Abstract
A highly enantioselective [2+2] versus a [4+2]-cycloaddition of 3-styrylindoles to N-allenamides catalyzed by identical gold(i)/chiral phosphoramidite complexes is presented, which provides facile access to synthetically valuable, optically active substituted cyclobutanes and tetrahydrocarbazoles. The cycloaddition mode unexpectedly depends on the electronic nature of the N-substituent 3-styrylindoles, the origin of which could be well rationalized using DFT calculations and experimental results. To the best of our knowledge, the present work represents the first example of such an impressive substituent effect in tuning the reaction mode with high chemo-, regio- and enantioselectivity in asymmetric gold catalysis.
Highlights
Over the past decade, gold-catalyzed intermolecular cycloadditions of two unsaturated components have shown their power for rapid access to various cyclic and polycyclic ring systems in an extremely efficient and stereoselective manner.[1]
Gold-catalyzed cycloadditions still pose challenges with respect to the cycloaddition mode as well as the enantioselectivity,[7] only a few elegant examples of enantioselective cycloadditions involving N-allenamides have been reported to date
An efficient asymmetric [3+2]-cycloaddition with nitrones was successfully realized by Chen.4b Very recently, Rossi, Vicente and their co-workers developed a seminal gold(I)catalyzed intermolecular [4+2]-cycloaddition of 2-vinylindoles with N-allenamides, furnishing good yields of racemic tetrahydrocarbazoles (Scheme 1a).5b to the best of our knowledge, a cycloaddition mode which depends on the electronic nature of the substituent is unprecedented in asymmetric gold catalysis
Summary
Gold-catalyzed intermolecular cycloadditions of two unsaturated components have shown their power for rapid access to various cyclic and polycyclic ring systems in an extremely efficient and stereoselective manner.[1]. Mascarenas and Lopez5c explored the elegant asymmetric [4+2]-cycloaddition with 1,3-dienes, leading to cyclohexenes and a cascade cycloaddition with the alkene tethered ketone.[6] An efficient asymmetric [3+2]-cycloaddition with nitrones was successfully realized by Chen.4b Very recently, Rossi, Vicente and their co-workers developed a seminal gold(I)catalyzed intermolecular [4+2]-cycloaddition of 2-vinylindoles with N-allenamides, furnishing good yields of racemic tetrahydrocarbazoles (Scheme 1a).5b to the best of our knowledge, a cycloaddition mode which depends on the electronic nature of the substituent is unprecedented in asymmetric gold catalysis. As a part of our continual program in developing asymmetric gold-catalyzed cycloadditions,[9] we report the gold catalyzed enantioselective [2+2] and [4+2] cycloadditions of 3styrylindoles with N-allenamides (Scheme 1b), furnishing synthetically valuable tetrahydrocarbazole[10,11] and 3-cyclobutylindole[12,13] scaffolds, respectively, which frequently appear in natural products and bioactive compounds (Fig. 1).
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