Radicals fragmentation routes

The compounds obtained by the replacement of ring sulfur by carbon, as in the case of penicillins, show somewhat improved antibiotic properties. A free radical— based route has been described for the conversion of fermentation derived cephalosporins to their carbocyclic derivatives. The first step in this sequence consists of the condensation of the cephalosporin sulfone (36-1) with formaldehyde and dimethyla-mine the initial product from the Mannich-like reaction consists of the exomethylene derivative at the position adjacent to the activating sulfone. The product is treated in situ with phenylselenol to give the Michael adduct (36-2). This fragments with an extmsion of sulfur dioxide when heated with the free radical initiator AIBN in the presence of tributyltin hydride the reaction can be envisaged as leading to the... 

Finally, a novel three-component radical cascade reaction involving isonitriles has just been published [6]. In this paper, aromatic disulfides, alkynes, and isonitriles have been reported to react under photolytic conditions to afford -arylthio-substituted acrylamides 49 or acrylonitriles 50 in fair yields as mixtures of the E and Z geometric isomers (Scheme 21). The procedure entails addition of a sulfanyl radical to the alkyne followed by attack of the resulting vinyl radical on the isonitrile. A fast reaction, for example, scavenging by a nitro-derivative (route a) or f-fragmentation (route b), is necessary in order to trap the final imidoyl radical, since addition of vinyl radicals to isonitriles seems to be a reversible process. The reaction provides very easy access to potentially useful poly-functionalized alkenes through a very selective tandem addition sequence. 

This technique has, for example, shown that HNC rather than HCN is lost from metastable aniline molecular ions and that ionized methyl acetate loses CH20H radicals rather than OCHj as the lowest-energy fragmentation route. 

Isocyanides can be reduced to the corresponding hydrocarbons by (TMS)3SiH. The reaction can be considered as a smooth route for the deamination of primary amines. An example is given in Reaction (20). The key step for these chain reactions is expected to be the fragmentation of the intermediate radical derived from the fast addition of (TMSlsSi radical to the terminal carbon atom. 

Characteristic for the fragmentation of the title compounds (237-243) are a very facile N—S bond cleavage (route A in equation 50 giving rise to 244) as in sulfonylhydrazides (Section V.C) and skeletal rearrangements accompanied by the extrusion of SO2 from the M and [M — 1] ions to the ionized JV-aryliminopyridinium betaines (245, compounds 237-239) or AT-imino-2-benzylpyridinium betaines (247, compounds 240-243 in equation 52) and azacarbazoles (246), respectively . Ion 244 decomposes further by elimination of HCN and via the loss of R N and HCN, as shown in equation (51). The sequence M " (240-243)- 247 ->248 (equation 52) is supported by the fact that AT-imino-a-alkylpyridinium betaines can lose an NH2 radical due to the operation of an ortho effect . 

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