Ring-fused derivatives

Unusual heterocyclic systems can be obtained by photodimerizations and for five-membered heterocycles with two or more heteroatoms such dimerizations need be effected on their ring-fused derivatives. Cyclobutanes are usually obtained as in the photodimerization of the s-triazolo[4,3-a]pyridine (540) to the head-to-head dimer (541). These thermally labile photodimers were formed by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another (77T1247). Irradiation of the bis( 1,2,4-triazolo[4,3-a]pyridyl)ethane (542) at 300 nm gave the CK0ifused cyclobutane dimer (543). At 254 nm the cage-like structure (544) was formed (77T1253). 

The scope for further developments in the chemistry of seven-membered heterocyclic systems is considerable, particularly with respect to multi-heteroatom component systems. New synthetic methods are needed for these systems and ring-fused derivatives. The demand for such systems is likely to be largely driven by the search for structurally novel drug leads. 

The distribution of the total spin density in radicals was calculated using the UB3LYP16-31g method. In systems 1, 3, 4, and 8-11, the spin density is localized on the -S-N-S- fragment with minor spin distribution onto the Jt-framework, presumably through a spin polarization mechanism. For instance, in 1,3,2-dithiazolyl 1, virtually all positive spin density is localized on the -S-N-S- array due to the two polar resonance structures shown in Equation (2). A very similar spin distribution is observed in ring-fused derivatives 3, 4 and 9-11 <2001PCA7615>. 

Ring-fused derivatives can also be prepared using this method (Equations 50 and 51) <1994JOC1011>. 

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