Catalyst free one-pot synthesis of densely functionalized pyridocarbazolone annulated furan derivatives

Abstract

Furans represent an important subclass of five-membered aromatic heterocycle that has been used for the synthesis of many pharmaceutical molecules and industrial materials [1]. The functionalization of furans often allows for further structural elaboration and adornment. As a consequence, many valuable synthetic protocols have been devoted for the constructions of polysubstituted furans and these are based either on introducing substituents onto the existing furan ring or furan ring construction from acyclic precursors [3]. The development of a facile new method for the efficient synthesis of tetra-substituted furans remains a challenging task in the field of Organic and Medicinal Chemistry. We present herein a unique and general synthetic strategy for accessing tetrasubstituted furans i.e; ethyl 2-(6-phenyl-7-oxo-7H-furo [2', 3':4, 5] pyrido [3, 2, 1-jk] carbazol-5-yl) acetate and its derivatives. All the synthesized compounds were shown the medium to good quantum yields.The Morita–Baylis–Hillman (MBH) acetates derived from nitroalkenes have been treated with 4-hydroxy-6H-pyrido [3, 2, 1-jk] carbazole-6-one in DMSO medium at room temperature in the absence of catalyst. This domino reaction provided good to excellent yields (85-93%) of previously unknown class of biologically interesting poly-functionalized pyridocarbazolone fused furan derivatives with short span of times (6- 11h) as shown in Scheme 1. The reaction may proceed through Feist– Bénary type addition-cyclization-elimination sequence process under catalyst-free conditions. All the derivatives excited by UV light produced blue emission, further studies revealed that the quantum yield of all these compounds are better in DCM than chloroform. The value of quantum yield varies according to the substituent groups attached on aryl rings. The planar structure of the pyridocarbazole-fused ring resulted in π͢͢͢͢-π*electronic transitions within the main frame, with an additional transition from the n(O) of carbonyl to the π٭ of the main frame. The excited states that arose from these transitions were responsible for the blue luminescence.Scheme 1. One-pot Feist-Bénary reaction between aryl-substituted nitro-allylic acetates and 4-hydroxy-6H-pyrido [3, 2, 1-jk] carbazole-6- one The products formed 3a-3i are highly fluorescent owing to rigid framework of the pyridocarbazolone scaffold and here conjugation is emanating from electron donating ‘N’ of carbazole moiety to electron withdrawing keto group in C6 position.