Azido-, fragmentation

The molecular structure of [Ti2Cp4(N3)2](/x-0) has been determined by X-ray diffraction, showing a binuclear complex with two bis-Cp azido fragments connected through the oxygen bridge.1510... 

The intrinsic reactivity of the azido fragment in the presence of an inert metal site was used by Hahn et al for the synthesis of benzannulated A-heterocyclic carbene ligands. 

The most important chemistry of azidoazoles is the fragmentation of derived nitrenes of which the prototypes are (453) (454) and (455) (456). Thus 5-azido-l,4-diphenyltriazole (457) evolves nitrogen at 50 °C (70JOC2215). 4-Azido-pyrazoles and -1,2,3-triazoles (458) undergo fragmentation with formation of unsaturated nitriles (8lAHC(28)23l). 

Thiophene, 2-amino-3-cyano-5-phenyl-synthesis, 4, 888-889 Thiophene, 3-amino-4,5-dihydro-cycloaddition reactions, 4, 848 Thiophene, 2-amino-3-ethoxycarbonyl-ring opening, 4, 73 Thiophene, 2-amino-5-methyl-synthesis, 4, 73 Thiophene, 2-anilino-synthesis, 4, 923-924 Thiophene, aryl-synthesis, 4, 836, 914-916 Thiophene, 2-(arylamino)-3-nitro-synthesis, 4, 892 Thiophene, azido-nitrenes, 4, 818-820 reactions, 4, 818-820 thermal fragmentation, 4, 819-820 Thiophene, 3-azido-4-formyl-reactions... 

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. 

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

As can be seen in Scheme 3.249, a-azido oximes (465) can be involved with advantage in the selective reduction of the azido group in the presence of the oximino fragment and also in the selective or nonselective reduction of the oximino fragment (525). 

A similar series of reactions was performed on a 6-azido-6-deoxy-2-ketose.246,252 In most of the foregoing syntheses, the piperidine formed is accompanied by the tautomeric furanose. The tautomers can, however, be separated without occurrence of equilibration (which can be induced by changing the pH of the medium). For the ketose derivative 183, the equilibrium mixture includes246 compound 184 and a dehydration product (182). The conformation and anomeric effect in these piperidines have been studied by n.m.r. spectroscopy,253 and their fragmentation pathways have been determined by mass spectrometry.254... 

The facile formation of the iodo compound, and the subsequent reduction with tributyltin hydride opens a new access to 2-amino-2,3-dideoxy-D-glucose (D-lividosamine) present in various aminoglycoside antibiotics [92], The same strategy been used in the synthesis of C-lO -C-l fragment of boromycin [93]. Displacement of the C-3 imidazylate ester of a 2-azido-2-deoxy-a-D-altropyranose derivative with benzoate occurred readily [94], compared with the corresponding tosylate [95],... 

Based on results presented in Scheme 37, Logothetis suggested that the thermal decomposition products from the olefinic azides in the scheme are derived from triazoline intermediates formed by an intramolecular cycloaddition reaction and not by fragmentation of the azido group to a nitrene.100 However, allyl azide and 4-azido-l-pentene do not undergo internal cycloaddition because of the strain in the corresponding triazoline they fail to give aziridines and imines upon thermolysis.100... 

Carbon atom 2 of the oxazole ring is also supplied by aldehydes in their reaction with a- (hydroxylamino) ketones, which proceeds in the presence of sulfuric acid and acetic anhydride (equation 109). Three further oxazole syntheses involving incorporation of a C(2) fragment are the condensation of triethyl orthoformate with the hydrochlorides of a-aminoacetophenones (equation 110), the reaction of acyl chlorides with a-azido ketones or a-azido esters in the presence of triphenylphosphine (equation 111), and the preparation of 2-aminobenzoxazole and benzoxazoleimines from o-aminophenols and cyanogen bromide (equation 112). 

Intramolecular cyclizations. Cyclizations of a>azido-/3-ketoesters, such as 138 (obtained by a multistep reaction starting from cyclohexanone), produced eight-membered enamines 139 (Scheme 57 <2004JOC997>). Preparations of 138 are based on a method developed from the formal ring expansion of cyclic ketones via retro-Reformatsky fragmentation <2004JOC997>. 

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