Systems, parent

Charge density values calculated by Castle for all the pyridopyridazine parent systems are tabulated in (73HC(27)968 p. 989). 

Irradiation of the 3-pyridyltetrazolium salt (367) similarly gives a mixture of 5,6-tetrazolo fused derivatives of these systems (368), which can be converted to the parent systems by various reducing agents (75TL569,56CB563). Use of the 4-pyridyl isomer gave the pyrido[4,3-cjcinnoline anologues. 

C NMR data have been used to determine the substitution pattern in complex isoxazoles by comparison with simply substituted molecules (75JCS(P1)2115, 760MR226, 80JOM(195)275). Additional data on the parent system and derivatives are available 77H(7)20l). 

Another system in which the factors constrolling the direction of reagent approach has been studied systematically is the bicyclo[2.2.1]heptane ring system. The stereochemistry of a number of reactions of the parent system and the 7,7-dimethyl derivative have been examined.Some of the results are given in Table 3.13. These reactions reveal... 

In 1955 Boyer d al challenged this formulation, and suggested a static, mesomeric system rather than a dynamic, tautomeric one, with Contributing structures of type 9 and 10 to a symmetrical resonance hybrid, proposing the name -o-dinitrosobenzene for the parent System. This notion, however, raised more problems than it solved,... 

As was pointed out earlier (76AHCS1, p. 217), tautomeric equilibria for substituted isoindole-isoindolenine systems depend critically upon the substituents. Isoindole exists in the o-quinoid form 6. Computational results for the parent systems are given in Table III (99UP1). The results indicate that within the B3LYP functional only large basis sets provide reliable energy differences. 

Hydroxythiophenes 56 have two possible keto tautomers (57 and 58) [for review see (86HC1)]. As has been pointed out earlier (76AHCS1, p. 229), the tautomerism of 5-substituted eompounds was extensively studied by Lawesson and eoworkers (63T1867) and by Hornfeldt (63MI1 68MI1). For 5-alkyl eompounds, only the keto forms were present, whereas with R = phenyl, thienyl and ethoxyearbonyl substantial amounts of the enol forms were deteeted. Computations for the parent system (R = H) showed that the 4 -thiobutenolide of type 57 is most stable (Table VII). 

A fairly large number of l,2,4-triazolo[l,5-c]pyrimidine derivatives were prepared via transformation of substituents attached to the skeleton of the parent system. The reported examples of such substituent transformation may collectively be categorized into the following types. 

Stable than the parent system 4. Hereinafter, for practical reasons, all five formal dihydroisomers 1-5 will be referred to as indolocarbazoles. 

Shortly thereafter, the first synthesis of the parent system 1 was reported, wherein Fischer indolization involving the carbazole derivative 12 furnished the desired product in 51% yield (Scheme 2). This achievement was correlated neatly with an alternative synthesis of 1 in 45% yield via the reaction of 1,2-cyclohexanedione mraiophenyUiydrazone and rtienylhydrazine in refluxing acetic acid, followed by treatment of the mixture with hot concentrated hydrochloric acid (57CIL363). 

During studies on ditryptophan derivatives, an interesting acid-induced cy-cHzation has been discovered. The 10-membered ring 37 was thus subjected to acidic conditions to produce the indolocarbazole derivative 38 (Scheme 6). Interestingly, calculations performed on the precursor 37 indicated that the lowest energy conformation resembled that of the diastereomer of 38, which was never observed. An additional experiment furnished the parent system 1 on treatment of 38 with a catalytic amount of acid. A TFA-induced formation of an indolo[2,3-<3]carbazole was also observed from a related acyclic 2,2 -connected tryptophan dimer (99JOC8537). 

Indolocarbazole glycosides have attracted considerable interest over the years, as most indolo[2,3-fl]pyrrolo[3,4-c]carbazole alkaloids possess this structural feature. These systems are beyond the scope of this review however, there are several studies where glycosides of the parent system 1 have been investigated, in some... 

In a recent communication, the parent system 2 has been obtained in poor yields (16% and 10%, respectively) from the double cyclization of N-diphenyl-1,3-phenylenediamine either by using two equivalents of palladium acetate in refluxing acetic acid or by irradiation in methanol in the presence of a catalytic amount of iodine (00SC3651). All the available approaches sununarized so far were marred by harsh reactioi conditions or troublesome-to-prepare starting materials, leading to low overall yields of the desired products and difficulty in introducing sensitive substituents. 

The parent system 3 was reported fw the first time as a product obtained in low yield originating from the vapor-phase cyclodehydrogenaticMi of /V,/V -diphenyl-m-phenylenediamine (137) over a platinum-on-magnesium catalyst (Scheme 17) (61JOC1509). 

The application of double Fischer indolization has been demonstrated to provide larger amounts of the indolocarbazole 4 from the treatment of the hisphenylhydra-zone 151 with aqueous sulfuric acid (63JCS2504). A considerably more efficient protocol has been devised by Robinson, who obtained 4 in 35% yield on treatment of 1,4-cyclohexanedione bisphenylhydrazone (151) in a mixture of acetic acid and sulfuric acid (63JCS3097). In spite of many recent developments, this is still the method of choice for the preparation of the parent system 4 as the starting materials are cheap and readily available. Cyclization of 151 in ethanol-water containing... 

A preparation of the parent system 4 has been claimed starting from the precursor 133. Thus, when treated with triethyl orthoformate under acidic conditions (H2SO4), 133 underwent transformatirai into 4 in 80% yield. This experiment did not lead to the formation of the isomeric system 167. This outcome was rationalized in terms of the initial formation of an indoleninium ion, followed by rearrangement... 

