One-shot photochemical synthesis of 5-(thiophen-3-yl)pyrano[2,3-c]chromen-2(3H)-ones from 3-propynyloxy-chromenones: a case of an intramolecular Paterno–Buchi reaction

Abstract

5-(Thiophen-3-yl)pyrano[2,3-c]chromen-2(3H)-ones (2), angular tricyclic compounds, were synthesized in significantly high yields through the photoinduced intramolecular coupling of the acetylenic group with the carbonyl centre in 3-(prop-2-ynyloxy)-2-(thiophen-3-yl)-4H-chromen-4-ones (1). This photoreaction is a case of an intramolecular Paterno-Buchi reaction and is unprecedented in 3-propynyloxy-chromenones. The structure of 2 has been determined by spectroscopic (FTIR, NMR and mass) and single crystal X-ray crystallographic studies.

Phototransformation of 3-alkoxychromenones: regioselective photocyclisation and dealkoxylation

“…The 1 H NMR spectral analysis of the photolysate also indicated the formation of a trace amount of pyranochromenones 9 (singlet at δ 6.8 ppm and doublet at δ 5.3 ppm), with no formation of xanthenones 10, similar to those obtained from the photolytic studies 15,17,35 on 3-( prop-2nyloxy)-2-(thiophen-3-yl)-4H-chromen-4-ones. However, these products, being in trace amounts, could not be isolated in spite of our best efforts.…”

“…The exceptional behaviour of the propargyl 1,4-biradical in forming photoproducts via unsaturated carbenes 12,13 has been investigated by Agosta and Margaretha, 14 and has encouraged us to study the photochemistry of chromenones containing the propargyl group. [15][16][17] In these studies, it was observed that the propynyloxy group behaved in many ways, such as as a suitable candidate for H abstraction followed by ring closure, in tandem electrocyclisation and rearrangement, and in intramolecular Paterno-Buchi cyclisation. In most of these reactions, xanthenones are the main photoproducts.…”

“…[23][24][25][26][27][28][29] In view of these and our continuous effort in developing the photochemistry of such molecules, in the present paper we wish to report the results of our investigations on the photolysis of some 4H-chromen-4-ones bearing the propynyloxy moiety at the 3-position and the 3-methoxyphenyl moiety at the 2-position, with the motive to determine the regioselective photocyclisation and other reactions, as elicited earlier in other similar molecules. [15][16][17][30][31][32]…”

Absorption and Fluorescent Studies of 3-Hydroxychromones

“…These molecules have also been found to show fascinating photochemistry furnishing the oxirane scaffold [22]. The 3-HCs have also been used as important substrates for the synthesis [23][24][25][26][27] Keeping in view the applications and significant photochemistry elicited by these 3-HCs, in the present communication, we have made an attempt to synthesise, determine and analyse their absorption and fluorescent properties. And also the effect of different substituents located at 4′-position of 3-HCs and intensity ratio of N * and T * emission bands was assessed.…”