Nitro-group photocyclizations

Nitro compounds have also been reported to undergo photocyclizations. The intermediacy of an isoxazoline in the photorearrangement of o-nitro-benzaldehyde to o-nitrosobenzoic acid is now in doubt,318 but intramolecular hydrogen abstraction by an excited nitro group in nitrobenzene derivatives can result in the formation of heterocycles. 4-tm-Butyl-3-methoxy-2,6-dinitrotoluene (384) on irradiation in methanolic sodium hydroxide solution... 

The photocyclization has been found not to occur with stilbenes substituted with acetyl, dimethylamino, or nitro groups, lodo substituents are replaced by hydrogen by photolysis in cyclohexane solution. w-Substituted stilbenes give mixtures of 2- and 4-substituted phenanthrenes which generally are difficult to separate. 

The photochemistry of substituted furans can be very complex indeed and may differ considerably from that found for benzenoid compounds. However, the photocyclization of 2,2 -ethylenedifuran to a benzodifuran is a simple reaction that is an exact counterpart to the photocyclization of stilbene to phenanthrenes.19 Some side chains allow ready dissociation to radicals typical is the nitro group and Scheme 4 displays what happens to irradiated... 

Two DHPs were obtained from tetramethoxystilbene [279]. Substitution by a nitro group in the meta or para position reduces distinctly [82], Saltiel et al. have questioned whether the values may be erroneous [105], On the basis of quenching measurements with azulene they proposed additional routes for bromostilbenes from c via excited states of DHP which may relax back to 3c or c. A consequence of a higher value of for the mechanism of cis -> tram isomerization is that the ratio of c decaying to the trans isomer may have to be reexamined. For trans-ct-bromostilbene and the / -phenyl substituted derivatives several photoreactions (e.g., debromination) compete with photocyclization [475]. Interestingly, no evidence for photocyclization could be found for several fluorinated stilbenes [481]. Rotamers can be distinguished in the cyclization of c/s-2,2 -DNE [482],... 

Syntheses of [n]helicenes (and [n]thiahelicenes) via oxidative photocyclizations of stilbenes have serious constraints. They must be carried out in dilute solutions ( 1 mM) and the functional groups that significantly affect relaxation of the singlet excited states of stilbenes such as bromo, iodo, keto, amino and nitro, are typically not compatible for longer helicenes, problems with regioselectivity of photocyclization may be encountered [61]. These limitations have spurred the recent developments of nonphotochemical syntheses for efficient preparation of highly functionalized, nonracemic helicenes. 

Further studies have been carried out on the photocyclization of 2-aroyl-l-methylene-tetrahydroisoquinolines (cf. Vol. 4). Under pregassed conditions, 8-oxoberbine derivatives have been obtained in excellent yield.The modification of this photocyclization in which an ort/to-methoxy- or halogeno-substituent is eliminated (cf. Vol. 5) has been further exploited, and it has been found that nitro- or methylthio-groups are also eliminated. 

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