Abstract
The chiral (salen)Cr(III)/BF3·OEt2 catalytic combination was found to be an effective catalyst for intramolecular Friedel–Crafts cyclization of electron-deficient Morita–Baylis–Hillman adducts. In presence of mild reaction conditions the chiral (salen)Cr(III)/BF3·OEt2 complex affords 2-substituted-1H-indenes from unique substrates of Morita–Baylis–Hillman adducts via an easy operating practical procedure.
Highlights
Reactions associated with carbon–carbon bond formations are explored for their synthetic utility in extending the carbon framework in organic molecules [1,2,3]
Inter/intramolecular Friedel–Crafts reactions of Morita–Baylis–Hillman adducts leads to a variety of products such as quinolinones [8], cycloheptene-6-carboxylates [9], indenes [10-13] and indanones [14]
We further investigated the role of co-catalysts (2.5 mol %), viz., AlCl3, SnCl2, FeCl3 and ZnCl2 with [Cr(salen)Cl] (15 mol %) in promoting the Friedel–Crafts cyclization in MBH adduct 5a (Table 1, entries 14–17)
Summary
Reactions associated with carbon–carbon bond formations are explored for their synthetic utility in extending the carbon framework in organic molecules [1,2,3]. The first report was by Basavaiah et al [10] on the phosphorous pentoxide-catalysed synthesis of indene using alkoxy-substituted MBH adducts. Thereafter Lee et al [12] and Xu et al [13] synthesised indenes in presence of a palladium catalyst from allylic acetates of MBH adducts.
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