Ring opening and closure

Application of the same procedures to other ring closures shows that 4 + 4 and 2 + 6 ring closures and openings require photochemical induction while the 4 + 6 and 2 + 8 reactions can take place only thermally (see 15-52). In general, cycloaddition reactions allowed thermally are those with 4n + 2 electrons, while those allowed photochemically have 4n electrons. 

The cyclohexane ring is too big for the dual-site ring closure and opening mechanism even over platinum. With Pt/Au alloys, however, such a reaction is not excluded (Fig. 13). The higher spacing of chromia (145) may be the reason why it promotes Cg, C7, and even Cg cyclization (132-132b). 

The reaction, unfortunately, does not appear to be completely reversible. The conrotatory photochemical ring closure and opening in the reaction below seems to be attractive (Darcy et al, 1981) ... 

Olefin-forming elimination reactions permit a similar division into cis and trans processes, and will be discussed in these two groups. The related reactions of epoxide ring closure and opening have sufficient similarity to the formation and reactions of olefins to warrant their inclusion in the same chapter. 

Table 8. Selection rules for ring closures and openings...

Perhaps the largest class of organic photochromic molecules are those based on symmetry-allowed electrocyclic ring closure and opening. A variety of examples exist spiropyrans, naphthopyrans, fulgides, and diarylethenes. 

Several types of thermally induced monomolecular reactions of silenes have been reported (Chart 8) (a) geometrical isomerization, (b) 1,2-shift, (c) other sigmatropic shifts, (d) electrocyclic ring closure and opening, (e) ene and retro-ene processes, (f) hydrogen atom transfer and (g) nucleophilic attack. 

Imidazolidine ring closure and opening Modified Curtius degradation... 

Enzymatic ring closure and opening lactone formation from hydroxy esters ring opening of anhydrides LPL, PFL lipase P, PFL... 

Fig. 6.9 a Molecular structure of a nematic GLC (30) with a dithienylethene core undergoing ring closure and opening with irradiation at 365 and 577 nm, respectively, and b reversible photochromism using a 4-pm-thick film at room temperature. Used with permission [45]... 

Fig. 6.10 Molecular structure of a smectic C -cholesteric GLC (31) for photoswitchable absorptive coloration via ring closure and opening on top of reflective coloration in a 5-pm-thick film. Used with permission [46]...

A comprehensive discussion of the factors that influence the kinetics for ring closure and opening has been made by Tobe, Romeo, and co-workers. They studied the formation and cleavage of six- and seven-membered rings under the large rra/w-effect of DMSO, reaction (5),... 

Pyrido(l, 2 2,3)-l-oxa-2,4-diazol-5-one ring closure and opening s. 12, 337 3-Subst. furans... 

Acetic anhydride (s, a. under CH COONa, CHfiOCl and TsOH) Enollactone ring closure and opening... 

ConceUon JM, Bemad PL, PCTez-Andres JA. NucleophiUc ring closure and opening of aminoiodohydrins. Tetrahedron Lett. 2000 41(8) 1231-1234. 

Phosphorus oxide chloride N-Condensed isoxazoline ring closure and opening... 

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