1.3- transposition reactions

The sensitized irradiation of 2-methoxyfuran gave a product (34) deriving from a transposition reaction of the methyl group (Scheme 13) (69TL2767). The 5-methyl derivative 35 was obtained starting from 2-acetoxyfuran (Scheme 13). 

Ring transposition reactions have also been described in diazoles. 1,4-Dimethylimidazole is converted on irradiation in terf-butyl alcohol to the... 

In agreement with other processes occurring through carbocations, elimination and transposition reactions can compete with the addition process which also depend on solvent. Finally, solvent properties can affect the stereochemical course of the addition reactions for example, the stereoselectivity of bromine addition to cis- and /r[Pg.392]

In the reactions described in Section III,C, a three-atom fragment in the heterocycle is replaced by the same atoms of the reagent and with the same sequence of atoms. This type of reaction has been referred to as an inter-molecular transfragment reaction (67JA6911) and therefore can be clearly distinguished from molecular rearrangements, such as the Dimroth reactions (68MI1 see also Chapter IV) and photochemical transposition reactions. 

Transposition reactions of larger ring systems sometimes produce azapentalenes. Of these, the formation of pyrrolo[l,2-a]imidazoles from 1,4-diazocines is probably the most important, and this is discussed below (Sections III,B,4,a and IV,B,2). 

Transposition reactions of diazocinediones (Sections III,By4,a and IV,B,2) can lead to compounds that are aza or benzo derivatives of two basic systems 453 and 454.165,314,321,323 331,424 Two groups of workers323c, 323d examined the reduction of 327 with lithium aluminum hydride, and the results of the earlier, more complete study323c are shown in Scheme 21. 

The only comparable sulfur-containing system investigated is that found in thiabenzene187 and thianaphthalene.188 In both cases, 1,2-aryl migrations have been observed on irradiation, as shown, for example, in the triphenyl-thianaphthalene (237). No evidence for any ring transposition reaction in these compounds has been reported. 

Another regulatory mechanism, called trans-cleavage, is considered a quality control of the transposition reaction. Transposition requires the formation of a specific complex between the transposase and both transposon ends called a trans-pososome. In transcleavage regnlation, transposase bound at one end is constrained to cleave the opposite transposon end. This obliges prior formation of the transpososome before DNA strand... 

For the widely dispersed Tc-Mariner transposon group, the transposase first cleaves within the 5 end of the transposon—the nontransferred strand. This activity resembles the nuclease activity (see above) that simply terminates at the cleavage step and does not take in charge the strand transfer step. Moreover, unlike transposition reactions, this cleavage does not require formation of a synaptic complex (29). The transferred strand is cleaved at the very tip of the TE. The fact that the nontransferred strand is cleaved within the transposon results in retention of the few TE-speciflc bases in the donor molecule after TE excision. After resealing and repair, the donor backbone retains several additional base pairs derived from the TE (called a scar) that marks the passage of the transposon (2). 

Interestingly, transposon Tn7 behaves in a similar way but, in this case, the 5 endonuclease activity is supplied by a separate enzyme whose structure resembles that of a type II restriction enzyme (30), and cleavage occurs cleanly at the transposon tip rather than within the TE. Transposition of Tn7, like most bacterial elements, does not leave a scar. In both the Tc-Mariner transposon group and the Tn7 family of transposons, the transposase then cleaves and transfers the 3 end in a true DDE transposition reaction (Fig. le). 

An analogous process has been developed for unbranched aldehydes which can be transformed into a-amino ketones when oxidized in the presence of an secondary amine and iodine, as the mediator, in aqueous terf-butanol. The actual reactive species is probably the enamine which is attacked by iodine cations and subsequently by water. Carbonyl transposition reaction releases iodine anions which can be anodically reoxidized [197]. 

---