Ring contraction cyclic

A variety of mono- and bi-cyclic /3-lactams have also been prepared by oxidative ring contraction of pyrrolidine-2,3-diones (180) (75JOC2356), and by the photolysis of pyrazolidin-3-ones (181) (78T1731, 75JOC3510, 75JOC3502, 75CC725). 

The procedure described is essentially that of Shioiri and Yamada. Diphenyl phosphorazidate is a useful and versatile reagent in organic synthesis. It has been used for racemlzatlon-free peptide syntheses, thiol ester synthesis, a modified Curtius reaction, an esterification of a-substituted carboxylic acld, formation of diketoplperazines, alkyl azide synthesis, phosphorylation of alcohols and amines,and polymerization of amino acids and peptides. - Furthermore, diphenyl phosphorazidate acts as a nitrene source and as a 1,3-dipole.An example in the ring contraction of cyclic ketones to form cycloalkanecarboxylic acids is presented in the next procedure, this volume. 

The present preparation illustrates a general and convenient irethod for ring contraction of cyclic ketones. The first step is the usual procedure for the preparation of enamines. The second step involves 1,3-dipolar cycloaddition of diphenyl phosphorazidate to an enamine followed by ring contraction with evolution of nitrogen. Ethyl acetate and tetrahydrofuran can be used as a solvent in place of toluene. Pyrrolidine enamines from various cyclic ketones smoothly undergo the reaction under similar reaction conditions. Diphenyl (cycloalkyl-1-pyrrolidinylmethylene)phosphoramidates with 5,6,7, and 15 members in the ring have been prepared in yields of 68-76%. 

D. Cyclic a-Diazoketones Ring-Contraction in Photolytic Wolff Rearrangements... 

Retropinacol rearrangement, 416 Ring contraction in photolytic cyclic a-diazoketones rearrangements, 316 12- p/-rockogenin mesylate, 400... 

The reaction of cyanogen azide with enamines of cyclic ketones to yield a cyanoamidine with one less member in the carbocyclic ring represents a potentially valuable method of ring contraction under mild conditions (199a). The reaction probably proceeds first by 1,3 cycloaddition of the azide to the enamine followed by rearrangement and elimination of a molecule of nitrogen. 

With cyclic a-halo ketones, e.g. 2-chloro cyclohexanone 6, the Favorskii rearrangement leads to a ring contraction by one carbon atom. This type of reaction has for example found application as a key step in the synthesis of cubane by Eaton and Cole for the construction of the cubic carbon skeleton ... 

The photolytic cleavage of cyclic ketones 14 leads to formation of a diradical species, that can undergo analogously the various reactions outlined above. The decarbonylation followed by intramolecular recombination yields a ring-contracted cycloalkane 15 ... 

With cyclic a-diazo ketones, e.g. a-diazo cyclohexanone 9, the rearrangement results in a ring contraction by one carbon " ... 

Though the reaction is not general, certain cyclic ketones can be photolyzed to give ring-contracted products.In the example above, the cyclobutanone (55) was photolyzed to give (56), where BZ is benzoyl. This reaction was used to synthesize tetra-r rf-butyltetrahedrane (57). 

In a reaction similar to 17-37, certain sulfones, both cyclic and acyclic, extrude SO2 on heating or photolysis to give ring-contracted products." An example is the... 

Ring contraction of cyclic diazo ketones (Wolff)... 

Reaction of the cyclic sulfoxide 1176 with trimethylchlorosilane (TCS) 14 in CH2CI2 affords the unsaturated sulfide 1177, in 86% yield, and HCl and HMDSO 7 whereas the ester 1178 gives rise to 72% 1179, 16% isomer 1180, and, via 1181, the chloro compounds 1182 and the ring contracted 1183 in 7% yield [17, 18] (Scheme 8.5). 

More recently, several similar thermal rearrangements of other cyclic sulfoxides involving reversible intramolecular cycloelimination of sulfenic acid have also been reported . A detailed study of this rearrangement which, in certain cases, is accompanied by ring expansion or ring contraction has been performed by Jones and coworkers. These authors found that at 140 °C in xylene, conditions under which acyclic sulfoxides readily decompose , thian 1-oxide (180a) was inert after 6 days. 

The photolysis of cyclic a-diazoketones results in ring contraction to a ketene, which can be isolated as the corresponding ester. 

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