations on the rationalization of the observed regioselectivity are also reported. Push and pull: The extension of the scope of cyclization reactions of donor–acceptor cyclopropanes with particular focus on the catalytic formal homo‐Nazarov process is reported (see scheme). An ester group or a heteroatom was introduced on the cyclopropane to enhance its acceptor‐ or donor‐mediated polarization. Control over regio‐ and diastereoselectivity was achieved, and a first proof of principle for asymmetric induction is described.