Synthesis of chromans and kinetic resolution of 2-aryl-3-nitro-2H-chromenes via the NHC-bound azolium homoenolate pathway

Abstract

Herein, an effective kinetic resolution (KR) of racemate 2-aryl-3-nitro-2H-chromenes to afford synthetically valuable chromans and 2H-chromenes through an NHC-bound acyl azolium homoenolate pathway has been demonstrated.

Carbene-catalyzed kinetic resolution of bispirooxindoles: selective lactam amidation and access to challenging chiral scaffolds

“…Kinetic resolution (KR) is one of the most convenient and reasonable methods for preparing pure chiral monomeric compounds from racemic mixtures. [1][2][3][4] Importantly, strategies involving N-heterocyclic carbene (NHC)-catalyzed KR have recently been demonstrated to be promising vehicles for the fabrication of chiral cyclic amines, esters, alcohols, and sulfoximines as well as phenols, as they effectively minimize the possibility of competitive racemic background reactions encountered with the application of other non-selective reagents such as acyl chlorides and anhydrides (Fig. 1a).…”

Highly diastereoselective annulation of 2-substituted 3-nitro-2H-chromenes with hemicurcuminoids and curcuminoids via a double and triple Michael reaction cascade

“…Being readily available and highly reactive building blocks, 3-nitro-2 H -chromenes are widely used in organic synthesis to obtain various chromene and chromane derivatives. 11,12 The electron-withdrawing nitro group in the 3-position not only activates the 4-position with respect to nucleophilic reagents, but also stabilizes the anion formed as a result of nucleophilic addition. In this regard, 3-nitro-2 H -chromenes are able to take part in cascade and multicomponent processes of carbo- and heterocyclization.…”

N-Heterocyclic carbene-catalysed homoenolate addition reaction to 3-cyano-2-imino-2H-chromenes: synthesis of C4-functionalized 2-amino-3-cyano-4H-chromene

“…In other words, N-heterocyclic carbenes (NHCs) as Lewis base catalysts have been proven to react with α,β-unsaturated aldehyde (enal) to form an in situ homoenolate intermediate which is subsequently trapped by a wide range of Michael acceptors to furnish synthetically useful cyclic or acyclic scaffolds in a highly stereoselective manner. 12 In this context and as a part of our NHC-carbene catalysed organic transformations, 13 we proposed to investigate a reaction between enals and 2-iminochromenes which resulted in a new type of C 4 -functionalized 2-amino-3-cyano-4 H -chromene (Scheme 2). Moreover, owing to the biological importance of the privileged core, the functionalization at the C 4 -center alongside other regions at the C 2 /C 3 -centers and the aryl part of the benzopyran ring through diversity oriented synthesis is highly significant and desirable.…”

“…Satisfyingly, when acetonitrile was used as the solvent, the homoenolate addition product 4a was formed in a good yield (73%) and with good diastereoselectivity (dr = 7 : 1). In order to improve the yield and selectivity of 4a , the reactions were also conducted in the presence of the additive sodium 4-chlorobenzoate 13 and an excess of base (100 mol%); however, no further improvement in terms of yield and selectivity was observed (entries 17 and 18).…”