2,1,3-Oxadiazol

Any heterocycle containing the OCH=CH moiety can in principle extrude the superfluous fragment and form oxirene, as illustrated for a five-membered ring in Scheme 105. Probably the most propitious AB fragment would be nitrogen, but the required 1,2,3-oxadiazole (123) is unknown (see Chapter 4.21), probably because of ready valence tautomerization to diazoethanal (Scheme 106) (this approach has been spectacularly successful with the sulfur analogue of (2) (8UA486)). The use of (123) as an oxirene precursor is thus closely linked to the important diazo ketone decompositions discussed in Section 5.05.6.3.4(f). 

Polyfluoroalkyl- andperfluoroalkyl-substituted CO and CN multiple bonds as dipolarophiles. Dmzo alkanes are well known to react with carbonyl compounds, usually under very mild conditions, to give oxiranes and ketones The reaction has been interpreted as a nucleophilic attack of the diazo alkane on the carbonyl group to yield diazonium betaines or 1,2,3 oxadiazol 2 ines as reaction intermediates, which generally are too unstable to be isolated Aromatic diazo compounds react readily with partially fluorinated and perfluorinated ketones to give l,3,4-oxadiazol-3-ines m high yield At 25 °C and above, the aryloxa-diazolines lose nitrogen to give epoxides [111]... 

Simple 1,2,3-oxadiazoles are unknown (spontaneous ring opening to give diazoketones) and hence 4-hydroxy- 147 and 5-hydroxy-l,2,3-oxadiazoles 148 are not studied (Scheme 55). For 2-hydroxy-l,3,4-oxadiazoles, the oxo tautomers 149b are usually considered the major forms [84CHEC-I(6)427, 96CHEC-II(4)267]. 

The mesoionic tetrazole dehydrodithizone is transformed by iron penta-carbonyl into 4-phenyl-2-phenylazo-A2-l,3,4-thiadiazolin-5-one, presumably by a mechanism of ring opening, complexation, carbonyl insertion and subsequent ring closure (Scheme 128).193 Unfortunately, analogous processes do not occur on other mesoionic compounds in the 1,2,3-oxadiazole, s-triazole or tetrazole series, and the scope of this unusual carbonylation is probably limited. 

Multinuclear NMR data have been recorded for molsidomine 12 and its metabolite SIN-1 13, confirming that both are closed ring structures and that the positive charge is accommodated at the N3 position <1996CHE1358>. The c C4-C5 coupling constants for selected 1,2,3-oxadiazoles lie between 69.4 and 89 Hz, but correlation of these values with measures of bond order and aromaticity is difficult due to substituent effects <2000MRC617, 2002JST269>. 

The dipole moments of both 3- and 4-phenylsydnones containing dimethylamino and nitro substituents were calculated (ah initio 3-21G basis set) the magnitude increases with the electron donor attached to the phenyl ring <1995JPC1923>. Quadrupole moments, octopole moments, and polarizability of 1,2,3-oxadiazole have been determined by ab initio calculations and simple models <1996JPC8752, 1999JPC10009>. 

There have been no reports of 1,2,3-oxadiazoles synthesized by transformation of another ring since the last survey <1996CHEC-II(4)165> other than those involving cyclization of substituents attached to benzene (Section 5.03.9). 

The chemistry of 1,2,3-oxadiazoles is dominated by the sydnones, sydnonimines, and their analogs. There are many examples that employ the standard route to sydnones the method remains unsurpassed. Methods for cyclization have been described that bypass the need for mixed anhydride formation, helping to shorten the standard route (Section 5.03.9.2). 

Numerous transformations of functional groups attached to 1,2,3-oxadiazoles have been reported in the last 10 years. The reactivity of substituents attached to ring carbon atoms was thoroughly discussed in CHEC(1984) and CHEC-11(1996). 

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