Nitrogen Ring Systems

Five Ring Nitrogen Systems.—Pynolines and Pyrrolidines. Ketene dithioacetals react with aziridines to give vinyl aziridines which undergo an iodine catalysed rearrangement to pyrrolines yields are high (Scheme 61). The novel spiro-com-pound (172) has been obtained by this route. A hetero-vinyl cyclopropane... 

Six Ring Nitrogen Systems.—ReducedPyridines. A -Substituted-l,2-dihydropyridines are of interest since they are implicated in indole-alkaloid biosynthesis. One of the few accessible derivatives is the carbamate (209), which on photolysis gives the 2-azabicyclo[2,2,0]hex-5-ene ester (210). Conditions for the conversion of the latter into the amine (211) have now been found, and several alkylations of this compound have been carried out thermolysis of these compounds has given jV-substituted... 

Fused Five and Six Ring Nitrogen Systems.—The acid catalysed reactions of alkyli-dene bisurethanes with conjugated dienes gave various azacyclic and azabicyclic molecules formed by 1,4 (or 1,6)-addition to the conjugated system by the acylimine (241). > In new work the first example of a formal 1,3-cycIoaddition during the reaction of (241) with dienes has been observed (Scheme 107). 

Much of the reactivity shown by the ring atoms and substituents of pyrimidine is akin to that of the corresponding parts of 1,3-dinitrobenzene and 3-nitropyridine. This arises from the quantitatively similar electron-withdrawing effects of doubly-bound ring nitrogen atoms and of nitro groups in reducing sharply the aromaticity of the cyclic system. 

This method of ring closure is the most convenient for the synthesis of ring-fused 1,2,4-triazoles with this arrangement of nitrogen atoms. It has been used extensively to form the analogous [5,6] ring-fused systems, and structural ambiguity is only encountered... 

The use of azide reagents is also important for the synthesis of cyclic sulfur(VI)-nitrogen systems. The reaction of SOCI2 with sodium azide in acetonitrile at -35°C provides a convenient preparation of the trimeric sulfanuric chloride [NS(0)C1]3 (Eq. 2.16). " Thionyl azide, SO(N3)2 is generated by the heterogeneous reaction of thionyl chloride vapour with silver azide (Eq. 2.17). This thermally unstable gas was characterized in situ by photoelectron spectroscopy. The phenyl derivative of the six-membered ring [NS(0)Ph]3 can be prepared from lithium azide and PhS(0)Cl. ... 

The aromaticity of the pyrimidine and purine ring systems and the electron-rich nature of their —OH and —NHg substituents endow them with the capacity to undergo keto-enol tautomeric shifts. That is, pyrimidines and purines exist as tautomeric pairs, as shown in Figure 11.6 for uracil. The keto tautomer is called a lactam, whereas the enol form is a lactim. The lactam form vastly predominates at neutral pH. In other words, pA) values for ring nitrogen atoms 1 and 3 in uracil are greater than 8 (the pAl, value for N-3 is 9.5) (Table 11.1). 

Ethyl iodide and 5-amino-2-methyl-l,3,4-thiadiazole react at 110° to give the N-3 salt (78 R = Me, R = NH2, R" = Et), as shown by the presence of the very reactive methyl group this salt is also used to prepare cyanine dyes. The slow quatemization at the ring-nitrogen atom furthest from the amino group is consistent with the reactions observed in other ring systems. As would be e pected, 5-alkylthio-2-methyl-l,3,4-thiadiazoles form salts at the N-3 (78 R = Me, R - S-alkyl).i ... 

The low reactivity (Section IV,B,3,a) of 3-chlorocinnoline compared to that of its isomer 407 is the combined result of decreased activation by an ortho ring-nitrogen and of the effect of the bicyclic system on charge stabilization in 2,3-orientations. The 2-Le-3-aza effect alone is the reason that the reactivity of 3-chlorocinnoline (396) is much lower than that of its benzopyridazine isomer 403. 

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