Benzo heterocycles, structure

A summary of bond lengths and angles from X-ray structures of heterocyclic compounds studied up to 1970 has been published <1972PMH(5)1>. This compilation contains many examples, particularly of furans, thiophenes, and pyrroles and their benzo derivatives further examples are contained in the appropriate chapters of the three editions of Comprehensive Heterocyclic Chemistry. The Cambridge Structural Database (CSD) maintains a comprehensive collection of heterocyclic structures. 

In b and c benzo heterocycles, the b heterocycle is much closer to the benzene structure (Figure 33) in particular, a large distortion is obtained for benzo[r]thiophene. 

For a brief discussion of substitution reactions on pyridines, sec Eichcr, T., Hauptmann, S. The Chemistry of Heterocycles Structures, Reactions. Synthesis, and Applications, 2nd ed. Wiley-VCH Weinheim, Germany, 2003 pp 269-310. For a more comprehensive discussion, sec Comins, D. L., Joseph, S. P. Pyridines and their Benzo Derivatives Reactivity at the Ring. In Comprehensive heterocyclic Chemistry H Katrizky, A. R. Rees, C. W. Scriven, E. F. McKillop, A. Eds. Pergamon Oxford, 1996 Vol. 5, p 37. 

Currently available materials with highly positive Ae are mainly based on benzo-nitrile structures (Tables 4 and 5). Introduction of heterocyclic ring systems (Structures 2.5 and2.6)or ester links(Structures2.7 and 3.5) lead to an increase of Ae compared with the corresponding non-heterocyclic and directly connected systems. Unfortunately, the effectivity regarding Ae of the benzoni-trile structures is decreased by a local antiparallel ordering of the dipole moments... 

The major internal comparisons to be made within this chapter are between (13) pyrrole (1), furan (2), thiophene (3), selenophene (4) and tellurophene (5) b) pyrrole (1) and indole (6) (c) indole (6), benzo[6 Jfuran (7) and benzo[6]thiophene (8) d) indole (6), isoindole (9) and indolizine (10) and (e) benzo[6] and benzo[c] fused systems. The names of relevant heterocyclic radicals are given with the structures of the parent heterocycle. 

Benzo[c]phenanthridine alkaloids are widespread in Papaveraceae, Fumariaceae, and Rutaceae. Fagaridine (118), the structure of which had to be revised, is a derivative of the unknown 5-methyl-benzo[c]phenan-thridine-8-olate (119) which is isoconjugate with the 2-methyl-chrysene anion (Scheme 43). Thus, Fagaridine is a member of class 1 of conjugated heterocyclic mesomeric betaines, which are isoconjugate with odd alternant hydrocarbon anions. 

Eight isomeric structures can be drawn for the 1,2,4-triazino-quinolines, which have fusion only on the heterocyclic ring. Fusion could also exist on the benzo ring. Only six isomers were reported during the period of this review. 

Compounds are generally classified according to their fully unsaturated parent compound (but see below). Thus substituted, partially saturated, and fully saturated derivatives of, for example, pyrrole are all indexed under pyrrole. Benzo and similar derivatives are included under the most unsaturated parent system (e.g., quinoline, thienofuran, etc.). For any given heterocyclic parent only one indicated hydrogen isomer appears in the text, typically the most stable or the lowest numbered form thus all instances of pyran, whether of the 2H- or 4H-form, are indexed under 2H-pyran. The charges and additional valences for any heterocyclic parent structure are not indicated. 

The reaction mode depends mainly on the choice of the base and reaction conditions, but route B is usually more common. Thus, tetrahydro-l,4-thiazine derivative 157 the structure of which was proved by X-ray crystal structure analysis (Fig. 7) [22b] was obtained in the reaction of the chloro ester 1-Me with 2-aminoethanethiol (156) using K2CO3 or EtgN as a base in 43 and 20% yield, respectively (Scheme 49) [22b, 26]. In the latter case, the secondary amino group in the primary tetrahydrothiazine product 157 underwent Michael addition to a second molecule of 1-Me to give 2-[r-(spirocyclopropanetetrahydrothiazinyl) cyclopropyl]-2-chloroacetate 158 (14% yield). When KOH in the presence of di-benzo-18-crown-6 was employed, however, the seven-membered heterocycles 155 (42%) and 159 (48%) were obtained upon reaction of 1-Me with 1,3-pro-panedithiol and 2-aminoethanethiol, respectively (Scheme 49) [26]. 

The solid-state structure of one benzo derivative of 1,3-oxathiane 96 has been studied the heterocyclic moiety was found to adopt a half-chair conformation with the aryl substituent in an equatorial position <1998JFA4002>. 

Fluorescence spectroscopy is reported for a benzo[4,5]thieno[2,3- / thiazolium iodide salt <2004SAA129>. A wide variety of structural variants of the nonconjugated heterocycles have reported X-ray crystallographic data however, perhaps surprisingly, only very limited X-ray structural information has been reported for the fully conjugated heterocycles. 

H NMR spectroscopy has provided useful structural information on the unstable benzo[c]-fused heterocycles (Table 6). The protons H-l and H-3, adjacent to the heteroatom, are observed at lower field than are the corresponding ones in the parent heterocycles or their benzo[b] isomers. 

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