Amino-1, 4-oxadiazole

Nucleophilic attack at substituted ring carbon is probably the most common reaction of 1,3,4-oxadiazoles. However, few examples have been reported of nucleophilic attack at unsubstituted carbon since such compounds (19a) are relatively uncommon. The mechanism of the well-known conversion of 2-amino-oxadiazoles (in aqueous alkali) into triazoles has been studied in the case of the reaction where (19a R = NHPh) is converted to (20). This proceeds via the anion of semi-carbazide PhNHCONHNHCHO and is initiated by hydroxide attack at C-5 <84JCS(P2)537>. A similar nucleophilic attack by hydroxide on oxadiazole (19a R = 5-pyrazolyl) was followed by cyclization to the pyrazolo-triazine (21). Hydrolytic cleavage of 2-ary 1-1,3,4-oxadiazoles to aroyl-hydrazides allows use of the former as protected hydrazides. Oxadiazole (19a R = 4-... 

R CONHNHCOR (ii) the formation of oxadiazoles by oxidation of acylhydrazones RCH=NNHCOR, and (iii) the formation of oxadiazolinones, oxadiazolinethiones, and amino-oxadiazoles by the action on hydrazides RCONHNH2 of phosgene, carbon disulfide (or thiophos-gene), and cyanogen bromide respectively. 

From N-cyan-imidates, 5-amino oxadiazoles are similarly obtained [33c). 

The reduction of 3-phenyl-5-amino oxadiazole with Pd on C has jdelded N-carbamidobenzamidine 61c). 

Nucleophilic substitutions are particularly easy with 3-trichloro-methyl derivatives, which are immediately transformed into hydroxy and amino-oxadiazole. 

They have not been isolated but immediately transformed into N-phenyl-amino-oxadiazoles by treating with aniline. 

Amino-oxadiazoles have been prepared from oxyguanidine. The yields do not exceed 10% 2,121) (s. p. 815). They have also been prepared from ethyl a-amino-a-isonitroso-acetate, which condenses with acylating agents to form 3-carboethoxy-oxadiazoles. 

Amino-oxadiazoles are stable towards acids and bases. 

Phenyl-5-amino-oxadiazole can also easily be obtained from phenylchloroxime and guanidine (ISa)... 

Via a same intermediate, the action of sodium cyanamide on phenylchloroxime or the action of hydroxylamine on ethyl-N-cyano-benzimidate, yields 3-phenyl-5-amino oxadiazole (61a, 33c). 

The Beckmann rearrangement of substituted aminoglyoximes has been reported to give 5-amino-oxadiazoles (14, 73, 122) (see p. 827) which can be hydrolyzed into 5-hydroxy derivatives. 

A related series of 5-substituted-2-amino-oxadiazole compounds have also been prepared in a one-pot procedure using a microwave-assisted cyclisation procedure (Scheme 6.26)164. Rapid preparation of the pre-requisite ureas from the mono acyl hydrazines and various isocyanates (or the isothiocyanate) was easily achieved by simple mixing. The resulting products were then cyclo dehydrated by one of the two procedures either by the addition of polymer-supported DMAP and tosyl chloride or alternatively with an immobilised carbodiimide and catalytic sulphonic acid. Purity in most cases was excellent after only filtration through a small plug of silica but an SCX-2 cartridge (sulphonic acid functionalised - catch and release) could be used in the cases where reactions required additional purification. 

These compounds provided the source material for the first blue OLEDs.72 However, these devices were short-lived. Yet devices fabricated with improved blue-emitting amino oxadiazole fluorene did exhibit greater efficiency and stability,9 although their performance was still inferior to that of polyfluorene-based PLEDs (see below). 

A ten-fold increase in Coli-Staph. activity (to l-2mcg/ml) resiilted on cyclization of the acyl semicarbazide to the 2-amino-oxadiazole XIII. The 2-0X0 analog had Coli-Staph. activity at Imcg/ml while its 2-ethoxy and... 

Oxadiazoles. Mesitonitrile oxide adds j A -dicyanomethylamine, MeN-(CN)2, to yield the oxadiazole (471 Ar = 2,4,6-Me3C6H2). The action of hydroxylamine on the cyanoimino-ether ArCH2C(OEt)=NCN (Ar = 2,6-CI2C6H3) unexpectedly leads to the 3-amino-oxadiazole (472). 3-Azido-... 

Oxadiazoles are difficult to alkylate, unless the ring contains a strong electron donor group such as an amino substituent. 

Amino-l,3,4-oxadiazoles (423) ring-open and the products immediately recyclize to triazolinones (424) (66AHC(7)183). 

Oxadiazole, 2-amino-5-aryl-biological activity, 6, 445 reactions, 6, 439... 

Oxadiazole, 2-amino-5-phenyl-mass spectra, 6, 429 methylation, 6, 431... 

Oxadiazoles, amino-alkylation, 6, 431 as photographic sensitizer, 6, 446 ring cleavage, 6, 434 tautomerism, 6, 381... 

---