Exocyclic nitrogen

J lie decarboxylation is frequently the most troublesome step in this sequence. Attempts at simple thermal decarboxylation frequently lead to recycliz-ation to the lactam. The original investigators carried out decarboxylation by acidic hydrolysis and noted that rings with ER substituents were most easily decarboxylated[2]. It appears that ring protonation is involved in the decarboxylation under hydrolytic conditions. Quinoline-copper decarboxylation has been used successfully after protecting the exocyclic nitrogen with a phthaloyl, acetyl or benzoyl group[3]. 

H transfer from the methyl to the exocyclic nitrogen of the acetamido group. 

The nature of the substituent on the exocyclic nitrogen also influences the ambident activity of anion 48 in DMF (Scheme 35) when R is an heterocyclic ring, nitrogen alkylation predominates (189) when R is a methylamino group, a mixture of the two isomers is reported (190) when... 

Nitraminothiazoles are sufficiently acidic to be alkylated by diazomethane the methyl substituent is introduced on the exocyclic nitrogen (194). When sulfathiazole is methylated with diazomethane in ether, a mixture of ring-methylated and amino-methylated products is obtained, the ratio being 30 70 (85). With anion 31 (R = p-NO CsH4SO -) the ratio becomes 15 85 (195). 

The exocyclic nitrogen atom is involved when 2-aminothiazoles and aromatic aldehydes react under mild conditions yielding 61 (Scheme 42)... 

Both carbonyl groups of terephthaldehyde are reported to react with the exocyclic nitrogen of 2-aminothiazole yielding 1.4-phenylene bis(2-methyleneamino)thiazole. The same report describes the reactions of 2-amino-4-phenylthiazole with terephth aldehyde and salicylaldehyde as yielding 64 and 65, respectively (Scheme 45) (215), whose structures are based on ultraviolet and infrared spectra. 

The metabolite of 2-amino-4-phenylthiazole (used as an anaesthetic for fish) was identified (223) as 2-amino-4-phenylthiazole 2-N, -d-glucopyranosiduronic acid (71) (Scheme 50). The formation of this compound probably involves the reaction of the exocyclic nitrogen on the Open-chain form of the acid. The isolation of this metabolite is part of a very Systematic study by Japanese researchers related to the anaesthetic... 

Small amounts of salt-like addition products (85) formed by reaction on the ring nitrogen may be present in the medium. (Scheme 60) but. as the equilibrium is shifted by further reaction on the exocyclic nitrogen, the only observed products are exocyclic acylation products (87) (130. 243. 244). Challis (245) reviewed the general features of acylation reactions these are intervention of tetrahedral intermediates, general base catalysis, nucleophilic catalysis. Each of these features should operate in aminothiazoles reactivity. 

Zugravescu et al. (263) showed that ethyl chloroformate reacts on the exocyclic nitrogen of 2-amino-4-methylthiazole to yield the carbamate (101) (Scheme 70) (see also Refs. 264 and 265). With an excess of chloroformate (2 moles for one of the thiazole) under Schotten-Bauman conditions the jV.A -dicarbamate of 2-imino-4-methylthiazoline (102) is obtained (263),... 

The exocyclic nitrogen is reactive even when already substituted 2-anilinothiazole (110) is acetylated by acetic anhydride (120). other examples of this reactivity are given in the tables (Section VII). 

Acetylation of 2-phenyl-4-amino-5-benzoylthiazole takes place on the exocyclic nitrogen (49). This exocyclic nitrogen remains the reactive center even with 2-imino-3-aryl-4-amino-5-carboxamido-4-thiazoline (111). Its acetylation with acetic anhydride gives the 4-acetamido derivative (112), which reacts further on heating to yield 2-(acetylimino)-(3H)-3-aryl-5-methylthiazolo[4,5-d]pvrimidin-7-(6H)-one (113) (Scheme 76) (276). 

Vollmann found that the reaction between l-imino-3-amino isoin dolenine (124) and 2-amino-4-methylthiazole is catalyzed by ammonium chloride and involves the exocyclic nitrogen (285). This reaction (Scheme 82) was later used to prepare dyes (286). 

Picryl halides react with 2-amino-4-methylthiazole. Again, the exocyclic nitrogen is the reactive center (288). and the product formed (128) is... 

Treatment of 2-imino-3-phenyl-4-amino-(5-amido)-4-thiazoline with isocyanates or isothiocyanates yields the expected product (139) resulting from attack of the exocyclic nitrogen on the electrophilic center (276). Since 139 may be acetylated to thiazolo[4,5-d]pyrimidine-7-ones or 7-thiones (140). this reaction provides a route to condensed he erocycles (Scheme 92). 

The exocyclic nitrogen is more reactive toward the sp-C (.A) than toward the sp C. but selectivity is lower than in the case of the ring nitrogen. 

The exocyclic nitrogen is slightly more reactive toward the electrophilic center A than is its ring counterpart. 

Amino-5-methylthiazole does not react with diazotized p-nitroaniline in solutions acidified with acetic or hydrochloric acid (391). 2-Amino-4,5-dimethylthiazole with the diazonium salts of para-substituted anilines, however, gives product 193, involving reactivity of the exocyclic nitrogen (Scheme 122) (399). 

