Two tert-butyldiphenylsilyl-protected synthons 7 and 9 are prepared in excellent yields and used in a one-pot synthesis of higher generation polyether dendrons.

Bulletin des Sociétés Chimiques BelgesVolume 105, Issue 1 p. 53-54 Article Crystal structure X-ray analysis of 5-thiobenzoyl-1,2,3-thiadiazole S-oxide Gerrit L'Abbé, Gerrit L'Abbé Department of Chemistry, University of Leuven, Celestijnenlaan 200F, 3001 Leuven (Heverlee), BelgiumSearch for more papers by this authorWim Dehaen, Wim Dehaen Department of Chemistry, University of Leuven, Celestijnenlaan 200F, 3001 Leuven (Heverlee), BelgiumSearch for more papers by this authorLuc van Meervelt, Luc van Meervelt Department of Chemistry, University of Leuven, Celestijnenlaan 200F, 3001 Leuven (Heverlee), BelgiumSearch for more papers by this author Gerrit L'Abbé, Gerrit L'Abbé Department of Chemistry, University of Leuven, Celestijnenlaan 200F, 3001 Leuven (Heverlee), BelgiumSearch for more papers by this authorWim Dehaen, Wim Dehaen Department of Chemistry, University of Leuven, Celestijnenlaan 200F, 3001 Leuven (Heverlee), BelgiumSearch for more papers by this authorLuc van Meervelt, Luc van Meervelt Department of Chemistry, University of Leuven, Celestijnenlaan 200F, 3001 Leuven (Heverlee), BelgiumSearch for more papers by this author First published: 1996 https://doi.org/10.1002/bscb.19961050109Citations: 2AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume105, Issue11996Pages 53-54 RelatedInformation

The title compounds were synthesized by two approaches, starting either from a 1,2,3-thiadiazole or a 1,2,3-thiadiazole derivative, and were shown try X-ray analysis of a selected example 9 to have two S-N bonds around 1.9 Angstrom.

The title reactions give 4,5-dihydrooxazoles containing two orthogonal fluorene groups suitable for the elaboration of semi-rigid macromolecules.

Thermolysis in solution of 5-azidoisoxazoles with formyl, acetyl or N-phenylimino in the 4-position led to the formation of bicyclic products; in the absence of such 4-substituents, ring-opening resulted and in two instances the expected nitrosoalkene intermediate was successfully trapped with 2,3-dimethylbuta-1,3-diene.Rate measurements indicate that there are small neighbouring group effects exerted by those 4-substituents which lead to cyclization, and it is argued that these effects are necessarily small in these 5-azidoisoxazoles. Nevertheless, these small effects appear to play a key role in directing the reaction towards cyclization instead of ring-opening.With ring substituents which cannot exert neighbouring group effects, electron-withdrawing groups at position 4 reduce the reaction rate, while electron-donating groups increase it. Substituents at position 3 have Little effect on rate. These results are interpreted in terms of a nitrene-like transition state which is substantially stabilized by electron release from the heteroaromatic ring.

Fused 1,2,4-thiadiazoles can be prepared in high yields from cyano substituted amines and thiocarbamoyl isothiocyanates by oxidation of the intermediate dithiobiurets.

The title compound 3 is readily available from 1,3,5-triacetylbenzene in a three-step procedure and is an efficient trifunctional core reagent for the synthesis of dendrimers.

1H-Pyrazolo[3,4-d]thiazole-5-carbonitriles 9 are obtained by treating 4-unsubstituted 5-aminopyrazoles with Appel's salt at ro om temperature in the presence of 2,6-dimethylpyridine.

The 1,2,3-thiadiazolium chlorides 4a,b react with benzylamine, aniline, phenylhydrazine, hydroxylamine and N-methylhydroxylamine at low temperature to yield the 1-arylamino-1,4-diazadienes 5-9 which have been fully characterized by NMR spectroscopy.

Acyl isothiocyanates 4a-e react with two equivalents of diphenyldiazomethane at room temperature to give the 4,5-dihydro-1,3-oxazole-4-spiro-2'-thiiranes 5a-e which isomerize thermally to the thietan-3-imines 6a-e.

A general method is described to transform the readily available title compounds into tri- and tetracyclic heterocycles, first by substituting the chlorine atom by an unsaturated thiolate or alkoxide, and then by modifying the aldehyde function into a 1,3-dipole. As 1.3-dipoles, nitrile oxide, nitrone, nitrile imine, azomethine ylide and azomethine imine groups were generated from the 5-allylsulfanyl-4-formylpyrazole 7, which resulted in intramolecular cycloaddition and formation of the heterocycles shown in Scheme 2. The other pyrazoles 22, 23, 26, 30 and 33 were converted via intramolecular nitrile oxide cycloaddition (INOC) into fused dihydroisoxazoles. A limitation to the method is the Claisen rearrangement which occurs when the allyl ether 26 or the prop-2-ynyl ether 30 is used.

