Fused six-membered rings

When two fused six-membered rings (naphthalene analogues) are considered, possibilities become very numerous, partly on account of the reduced symmetry of naphthalene, compared with benzene, and also because of the larger number of positions available for substitution. Thus, there are two monoazanaphthalenes, quinoline (8) and isoquinoline (9), four benzodiazines [cinnoline (10), phthalazine (11), quinazoline (12) and quinoxaline(13)], with the two nitrogen atoms in the same ring, and six naphthyridines (e.g. (14), named and... 

Both of these approaches have been attempted, and both are substantially equivalent for heterocyclic (e.g. quinoline and isoquinoline) and homocyclic (naphthalene) systems. Consequently, in the subsequent discussion it is fruitful to include the available work on naphthalene derivatives. In the case of the fused six-membered rings, Eq. (3) is not applied because it does not permit treatment of the 5- and 8-positions, and the available series as a whole are too short to make this treatment useful. 

Compounds in this section comprise indolizines and their hetero-substituted analogues which are fused through the /-edge to a six-membered heterocycle. Where the fused six-membered ring is pyridine, however, these compounds are named not as fused indolizines but as pyrrolonaphthyridines. 

Many versatile approaches to the construction of fused heterocyclic systems (6 5 6) with ring junction heteroatoms have been reported. More general reactions which can be used for synthesis of derivatives of several tricyclic systems, and transformations which have potential for use in the preparation of a series of substituted compounds, are discussed in this section. Formation of the five-membered ring is presented first because it is a conceptually simple approach. It should be noted, however, that the addition of a fused six-membered ring to a bicyclic component offers much more versatility in the construction of a (6 5 6) system. Each subsection below starts with intramolecular cyclization of an isolated intermediate product. Reactions which follow are one-pot intermolecular cyclizations. 

The forward reaction is extremely easy bevause of aromaticity recovery. These aspects of the intramolecular Diels-Alder reaction are generally very useful and able to provide polycyclic fused six-membered ring compounds which are otherwise difficult to realize. The controlling factors, geometry and mechanism of intramolecular Diels-Alder reactions have been comprehensively reviewed elsewhere i°4,106), and it is not our intention to discuss these in details. However, the synthetic utility of the reaction is demonstrated by the following examples107). 

Pentalenolactone E methyl ester (46), an angularly fused sesquiterpene lactone, was first isolated and characterized by Cane and Rossi [38]. One approach to the synthesis of this material is illustrated in Scheme 5. Key to the successful implementation of the plan is the synthesis of butenolide 49, the electrochemi-cally promoted cyclization of 49 to the tricyclic y-lactone 48, ring opening of the latter to convert the linearly fused system to the angularly fused six-membered ring lactone 47, and functional group elaboration leading to the natural product 46 [36,37]. 

If the ligand has an aromatic portion (normally corresponding at least to three fused six-membered rings), it can position itself between base-pairs, in... 

Relevant synthetic methods for the annulation of a benzo-fused seven-membered ring onto an existing benzo-fused six-membered ring are available with M-, N-, and P-types. 

From the satellites in the ESR spectra, the symmetry of the radicals as well as the information about the localization of the unpaired electron in RGgp can be obtained (Figure 6.2). In BuGgQ, for example, ten pairs of G satellites are clearly resolved (Figure 6.1). A Q-symmetry can be deduced from their relative intensities and splittings [1, 12]. The unpaired electron is mostly located on two fused six-membered rings of the Gjq surface, which rules out an extensive delocalization on the Ggo framework. 

Fused Six-Membered Ring Nitrogen Heterocycles under Fused Six-Membered Ring Nitrogen Heterocycles Glycols... 

Pyrazolo[4,3- ][l,3,4]oxadiazine 78, whose preparation is given in Section 10.13.9.1.2(iv), was condensed with various amines to give pyrazolo[3,4- ][l,2,4]triazines 101 (Scheme 78 Table 9) <2002IJB664>. This ring system was earlier reported by synthesis of the fused six-membered ring, through condensation onto pyrazolediones <1996CHEC-II(7)489>. 

The synthesis of fused 1,2,3-triazoles is nearly always achieved by treating 1,2-diamines attached to diazines with nitrous acid (e.g., [l,2,3]triazolo[4,5-f]pyridazine (Section 10.13.9.2.1(iii)) and [l,2,3]triazolo[4,5-rf pyrimidine (Section 10.13.9.2.1(iv)), <1996CHEC-II(7)489>). When a heterocyclic system containing a diazole fused onto 1,2,4-triazine is required, annulation of the five-membered heterocycle is nearly always the most facile route (e.g., imidazo[4,5-r ][l,2,4]triazine (Section 10.13.9.2.l(i)) and pyrazolo[4,3-r ][l,2,4]triazine (Section 10.13.9.2.1(ii))). In support of the latter, the synthesis of the fused six-membered ring of pyrazolo[3,4-r ][l,2,4]triazine from the pyrazo-ledione was reported as low yielding <1996CHEC-II(7)489>. 

Tricyclic Systems Central Carbocyclic Ring with Fused Six-membered Rings... 

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