Bond Cleavage and Rearrangement

Examples of 47i electrocyclization and related ring openings in oxygen-and sulfur-containing heterocycles have been widely reported. The effect of substituents on the ease of photocyclization of 1-benzoxepins (1) to cyclo-buta[b]-l-benzofurans (2) has been studied.3 1,3-Dioxepin (3) is readily converted to 2,4-dioxabicyclo[3.2.0]hept-6-ene (4) on irradiation in diethyl 

6-Acetoxy-l,4-dithiocin (10), like benzo-l,4-dithiocin, is converted on irradiation to the bicyclic isomer (11).14 Analogous cyclizations have been observed in certain 4-phenyl-l-benzothiepins (12)1 s and in 1-benzothiepin itself.16 Further rearrangement of the photoproduct (13) to isomer 14 is believed to occur via the diradical intermediate (15). 

Examples of photochemically induced im electrocyclizations in oxygen-containing systems are relatively rare. 2//-Phenanthro[9,10-h]-pyran-4-carboxamides (16), for example, have been obtained in this way by irradiation of dienones (17).17 Similarly, the a-pyran (18) is formed on irradiation of ( )-//-ionone (19)18 a triplet excited state is believed to be involved and the reaction proceeds via the Z-isomer (20). The reverse process involving ring opening is more common and can lead to a wide variety of photochemically derived products. Thus, the products of irradiation of 2,2-dimethylchromene 

Certain thiin derivatives undergo similar transformations. The formation of the sulfonate ester (31) on irradiation of 3-phenyl-27/-lhiopyran 1,1-dioxide (32) in methanol strongly supports the intermediacy of the sulfene (33).2e Initial formation of a sulfene has also been proposed to account for 

Numerous examples of stilbene-to-dihydrophenanthrene photocyclization incorporating oxygen and sulfur heterocycles have been reported. Oxidation to the phenanthrene is usually effected by added iodine or by oxygen. Thus, irradiation of 2,3-diphenylchromone (37) results in the formation of phenanthro[9,10 -2,3]chromone (38)29 analogous photocyclizations have 

0-xylylene (24) (R = H), obtained by irradiation of 4-methylisochromene (25), to give the ketene (26) attempts to trap the o-xylylene with dienophiles have been unsuccessful. Analogous photochemically induced ring openings in benzoindolinospiropyrans, widely studied for possible application in imaging systems, are responsible for their photochromic properties. 

The increase in energy in a molecule on absorption of UV light is sufficient to bring about bond cleavage. As a result, fragmentation and rearrangement of the molecule can occur. The effect on heterocycles is discussed in this section and, for simplicity, the transformations are classified, somewhat arbitrarily, on the basis of ring size pyrazolines are treated separately. Heterocyclic dienes and heteroaromatic compounds are also discussed separately, and the section is completed by consideration of the photochemistry of heteroaromatic A-oxides. 

Thiiranes appear9 to undergo carbon-sulfur bond cleavage more readily, but this process is less well investigated. The only photoproduct so far obtained from methylthiirane (1) is the dimeric allyl 

Photofragmentation of phenyl-substituted oxiranes has been shown12 to result in the formation of carbenes triphenyloxirane (3) on irradiation in methylcyclohexane at 77°K affords benzaldehyde (4) and diphenylmethylene (5), identified by fluorescence and electron paramagnetic resonance (EPR) absorption studies. The most convenient precursor of phenylcarbene (6) is stilbene oxide (7),13 and the 

Formation of these carbenes may well involve a two-step homolytic cleavage, but the extension of this process to the formation of phenyl-cyanocarbene and phenylmethoxycarbonylcarbene from the appropriately substituted oxirane led to the suggestion that heterolytic cleavage of the carbon-carbon bond might be the initial step.15 This would account for the formation of phenylmethoxycarbonylcarbene in preference to diphenylmethylene in the photolysis of 2-methoxy-carbonyl-2,3,3-triphenyloxirane [Eq. (2)]. 

Recently, considerable interest has been shown in the photochemistry of a,/9-epoxyketones.16 Although the photochemistry of this system is undoubtedly the result of an n- ir excitation in the carbonyl function, the orbital overlap with the bent bonds of the three-membered ring, for which there is considerable evidence,16 is also implicated in the process. The major product of irradiation of an a,/8-epoxyketone is the corresponding /8-diketone, the result of oxirane ring cleavage and migration of a /9-substituent to the a-position [Eq. (3)]. Other photoproducts arise mainly from the /9-diketone. 

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N—N Bond Cleavage and Rearrangements of Arylhydrazones and Arylhydrazides — Recent Developments R. Fusco and F. Sannicolo, Tetrahedron, 1980, 36, 161-170. An Application of Beta-Lactam to the Synthesis of Heterocyclic Compounds S. Kano, Yuki Gosei Kagaku Kyokaishi, 1978, 36, 581-594. 

Formation of the high energy cation-radical species by PET produces C—C bond cleavage and rearrangements to form more stable cation-radical structures. 

B. Peptide Bond Cleavage and Rearrangement to the Native State... 

In the 1960s, a more promising expansion of the electrodecarboxylation reaction in terms of synthetic utility was recorded in which a carbenium intermediate R" " formed at the anode plays an important role [Eq. (7)] [4]. Depending on the structural characteristics of the carboxylates and/or electrochemical variables, the cation intermediate may undergo the so-called non-Kolbe reactions, for example, substitution, deprotonation, C-C bond cleavage, and rearrangement to provide alcohols, ethers, esters, amides, olefins, and others. 

In many biosynthetic pathways, an initially formed aromatic product undergoes a profound structural reorganization with C—C bond cleavages and rearrangements to give a complex new skeleton. Use of doubly labeled acetate has given biosynthetic chemists a powerful tool to probe and prove such structural changes. 

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