Heterocycles 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. 

Although 1,3,2-diazaphospholenium cations are usually prepared from neutral NHPs or 1,3,2-diazaphospholes via Lewis-acid induced substituent abstraction or A-alkylation, respectively (cf. Sect. 3.1.2), the group of Cowley was the first to describe a direct conversion of a-diimines into cationic heterocycles by means of a reaction that can be described as capture of a P(I) cation by diazabutadiene via [4+1] cycloaddition [31] (Scheme 4). The P(I) moiety is either generated by reduction of phosphorus trihalides with tin dichloride in the presence of the diimine [31] or, even more simply, by spontaneous disproportionation of phosphorus triiodide in the presence of the diimine [32], The reaction is of particular value as it provides a straightforward access to annulated heterocyclic ring systems. Thus, the tricyclic structure of 11 is readily assembled by addition of a P(I) moiety to an acenaphthene-diimine [31], and the pyrido-annulated cationic NHP 12 is generated by action of appropriate... 

Pyrimidine annulated heterocycles fused at positions 5 and 6 to uracil were synthesized via a three-step sequence starting from uracil 63 [20]. Firstly, the reaction with 3-bromocyclohexene gave the AT-allyl-vinyl core system 64 in 80% yield. Upon heating 64 in EtOH in the presence of HCl, aza-Claisen rearrangement gave rise to the C-cyclohexenyl uracil 65 in 38% yield. Final bromination ( 66) and dehydrogenation steps ( 67) allowed synthesis of the desired tricyclic fused uracil systems (Scheme 15). 

Table 23 The Lowest Vertical Ionization Potentials (eV) of Benzo Annulated Heterocycles...

The arrangement of the material in this chapter follows closely that for the bicyclic analogues it is ordered first by the mutual relationship of the fused rings, second by the number of heteroatoms and finally by the size of the heterocyclic ring. We define adjacent as in (1) and non-adjacent fused rings as in (2). Rings of type (3) form a class of pen -annulated heterocyclic systems which are of growing importance. 

Huisgen and coworkers have also described the cycloaddition behavior of the munchnones , unstable mesoionic A2-oxazolium 5-oxides with azomethine ylide character.166 Their reactions closely parallel those of the related sydnones. These mesoionic dipoles are readily prepared by cyclodehydration of N-acyl amino acids (216) with reagents such as acetic anhydride. The reaction of munchnones with alkynic dipolarophiles constitutes a pyrrole synthesis of broad scope.158-160 1,3-Dipolar cycloaddition of alkynes to the A2-oxazolium 5-oxide (217), followed by cycloreversion of carbon dioxide from the initially formed adduct (218), gives pyrrole derivative (219 Scheme 51) in good yield. Cycloaddition studies of munchnones with other dipolarophiles have resulted in practical, unique syntheses of numerous functionalized monocyclic and ring-annulated heterocycles.167-169... 

Majumdar and coworkers used a combination of a 3,3-sigmatropic rearrangement followed by an intramolecular [1,6]-Michael addition for the synthesis of py-rimidine-annulated heterocycles as 4-308 from 4-307 (Scheme 4.66) [103]. 

Some examples in which quinazolines are obtained by a C-N bond cleavage in an annulated heterocycle are collected in Table 9. 

II. Perz -Annulated Heterocyclic Naphthalene Derivatives with... 

The specific feature of peri-annulation consists in the fact that the extraction of a heteromonocycle is impossible. Therefore, the minimum structural unit in this case is the tricyclic framework, for instance, (VII-IX). It is therefore obvious that peri-annulated heterocyclic systems possess qualitatively new structural features sufficient to separate these substances into an independent domain distinct from the array of the heteromonocyclic and ortho-fused heterocycles. 

Those yzurz-annulated heterocycles with a closed rc-system that possess a double bond or another tt- or /z-clcclron bridge situated in the pczz-position of the naphthalene ring opposite to the heterocycle should be set apart. Examples include the Idzi-electron /zcrz-fused heterocyclic acenaphthylene derivatives XII and XV, whose 14 p- and rc-clcclrons are situated on the perimeter of the heteroaromatic skeleton forming an aromatic contour (according to Huckel), while two re-electrons occupy an internal orbital. 

Volume 95 of Advances in Heterocycle Chemistry commences with Part I of an update of Peri-Annulated Heterocyclic Systems by Valerii V. Mezheritskii (Rostov-on-Don State University, Russia). This chapter comprises an update of a review published in Volume 51 of our series (1990) by the same author together with V. V. Tkachenko. The present chapter is concerned with naphthalene derivatives with a four-membered peri-annulated heterocyclic ring. 

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