Carbocyclic Fused Systems

Various Carbocyclic Fused Systems.—l,3-diphenyl-4//-5,6-dihydrocyclo-penta[c]thiophen-c/5-4,6-dicarboxylic acid has been prepared by the reaction of the appropriate 1,4-diketo-derivative with phosphorus pentasulphide. The reaction of (169) with tolane gave (170). Acylation of 2-methoxy- 

Aurousseau, and F, Nicoine, Chim. Ther., 1979,14, 467. 

5 Thiophens Fused to Five-membered Heteroaromatic Rings 

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It is noteworthy that, based on the sulfoxide-sulfenic acid rearrangement, the readily accessible 1,3-dithiolane systems (316) may be utilized (equation 116) as an efficient entry into the 1,4-dithiane series , including the construction of carbocyclic fused systems . The oxidation of the dithienes 318 to the corresponding sulfoxides (319 and 320) and sulfones is a simple, straightforward process. 

Various Carbocycle-fused Systems.- Azuleno[2,l-fc]thiophens have been prepared from ethyl 2-chloro-3-formylazulene-l-carboxylate and ethyl mercaptoacetate. The reaction of (261) with sulphur in quinoline gave (262). It is claimed that, in the cation of (263), the charge is largely localized on the carbon atom that is flanked by the two thiophen rings. 

Three different fused systems have been reported in the literature thieno[3, 2 4,5]thieno[2,3-c]pyridine (f/r-thiophenes), thieno[2, 3 4,5]thieno[2,3-c]pyridine (tram- thiophenes) and thicno[3,2-g]thicno [3,2-d pyridine derivatives. So far, the parent thienothienopyridines have only been prepared via the corresponding pyrylium salts (see below) all others are benzo or extended carbocyclic derivatives, for example, thienothienoquinolines. 

This chapter covers literature published since 1982 early materials are included here only where needed as a basis for describing further developments or where not previously mentioned in CHEC-1 <84CHEC-l(5)669>. The structural types surveyed here include 1,2,3-triazoles, benzotriazoles, their dihydro derivatives and carbocyclic fused compounds. Compounds with heterocyclic fused rings are not included. The nomenclature system was discussed in CHEC-T <84CHEC-I(5)670>. 

The most widely used method to synthesize 1,2,3-selenadiazoles is the reaction of semicarbazones with selenium dioxide. This reaction has been described in detail <1996CHEC-II(4)743, 1984CHEC(6)333, 2004HOU777>. This method has often been utilized for synthesis of a variety of 1,2,3-selenadiazoles, including carbocyclic and heterocyclic fused systems. 

Both the 1,2- and 2,1-benzisothiazoles are attacked by electrophiles in the carbocyclic ring. Electrophilic substitution in analogous azine fused systems is therefore dependent on the presence of strongly electron donating substituents. 

The most important IOtc carbocyclic system is naphthalene (19) in which two benzene rings are fused together. The fused systems anthracene (20) and phenanthrene (21) obey Hiickel s rule, where = 3, and have 14tc-electrons. All three compounds are typically aromatic and their chemistry is similar to that of benzene, as discussed in Chapter 12. 

Because of their aromatic character, pyrones undergo annulation reactions and the introduction of substituents less readily than do most cyclic a,B-unsaturated carbonyl compounds. Nevertheless, some suitable modifications have been developed that allow these compounds to be used effectively in annulation reactions. Some reviews about synthesis of 2-pyrones with 3,4-fused carbocyclic ring systems are currently available (82S337 86G109 92T9111). 

Table 1. Fused Carbocyclic Ring Systems from Flash-Vacuum Pyrolysis of Vinylcyclopropanes...

Six-Membered Heterocyclic Compounds with a Single Nitrogen Atom in the Ring to which are Fused Two or More Carbocyclic Ring Systems, and Six-Membered Ring Compounds where the Hetero-Atom is Phosphorus, Arsenic, Antimony or Bismuth. Alkaloids Containing a Six-Membered Heterocyclic Ring System... 

