5-Membered rings fused ring systems

A seven-membered fused ring system 66 (85-95%) has been built by variation of the amine component. Tliis example deals with the reaction of 33 with 3-(2-aminophenyl)amino-5,5-dimethylcyclohex-2-enone (65)... 

It is clear from the foregoing that ring closure to five-membered rings fused to the isoxazolidine, regardless whether part of a pyrrolizidine or of an indolizidine system, led mainly to the cis-anti isomers (see 243 a, b and 246 a, b)... 

Finally, the phosphinite-oxazole catalyst 29 (Fig. 29.16) was recently reported and used to hydrogenate a series of functionalized and unfunctionalized alkenes [31]. It was anticipated that the planar oxazole unit and the fused ring system would improve the enantioselectivity compared to the PHOX catalyst by increasing rigidity in the six-membered chelating ring [32]. Indeed, these catalysts... 

All polycyclic pigments, with the exception of triphenylmethyl derivatives, comprise anellated aromatic and/or heteroaromatic moieties. In commercial pigments, these may range from systems such as diketopyrrolo-pyrrol derivatives, which feature two five-membered heteroaromatic fused rings (DPP pigments) to such eight-membered ring systems as flavanthrone or pyranthrone. The phthalo-cyanine skeleton with its polycylic metal complex is somewhat unique in this respect. 

Other fused ring systems in which C-protonation in the five-membered ring leads to pyridinium or quinolinium aromatic rings, respectively, are 2-phenyl-2-pyrindine (Anderson and Harrison, 1964) and 3-acyl-l,2-dihydro-4H-j3-quinindines (Kholodov et al., 1969). 

Penicillins are the most widely used of the clinical antibiotics. They contain in their structures an unusual fused ring system in which a four-membered P-lactam ring is fused onto a five-membered thiazolidine. Both rings are heterocyclic, and one of the ring fusion atoms is nitrogen. These heteroatoms do not alter our understanding of molecular shape, since we can consider that they also have an essentially tetrahedral array of bonds or lone pair electrons (see Section 2.6.3). 

In the synthesis of cyclohexene oxide from cyclohexene shown, this does implicate the less favourable diaxial conformer in the epoxide-forming step. Cyclohexene oxide contains a c/s-fused ring system, the only arrangement possible, since the three-membered ring is necessarily planar (see Section 3.5.2). 

In a study of synthetic routes to the phorbol class of compounds, Lautens cyclised the stannane 307 (with 5 equiv MeLi, to avoid stannane reincorporation) to the 7,5-fused ring system 3 0 8.138 This reaction also works in the synthesis of analogous saturated five-membered nitrogen and sulfur heterocycles (see below). 

ANICS (ppm) difference between NIGS value in fused-ring system and NICS value in reference compound (benzene, pyrrole, furan, and thiophene) J]NICS (ppm) sum of NICS values of rings in compound. For NICS values, 5MR indicates five-membered ring while 6MR indicates six-membered ring. 

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