On the photochemical dimerization of some 5‐substituted 2‐styryl‐4‐pyrones. The effect of 5‐hydroxy‐/ 5‐methoxy‐substitution

Abstract

Irradiation of styryl‐4‐pyrones 1a‐1d or 2a‐2e (6‐9 × 10−3 M, methanol solution) with filtered (RAYONET photochemical reactor, 300 nm) or unfiltered uv‐light (high‐pressure mercury arc lamp) under aerobic conditions led mainly to dimeric products. Parent 5‐hydroxy‐substituted compounds 1a‐1d yielded exclusively “half‐cage” dimers 3a‐d characteristic for 4‐pyrone dimerization. 5‐Methoxy‐analogues 2a‐2e behave like typical stilbene structures and the mixture of tetrasubstituted cyclobutanes 4 and 5 accompanied with minor amount of phenanthrene‐like compound 6 were the only isolable products of the irradiation. The structure elucidation of products is based on spectral data obtained from MS, IR, 1H NMR and 13C NMR spectra applying COSY, APT, HETCOR, HMBC and NOESY techniques.

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

“…The regioselectivity could be completely reverted if the reaction was performed in the solid state . A diastereoselective [2 + 2] photodimerization was achieved with chiral amides. , The related [2 + 2] photodimerization of pyrano­[2,3- c ]­pyrazole-4-(1 H )-ones was studied by Pavlik et al, and the [2 + 2] photodimerization of 5-substituted 2-styryl-4-pyrones was investigated by Jakopčić and co-workers …”

“… 490 , 491 The related [2 + 2] photodimerization of pyrano[2,3- c ]pyrazole-4-(1 H )-ones was studied by Pavlik et al, 492 and the [2 + 2] photodimerization of 5-substituted 2-styryl-4-pyrones was investigated by Jakopčić and co-workers. 493 …”

“…490,491 The related [2 + 2] photodimerization of pyrano[2,3-c]pyrazole-4-(1H)-ones was studied by Pavlik et al, 492 and the [2 + 2] photodimerization of 5-substituted 2styryl-4-pyrones was investigated by Jakopcǐćand co-workers. 493 In some cases, the carbonyl group of [2 + 2] photocycloaddition products may give rise to further photoinduced reactions. Sabui and Venkateswaran for example observed in the intermolecular [2 + 2] photocycloaddition of chromone 270 that, apart from the desired product rac-271, the oxetane rac-272 was isolated (Scheme 86).…”

“…Lately, the same group further employed this method for the synthesis of fullerene anhydrides. 852 The Orfanopoulos group studied the stereochemistry and possible isotope effects of the [2 + 2] photocycloaddition between C 60 (493) and 4vinylanisole (Scheme 166). 853−855 The lack of any stereoselectivity (cis-495/trans-495 = 50/50) indicated that this transformation may involve the radical anion/radical cation complex 494 and a dipolar intermediate.…”

A [2 + 2] Cycloaddition Dimer and a Diels–Alder Adduct fromAlpinia katsumadai

“…Two possible cycloadducts, head-to-tail or head-to-head, exists (Figure ), which cannot be distinguished directly by NMR data. On the basis of the mass spectrometric studies on similar situations, , three typical fragments at m / z 350, 180, and 520 should be found for 1 in head-to-head mode, while only one fragment ion at m / z 350 could be found in head-to-tail mode. The EI-MS of 1 exhibited a very small molecular ion at 700 and the presence of a typical fragment at m / z 350 indicated 1 to be a head-to-tail dimer.…”