Photoinduced 6π-Electrocyclization of a 1,3,5-Hexatriene System Containing an Allomaltol Fragment

Abstract

For the first time, the possibility of photocyclization of the 1,3,5-hexatriene system containing a fragment of allomaltol was demonstrated. A preparative method for the synthesis of previously unknown benzo­[5,6]­chromeno­[8,7-d]­oxazole-2,7­(3H)-diones was developed based on the investigated photoreaction. A distinctive feature of this approach is the modification of the starting terarylenes aimed at blocking the competitive process leading to side reactions of the pyranone fragment. It was shown that the proposed photocyclization of substituted oxazol-2-ones can be used for the photogeneration of biologically active alcohols and various acids. The structure of one of the cyclization products was determined by X-ray diffraction.

A study of the photochemical behavior of terarylenes containing allomaltol and pyrazole fragments

“…Initially, based on the previously obtained results for terarylenes with an oxazolone bridge fragment, we chose pyrazole 16 containing a methoxy group in the allomaltol core as a model object for studying photochemical properties. As it was shown previously for similar oxazolone derivatives [ 26 ] the presence of a methoxyl substituent in the compound 16 must exclude the occurrence of ESIPT-promoted photoprocesses. Therefore, it could be expected that UV irradiation of such a compound would lead only to 6π-electrocyclization of the 1,3,5-hexatriene system.…”

“…As it was demonstrated earlier [ 26 ], UV irradiation of terarylenes containing oxazolone and 3-hydroxypyran-4-one fragments 9 can lead to two types of photoprocesses. Based on the aforementioned results we can assume that the key difference of the related pyrazole derivatives considered in this report is the absence of a pathway associated with 6π-electrocyclization.…”

“…The starting pyrazoles 12 containing the 3-hydroxypyran-4-one fragment were obtained by a literature method [ 33 ]. The methoxy derivative 16 was synthesized by methylation of the corresponding pyrazole 12a according to the standard procedure [ 24 , 26 ] ( Scheme 2 ). Initially, based on the previously obtained results for terarylenes with an oxazolone bridge fragment, we chose pyrazole 16 containing a methoxy group in the allomaltol core as a model object for studying photochemical properties.…”

“…At the same time, such processes can be complicated if additional photosensitive fragments are presented in the structure. For example, we previously studied the photochemical behavior of oxazolone terarylenes 9 containing the allomaltol fragment ( Scheme 1C ) [ 26 ]. It has been shown that UV irradiation of such systems leads to a complex mixture of products.…”

Recent synthetic methodologies for imidazo[1,5-a]pyridines and related heterocycles

“…Komogortsev et al 87 reported an unusual method for the synthesis of 1,3,5-hexatrienes 228 with an allomaltol fragment via the condensation of 1,2-diketones 227 with aldehydes and NH 4 OAc in acetic acid. Solutions of 228 in Petri dishes were exposed to UV radiation at 365 nm (Vilber Lourmat VL-6.LM lamp) for 30 h, which resulted in the formation of photoproducts 229 .…”