Oxidative cyclization, photo

Fig. (7). Reaction mechanism of photo-oxidative cyclization of morusin (3).

As described in Chapter III, morusin (3) has been found to be anti-tumor promoter in a two-stage carcinogenesis experiment with teleocidin. Considering the similarity of the structures between morusin (3) and artonin E (7), artonin E (7) was expected to be an anti-tumor promoter. Furthermore we found a novel photo-oxidative cyclization of artonin E (7) as follow photo-reaction of artonin E (7) in CHCI3 containing 4% ethanol solution with high-pressure mercury lamp produced artobiloxanthone (8) and cycloartobiloxanthone (9), and the treatment of artonin E (7) with radical reagent (2,2-diphenyl-1-picrylhydrazyl DPPH) resulted in the same products, Fig. (15), [84]. 

As described in Chapter III, we have reported the photo-oxidative cyclization on morusin (3). These results suggested that the photo-... 

Field et al [31] reported synthesis of fused aromatic system dibenzoanthracene-tetracarboxylic dianhydride by interesting photo-oxidative cyclization. 

Intramolecular Cyclization Reactions This Section is concerned with photo-induced cyclization reactions of aromatic compounds in which the final product is also an arene. Examples of intramolecular cyclization in which the product is a stable aliphatic system (e.g. intramolecular meta photocycloaddition of ethylenes to benzenes) are considered in Section 3 of this Chapter. The present type of process occurs for a wide variety of systems and may involve arene-arene and arene-ethylene photo-oxidative cyclization or reactions proceeding by loss of halogen acid. For several years now one of the major interests in these processes has been in their use as synthetic procedures and again within the review period there have appeared a number of accounts which illustrate this feature. Stilbene-... 

Sudhakar and Katz have devised an improved route to [7] helicene. The bromine atom is found to direct the stilbene cyclization away from occupied positions and those ortho to the substituent so that with (153) the bromo [7] helicene (154) is formed in 75% yield on irradiation of benzene solutions containing iodine, and less than 10% of the planar arene (155) is observed. Such results compare very favourably with those from irradiation of the hydrocarbon (156) when equal quantities of the [7] helicene and (157) are formed.Irradiation of (158), the meta isomer of (156), cyclizes selectively without bromine direction and (159) is formed in 75% from xylene solution in the presence of oxygen and iodine The origins of this selectivity may not be readily evident and it is difficult to appreciate why of the four possible conformers of the first intramolecular photo-oxidative cyclization product only (160) undergoes further reaction. The authors suggest that this feature reflects that of the four conformers (160) has the smallest degree of steric interactions between the two aryl moieties. The photo-cyclizations of halostilbenes have been examined by other workers who report that various o-chloro and bromo-compounds (161) undergo... 

Last year Rawal and Cava gave a preliminary account of their studies into the photocyclization of 1,2-dipyrrolyl-ethylenes and related systems and pointed out that such photo-oxidative cyclization may be employed as the key step to the tricyclic ring structure present in the antitumour antibiotic dipeptide CC-1065t The same workers have reported further on the photochemistry of... 

Aryl groups separated by moieties other than ethylene also undergo facile photo-oxidative cyclization and this type of reaction has been used to provide the regiospecific synthesis of... 

The absolute configuration of tylophorine (19) has now been rigorously established by exhaustive ozonolysis, which afforded S-pyrrolidine-2-acetic acid (25). This confirms that the photo-oxidative cyclization of L-septicine (26), for which the S-configuration at position 13a had previously been established, occurs with retention of configuration. Comparison of the o.r.d. curves of tylophorine, antofine (27), and (-I-)-isotylocrebrine (28), indicates that this last alkaloid, like antofine, has the -configuration at C-13a. 

The following three possible mechanisms (62) can be proposed for the primary step of the photo-oxidative cyclization of 24 as outlined in Fig. 12. 

Nomura, T., and T. Fukai Studies on the Constituents of the Cultivated Mulberry Tree IV. On the Reaction Mechanism of Photo-oxidative Cyclization of Morusin. Heterocycles 9, 635 (1978). 

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