Selectivity in 1,3-Dipolar Cycloadditions of β-Substituted Captodative Olefins – An Experimental and DFT Transition State Study

✍ Scribed by Rafael Herrera; Jorge A. Mendoza; Miguel A. Morales; Francisco Méndez; Hugo A. Jiménez-Vázquez; Francisco Delgado; Joaquín Tamariz

• Book ID: 102173410
• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 315 KB
• Volume: 2007
• Category: Article
• ISSN: 1434-193X
• DOI: 10.1002/ejoc.200600988

✦ Synopsis

Abstract

Captodative olefins 1‐acetylvinyl carboxylates substituted with alkyl groups at theposition, 12, strongly modified the regioselectivity of 1,3‐dipolar cycloadditions with respect to the behavior observed for their unsubstituted analogs, 1. When the reaction of 12 was carried out with diphenyl nitrone (7a), the corresponding C‐4 disubstituted isoxazolidines were obtained as a mixture or as single stereoisomers, in contrast to the isomeric C‐5 disubstituted heterocycles yielded by olefins 1. Nevertheless, olefins 12 reacted with benzonitrile oxide (2a) to give the C‐5 acetyl isoxazoles, as observed with dipolarophiles 1. This intricate behavior of the reactions between 1,3‐dipoles and β‐substituted captodative olefins was rationalized on the basis of DFT calculations [B3LYP/6‐31(d)] of the transition states (TSs) for nitrone 7a and olefin 12a. Thus, the observed C‐4 and endo regio‐ and stereoselectivities agreed with the most stable TSs, which are mainly stabilized by dipolar and electrostatic interactions.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)