Fragmentation intramolecular

An increase in the tether length requires substantially more severe reaction conditions. For tethers containing more than five atoms between the reacting fragments, intramolecular [3+ 2]-cycloaddition reactions remain unknown. 

When the alkoxyl radical and the hydrogen to be abstracted are not properly disposed for the Barton reaction, the reactions of the alkoxyl radical, for example -fragmentation, intramolecular addition to the double bond, disproportionation or a-hydrogen fission, and intermolecular hydrogen abstraction, compete with the Barton reaction or result in an exclusive reaction. Among these reactions, /l-frag-... 

Fragments Intramolecular Cyclizations of Hydroxamic Acids and Aminophenylthiocarbamides... 

Morphine alkaloids (ACNO fragment) Intramolecular HR of an aryl iodide [523]... 

The strategy of using intramolecular reactions to set up the correct relationslup between two groups is of more general importance. We obviously want to disconnect bonds a and b in TM 328 so that we add a four carbon fragment to PhOMe in the synthesis. 

Silyl ethers serve as preeursors of nucleophiles and liberate a nucleophilic alkoxide by desilylation with a chloride anion generated from CCI4 under the reaction conditions described before[124]. Rapid intramolecular stereoselective reaction of an alcohol with a vinyloxirane has been observed in dichloro-methane when an alkoxide is generated by desilylation of the silyl ether 340 with TBAF. The cis- and tru/u-pyranopyran systems 341 and 342 can be prepared selectively from the trans- and c/.y-epoxides 340, respectively. The reaction is applicable to the preparation of 1,2-diol systems[209]. The method is useful for the enantioselective synthesis of the AB ring fragment of gambier-toxin[210]. Similarly, tributyltin alkoxides as nucleophiles are used for the preparation of allyl alkyl ethers[211]. 

The use of a vinylphosphonium salt as the source of the QQ fragment instead of the more commonly employed 1,2-dicarbonyl substrate is illustrated by the pyrrole synthesis in Scheme 79b (8UOC2570). A particularly interesting feature is the intramolecular Wittig reaction with an amide carbonyl group. A very useful synthesis of pyrroles depends upon the addition of the anion of p-toluenesulfonylmethyl isocyanide (TOSMIC) to a,/3-unsatur-... 

Ring opening by intramolecular attack by an electron source located on a /3-carbon is represented by the Wharton reaction (Scheme 62) (8UCS(P1)2363) and the Eschenmoser fragmentation (Scheme 63) (81S276). 

Most diaziridines are not sensitive towards alkali. As an exception, diaziridines derived from 2-hydroxyketones are quickly decomposed by heating with aqueous alkali. Acetaldehyde, acetic acid and ammonia are formed from (162). This reaction is not a simple N—N cleavage effected intramolecularly by a deprotonated hydroxy group, since highly purified hydroxydiaziridine (162) is quite stable towards alkali. Addition of small amounts of hydroxybutanone results in fast decomposition. An assumed reaction path — Grob fragmentation of a hydroxyketone-diaziridine adduct (163) — is in accord with these observations (B-67MI50800). 

One of the most thoroughly studied examples of intramolecular tunneling is isomerization of malonaldehyde involving the transfer of an H atom in an OH O fragment. 

Intramolecular mobility of metal complexes of rotaxanes and catenanes with macroheterocyclic fragments 98ACR611. 

Scheme 13. Intramolecular radical addition/fragmentation in Boger s synthesis of (+)-CC-1065.

The outcomes of intramolecular cyclizations of hydroxy vinylepoxides in more complicated systems can be difficult to predict. In a study of the synthesis of the JKLM ring fragment of dguatoxin, epoxide 44 was prepared and subjected to acid-mediated cydization conditions (Scheme 9.24) [114]. Somewhat surprisingly, the expected oxepane 45 was not formed, but instead a mixture of tetrahydropyran 46 and tetrahydrofuran 47 was obtained, both compounds products of attack of the C6 and C5 benzyl ether oxygens, respectively, on the allylic oxirane position (C3). Repetition of the reaction with dimsylpotassium gave a low yield of the desired 45 along with considerable amounts of tetrahydropyran 48. 

The investigation of structural dynamics of CP is particularly topical in connection with the establishment of correlation between local intramolecular mobility and the reactivity of chain fragments. It has been established that groups located in the most mobile parts of the polymer chain exhibit the greatest reactivity [48], The chemical heterogeneity in relationship to local mobility is particularly... 

In the case of allyl peroxides (12 X= CH2, A=CH2, BO),1 1 1 intramolecular homolytic substitution on the 0-0 bond gives an epoxy end group as shown in Scheme 6.18 (1,3-Sn/ mechanism). The peroxides 52-59 are thermally stable under the conditions used to determine their chain transfer activity (Table 6.10). The transfer constants are more than two orders of magnitude higher than those for dialkyi peroxides such as di-f-butyl peroxide (Q=0.00023-0.0013) or di-isopropyl peroxide (C =0.0003) which are believed to give chain transfer by direct attack on the 0-0 bond.49 This is circumstantial evidence in favor of the addition-fragmentation mechanism. 

Peroxyacetals 58106 and peresters such as 61107 are also effective transfer agents, however, at typical polymerization temperatures ( 60 CC) they are thermally unstable and also act as initiators. Compounds such as 62 which may give addition and 1,5-intramolecular substitution with fragmentation have also been examined for their potential as chain transfer agents (l,5-SHi mechanism).108... 

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