The N-3 and N-2 methylated 1,2,3-thiadiazolium tetrafluoroborates 3 and 4 react with aliphatic activated methylene ketones and esters in the presence of a base to give the substitution products 7-11 and 18-24. Under similar conditions activated methylene azoles afford products formulated by NMR analysis as N-S...O rotamers (25, 26), N-S...N rotamers (12-15, 27-29), or a mixture of both (16, 17, 30). The X-ray crystal structure analysis of product 21, derived from the thiadiazolium salt 4 and 2,2-dimethyl-1,3-dioxane-4,6-dione, reveals a nearly linear N-S...O sequence (169 degrees)and a short S...O contact (2.37/2.34 Angstrom) for the two independent molecules.

The thermolysis rate of 4-methoxy-1-azidobenzene reveals that the methoxy group increases the rate much less than the endocyclic sulfur does in 3-azidothiophene. With 3-azidothiophene, the neighbouring groups 2-acetyl and 2-nitro enhanced the rate in decalin solution only 5- and 17-fold, respectively, whereas in azidobenzene the corresponding enhancements are 413 and 1060. It is argued that the preferred electron distribution in the transition state for decomposition of 3-azidothiophene is not consistent with that required for effective neighbouring group participation. 2-Cyano and 2-methoxycarbonyl substituents, which are not known as effective neighbouring groups in thermal decompositions of azides, have very little effect on the rate for 3-azidothiophene.

5-Azidotriazoles bearing a thiazole, benzothiazole or pyridine ring at the 4-position were synthesized and thermolyzed at 60-degrees-C. Whereas the 5-azido-4-(thiazol-2-yl)triazole 4 decomposed with extrusion of nitrogen and formation of the triazene 5 as the sole reaction product, the 5-azido-4-(benzothiazol-2-yl)triazoles 11a,b furnished mixtures of the triazenes 12a,b and tetrazoles 14a,b. In the case of the 5-azido-4-(2-pyridyl)triazoles 17a-d, the product distribution was found to depend strongly on the N-1 aryl substituent, favouring the tetrazole 20 by increasing the electron-withdrawing capacity of this group.

The title compounds have been obtained from 5-amino-1,2,3-thiadiazole 6 by methylation and reaction with arenediazonium tetrafluoroborates; their 13C NMR data and X-ray analysis are discussed in terms of two non-equivalent resonance contributors 11A and 11B.

Fused 1,2,4-thiadiazoles (14–18 and 26–31) are conveniently prepared by reacting the chlorothiadiazolone 12 with (ω-aminonitriles and aminoazoles. During these reactions the original thiadiazole ring is opened and a new, fused thiadiazole ring is formed, probably via a hypervalent sulfur intermediate. The products derived from the aminoazoles were analyzed by X-ray crystallography and found to have structures different from those published earlier.7

2-Methoxyazidobenzene has been used as a model compound for alpha-azido five-membered heterocycles, and undergoes thermal loss of nitrogen 16 times as fast as azidobenzene and 3.6 times as fast as its para isomer. These rates identify an important electrostatic stabilization within a charge-separated transition state. It is argued that this stabilization would be very much larger in thermolyses of alpha-azido five-membered heterocycles and removes the need to postulate that their high rates are due to appreciable ring-opening at the transition state. Electronic effects of ring substituents in 5-azidopyrazoles are consistent with this electrostatic model. The electron distribution required is not compatible with the operation of a neighbouring group effect, and none is found for 5-azido-4-nitro-pyrazole.

1,3-Oxathiol-2-imines 4–8 are conveniently prepared from 5-chloro-1,2,4-thiadiazol-3(2H)-one 3 and pentane-2,4-dione, methyl acetoacetate, dimethyl malonate and cyclohexane-1,3-diones, while the normal substitution product 9 is obtained from the thiadiazole 3 and Meldrum's acid.

The N-15 nmr spectra of a series of 1,2,3-thiadiazoles reveal the strong influence of substituents at C-5 on the N-2 resonance. Upon methylation, the two thiadiazole nitrogen resonances are shielded, but the most dramatic shift is observed for the methylated nitrogen, DELTAdelta > 140 ppm. The N-15 chemical shifts of some mesoionic thiadiazoles were also determined and explained by the dual effect of 5-substitution and salt formation. By disconnecting these effects, the N-15 chemical shifts of 10 and 11 were found to be unusual and to reflect a thiapentalene character.

The N-3 methylated thiadiazolium salt 3 reacts with cyclohexane-1,3-dione and 5,5-dimethylcyclohexane-1,3-dione to give mesoionic 1,2,3-thiadiazoles (5 and 6) with short intramolecular S...O contacts, whereas the N-2 methylated thiadiazolium salt 4 furnishes oxathiole derivatives (7 and 8) with short intramolecular S...N contacts.