Photodimerization and

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). 

In the crystal of 1,4-dicinnamoylbenzene (1,4-DCB) (see Fig. 12), the distances between the intermolecular photoadductive carbons are 3.973 and 4.086 A for one cyclobutane ring, and 3.903 and 3.955 A for the other. The two topochemical pathways may occur competitively in a single crystal of 1,4-DCB at the initial stage of reaction. Then, both intramolecular photodimerization and intermolecular photopolymerization of the diolefinic mono-cyclobutane intermediate occur competitively to give tricyclic dimer 21,22,23,24-tetraphenyl-l,4,ll,14-tetraoxo-2(13),12(13-diethanol, [4.4] para-cyclophane or oligomers (Hasegawa et al., (1985). On photoirridation at room temperature the 1,4-DCB crystal gives >90% of the tricylic... 

The subject of the first section of this chapter is the photodimerization and photocycloaddition reactions of olefins. In this category we include only those compounds in which the photoreactive olefin or polyene is not part of an aromatic system although it may bear an aromatic substituent, such as in styrene. 

All of the photodimerizations and photoadditions mentioned up to this point have one common feature. That is, they result from the addition of two 4w-electron systems ... 

Clearly all the answers to the questions of the mechanisms of both the photodimerization and photooxidation of anthracene are not yet known. Hopefully, what we have seen in this chapter will serve to convince the reader that photochemistry is still an exciting and challenging field of study in which there is ample room for further research. 

This chapter contains a review of some photodimerizations and photo-condensations leading to the formation of substituted cyclobutanes. Since the literature in this field is indeed vast, it would be impossible to present here a comprehensive review of all the reactions reported to lead to such products. Instead, it is our goal to present a general survey of the various types of compounds known to produce cyclobutane compounds upon irradiation and to discuss mechanisms which have been proposed to account for some of these products. 

PHOTODIMERIZATION AND PHOTOCYCLOADDITION REACTIONS OF ,j8-UNSATURATED CARBONYLS AND ACID DERIVATIVES... 

Many interesting examples of photodimerizations and photocycloadditions of a,/ -unsaturated carbonyls and acid derivatives yielding cyclobutanes have been reported. In this section, as in Section 10.1, we will first discuss photodimerizations and then photocycloadditions. 

---