Heterocycles imidazo-annulated

Scheme 8 Microwave-assisted synthesis of imidazo-annulated heterocycles...

Microwave and fluorous technologies have been combined in the solution phase parallel synthesis of 3-aminoimidazo[l,2-a]pyridines and -pyrazines [63]. The three-component condensation of a perfluorooctane-sulfonyl (Rfs = CgFiy) substituted benzaldehyde by microwave irradiation in a single-mode instrument at 150 °C for 10 min in CH2CI2 - MeOH in the presence of Sc(OTf)3 gave the imidazo-annulated heterocycles that could be purified by fluorous solid phase extraction (Scheme 9). Subsequent Pd-catalyzed cross-coupling reactions of the fluorous sulfonates with arylboronic acids or thiols gave biaryls or aryl sulfides, respectively, albeit it in relatively low yields. 

Scheme 6.48 Synthesis of imidazo[l, 2-a]annulated N-heterocycles via the Ugi reaction.

Diazadienes have been used in organic synthesis for the preparation of various heterocyclic compounds. Alkylation of 1,3-diazadienes 207 and the benz-fused analog 210 at the nitrogen atom by aryl acyl bromides provided the iV-alkyl amidinium bromides 208 and 211, which underwent annulation to the 2,3-dihydro-imidazo[2,l-A]thiazole 209 and imidazo[2,l- ]benzothiazoles 212, respectively (Equations 92 and 93) <2001S741, 2002J(P1)741>. 

When the synthesis of a heterocyclic system containing a diazole fused onto l,2,3-triazin-4-one (e.g., imidazo[4,5-i [l,2,3]tnazin-4-one (Section 10.13.9.1.l(i)) and pyrazolo[3,4-rf [l,2,3]triazin-4-one (Section 10.13.9.1.1(ii))) or 1,2,3-triazole fused onto pyrimidin-4(7)-(di)one (e.g., [l,2,3]triazolo[4,5-rf pyrimidin-4(7)-(di)one (Section 10.13.9.1.l(iv))) is required, the synthesis of the six-membered ring is the most reliable method. There are several recent accounts of six-membered annulations onto 1,2,3-triazoles to give [l,2,3]triazolo[4,5-r/ pyrimidines (Section 10.13.9.1.l(iv)), which are valuable alternatives to 1,2,3-triazole annulation using nitrosation techniques <1996CHEC-II(7)489>. 

Imidazo[l,2-a][l,3,5]triazines have been prepared by annulation of the imidazole onto an existing triazine as well as by annulation of the triazine onto an imidazole precursor. Depending upon the desired substituent pattern, both routes allow effective preparation of variously substituted heterocycles. Research on this ring system has been minimal until recently, when several papers have focused on the synthesis of nucleoside derivatives, which can be thought of as 5-aza-7-deazapurine nucleosides. 

This review presents purines annulated with fused five-membered rings, as pyrrolo-, oxazolo-, thiazolo-, imidazo-, pyrazolo-, triazolo-, and tetrazolo-purines. Aiming to eliminate some misinterpretations (numbering of the skeleton in some papers did not match those in Chemical abstracts) atoms of the first scheme in each chapter will be numbered. Each group of compounds includes the synthesis, reactions of the heterocyclic system in question, biological effects, and, if accessible, also references about the detailed spectral data, X-ray analysis and so on. Papers were abstracted up to December 2002, inclusive vol. 137 of Chemical Abstracts. 

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