Beilstein J. Org. Chem.2021,17, 2186–2193, doi:10.3762/bjoc.17.140
-substituted-1H-indenes from unique substrates of Morita–Baylis–Hillman adducts via an easy operating practical procedure.
Keywords: boron trifluoride etherate; chiral (salen)chromium(III); intramolecular Friedel–Crafts cyclization; Morita–Baylis–Hillman adducts; substituted-1H-indenes; Introduction
arrangement would facilitate the intramolecular Friedel–Crafts cyclization of the MBH adducts.
Cycloaddition of azomethine imine 7a with 2-substituted-1H-indenes 6b and 6c was attempted to ascertain the structure of the synthesized 1H-indenes (Scheme 3). The azomethine imine, 1-benzylidene-3-oxopyrazolidin-1
-ium-2-ide (7a) was synthesised at room temperature by treating methyl acrylate, hydrazine hydrate and benzaldehyde in a yield of 67%. On treating the synthesised azomethine imine 7a (1.2 mM) and 2-substituted-1H-indenes 6b and 6c (1 mM) in toluene at 70 °C affords 8a and 8b via [3 + 2] cycloaddition
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Graphical Abstract
Scheme 1:
Literature-reported approaches to synthesise indene from MBH adducts [10-14].
Beilstein J. Org. Chem.2011,7, 717–734, doi:10.3762/bjoc.7.82
quantitatively catalyzed by AgClO4 (2 mol %, C6H6, rt) [33].
Wang et al. also examined the behaviour of 3-arylcyclopropenes bearing a protected hydroxymethyl group at C3: only acetates 29 underwent clean conversion to 1-methylene-2-substituted-1H-indenes 30 [21]. The yields were improved by the addition of DBU
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Graphical Abstract
Scheme 1:
General reactivity of cyclopropenes in the presence of gold catalysts.