Acyl fission

On the other hand, in cyclic ethers (alkene oxides, oxetans, tetrahydrofuran) and formals the reaction site is a carbon-oxygen bond, the oxygen atom is the most basic point, and, hence, cationic polymerization is possible. The same considerations apply to the polymerization of lactones Cherdron, Ohse and Korte showed that with very pure monomers polyesters of high molecular weight could be obtained with various cationic catalysts and syncatalysts, and proposed a very reasonable mechanism involving acyl fission of the ring [89]. 

In both schemes of V,l, the bond A between the carbonyl carbon and the ethereal oxygen is broken i.e., acyl fission is involved. That the same should apply to the reactions catalyzed by ChE s had been derived from theoretical considerations by Burgen (27) and by Wilson and Bergmann (28). Direct proof was advanced by experiments which were designed in analogy to those which had served in the elucidation of the pathway of chemical hydrolysis. 

Similarly, the Norrish Type I acyl fission process was depicted 1,3 5) as proceeding by gradual disengagement of the alkyl moiety initially bonded to the carbonyl carbon thus leaving an acyl radical. This process was noted as arising by overlap of the electron deficient pv orbital with weakening of the alkyl to carbonyl carbon... 

Also, beginning in 1961 it was noted by the author 1 3,4) that there are two general kinds of n-7t reactions, those leading directly to ground state of photoproduct and those involving several steps before ground state is reached. The present example of the Norrish Type I (i.e. acyl fission) reaction is one of several of the first variety. 

Acyl fission merely regenerates starting materials. The acid-catalyzed hydrolysis of tcrt-butyl acetate proceeds by an SnI process (stable carbonium intermediate) with alkyl fission. 

Indene derivatives 264a and 264b are formed by the intramolecular reaction of 3-methyl-3-phenyl-l-butene (263a) and 3,3,3-triphenylpropylene (263b) [237]. Two phenyl groups are introduced into the /3-substituted -methylstyrene 265 to form the /3-substituted /3-diphenylmethylstyrene 267 via 266 in one step[238]. Allyl acetate reacts with benzene to give 3-phenylcinnamaldehyde (269) by acyl—O bond fission. The primary product 268 was obtained in a trace amount[239]. 

Thiazolidine-2,4-dione, 2-dialkylamino-bisimide synthesis, 5, 129 Thiazolidine-2,4-diones IR spectroscopy, 6, 242 tautomerism, 6, 270 Thiazolidine-2,5-diones synthesis, 5, 138 Thiazolidine-4,5-diones synthesis, 5, 129 6, 316-317 Thiazolidine-2,4-dithiones tautomerism, 6, 270 Thiazolidines "C NMR, 6, 243 conformation, 6, 242, 247 dihydrothiazines from, 2, 93 hydrolysis, 6, 273 IR spectra, 6, 242 ring fission, 5, 80 synthesis, 5, 118 6, 316-321 Thiazolidines, imino-tautomerism, 6, 273 Thiazolidines, methyl-conformation, 6, 242 Thiazolidine-2-thione, 3-acyl-reduction, 1, 469 Thiazolidine-2-thione, 4-alkyl-synthesis, 6, 318... 

Isotopic substitution. A classic example is the demonstration by Polanyi and Szabo (37) of acyl-oxygen fission in the alkaline hydrolysis of -amyl acetate. An ester could undergo cleavage at two locations, as indicated in 3. 

The designation Ac signifies acyl-oxygen fission, whereas A1 is alkyl-oxygen fission. When n-amyl alcohol was hydrolyzed in 0-enriched water, the 0 appeared in the product acid rather than the alcohol, showing that alkyl-oxygen fission could not have occurred. Carpenter gives many examples of isotopic studies. 

The alkaline fission may be represented similarly. (8-Addition of hydroxide ion to the benzylidene double bond would lead to 130 which by a rather surprising elimination of the A-hydroxyl group could give the acyl dioxopiperazine (128). Cleavage of 128 by ammonia to give a benzamide is unexceptional. Birch et al. noted in 1966 that no... 

Transesterification occurs by mechanisms that are identical with those of ester hydrolysis—except that ROH replaces HOH—that is, by the acyl-oxygen fission mechanisms. When alkyl fission takes place, the products are the acid and the ether. 

Xun et al. 1999 Ohtsubo et al. 1999). The degradation of 2-chlorohydroquinone, which is produced during the degradation of y-hexachlorocyclohexane, is carried out analogously by dioxygenation followed by hydrolysis of the acyl chloride to 3-hydroxymuconate, which is mediated by an unusual extradiol fission enzyme encoded by linE (Miyauchi et al. 1999 Endo et al. 2005). 

Isotopes can also be used to solve mechanistic problems that are non-kinetic. Thus the aqueous hydrolysis of esters to yield an acid and an alcohol could, in theory, proceed by cleavage at (a) alkyl/ oxygen fission, or (b) acyl/oxygen fission ... 

If the reaction is carried out in water enriched in the heavier oxygen isotope 180, (a) will lead to an alcohol which is 180 enriched and an acid which is not, while (b) will lead to an l80 enriched acid but a normal alcohol. Most simple esters are in fact found to yield an lsO enriched acid indicating that hydrolysis, under these conditions, proceeds via (b) acyl/oxygen fission (p. 238). It should of course be emphasised that these results are only valid provided that neither acid nor alcohol, once formed, can itself exchange its oxygen with water enriched in 180, as has indeed been shown to be the case. 

Elliott DF. Specific chemical methods for the fission of peptide bonds. I. /V-acyl to O-acyl transformation in the degradation of silk fibroin. Biochem J 542, 1952. 

Thus, the best compromises for Boc and Fmoc chemistries seem to be cyclohexyl and 2,4-dimethylpent-3-yl (Dmpn), which is of intermediate stability, and the removal of which by trifluoromethanesulfonic acid with the aid of thioanisole (see Section 6.22) leads to minimal imide formation (see Section 6.13). Points to note are that acidolysis of esters by hydrogen fluoride can lead to fission at the oxy-car-bonyl bond instead of the alkyl-oxy bond, thus generating acylium ions that can react with nucleophiles (see Sections 6.16 and 6.22), and that benzyl esters may undergo transesterification if left in methanol. The side reactions of cyclization (see Section 6.16) and acylation of anisole (see Section 6.22) caused by acylium ion formation do not occur at the side chain of aspartic acid.47-51... 

Thermal decomposition in three different ways, i.e. homolytic, polar and radical induced decomposition, as well as intermolecular reaction of sulfonyl peroxides are the main reactions displayed by sulfonyl peroxides. When bis(arylsulfonyl) peroxides are allowed to decompose at 25-40 °C in chloroform, homolytic 0—0 bond fission followed by hydrogen abstraction from the solvent results in the formation of the corresponding arylsnlfonic acids. Mixed acyl sulfonyl peroxides undergo complicated thermal decomposition in solution, and have been used commercially as polymerization initiators, since they provide a source of free radicals at a relatively low temperature . 

Unhindered 3,4-diacylfuroxans prefer ring opening by N—O bond fission of the oxadiazole ring with concomitant migration of an acyl group to give an a-(acyloximino)-p-ketonitrile oxide [type (b)] (103) (Scheme 6.8). 

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