In general, if the atoms that make up the ring contain heteroatoms, such rings become heterocycles, and organic compounds containing heterocycles are called heterocyclic compounds. An article called Synthesis of 1,8-naphthyridine homologs and their hydrogenation, published in 1941, which mentions a compound: 1569-17-1, Name is 4-Methyl-1,8-naphthyridine, Molecular C9H8N2, Synthetic Route of C9H8N2.
In earlier work (CA 33:2525.5) it was found that Me 1,4-dihydroxy-2,5-naphthyridine-3-carboxylate (C. A. numbering, 5,8-dihydroxy-1,6-naphthyridine-7-carboxylate) and the 1-Cl compound on catalytic hydrogenation take up H only on the nonsubstituted pyridine ring. In continuation of this work, 2,4-dimethyl- (I) and 4-methyl-1,8-naphthyridine (II) have been synthesized and a similar phenomenon on hydrogenation has been observed. In the meantime some other 1,8-naphthyridines described in this paper have been prepared by analogous methods by Mangini (preceding abstract). 7-Amino derivative of I (0.5 g. from 2 g. 2,6-diaminopyridine, 2 g. CH2Ac2 and 1 g. fused ZnCl2 heated 3 hrs. at 120-30°), m. 220° (Ac derivative, pale yellow, m. 300°), converted by diazotization in 40% H2SO4 into the 7-HO compound, m. 251°, which, heated 30 min. in a sealed tube at 140° with POCl3, gives the 7-Cl compound, m. 146-7°; this, boiled 30 min. with 20% MeONa in MeOH, gives the 7-MeO compound, m. 65° (picrate, m. 188-9°). Hydrogenation of 1 g. of the HO compound in 20 g. alc. with 1 g. Ni-kieselguhr under 110 atm. of H for 10 hrs. at 170-80° gave, along with 0.6 g. unchanged material, 0.2 g. of a dihydro derivative, C10H12N2O, m. 175-80°. The Cl compound (0.5 g.), shaken in 10% KOH-MeOH with 0.2 g. of 20% Pd-charcoal and H until about 1.2 mols. H had been absorbed, and the product chromatographed in benzene through Al2O3, yielded about 0.05 g. I, m. 85-6° (HCl salt, decomposes 240°; picrate, decomposes 204-6°; methiodide, yellow needles with 1 H2O, m. 93-4; chloroplatinate, I.H2PtCl6, decomposes 242-4°; chloroaurate, decomposes 166-7°). When 0.1 g. of the Cl compound in 10 cc. of 10% KOH-MeOH was hydrogenated to saturation with 0.5 g. of 20% Pd-charcoal it yielded the tetrahydro derivative (III) of I described below. With 1.2 g. of the Cl compound in 20 cc. of 5% KOH-MeOH, 0.5 g. PdO-CaCO3 and a trace of Pd-charcoal, the hydrogenation stopped in 30 min. (about 170 cc. H absorbed) and 0.8 g. I was obtained. Shaken in 10 cc. AcOH with 0.1 g. Pt oxide and H to saturation, 0.5 g. I absorbed about 160 cc. H and yielded 0.5 g. of a tetrahydro derivative (III), m. 118°, giving a pos. Liebermann reaction (picrate, m. 207°; Ac derivative, m. 42-3°); III was also obtained in 0.85-g. yield from 1 g. I in 50 cc. cyclohexane and 5 cc. alc. with 1 g. Raney Ni heated under an initial H pressure of 70 atm. 2 hrs. at 120° and 2 hrs. at 190°. III was unchanged by 4 hrs. treatment in AcOH with Pt oxide and 110 atm. H pressure, at room temperature With Na in boiling alc., however, it yielded the decahydro derivative of I, easily subliming needles, m. 92-3° (di-Ac derivative, thick oil, b0.02 135-45°). 2,7-Dichloro-4-methyl-1,8-naphthyridine in 10% KOH-MeOH hydrogenated with PdO-CaCO3 and a trace of Pd-charcoal gave, together with a mono-Cl compound, C9H7ClN2, m. 104°, chiefly (about 70%) II, b0.05 147-8° (picrate, decomposes 204-5°; perchlorate, m. 180-1°). II (1 g.) in 10 cc. AcOH with 0.5 g. Pt oxide and H yielded a mixture of 2 isomeric tetrahydro derivatives, separated by fractional crystallization from petr. ether: 0.2 g. of a more soluble isomer A (IV), m. 62-3°, giving a pos. Liebermann reaction (Bz derivative, m. 86-7°), and about 0.8 g. of a less soluble isomer B (V), m. 102-3° (picrate, decomposes 248°; Bz derivative, m. 105-6°; nitro derivative, m. 217-18° and giving a pos. Liebermann reaction, prepared by treating the tetrahydride in cold H2SO4 (dry ice-acetone) with fuming HNO3 (d.1.6), pouring on ice, crystallizing from alc., heating the crystals (m. 124-5°) in concentrated H2SO4 at 60°, again pouring on ice, filtering, making alk. with Na2HPO4 and extracting with ether). V is unchanged by hydrogenation in AcOH with PtO and 65 atm. H pressure. With Na in boiling AmOH, both isomers yield the same (racemic) decahydro derivative of II, b0.1 70-80°, m. 87°, gives a pos. Liebermann reaction (picrate, decomposes 210°). The structures of III, IV and V have not been definitely established but the following considerations make it highly probable what they are. The work of earlier investigators on the hydrogenation of quinoline homologs with Ni and H under pressure and with Sn and HCl has shown that Me groups have a disturbing influence on the hydrogenation of the ring half on which they are substituted whereas Na and alc. readily hydrogenate the Me-substituted rings. This disturbing effect of Me groups is ascribed to the inductive effect of the Me group. III is considered to be the 5,6,7,8-tetrahydro compound To further confirm this, III was heated in a little alc. with an excess of ClCH2COMe for 4 hrs. at 100°; the resulting addition product, C15H21ClN22O2, m. 181-2°, allowed to stand 1 day in a little water with 2 drops of 10% Na3CO3, gave, in addition to unchanged III, a resin whose blue Ehrlich reaction pointed to the presence of an indolizine ring. Such a ring can be formed only from a nonhydrogenated 2-methylpyridine. IV is considered to be the 1,2,3,4- and V the 5,6,7,8-tetrahydro compound because the latter is formed in the larger amount; its higher m. p. is also in harmony with such an assumption.
This literature about this compound(1569-17-1)Synthetic Route of C9H8N2has given us a lot of inspiration, and I hope that the research on this compound(4-Methyl-1,8-naphthyridine) can be further advanced. Maybe we can get more compounds in a similar way.
Reference:
1,8-Naphthyridine – Wikipedia,
1,8-Naphthyridine | C8H6N2 – PubChem