Heterocycles naming

As might be anticipated from the behaviour of the parent heterocycles, C-2 of indole, benzo[i]furan and benzo[i]thiophene (Table 13) is shifted to lower field than C-3. However, the shifts for C-2 (O, 144.8 Se, 128.8 S, 126.1 NH, 124.7 Te, 120.8) and C-7a (O, 155.0 Se, 141.3 S, 139.6 NH, 135.7 Te, 133.0) in the benzo[i] heterocycles vary irregularly (80OMR(l3)3l9), and the sequence is different to that observed for C-2 in the parent heterocycles, namely 0>Se>Te>S>NH. Also noteworthy is the upheld position of C-7, especially in indole and benzofuran, relative to the other benzenoid carbons at positions 4, 5 and 6. A similar situation pertains in the dibenzo heterocycles (Table 14), where not only are C-1 and C-8 shifted upheld in carbazole and dibenzofuran relative to the corresponding carbons in dibenzothiophene and fluorene, but similar, though smaller, shifts can be discerned for C-3 and C-6 in the former compounds. These carbon atoms are of course ortho and para to the heteroatom and the shifts reflect its mesomeric properties. Little variation in the carbon-hydrogen coupling constants is observed for these dibenzo compounds with V(qh) = 158-165 and V(c,h) = 6-8 Hz. 

Pyrano-fused heterocycles, namely pyrano[3,2-f]quinoline-2,5(6//)-diones, pyrano[3,2-f]benzopyran-2,5(6//)-dione, and pyrano[3,2-f]pyridine-2,5(67T)-diones, have been efficiently prepared by the condensation of 4-hydroxy-2-(l//)-quinolines, 4-hydroxycoumarin, or 4-hydroxy-(17/)-pyridone with a-acetyl-y-butyrolactone or the sodium salt of a-formyl-y-butyro-lactone in the presence of ammonium acetate <1999JHC467>. 

This chapter concerns a rather special, relatively rare species of heterocycle, namely those molecules incorporating more than eight peripheral atoms but which are neither heterocyclophanes nor crown ethers. Also excluded from coverage in this review is the large body of naturally occurring substances, or synthetic simulations thereof, incorporating large heterocyclic frames such as macrolides, alkaloids and, possibly, others. 

A six-membered nitrogen-sulfur heterocycle, namely, a 2,3-dihydro-2-imino-4//-benzo[e thiazin-4-one, is produced on reaction of methyl 2-chloro-3,5-dinitrobenzoatc with thiourea in basic medium 373 furthermore, a tetrahydrospirobenzothiazine 213 is formed in addition to its hexahydrospiroquinazoline analog on interaction of cyclohexanone with thiourea in hydrochloric acid.329... 

Another simplified nomenclature adopted here relates to natural spiro-2,3-dihydrobenzofuran derivatives, such as griseofulvin. The fundamental heterocycle, named grisan, being spirobenzofuran-2(3ff)-l -cyclohexane (5), griseofulvin (7-chloro-4,6,2 -trimethoxy-6 -methyl-spirobenzofuran-2(3//)-l -[2]cyclohexene-3,4 -dione) (6) becomes 7-chloro-4,6,2 -trimethoxy-6 -methyl-2 -grisene-3,4 -dione. 

The nomenclature of peri-naphthalene heterocycles does not follow a common principle. In many original papers, the names of heterocyclic systems are derived from the corresponding peri-annelated hydrocarbon derivatives (1,2-diazaacenaphthylene, 1-oxaphenalene, etc.), from monoheterocycles with an indication of linked positions (naphtho[l,8-6c]furan, naphtho[l,8-de]azepine, etc.), and from benzoannelated heterocycles (benzo[o/]indole, benzo[heterocyclic systems and some compounds have trivial names, for instance, perimidine, naph-thostyryl, and naphtholactone. Moreover, it is necessary to remember some peculiarities in the electronic structure of peri-annelated heterocycles, namely the absence of independent existance of the 7r-closed-loop monoheterocycles which could be a fragment of peri-annelated heterocyclic systems. Therefore, the separation of a heterocycle from the united 7r-system is impossible. In this case, the simplest structure and the tt-electron unit is the whole peri-heterocyclic nucleus. 