The parent system 4 has also been jnepared employing a double reductive ring closure of a dinitrodiphenylbenzene derivative (Scheme 25). The required starting material, 195, was prepared using a palladium-catalyzed coupling of 1,4-dibromo-2,5-dinittobenzene with phenylboronic acid and after reaction with tri-ethylphosphite in hot tert-butylbenzene, 4 could be isolated in a moderate yield (92JHC1237). 

According to electronic structure calculations in parent systems, we observe a increase of U when the platinium concentration increases. Nevertheless this evolution seems to be small. 

Unsubstituted poly(/ -phenylene) PPP 1 as a parent system of a whole class of polymers is an insoluble and intractable material, available by a variety of synthetic methods [3, 4]. The lack of solubility and fusibility hinders both unequivocal characterization and the processing of PPP 1. Moreover, the intractability of unsubstituted PPP materials has thwarted any serious commercial development of the polymer. 

Oxabicyclo[4.1.0]hept-3-enes with a bromo substituent in position 2 can be converted to oxepins 11 by reaction with an appropriate base such as potassium ter+butoxide or triethylamine (see the experimental procedures for the preparation of the parent system in Houben-Weyl, Vol. 6/ld, pi78 and Vol. 6/4, p462).12,156,157 Usually the reaction products are mixtures of oxepin 11 and benzene oxide 12. In the case of ZerZ-butyl 7-oxabicyclo[4,1.0]hept-3-ene-2-carboxylate, the equilibrium lies completely on the benzene oxide side 12a.158... 

Bromo-6-ethoxy-7//-dibenz[/>,d]azepin-7-one(28, R1 = OEt Rz = Br). prepared by oxidation of the 5,6-dihydro derivative 27 with manganese(IV) oxide, is one of the few examples of unsaturated dibenz[/>,d]azepines to be reported.4 Earlier reports concerning the preparation of the parent system 28 (R = R2 = H) are incorrect the product is in fact a dimer.115... 

Unlike the parent system, 5-methyl-5//-dibenz[c,e]azepine (1, R1 = Me R2 = H) on treatment with lithium diisopropyl amide fails to yield the tautomeric phenanthridine-imine (see Section 3.2.1.5.4.2.), but forms the 5-carbanion, which on quenching with deuterium oxide furnishes 5-methyl-[5-2H,]-5//-dibenz[e,e]azepine (l).83 5,7-Diphenyl-5//-dibenz[r,e]azepine (1. R1 = R2 = Ph) behaves similarly. In contrast, however, 5,7-dimethyl-5//-dibcnz[c,e]azepine (1, R1 = R2 = Me) yields theazaallyl anion 3, which on addition of deuterium oxide deuterates regiospecifically at the 7-methyl group to give derivative 4. 

In contrast to most other azacyclooctatetraenes, for azocine a synthesis of the parent system 1 is known. Substituted derivatives are much more stable and, therefore, have been preferentially investigated. 2-Alkoxyazocine systems have attracted particular interest due to the fact that they are available by an easy synthesis and are rather stable compounds.6... 

The first and to date only synthesis of the parent system 2 uses a flash-vacuum pyrolysis (FVP) of 7,8-diazapentacyclo[4.2.2.02-5.03 9.04 10]dec-7-ene (diazabasketene, 1). After condensation at -196 °C, the pyrolysis product is distilled in vacuum to give pure azocine in ca. 60% yield.12... 

To date, the chemistry of 1,3-diazocines is poorly understood. Neither the parent system 1, nor simple benzannulated derivatives, have been described.1... 

Diazocines can formally exist in two bond-isomeric forms. Hitherto, however, it has not been possible to prepare the parent system 1 only benzannulated systems are known. Moreover, many of the systems originally claimed as benzannulated 1,4-diazocines were subsequently identified as the rearranged isoindolobenzimidazoles (see Section 1.4.1.1.1.). 

The benzo-fused derivative 8 of l,4-dimethyTl,4-dihydro-l,4-diazocine can be prepared by gas phase thermolysis of the cyclobutene derivative 7,31 a reaction that can be reversed photo-chemically. In contrast to the parent system, the heterocyclic ring of the benzannulated compound displays no aromatic character. 

To date there is no synthesis of the parent system or of simple benzannulated 1,2,5-triazocine derivatives. It is very likely that the 1,2,5-triazocine system exists preferentially as the valence isomeric triazapentalene betaine.7... 

In an attempt to prepare the parent system via the valence isomerization of the. ryn-benzene-bisepisulfide 2, which in turn was generated from the syw-benzenebisepoxide 3 via diol 4, no valence isomerization to 1,4-dithiocin (1) was detected.3 The bisepisulfide 2 is a thermally rather unstable compound and decomposes in solution even at 20 C, leading to benzene and sulfur as the only isolated products. 

Besides the parent system oxonin (l),5,6 which is thermally unstable and undergoes a valence isomerization to 3a,7a-dihydrobenzofuran (2) above 35 C, some annulated derivatives are known. All reported systems are nonaromatic and exist in nonplanar conformations. 

The parent system oxonin (1) is generated by benzophenone sensitized irradiation with a Hanovia light source3 5 or direct irradiation6 of the monoepoxide of cyclooctatetraene (9-oxabicyclo[6.1.0]nona-2,4,6-triene, 6). 

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