Photochemical ring-cleavage

Furoxans and benzofuroxans undergo thermal and photochemical ring cleavage, reactions with nucleophiles, Boulton-Katritzky rearrangement, reduction and deoxygenation, ring transformation, etc. (see also Section 5.05.6.2). 

The procedure developed in the initial NMR study of 12 has been used in subsequent studies of 61, 62 ° of 41 and 44 of and of 64 The DHP derivative can be obtained only in photostationary concentrations (at most), in reaction mixtures containing both cis- and trans-isomers of the 1,2-diaryl ethylene. Under such conditions the NMR signals due to the nuclei of the DHP derivative are identified as those which disappear following photochemical ring cleavage (process B). This process yields only the cis-isomer of the parent ethylenic compound. In addition to their structural value, NMR studies in this field allow to verify the conversion estimates obtained from optical studies In 61, 62 and 64 proton NMR proves that photocyclization takes place between 1 and 1 atoms and not for instance between atoms 1 and 8 or between atoms 8 and 8 (numbering as in 61 in Table 8). Table 10 provides a summary of the chemical shifts of 12, 25, 41, 44, 61, 62, and 64. The atoms of the DHP moiety are numbered as in 7 (see Tables 1—9 for details). 

As stated previously 4a,4b-dihydrophenanthrenes cannot be isolated pure but can be obtained only in reaction mixtures containing both cis- and trans-isomers of the parent diarylethylene. Thus studies of 4a,4b-dihydrophenanthrenes always depend on the prior development of conditions providing considerable conversion of diaryl-ethylenes into their DHP photoisomers. One requirement for obtaining maximum conversion consists in minimizing the rates of decomposition processes such as photochemical ring cleavage,... 

Oxadiazoles undergo thermal and photochemical ring cleavage at the 0(1)-N(2) and C(3)-C(4) bonds to yield nitrile and nitrile oxide fragments, and products derived therefrom. Thus, diphenylfurazan (30, X = 0) decomposes under flash vacuum pyrolysis conditions (600°C, 10-3 mm... 

Photochemical ring cleavage also occurs readily in systems where stable fragments can be extruded. Photolysis of the 1,3,4-oxadiazinone (127) gives an E/Z mixture of stilbenes by initial extrusion of CO2 followed by rearrangement and loss of N2 as shown in Scheme 7 (68JA1061). 

The most studied reactions of A3-l,3,4-oxadiazolines are thermal and photochemical ring cleavage, generally with loss of a molecule of nitrogen. 

In an interesting and novel synthesis of dimethylcrocetin (68) (Scheme 2) much of the carbon skeleton was derived from 2,6-dimethylphenol (64). Condensation of this with ( )-l,4-dibromobut-2-ene (65) gave as a main product the compound (66) which underwent photochemical ring cleavage in sunlight to give the acyclic product (67) and thence dimethylcrocetin (68). 

The analogous hydroxybenzo[6]thiepin (234 R = OH) undergoes a secondary photoreaction to give the isomer (236), presumably via the stabilized biradical (237). The sulphene (238), arising by photochemical ring-cleavage, is... 

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