These halogenation reactions all take place in the 5-position (408. 409. 430) even when there is a phenyl or a 2-pyridyl (382) substituent on the exocyclic nitrogen. Crystalline perbromides have been isolated (166. 320. 

Bromination of 2-dialkylaminothiazoles has been reported to be successful by one author (415) and to fail by others (375. 385). If the mechanism of direct electrophilic substitution is accepted for these compounds, it is difficult to understand why alkyl substitution on such a remote position as exocyclic nitrogen may inhibit this reaction in the C-5 position. 

This genera] scheme could be used to explain hydrogen exchange in the 5-position, providing a new alternative for the reaction (466). This leads us also to ask whether some reactions described as typically electrophilic cannot also be rationalized by a preliminary hydration of the C2=N bond. The nitration reaction of 2-dialkylaminothiazoles could occur, for example, on the enamine-like intermediate (229) (Scheme 141). This scheme would explain why alkyl groups on the exocyclic nitrogen may drastically change the reaction pathway (see Section rV.l.A). Kinetic studies and careful analysis of by-products would enable a check of this hypothesis. 

The principal reactions of this class of compounds are summarized in Scheme 172. In most of these reactions the reactive nucleophilic center is the terminal NHj group, although the other exocyclic nitrogen may also be involved, as shown by acetylation, which yields 284 and 285. However, the structure of compound 281 is not the one proposed in a recent report (1582) that attributes the attack to the other exocyclic nitrogen. The formation of osazones (287) from sugars, 2-hydrazinothiazoles, and hydrazine has been reported (525, 531). 

The nitro group increases the acidity of the hydrogen born by the exocyclic nitrogen, and alkylation of 2-nitraminothiazole with diazomethane is possible (87), The formed 2-(A"-methylnitramino)-thiazole also may be obtained from the reaction of 2-nitraminothiazole with dimethylsulfate in basic medium (194). 

The exocyclic nitrogen of 2-aminothiazole reacts with arylsulfenyl chlorides (32. 456. 457. 621-624) to yield 2-sulfenamidothiazoles (335) (Scheme 192). 

Imino-4-thiazolines are far more basic than their isomeric 2-aminothiazoles (see Table VI-1). They react with most electrophDic centers through the exocyclic nitrogen and are easily acylated (37, 477, 706) and sulfonated (652). The reaction of 2-imino-3-methyi-4-thiazoline (378) with a-chloracetic anhydride yields 379 (Scheme 217) (707). This exclusive reactivity of the exocyclic nitrogen precludes the direct synthesis of endocyclic quaternary salts of 2-imino-4-thiazolines. although this class of compounds was prepared recently according to Scheme 218 (493). 

Left bond bonded to exocyclic nitrogen right, linked to sp"-C-... 

Azole iV-oxides, iV-imides and iV-ylides are formally betaines derived from iV-hydroxy-, iV-amino- and iV-alkyl-azolium compounds. Whereas iV-oxides (Section 4.02.3.12.6) are usually stable as such, in most cases theiV-imides (Section 4.02.3.12.5) andiV-ylides (Section 4.02.3.12.3) are found as salts which deprotonate readily only if the exocyclic nitrogen or carbon atom carries strongly electron-withdrawing groups. 

Canonical forms of type (442b) facilitate proton loss from the amino groups the anions formed react easily with electrophilic reagents, usually preferentially at the exocyclic nitrogen atom e.g. 443 -> 444) (79HC(34-2)9). 

The chemical behaviour of the mesoionic pyrazole (459) has been studied by Boyd et al (74JCS(P1)1028). Protonation and alkylation take place on the exocyclic nitrogen atom and a thermal rearrangement of a methyl group is observed when (459) is boiled in benzonitrile for several hours giving (460). 

In addition to (461), Dorn has described the imine (463) isolated from 5-amino-l-methylpyrazole and arenesulfonyl chloride (80CHE1). Upon heating, or in the presence of triethylamine, it undergoes rearrangement to the more stable 5-bis(arylsul-fonamido)pyrazoles (464). 5-Iminopyrazolines (461) react with acyl chlorides at the exocyclic nitrogen atom to afford amidopyrazolium salts (B-76MI40402). 

Below 100 °C tri-t-butyldiaziridinimine (176) only undergoes inversion at the exocyclic nitrogen, as evidenced by coalescence of NMR signals at about 50 °C. Heating for 1 h to 150 °C, however, results in clean formation of ( )-azoisobutane and t-butyl isocyanide. 

In spite of the fact that the vast majority of quaternizations of amino-heterocyclic compounds are reported as occurring on the ring nitrogen atom only, it seems quite likely that salt formation may also take place on the exocyclic nitrogen in other cases but that it has been overlooked in the absence of a test such as was available for 4. 

In all cases investigated, the cycloacylation reaction also occurred at the exocyclic nitrogen atoms. In order to obtain precursors for oligomeric/poly-meric heterofulvalenes the TAFs of type 85 were condensed with ortho es-... 

Pyridinium imides lacking a substituent on the exocyclic nitrogen atom do not yield di-azepines. 

In this exothermic reaction (Fig. 8.8) the [BN2] ions undergo a trimerization to form a cyclic [BsNs] ion with three exocyclic nitrogen atoms (Fig. 8.9). Afterwards La3(B3N6) is washed with water in order to remove liCl. 

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