Ring contraction azides, cyclic

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 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. 

Indoles (103), where R2R3 is a cyclic substituent, react with azides to give ring-contraction or -expansion products, depending on the arylsulfonyl azide and the solvent.499,502,503,512-517 Dihydroquinolines and dihydroiso-quinolines518 as well as indolizines519 show analogous behavior. 

Aromatic sulfonyl azides react with enol ethers of cyclic ketones to form arenesulfonyl imidate esters (96) with ring contraction, the addition-rearrangement process is very stereospecific (Scheme 63). 

Arenesulfonyl azides react at ambient pressure with enol ethers of simple cyclic ketones to give ring contracted arenesulfonylimidate esters in good yields (equation 99)90. The addition-rearrangement is highly stereoselective as shown in equations 100-102. 

Ring contractions. This azide can function in 1,3-dipolar cycloaddition to enamines of cyclic ketones (1). The resulting AMriazolines (a) have not been isolated since they undergo loss of nitrogen with ring contraction to form the ring-contracted products (2). These can be converted into cycloalkanoic acids (3). ... 

A previously known thermal ring-contraction, which follows an initial 1,3-dipolar cycloaddition between azides and cyclic enones, can be smoothly promoted by TMSOTf to give enam-inones (eq 59). ... 

Dipolar Cycloaddition. The principal use of p-bromobenzenesulfonyl azide is in 1,3-dipolar cycloaddition reactions with functionally substituted alkenes. The reagent has been used at ambient temperature and pressure to convert simple trimethylsilyl and methyl enol ethers of cyclic ketones to ring-contracted p-bromobenzenesulfonimidates, and thence to the corresponding amides, esters, or acids (eqs 1 and 2). 

Another advantage of sterically demanding trisyl azide is the selective azidation of cyclic 8-ketoesters (eq 32). Thus, the desired azidation product A was slowly formed as a single product in either THF or MeCN. In contrast, more reactive sulfonyl azides such as 4-methoxybenzenesulfonyl azide, 4-nitrobenzenesulfonyl azide, mesyl azide, triflyl azide, and tosyl azide provided not only the azidation product A but also a ring-contraction product B, a ring-opened diazo-transfer product C, and a mixture of ring-opened olefins D. The amounts of by-products B, C, and D were dependent upon the azide reagent. 

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