Two new alkaloids designated as pteropodine and isopteropodine have been isolated from Uncaria pteropoda and characterised. They are stereoisomeric with mitraphylline and uncarine-A and -B and have been shown to have the same structure as these alkaloids on the basis of structural and degradative evidence.

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V. spectra in a range of solvents, i.r. and l H n.m.r. spectra, and ionization constants in water have been determined for the title compounds (I) to (IV) and their 0and N-methylated derivatives (V) to (VIII). Replacement of the annelated five-membered ring by a six-membered one (a) reduces the ratio [pyridol]/[pyridone] (KT) by a factor of ca. 15-30; ( b ) raises the base strength of the 2-methoxypyridine; (c) scarcely changes the base strength of the 2-pyridone; (d) lowers the acid strength of the 2-pyridone. The greater part of the ring-size effect on the tautomeric equilibrium is attributable to a Mills-Nixon-Brown type of ring strain, which is present in the five-membered ring in (11) but absent from the six-membered one in (IV) ; this strain is relieved by conversion into the pyridol (I), and by ionization; in part the ring-size effect is due to electron-withdrawal by the (strained) five-membered ring. In cation formation by the 2-pyridone (11) this electron-withdrawal and relief of ring strain oppose and almost cancel one another. 2-Hydroxy-6-methoxypyridine is intermediate between [(I), (11)] and [( 111), (IV)] in both tautomeric equilibrium constants and ionization constants, but when allowance is made for the effect that a 4-and 5-alkyl group is expected to have in the first-named compound it is found that the properties of the pyrano-compound are quite similar to those of the ' open-chain ' counterpart, whereas those of the furocompound are normally different. In the solid state all three above compounds exist in the pyridone form. Differences in zwitterionic character between the furopyridones and the pyranopyridones are shown not to be significant.In an Appendix attention is drawn to the relatively large errors in KT values derived by some common pK -KT relations.

U.V. spectra, in a range of solvents, i.r. spectra, and ionization constants in water have been determined for the title compounds (I) to (VI), their 0and N-methylated derivatives (VII) to (XII), counterparts that correspond to 0and S-ring fission products (XIII) to (XVI), and their methylated derivatives (XVII) to (XIX). All the the title compounds exist to an appreciable extent in the pyridol form in solution in dioxan. By a comparison of the cyclic sulphide with, on the one hand the cyclic sulphone, and on the other the cyclic ether, the effects of electron withdrawal and of ring strain have been assessed almost independently of one another. Replacement of the sulphide ring by the (smaller) ether ring (a) increases the ratio [pyridol]/[pyridone] (KT) in dioxan by a factor of 2.7, (b) lowers the base strength of the 2-methoxypyridine, (c) lowers the base strength of the 2-pyridone, but by a much smaller amount, ( d ) raises the acid strength of the 2-pyridone. Replacement of the sulphide by the sulphone ring (a) produces a 2.8-fold increase in KT in dioxan, and (6). (c), and ( d ) reduces the base strengths and raises the acid strength, all by much greater amounts than in the above comparison. Analysis shows that no more than a third of the difference in KT between the cyclic ether and the cyclic sulphide is attributable to greater electron withdrawal (inherent plus ring-strain-induced) by the oxygen ring; about two thirds of it is due to a Mills-Nixon-Brown type of ring strain in the pyridone (11). which is relieved by conversion into the pyridol (I) or by ionization. The effects of ' ring closure ' of the di-(methyl ether) to the cyclic ether, e.g. of [(XIII), (XIV)] to the [(I), (11)], show clear evidence of ring strain in the latter, whereas the effects of ' ring closure ' of the dimethylthio-compounds to the cyclic sulphides indicate only a small amount of ring strain in the last-named All the 2-hydroxypyridines listed are in the pyridone form in the solid state.IN the preceding study of 2,3-dihydro-6-hydroxy-4-mechanisms : (a) a Mills-Nixon-Brown type of ring methylfuro[2,3-b]pyridine it was found that the five-strain,2 referred to as a ' direct ring-strain effect,' (b) by membered ring annelated at the [5,6] position was of an electron-withdrawing effect which is an indirect major importance in favouring the pyridol over the pyridone tautomer, and that this occurs by two distinct Spinner and G.Yeoh, preceding paper.2 Refs. 17-20 of preceding paper.

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