Ring systems, photolysis

When di- or trisilanes are part of a ring system photolysis commonly results in the extrusion of a silylene resulting in ring contraction. Two typical systems are shown in equation 4474,75. 

Wender, P. A. and RawUns. D. B., Toward the synthesis of the taxol C,D ring system photolysis of a-methoxy ketones. Tetrahedron, 48, 7033,1992. 

A wide variety of such derivatives is known their properties are usually those of the individual ring systems. However, some unique reactions arise from the special juxtaposition of the two rings, e.g. pyridotetrazoles (547) on photolysis yield cyanopyrroles (548) (81AHC(28)23l). 

The prospect of obtaining new ring systems of unusual structure has led to much work on the photolysis of cyclopentadlene-splropyrazolenlnes and related compounds. The results obtained have justified the expectations. Figure 26 contains some structures prepared in this way. 

The reaction of vinylogous amides, or ketoaldehydes, with hydroxylamine produced 4,5,6,7-tetrahydro-l,2-benzisoxazole. A side product is the 2,1-benzisoxazole (Scheme 173) (67AHC(8)277). The ring system can also be prepared by the reaction of cyclohexanone enamines with nitrile oxides (Scheme 173) (78S43, 74KGS901). Base treatment produced ring fission products and photolysis resulted in isomerization to benzoxazoles (76JOC13). 

The bicyclo[2.2.0]hexa-2,5-diene ring system is a valence isomer of the benzene ring and is often referred to as Dewar benzene. After many attempts to prepare Dewar benzene derivatives failed, a pessimistic opinion existed that all such efforts would be finitless because Dewar benzene would be so unstable as to immediately revert to benzene. Then, in 1962, van Tamelen and Pappas isolated a stable Dewar benzene derivative by photolysis of 1,2,4-tri(/-butyl)benzene. ... 

Photolysis of 2,3,5,6-tetrakis(trifluoromethyl)-l,4-di hoshabenzene gives 1,3,4,6 tetrakis(tnfluoromethyl)-2,5-diphosphatncyclo[3 1 0 0 ]hex-3-ene, an ana logue of benzvalene containing phosphorus atoms in the ring system [267, 16S] (equation 40)... 

Interestingly, the azepinoazepines 16 and 20 are produced by irradiation of the azidobenz-azcpines 15 and 19 for a short time (5-10 min), whereas prolonged irradiation of the diazidcs results in decreased yields of the azepinoazepinc systems. Photolysis of 1.2-diazido- and 2,3-diazidonaphthalene yield only ring-opened or rearranged products. 

When the ring system containing the sulfone moiety is unsaturated, a quite different set of products has been observed on photolysis, apparently arising from an initial cycloreversion to a short-lived sulfene intermediate. For example, irradiation of 2H-1-benzothiopyran 1,1-dioxide (41) in dichloromethane or methanol gave the cis- and trans-sultines (42) and the ring-expanded sultine (43) in roughly equal amounts, accompanied by minor amounts ( 5%) of the products of formal loss of S02, i.e. indene, and SO, i.e. 4472... 

In contrast to the strained ring systems discussed above, photolysis of the larger ring system in (7) results in olefin (8) as the major product(14> ... 

The sydnone and sydnonimine ring systems are stable in acid solution at room temperature but are rapidly hydrolyzed in basic media <1984CHEC(6)365>. 3,4-Diarylsydnones 67 lose carbon monoxide on photolysis giving nitrile imines 68 that can be intercepted by dipolarophiles (Equation 1) <2004TL9057>. 

The trans-cis isomerization process observed in the eudesmane/germacrane ring system has been utilized for the synthesis of the c/s-fused sesquiterpene occidentalol, 48a, and its 7-epi isomer, 48b (Scheme ll)22. Photolysis of the traws-fused diene 45 at —78°C afforded triene 46, which upon warming underwent disrotatory electrocyclization to give 47a and 47b as a 1 2 mixture of diastereomers. Apparently, the carboalkoxy group imparts... 

The photolysis of epoxides containing a tethered dipolarophile can result in the intramolecular capture of the transient ylide and lead to the synthesis of more highly complex ring systems. Eberbach and co-workers (28-30) conducted several studies regarding this type of process to probe the viability of the method (Scheme 4.22). 

Nitrofuran antibacterials are synthetic compounds that are substitution products of the 5-nitrofuran nucleus, differing in the substituent at position 2. This substituent may be an azomethine group connected with other ring systems, or an alkyl, acyl, hydroxyalkyl, or carboxyl group, free or esterified. All these antibacterials are susceptible to photolysis, particularly by sunlight, and manipulations must be carried out under subdued light. 

Photolysis of , 4,5-triphenyl-1,2,3-triazole 1-oxide gives the 1,3,4,5-oxatriazine (13) (80AJC2447). This is the only reported example of this monocyclic ring system, and the mechanism of its formation (Scheme 35) probably involves the intermediacy of an oxaziridine and an oxygen walk process not uncommon in the photochemistry of heterocyclic iV-oxides. 

Photolysis of 2-substituted cyclopropyl carbyne complexes afforded 4-substituted cyclopentenones (equation 115). Irradiation of the diastereomeric mixture of either cis- or /ra j,-2,3-dimethylcyclopropyl carbyne complexes gave selectively the irons-3,4-dimethylcyclopentenone, indicating a rapid cyclopropane photoisomerization prior to photooxidation (equation 116). The cis configuration is, however, retained in fused ring systems. Thus photooxidation of bicyclo[4.1.0]hept-7-yl carbyne complex results in... 

The previously unknown benzocyclopropene system was obtained by a similar ring contraction. Photolysis of methyl-3,3-dimethyl-3 -indazole-6-carboxylate (104) in pentane solution gave the moderately stable methyl-1,l-dimethylbenzocyclopropene-3-carboxylate (105).88... 

Bcnzofuran-4,7-dioncs have been synthesized regioselectively by [3 + 2] photoaddition of 2-hydroxy-1,4-benzoquinones with a range of alkenes (equation 185)664. The reaction occurs in 30-60% yield and is a useful method for the synthesis of the benzofuran ring system, which is important in natural products like acamelin665. Substituted naphthoquinones may also be used in this reaction666,667 and this has lead to a very simple two-step synthesis of maturinone. In a similar reaction, a [3 + 2] photoaddition reaction of 2-amino-1,4-naphthoquinones with electron-rich alkenes gave 13-82% yields of 2,3-dihydro-177-bcn/ /]indole-4,9-diones in a single-step process which involved photolysis followed by oxidation (equation 186)668,669. 

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