Alkaloids pyridine ring

Non-celastraceous sesquiterpene pyridine alkaloids are those compounds which have been isolated from other plants not belonging to the Celestraceae family. Rotundine, for example, has been isolated from Cyperus rotun-dus (Cypraceae). This is a structurally interesting alkaloid because it has a sesquiterpene skeleton containing a cyclopentane ring attached to the pyridine ring . 

Nakai 124-126). UV and IR spectra of 78 and 81-83 are characteristic of lupinine alkaloids of the cytisine series containing an a-pyridine ring. MS fragmentation patterns are similar to those of cytisine alkaloids. The structures of these alkaloids were confirmed by synthesis from cytisine by reaction with HCOOH (81), (CHjCO) (78), C2H5Br (82), or CH2=CH—COCH3 (83). 

Whilst many alkaloids contain the pyridine ring system, the combination of two pyridine rings as exemplified in the naphthyridines is rather uncommon in nature. The alkaloid jasminine (436), a 2,7-naphthyridine derivative, has been isolated from Jasminium species (68AJC1321) whilst 4-methyl-2,6-naphthyridine (437) occurs in the aerial parts of Antirrhinum majus and aronticus (71P2849). 

Root bark of P. callicarpoides contains, beside harman, two main alkaloids in which a 3-carboline group is linked to a pyridine ring, namely, pauridianthine (7) and its pentacyclic isomer pauridianthinine (8) (75). These two compounds are absent in P. Lyallii, in the root and stem bark of which occurs a reduced analog, pauridianthinol (9) (78). The chemotaxonomic relationship between P. callicarpoides, a plant found in the Congo, and P. Lyallii, growing exclusively in Madagascar, is thus established. 

Piperidine-type metacyclophane alkaloids have four chiral carbon atoms C-3, C-5, C-9, and C-ll. With 96 in hand, Fujita et al. hydrogenated its pyridine ring over Adams catalyst and Raney nickel in order to define the relative stereochemistry of C-5 and C-9. They obtained a single crystalline hexahydro derivative in quantitative yield. Catalytic hydrogenation of substituted pyridines generally results in cis products. Therefore, one can assume the cis relationship of C-5 and C-9 hydrogens in the foregoing... 

Ursuline (162) appears to have been discovered simultaneously in two different laboratories as a constituent of the stem bark of two Oxandra xylopioides accessions. The same plant material from Colombia that gave macondine (161) yielded a small amount of ursuline, which was separated from the former alkaloid as its 0-acetyl derivative (92). The two acetyl esters were found to be isomeric, but 0-methylursuline was shown to differ from 0-methylmacondine. Beside the usual methylated pyridine ring signature and the acetyl resonance, a methoxyl signal at 4.09 ppm (cf. 4.14 ppm in 0-acetylmacondine) and an ortho-... 

Quinoline forms part of quinine (structure at the head of this chapter) and isoquinoline forms the central skeleton of the isoquinoline alkaloids, which we will discuss at some length in Chapter 51. In this chapter we need not say much about quinoline because it behaves rather as you would expect—its chemistry is a mixture of that of benzene and pyridine. Electrophilic substitution favours the benzene ring and nucleophilic substitution favours the pyridine ring. So nitration of quinoline gives two products—the 5-nitroquinolines and the 8-nitroquinolines—in about equal quantities (though you will realize that the reaction really occurs on protonated quinoline. 

Like the pyrrolidine ring, the piperidine and pyridine rings are found in a number of alkaloids, including nicotine, strychnine, cocaine, and reserpine (see p. 1004). 

Nicotiana Alkaloids.—Experiments with C02 have also been used in consideration of the origin of the pyridine ring of nicotine (27) and correlated with similar work on the pyrrolidine ring of this alkaloid. When Nicotiana glutinosa... 

This section covers alkaloids which have piperidine or pyridine rings as the distinguishing skeletal features, including those which are related to mono- or sesqui-terpenes. 

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