Fused-ring compounds isomerism

Reaction of the 2-trimethoxybenzyl glycosylating agent 149 with IDCP gave the fused-ring compound 150 which with BFs.EtiO isomerized to the trans-fused P-anomer which was taken through to lactone 151, the triacetate of tri-0-methylnorbergenin (Scheme 15) ... 

Another class of compounds is called condensed-ring or fused-ring systems. These structures contain two or more aromatic rings that share a pair of carbon atoms. Examples include naphthalene, anthracene, and phenanthrene, the latter two being isomeric structures. 

Caryophyllenes, as an example of two naturally occurring isomeric sesquiterpenes containing a medium-sized ring, in which the success of the total syntheses lies in the stereoselective control of a chiral centre, in a common synthetic key intermediate, which governs the configuration (JE or Z) of the double bonds present in each one of the two isomers. In this context, a brief reference to Cecropia Juvenile Hormone synthesis by the Syntex group, as well as to Johnson s cationic cyclisation of unsaturated polyolefins to fused polycyclic compounds, is made. 

Fused-ring triazolines derived from 3,3-dimethylcyclopropene dicarboxyl-ate,442 Dewar thiophene (as was shown in Scheme 20)161 and cyclobutadiene undergo thermal isomerization to diazoimino compounds in a retro-1,3-cycloaddition (Scheme 146). 

A new bicyclic betaine, a pyrrolo[l,2-.7 [l,2,4]triazin-2-ium -olate 1347, was prepared by the thermolysis of a ring-expansion reaction product 1346, 2,3-di(/-butyl)-l-hydroxy-2,3-dihydropyrrolo[l,2-.7 [l,2,4]triazin-4(17/)-one, of 1,2-di(/-butyl)-l,2-diaziran-3-one 1345 with 177-pyrrole-2-carbaldehyde (Scheme 256) <1995MI1>. Its cyclization reaction with a dipolarophile such as dimethyl 2-butynedioate leads to a fused ring-enlarged compound, a triazocinone derivative 1348 in rather low yield. Heating a solution of triazocine 1348 in benzene- 6 in an NMR tube at 140 °C for 0.5 h leads to its thermal transformation into compound 1349 in 21% yield via skeletal isomerization <1999T13703>. 

We observe another structural variation for a piperidine alkaloid in the compound lupinine (3.16), which is isolated from seeds of Lupinus and Anabasis plants. Here, nitrogen is found at the junction of two fused rings. There are two chiral carbon atoms, so there will be four possible stereoisomers. Natural lupinine has only one of these isomeric stmctures, with the attached H atoms cis to each other. Nitrogen is not chiral because of the phenomenon of pyramidal inversion. This will be discussed in Chapter 10. 

With 3-alkyl-2-cyclohexenones and related fused ring systems, " the irradiation affords deconjugated compounds only when the reaction is performed in the presence of small amounts of a weak acid Kke acetic acid. This occurs irrespective of the solvent. The course of the process is depicted in Scheme 8. The isomerization is not a concerted process but occurs by a fast protonation of the highly... 

Indole is a heteroaromatic compound consisting of a fused benzene and pyrrole ring, specifically ben2o[ ]pyrrole. The systematic name, IJT-indole (1) distinguishes it from the less stable tautomer 3JT-indole [271-26-1] (2). Iff-Indole [120-72-9] is also more stable than the isomeric ben2o[ ] pyrrole, which is called isoindole, (2H, (3) and IH (4)). A third isomer ben2o[i ]pyrrole is a stable compound called indoli2idine [274-40-8] (5). 

Thiepin, as a seven-membered conjugated system with sulfur as heteroatom, is a member of the 8 7t-electron heteroannulenes which are antiaroinatic according to Hiickel s rule. In contrast to oxepin, thiepin is not stable at room temperature and no valence isomerism with an arene sulfide has been observed. Stable thiepins are obtained only when two bulky substituents, e.g. /ert-butyl, are introduced into positions 2 and 7. In benzothiepins the annellation effect of the aromatic rings contributes decisively to the stability of these compounds stability increases with an increasing number of fused benzene rings. 

Diazo compounds also undergo cycloaddition with fullerenes [for reviews, see (104),(105)]. These reactions are HOMO(dipole)-LUMO(fullerene) controlled. The initial A -pyrazoline 42 can only be isolated from the reaction of diazomethane with [60]fullerene (106) (Scheme 8.12) or higher substituted derivatives of Ceo (107). Loss of N2 from the thermally labile 42 resulted in the formation of the 6,5-open 1,2-methanofullerene (43) (106). On the other hand, photolysis produced a 4 3 mixture of 43 and the 6,6-closed methanofullerene (44) (108). The three isomeric pyrazolines obtained from the reaction of [70]fullerene and diazomethane behaved analogously (109). With all other diazo compounds so far explored, no pyrazoline ring was isolated and instead the methanofullerenes were obtained directly. As a typical example, the reaction of Cgo with ethyl diazoacetate yielded a mixture of two 6,5-open diastereoisomers 45 and 46 as well as the 6,6-closed adduct 47 (110). In contrast to the parent compound 43, the ester-substituted structures 45 and 46, which are formed under kinetic control, could be thermally isomerized into 47. The fomation of multiple CPh2 adducts from the reaction of Ceo and diazodiphenylmethane was also observed (111). The mechanistic pathway that involves the extrusion of N2 from pyrazolino-fused [60]fullerenes has been investigated using theoretical methods (112). 

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