Highly enantioselective catalytic conjugate addition of A-heterocycles, namely purines, benztriazole, benzimidazole, and 5-phenyltetrazole, to a,ft-unsaturated ketones and imides has been attained with chiral, salen-type (Jacobsen) aluminium complexes as catalysts.142... 

Although the number of dicoordinated phosphorus compounds with a -Pxix=C bond is fairly large, compounds with a-Pxn=N bond are still few in number since only one heterocycle, namely 1,2,4,3-triazaphosphole ( 1 ), and two types of acyclic compounds, namely the diazaphosphapropenes (2) and the tetrazaphosphapentenes ( ), are known ... 

The names aziridine and azetidine are derived from a reasonably logical system of nomenclature, which assigns three-part heterocycle names according to (a) the heteroatom ( az- = nitrogen, ox- = oxygen, thi- = sulfur) (b) the ring size f-ir- = 3, from trl -et- = 4, from tetra ... 

The assumption that the azomethine 34 is formed by an intermolecular hydride transfer from aldehyde 30 to the nitrilium salt 31 is not confirmed because the different Schiff bases 34 obtained carry the group R which are originated from the aldehyde 30. In this way, the process described in equation 14 can be reversed and applied as an enamide synthesis by acylation of imines2,3. However, the 7V-ethylbenzonitrilium salt 35 reacts with benzaldehyde to give the more stable Af-benzoyl-7V-ethyliminium ions 3647,49 which add to trimethylethylene to form 5,6-dihydro-4i/-l,3-oxazinium salt 37. On the other hand, the reaction of ions 36 with phenylacetylene leads to another type of 1,3-oxazinium heterocycle, namely to 4i/-l,3-oxazinium hexachloroantimonate 38 (equation 15). 

The principal mechanisms proposed for the heterogeneous HDS of a prototypical 5-heterocycle, namely benzo[6]thiophene (BT), are illustrated in Scheme 1. The plain hydrogenation of C-C double bonds occurs in step a, involving the regiose-lective reduction of BT to dihydrobenzo[/ ]thiophene (DHBT), as well as in step e, where styrene is reduced to ethylbenzene. 

The second part of Chapter 3 discusses six groups of heterocycles, namely 2,6-disubstituted-... 

By the same token one may find a dependence of Fq factors on AGg for other heterocycles, namely 1,3-dioxane, tetrahydropyran, piperidine, and... 

Analogously, employing 6-aminouracils 101 as the enamino component for cyclocondensation provided a facile entry into another biologically important class of nitrogen heterocycles, namely the pyrido[2,3-i>]pyTimidines 102 as shown in Scheme 1.4.19 (Synthetic sequence, shown for Sasrin resin). 

For the time being, the nomenclature of crowns and other heteromacrocycles will likely remain imperfect due to the inherent complexity of the structures. Nevertheless, standard heterocycle naming practices should be applied but often have not been. For example, crowns containing O, S, and/or N should be numbered starting with O rather than one of the atoms differentiating the compound from a simple crown. The compound should likewise be named so the numbers are minimized and the numbering of heteroatoms is in the order O, S, N, if possible. Structures (3)-(5) illustrate this. 

There are also examples citing snbstructure F in the total synthesis of natural products containing 0-heterocycles, namely in a synthetic approach to (205 )-camptothecin [106] or in the synthesis of the Amaryllidaceae alkaloids ( )-tazettine and ( )-6a-epipretazettine (Scheme 6.39) [107]. 

El Maatougui et al. (2011) described the synthesis of three different libraries of pyridazin-3-ones using silica-supported aluminum trichloride (Si-AlCl3) as a heterogeneous and reusable catalyst. In previous work, the same authors discovered a novel family of potent platelet aggregation inhibitors derived from the pyridazin-3-one scaffold and incorporated diverse a,p-unsaturated residues at position 5 of the heterocycles, namely chemotypes A, B, and C (Figure 1.3) (Coelho et al., 2007). 

Due to a number of advantages offered by microwave heating and irradiation, it was also employed in the synthesis of bioactive heterocycles. Although numerous literature is available on heterocyclic chemistry [52], heterocyclic name reactions [53], catalysed heterocyclic reactions [54], fluorinated heterocyclic compounds [55], heterocyclic carbene complexes [56], etc., green chemical pathways such as microwave synthesis has been found to be the superior one. 

The designation ozonide should be used only if no exact structure of the compound in question is known otherwise a heterocyclic name is to be preferred. 

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