Mild Base‐Promoted Indole Annulation–Oxidative Cross‐ Coupling of 2‐Nitrocinnamaldehydes with β‐Tetralones for 3‐Naphthylindole and 3‐Naphthylbenzo[g]indole Fluorophores

Abstract

This paper describes the transition metal‐free indole annulation–oxidative cross‐coupling of 2‐nitrocinnamaldehydes or (E)‐3‐(1‐nitronaphthalen‐2‐yl)acrylaldehyde with β‐tetralones for the construction of diverse 3‐naphthylindole and 3‐naphthylbenzo[g]indole fluorophores in moderate to good yields. This unique annulation reaction proceeds via the domino Michael addition/hemiacetalization/intramolecular addition of an enolate to a nitro group/decarbonylation/oxidative aromatization sequence under transition metal‐free conditions without the use of an external reductant and oxidant. The synthetic utility of this protocol has been also demonstrated by evaluating the photophysical properties of the synthesized 3‐naphthylindole and 3‐naphthylbenzo[g]indole derivatives.magnified image

Annulation strategies for diverse heterocycles via the reductive transformation of 2-nitrostyrenes

“…In 2017, our group synthesized 3-naphthylindoles by the mild base-promoted oxidative coupling of 2-nitrocinnamaldehydes 1a with β-tetralones 108 . 62 This reaction was promoted by cesium carbonate, and tetrahydrofuran (THF) was used as the solvent under reflux conditions to obtain the corresponding indoles 109 in moderate to good yields (Scheme 36). The versatility of this methodology was demonstrated by the successful coupling of various o -nitrocinnamaldehydes with β-tetralones bearing diverse substitution patterns.…”

Ag(i)-catalyzed cyclization of o-alkynylacetophenones facilitated through acetal formation: synthesis of C3-naphthyl indole derivatives

“…7 Later, the same group, realized the synthesis of C3-naphthyl indoles and 3-naphthylbenzo[g]indole fluorophores via a base-promoted indole annulation-oxidative cross-coupling of 2-nitrocinnamaldehydes with β-tetralones (Scheme 2, II). 8 Synthesis of C3-naphthyl indoles was achieved from o-alkynyl-phenyl diazoacetate and indoles under gold(I)-catalysis. This reaction happens through the cyclization of the diazo on the goldactivated alkyne to form a gold carbene, which is trapped by indoles (Scheme 2, III).…”

Domino C−S/C−N Bond Formation Using Well‐Defined Copper‐Phosphine Complex Catalyst: Divergent Approach to 3‐Sulfenylated Indoles

“…In addition, although numerous advances have been made for C−S bond formation using well‐defined complexes, limited success has been achieved with Au, Ag, and Cu metals . Recently, we reported on the construction of heterocycles through a base‐ or CuI/base‐promoted cascade reaction of 2‐nitrocinnamaldehydes with β ‐ketoesters or benzyl cyanides –. While intramolecular N ‐heteroannulation has been employed as a powerful arsenal to construct indoles, one‐pot tandem sulfenylation/indolization which exploits readily available 2‐nitrocinnamaldehydes has not yet been achieved .…”