Carboxylic acid esters azides

The (aminomethyl)thiophene-2-carboxylic acid isomers were prepared from the corresponding methylthiophene-2-carboxylic acid esters, as exemplified for the 3-(aminomethyl) isomer 79 (Scheme 22))66 The 4-(aminomethyl) isomer was prepared in the same way, but with a low yield for the bromination step (7%))66,111 For the 5-(aminomethyl) isomer the bromomethyl compound was reacted with sodium azide, followed by catalytic reduction)74 The corresponding furan and pyridine analogues were prepared similarly. 

The transformation of an active CH compound into the corresponding diazo derivative with -toluenesulfonyl azide has been designated a diazo transfer reaction and possesses a variety of preparative uses. The method has been useful for the syntheses of diazo derivatives of cyclopentadiene, 1,3-dicar-bonyl compounds, 1,3-disulfonyl compounds,1,3-keto-sulfonyl compounds, ketones, " carboxylic acid esters, and /3-keto imines. Further reaction of these diazo intermediates can lead to azo compounds,"- " 1,2,3-triazoles, and pyrazolinones. ... 

Carboxylic acid esters from carboxylic acid azides... 

For example, 2-arylazirines 13 can rearrange to indoles 14 when heated in xylene (Scheme 6.6) and the direct thermal conversion of alkyl 2-azidocinnamates into indoles has been exploited as a route to indole-2-carboxylic acid esters. The temperature has to be controlled when these vinyl azides were decomposed in solution in order to isolate 2H-azirines. ° Dilute solutions are also usually preferable in order to avoid bimolecular reactions. Photolysis is sometimes preferable to thermolysis for the generation of thermally unstable azirines. ... 

The transformation of bromides into azides and subsequent azide-alkyne Huisgen cycloaddition, as shown in the previous example, has become a common powerful modification technique. These sequential modifications are described in detail in the chapter by Kakuchi in this book [28] and were used to introduce a whole variety of fimctional groups, such as triple bonds, alcohols, carboxylic acids, esters or other polymer chains [29-31]. 

In synthetic target molecules esters, lactones, amides, and lactams are the most common carboxylic acid derivatives. In order to synthesize them from carboxylic acids one has generally to produce an activated acid derivative, and an enormous variety of activating reagents is known, mostly developed for peptide syntheses (M. Bodanszky, 1976). In actual syntheses of complex esters and amides, however, only a small selection of these remedies is used, and we shall mention only generally applicable methods. The classic means of activating carboxyl groups arc the acyl azide method of Curtius and the acyl chloride method of Emil Fischer. 

The methyl ester (100, R = CH3), derived from this A-nor acid by treatment with diazomethane, is different from the ester (102) obtained either by Favorskii rearrangement of 2a-bromo-5a-cholestan-3-one (101) or by the action of cyanogen azide on 3-methoxy-5a-cholest-2-ene (103) followed by hydrolysis on alumina. The ketene intermediate involved in photolysis of (99) is expected to be hydrated from the less hindered a-side of the molecule to give the 2j -carboxylic acid. The reactions which afford (102) would be expected to afford the 2a-epimer. These configurational assignments are confirmed by deuteriochloroform-benzene solvent shifts in the NMR spectra of esters (100) and (102). ... 

Intramolecular cycloaddition between an azide and an unsaturated ester (see 300) was the key step in the synthesis of triazole carboxylic acids 302 a, b, prospective anionic sugar mimics (Eq. 33) [79]. 

The spin label in question may as well be in the carboxylic acid fragment as in the alcohol moiety. Photoreactive esters bear azido groups in their carboxylic acid moieties. The esterification of nitroxide- or azide-bearing carboxylic acids with complex alcohols and CDI is illustrated in Table 3-6 by way of some examples. 

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