A Strategy To Obtain o-Naphthoquinone Methides: Ag(I)-Catalyzed Cyclization of Enynones for the Synthesis of Benzo[h]chromanes and Naphthopyryliums

Abstract

A new strategy to obtain o-NQM intermediates through a ring-formation strategy by Ag­(I)-catalyzed cyclization of 2-alkenylphenyl alkynyl ketones and its [4 + 2] annulations with styrenes has been developed. This reaction features high efficiency, mild reaction conditions, as well as flexible substitutions and atom economy. The obtained benzo­[h]­chromane products were further oxidized to naphthopyryliums, which displayed tunable photophysical properties.

Catalytic Benzannulation Reactions of Enynones for Accessing Heterocycle-Incorporating Diarylmethanes

“…8 Zhu's group reported an elegant work where enynones as o-NQM precursors were reacted with styrenes to Template for SYNLETT Thieme generate benzo[h]chromanes via Ag(I)-catalyzed [4+2] annulation. 9 Later, the same group synthesized unsymmetrical triarylmethanes via 1,4-addition between the in-situ-generation of o-NQM intermediates from Ag(I)-catalyzed annulation of enynones and electron-rich aromatics such as 1,3,5trimethoxybenzene, 1,2,3-trimethoxybenzene, 1-naphthol and 2-naphthol as nucleophiles. 10 However, nucleophilic indoles were ineffective in this approach (Scheme 1a).…”

“…Subsequently, the effect of the solvent was evaluated and some solvents such as toluene, acetonitrile (CH3CN) and 1,2-dichloroethane (DCE) were tested. The results showed that none of them gave positive results (entries [9][10][11]. The reaction also carried out in 2-methyltetrahydrofuran (2-MeTHF as solvent with a slight decrease in yield (entry 12).…”

Synthesis of Diversified Pyrazolo[3,4-b]pyridine Frameworks from 5-Aminopyrazoles and Alkynyl Aldehydes via Switchable C≡C Bond Activation Approaches

“…Recently, catalytic carbon-carbon bond activation has emerged as a useful tool to build complex molecules rapidly and efficiently [ 6 , 7 , 8 , 9 ]. There are versatile intermediates involved in these reactions, which could be trapped in situ by a second molecule that triggers subsequent tandem reactions [ 10 , 11 , 12 , 13 , 14 , 15 ]. The nucleophilic/electrophilic addition reactions of alkynes are well-known and provide a convenient way to synthesize functionalized molecules [ 16 , 17 , 18 , 19 , 20 ].…”

Copper-Catalyzed Annulation–Trifluoromethyl Functionalization of Enynones

“…In particular, we and collaborators have disclosed several radical addition–annulation–difunctionalization of 1,6-enynes for multiple ring formations (Scheme a) . Zhu and co-workers pioneered the preparation of a novel class of enynones by adjusting the carbonyl group position of 1,6-enynes and applied them as o -naphthoquinone methides ( o -NQMs) precursors to undergo intramolecular 6- exo - trig cyclization, leading to the formation of o -NQMs intermediate in the presence of silver catalysts, followed by o -NQMs’ [4 + 2] annulations with styrene to generate benzo­[ h ]­chromanes (Scheme b, path i ) . Moreover, o -NQMs could be trapped by electron-rich aromatic compounds (1,3,5-trimethoxybenzene, 1,2,3-trimethoxybenzene, 1-naphthol, and 2-naphthol) or heteroaryl compounds (indoles and tetronic acid-derived enamino lactones) to give unsymmetrical triarylmethanes through 1,4-conjugate addition (Scheme b, path ii ) .…”