With heterocyclic ring systems

We are familiar with heterocyclic ring systems with various elements, ring sizes, and numbers of hetero atoms. Here only systems with Si-Si bonds are discussed. The number of possible systems is very great in this field with the frequent possibility that different isomers are present. Only a small number of such ring systems have so far been synthesized. 

Antidepressant activity is retained when the two carbon bridge in imipramine is replaced by a larger, more complex, function. Nucleophilic aromatic substitution on chloropyridine 31 by means of p-aminobenzophenone (32) gives the bicyclic intermediate 33. Reduction of the nitro group (34), followed by intramolecular Schiff base formation gives the required heterocyclic ring system 35. Alkylation of the anion from 35 with l-dimethylamino-3-chloropropane leads to tampramine 36 [8]. 

The first synthesis of this heterocyclic ring system, 224, was carried out (86S71) by the amination of aminopyrazole 222 with H2N0S03H fol-... 

While these reports are mainly concerned with an evaluation of the peculiar structure of the iminophosphenium cation, this species also reveals cycloaddition properties. The cation shows a [2-t-l] cycloaddition behaviour towards alkynes [50]. With iminophosphanes and alkylazides the cations react with the formation of four- and five-membered heterocyclic ring systems [51], as shown for example in Scheme 9. 

Phenylthio)nitroalkenes are also excellent intermediates for the synthesis of other heterocyclic ring systems. For example, tetrahydropyran carboxylic acid derivatives are formed by the intramolecular addition of oxygen nucleophile to l-(phenylthio)nitroalkene predominantly as the m-isomer (9.1 1) (see Eq. 4.40). The reaction may proceed via the chair-like transition state with two pseudo-equatorial substituents.50... 

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... 

A fairly large number of protocols have been successfully applied to the synthesis of compounds with heterocyclic rings fused (5 5 5). Synthetic methods of this particular class of heterocycles reported in the literature vary widely. These heterocycles have been classified as shown in Tables 1-7 depending on structural pattern along with the number of heteroatoms present. Thus, the discussion starts with the synthesis of linearly fused (5 5 5) systems with two heteroatoms presented in Table 1, followed by Tables 2-7, respectively. 

Table 2 Fused heterocyclic ring systems (6 5 6) with two ring junction heteroatoms...

When 34d was generated in the presence of enol ethers 190, [4 + 2] cycloadducts 191a-c were isolated [47] (Table 17). The cycloadducts 191b and 191c proved to be pure stereoisomers, with the two heterocyclic ring systems cis connected. 

In order to study heterocyclic steroid analogues, such as the 7,11-dithiaazasteroid analogues, Fravolini developed the synthesis of new heterocyclic ring systems tri- and tetracyclic 2,1-benzothiazines <82JHC1045>. Intermediate 137 was prepared from 1-methyl-4-oxo-lH-2, -bcnzothiazinc-4(3f/)-onc 2,2-dioxide 37 and thioglycolic acid and could be converted into 6-methyl-4-oxo-3,4-dihydro-2//,6//-thiopyrano[3,2-c][2,l]benzothiazine 5,5-dioxide 138 by cyclization with polyphosphoric acid. The reaction of 138 with dimethyl... 

In a separate study, Ohberg and Westman applied the same PS-DMAP in a one-pot microwave-induced base-catalyzed reaction of N-aryl and N-alkyl amino acids (or esters) and thioisocyanates for the library synthesis of thiohydantoins (Scheme 7.115) [136]. Thiohydantoins are of interest due to their ease of preparation and the range of biological properties associated with this heterocyclic ring system. The use of PS-DMAP as the base in this reaction gave slightly lower yields compared to when triethylamine (TEA) was used, but it resulted in a cleaner reaction mixture and an easier purification procedure. Cyclizations of a number of N-substituted... 

Thiadiazolines are less stable compared to 1,2,4-thiadiazoles and this can be attributed to the loss of aromatic character. They are readily cleaved at the N-S bond under fairly mild conditions (H2S in pyridine) in some cases, the product from ring cleavage can recyclize to give new heterocyclic ring systems. The 3-imino-l,2,3-thiadiazoline 24 when reduced with H2S affords the two J-triazine derivatives 25 and 26 (Scheme 3) <1996CHEC-II(4)307>. 

H(53)323> and 1,2,4-triazinoquinolines have been reported. The preparation of a novel heterocyclic ring system, the indazolotriazinopurine 107, by the condensation of 8-hydrazinotheophylline (103) with 5-substituted isatins 104, via the hemiaminal 105 and hydrazone 106 intermediates, has been described <00JHC373>. 

The 1-benzoborepines 82 (R = Cl, Ph), representative of a previously unknown heterocyclic ring system, have been made by sequential reaction of the 1-benzostannepine 81 (R = n-Bu) with BCI3 and then PhBCl2. The 1-benzostibepines 83 (R = Ph, Me, n-Bu) could also be prepared from 82 . 

This is the case for the [Fe N6]2+ derivatives of 40 [50], 41 [51], 42 [52] and is consistent with the behaviour of the pyridyl-quinoline or 6-methyl-bipyridine systems. The incorporation of five-membered heterocyclic ring systems in this way could be readily achieved but does not seem to have been exploited in the generation of the crossover situation. 

There are little data on theoretical chemistry associated with such ring systems. In a series of 5,5-fused heterocycles, the inner salt 1 displays a substantial aromatic character as indicated by the aromaticity index 7(5,5) with a value of 82 <1987T4725>. 

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