Diels–Alder reactions of acyclic α-cyano α,β-alkenones: a new approach to highly substituted cyclohexene system
摘要:
The Diels-Alder reactions of a variety of acyclic a-cyano a,p-unsaturated ketones 8 have been investigated. With the assistance of boron trichloride, these compounds were found to undergo the cycloaddition readily with dienes to give the corresponding adducts 9 in good to high yields. In addition, some of 9 could be further subjected to the reductive alkylation reactions using lithium naphthalenide (LN) and an appropriate alkylating agent, thus allowing for the generation of highly substituted cyclohexenes 10 with the replacement of the cyano group with an alkyl substituent. (C) 2009 Elsevier Ltd. All rights reserved.
Phosphine-Catalyzed Addition/Cycloaddition Domino Reactions of β′-Acetoxy Allenoate: Highly Stereoselective Access to 2-Oxabicyclo[3.3.1]nonane and Cyclopenta[a]pyrrolizine
摘要:
Two classes of phosphine-catalyzed addition/cycloaddition domino reactions of beta'-acetoxy allenoate 1 have been developed. The reaction of 1 with 2-acyl-3-methyl-acrylonitrile 2 readily occurs to give 2-oxabicyclo[3.3.1]nonane 3, furnishing the beta'-addition/[4 + 4] cycloaddition domino sequence. In this sequence, beta'C of allenoate 1 is an electrophilic center, and its beta'C and gamma C serve as a 1,4-dipole. When the other reaction partner is switched to 2-acyl-3-(2-pyrrole)-acrylonitrile 8, a gamma-addition/[3 + 2] cycloaddition domino reaction is instead observed, in which allenoate 1 exhibits dual electrophilic reactivity of gamma C and 1,3-dipole chemical behavior of beta C and beta'C. Furthermore, both of these two asymmetric variants have also been achieved with up to 93% ee. The domino reactions presented in this report are valuable for highly stereoselective construction of complex structures under mild reaction conditions.