Ch. 1 Factors Influencing Chemical Reactivity and Substitution in the Thiophene Systems Ch. 2 Thianaphthene and Other Thiophene Compounds Containing One Carbocyclic Fused Ring Ch. 3 Thioindigo and Related Dyes Ch. 4 Dibenzothiophene and Its Derivatives... 

Ch. 6 Thiophene Systems Containing Three or More Carbocyclic Fused Rings... 

The completely saturated skeleton of carbazole 289 was used as a platform to investigate and develop selective iridium-catalyzed dehydrogenation reactions for the synthesis of net pyrroles bearing fused carbocyclic ring systems 290. Thus, complete reduction of the carbazole 288 skeleton with Pd/C at 120 °C followed by reaction with an iridium pincer catalyst underwent dehydrogenation in smooth order. These molecules were shown to have an activity window of 172-178 °C (14CC5987). 

Voliime 6 reviews the literature published during 1976. The format of the earlier volumes is retained, with individual chapters on three-membered rings four-membered rings five- and six-membered rings and related fused systems medium-and large-ring compounds and bridged carbocycles. 

The following syntheses of five-membered carbocyclic systems involve radical-induced epoxide fragmentation with radical translocation and cyclization. The resulting bicyclic alcohols are formed as a mixture of two epimeric esters with cw-fused rings.[71]... 

Finally in 2005, Hutt and Mander reported their strategy for the synthesis of nominine (Scheme 1.3) [29], The approach relies upon construction of the steroidal ABC carbocyclic ring structure followed by stepwise preparation of the fused aza-ring system. In the key sequence of the synthetic study, enone 50 was oxidized to dienone 51 with DDQ followed by Lewis acid-catalyzed intramolecular conjugate addition of the methylcarbamate to the newly formed dienone to deliver pyrrolidine 52. 

Intramolecular cycloadditions are among the most efficient methods for the synthesis of fused bicyclic ring systems [30]. From this perspective, the hetisine skeleton encompasses two key retro-cycloaddition key elements. (1) a bridging pyrrolidine ring accessible via a [3+2] azomethine dipolar cycloaddition and (2) a [2.2.2] bicyclo-octane accessible via a [4+2] Diels-Alder carbocyclic cycloaddition (Chart 1.4). While intramolecular [4+2] Diels—Alder cycloadditions to form [2.2.2] bicycle-octane systems have extensive precedence [3+2], azomethine dipolar cycloadditions to form highly fused aza systems are rare [31-33]. The staging of these two operations in sequence is critical to a unified synthetic plan. As the proposed [3+2] dipolar cycloaddition is expected to be the more challenging of the two transformations, it should be conducted in an early phase in the forward synthetic direction. As a result, a retrosynthetic analysis would entail initial consideration of the [4+2] cycloaddition to arrive at the optimal retrosynthetic C-C bond disconnections for this transformation. 

Also, the intramolecular [3+2] cycloaddition approach can be used to generate several tricyclic ring systems 66-68 when the azide and the cyanamide functionalities are bonded to a carbocyclic ring <20010L4091> (Scheme 14). The relative stereochemistry of the starting materials is preserved in the products. While the yield of the m-fused 5-5-6 tricyclic ring system 66 is very high, the yield of the trans-fuseA products 67 and 68 is considerably lower as expected based on the unfavorable conformation for the cycloaddition process. The even lower yield for the tosylated and therefore activated derivative 68 was rationalized by its decreased thermal stability. 

Polymers constituted of repeatedly fused or spiro carbocyclic systems... 

Free-radical cyclizations using ethyl radicals generated by EtsB/air system or stannyl radicals systems provide a range of carbocyclic and heterocyclic hydroxylamines (equation 77). Stereoselectivity in these reactions is variable but can be semiquaUtatively predicted by Beckwith-Houk models . Depending on the substitution pattern of the emerging cyclic system, stereoselectivity can be very high, especially in fused polycyclic systems (equation... 

Polycyclic parent hydrides. These are classified as bridged polyalkanes (also known as von Baeyer bridged systems, from the nomenclature system developed to name them), spiro compounds, fused polycyclic systems and assemblies of identical rings. The four systems may be either carbocyclic or heterocyclic. In developing their names, the following principles are used. 

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