Acylation of 3,4-Dihydropyrrolo[1,2-a]pyrazines

Теренин, В. И.; Kabanova, E. V.; Tselishcheva, N. A.; Kovalkina, M. A.; Плешкова, А. П.; Зык, Н. В.

doi:10.1023/b:cohc.0000028632.61226.cd

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“…In previous work [6], we showed that dipyrrolo[1,2-a:2',1'-c]pyrazines form ditrifluoroacetyl derivatives as the major reaction products with excess trifluoroacetic anhydride even at room temperature. On the other hand, much more complicated behavior is found for 3,4-dihydropyrrolo[1,2-a]pyrazines, for which the direction of the reaction depends on the structure of the starting compounds and substrate/reagent ratio [7].The α-position of the pyrrole ring in 1-methylpyrrolo[1,2-a]pyrazine (1) is unoccupied and we might expect that the most likely product in its reaction with trifluoroacetic anhydride would be 6-trifluoroacetylpyrrolo[1,2-a]pyrazine by analogy to acylation by acetic anhydride [1]. However, the reaction of 1 with a 2.5-fold excess of trifluoroacetic anhydride in benzene (method A) yielded three products.…”

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Trifluoroacetylation of pyrrolo[1,2-a]pyrazines

Теренин

Kabanova

Tselishcheva

et al. 2007

Chem Heterocycl Comp

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A study was carried out on the reaction of pyrrolo [1,2-a]pyrazines containing an alkyl, aryl, or aralkyl substituent at C-1 with trifluoroacetic anhydride. Trifluoroacetylation products may be formed either by reaction in the pyrrole ring or at the aryl or aralkyl groups at C-1. Products of electrophilic substitution at C-6 are formed in the trifluoroacetylation of pyrrolo [1,2-a]pyrazines containing at C-1 a substituent bulkier than a methyl group but lacking substituents at C-6 (the α-position of the pyrrole ring).The pyrrolo[1,2-a]pyrazine bicyclic aromatic system has not been studied extensively, perhaps due to the difficulties in its preparation and its relatively low reactivity. The relative stability of the pyrrolo[1,2-a]pyrazine system to the action of weak electrophilic agents is indicated, for example, by the formation of the 6-acetyl derivative in only 16% yield upon heating 1,7-dimethylpyrrolo[1,2-a]pyrazine with excess acetic anhydride at reflux for 24 h [1]. The bromination of unsubstituted pyrrolo[1,2-a]pyrazine leads to a 1:1 mixture of 8-bromo and 6,8-dibromo derivatives [2, 3], while formylation of this model leads to 8-formylpyrrolo[1,2-a]pyrazine in 60% yield as the authors [2] affirm, although the 1 H NMR spectrum given for this compound leaves doubt concerning its structural identification. Paudler and Dunham [3] were unable to formylate unsubstituted pyrrolo[1,2-a]pyrazine by the Vilsmeier-Haack procedure. Attempts to obtain nitroso or azo derivatives of pyrrolo[1,2-a]pyrazines in reactions with nitrous acid and phenyldiazonium chloride also proved unsuccessful [4].Hence, we undertook a systematic study of the behavior of pyrrolo[1,2-a]pyrazines 1-10 in reactions with electrophiles such as acylation with trifluoroacetic anhydride, which is a stronger acylating agent than acetic anhydride. The trifluoroacetyl cation is a rather strong electrophile. Thus, the acylation of pyrroles by trifluoroacetic anhydride proceeds rapidly at 0°C [5]. In previous work [6], we showed that dipyrrolo[1,2-a:2',1'-c]pyrazines form ditrifluoroacetyl derivatives as the major reaction products with excess trifluoroacetic anhydride even at room temperature. On the other hand, much more complicated behavior is found for 3,4-dihydropyrrolo[1,2-a]pyrazines, for which the direction of the reaction depends on the structure of the starting compounds and substrate/reagent ratio [7].The α-position of the pyrrole ring in 1-methylpyrrolo[1,2-a]pyrazine (1) is unoccupied and we might expect that the most likely product in its reaction with trifluoroacetic anhydride would be 6-trifluoroacetylpyrrolo[1,2-a]pyrazine by analogy to acylation by acetic anhydride [1]. However, the reaction of 1 with a 2.5-fold excess of trifluoroacetic anhydride in benzene (method A) yielded three products. NMR spectroscopy and mass spectrometry indicated the formation of 1-(3,3,3-trifluoro-2-oxopropylidene)-__________________________________________________________________________________________ M. V. Lomonosov

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confidence: 99%

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Trifluoroacetylation of pyrrolo[1,2-a]pyrazines

Теренин

Kabanova

Tselishcheva

et al. 2007

Chem Heterocycl Comp

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