Dimerization cyclobutanes

The spontaneous polymerization of styrene was studied in the presence of various acid catalysts (123) to see if the postulated reactive intermediate DH could be intentionally aromatized to form inactive DA. The results showed that the rate of polymerization of styrene is significantly retarded by acids, eg, camphorsulfonic acid, accompanied by increases in the formation of DA. This finding gave further confirmation of the intermediacy of DH because acids would have Httie effect on the cyclobutane dimer intermediate in the Flory mechanism. 

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 HPLC-MS/MS assay was also successfully applied to the measurement of UV-induced dimeric pyrimidine photoproducts [123, 124]. The latter lesions were released from DNA as modified dinucleoside monophosphates due to resistance of the intra-dimer phosphodiester group to the exonuclease activity during the hydrolysis step [125, 126]. The hydrolyzed photoproducts exhibit mass spectrometry and chromatographic features that allow simultaneous quantification of the three main classes of photolesions, namely cyclobutane dimers, (6-4) photoproducts, and Dewar valence isomers, for each of the four possible bipyrimidine sequences. It may be added that these analyses are coupled to UV detection of normal nucleosides in order to correct for the amount of DNA in the sample and obtain a precise ratio of oxidized bases or dimeric photoproducts to normal nucleosides. 

Irradiation of butadiene in isooctene solution is reported to yield cyclobutane dimers (8) and (9) in low yield<4) ... 

Cyclopentadiene adds to dimethylmaleic anhydride to produce cyclobutane dimer (84) and two products from l,4-cycloaddition<97) ... 

Irradiation of crystalline acenaphthylene1121 at temperatures below 25°C results in formation of the tra/w-cyclobutane dimer only. At temperatures above 40°C both cis and trans dimers are formed, with the cis dimer predominating at 60°C (Table 10.12). The formation of the trans dimer, which occurs with low quantum yield at all temperatures studied, is thought to... 

A computational study was concerned with the effect of solvation on the radical ion involved in CDP photolyase enzyme-catalysed reversion of thymine and uracil cyclobutane dimers stimulated by visible light <06T6490>. 

The object of this study is to develop new photoresists for deep-UV lithography, by using the reversible photoreaction of pyrimidine bases (17-19). Applicability of pyrimidine containing polymers to both negative and positive type photoresists is due to this photoreversible reaction in which cyclobutane dimers are either formed or cleaved depending on the exposure wavelength (Scheme 2). 

A cycloaddition reaction produces a ring of atoms by forming two new G-bonds, for example the formation of a cyclobutane dimer from two alkene molecules. The direct photoreaction involves the concerted reaction of the singlet Jtpt ) excited state of one alkene with the ground state of the other. Stereospecific reactions in which the dimers preserve the ground-state geometry occur when liquid cis- or trans-but-2-ene are irradiated at low temperature ... 

The cyclobutane derivatives can revert to the original bases when irradiated with shorter-wavelength light. This reaction is involved in the repair process, which helps keep damage caused by ultraviolet to a minimum. Some repair mechanisms involve enzymes that are important in the breakdown of cyclobutane dimers by longer-wavelength light. 

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

In the presence of catalytic amounts of tris(4-bromophenyl)ammoniumyl hexachloroantimonate in methylene chloride at 0°C, trani -anethole is smoothly converted in less than 5 min to the cyclobutane dimer (Bauld et al. 1996 Scheme 7.16). 

Orotic acid readily forms dimers even when irradiated in liquid medium [582, 583]. 5-Bromouracil (5-BrU) in DNA is dehalogenated, rather than forming cyclobutane-type dimers. Such DNA derivatives are more sensitive to ultraviolet irradiation than normal DNAs [584-594], Irradiation of 5-bromo-uracil and derivatives in aqueous medium produces 5,5 -diuracil [590, 591]. However, derivatives such as 3-sbutyl-5-bromo-6-methyluracil have been reported to yield cyclobutane dimers either by irradiation of frozen aqueous solutions, or by catalysis with free radical initiators, such as aluminium chloride, ferric chloride, peroxides or azonitriles [595]. 5-Hydroxymethyluracil is reported to dimerize very slowly in frozen water at 2537 A [596]. The fundamental research in the photochemistry of the nucleic acids, the monomeric bases, and their analogues has stimulated new experiments in certain micro-organisms and approaches in such diverse fields as template coding and genetic recombination [597-616]. 

The only stem materials examined were from the Setaria anceps and Digitaria decumbens grasses. The major substituted cyclobutane dimers in these walls appeared, from the mass spectral data, to be mixed dimers of ferulic acid and coniferyl alcohol (FA-ConAlc type). Further examination of the aromatics of stems is required but if such dimers are present then they may well be involved in the biosynthesis of lignin (41,42). 

A number of cyclobutane dimers of benzocycloalkenes, e.g., 1, have been investigated466. It transpires that the chemical shifts of all of the aliphatic carbons are 4 to 6 ppm smaller in the cis-anti-cis-isomer than in the c/.y-swi-cw-derivative466. This is in line with the abovementioned trends and is due to shielding y-effects. 

In a fluid environment the photodimerization of thymine and its derivatives involves cycloaddition at the 5,6-double bond to form one or more of the four possible stereoisomers of the cyclobutane dimer shown... 

In contrast to If/-azepines the photolysis of N-substituted 5f/-dibenz[Z>,/]azepine under sensitized (PI12CO) conditions yields [2 + 2] cyclobutane dimers (77 R = Ac, CC Et, CONH2, COPh). The N-p- tosyl derivative, however, undergoes photo-Fries reaction to the 2-p-tosylamino derivative (74CRV101). 

It is interesting to note that head-to-tail dimeric structures are proposed for these products. On the other hand, the head-to-head cyclobutane dimers 15 and 16 are obtained on isomerization of the propellanes 13 and 14. respectively.8 The bridgehead alkenes have been proposed as intermediates as is evident from trapping of the adducts with furan. An X-ray crystallographic study of 15 confirmed the configurations of both cyclobutanes 15 and 16 indicating syn head-to-head dimerization, as 15 could cleanly be converted to 16 by catalytic hydrogenation. 

Alkenes sometimes form cyclobutane-dimers by direct irradiation... 

Cyclobutane dimers can be produced from cyclohexenes or cyc-loheptenes on direct or sensitized irradiation for cyclohexene itself (2.79) the ratio of isomers is the same in either case, which is... 

Reaction with OH also leads to the splitting of the dimer (-30% efficiency), and there is evidence that one-electron oxidants such as S04 may also induce the splitting of the dimer (Heelis et al. 1992). The NO -radical-induced splitting of the tetramethyl-substituted Ura cyclobutane dimer has been investigated in acetonitrile (Kruger and Wille 2001). The -N03 radical has been generated photo-lytically from a Ce(VI) salt (Chap. 5.2). Under theses conditions, the 5-5 -linked intermediate is also trapped, possibly by a deprotonation or a Ce(IV)-mediated oxidation that competes with 3-fragmentation [reactions (310)—(313)]. 

Yamada H, Hieda K (1992) Wavelength dependence (150-290 nm) of the formation of the cyclobutane dimer and the (6-4) photoproduct of thymine. Photochem Photobiol 55 541-548 Yan M, Becker D, Summerfield S, Renke P, Sevilla MD (1992) Relative abundance and reactivity of primary ion radicals in y-irradiated DNA at low temperatures. 2. Single- vs double-stranded DNA. J Phys Chem 96 1983-1989... 

---