SummaryHeterocyclic compounds obtained by addition of acetylenedicarboxylic acid esters to thioureas, cyclic amidines and o-difunctionalized aromatic systems have been studied by 13C-NMR. In particular, C, H-spin-coupling constants over two and three bonds were used to differentiate between the various constitutional isomers and to establish the configuration of trisubstituted exocyclic C, C-double bonds. The configurational significance and diagnostic value of vicinal cis and trans C, H-spin coupling is again demonstrated in the present series.

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Isorhyncophylline, the isomer into which rhyncophylline is convertible, has been f o~~n d to occur in nature. Both bases are interconvertible. Isorhyncophylline on hydrolysis with dilute hydrochloric acid is converted to an aldehyde reducible to isorhy~~cophyllol. When the aldehyde is reduced in the Wolff-Icishner reaction, it is also isomerized and the product is isorhyncophyllane. This reduction product is oxidized by mercuric acetate to a neutral dilactam w h i~h still contains the oxindole carbonyl and further contains a new lactam carbonyl present in a six-membered ring. Reduction of the dilactam with lithium aluminum hydride gave a product having the spectroscopic properties of a n indole. This confirms the assumption previously made that in rhyncophylline, ring C is five-membered. The isomerization of rhyncophylline, mitraphylline, and formosanine is described. Formosanine has been shown t o be identical with uncarine-B and thus uncarine-A is the iso base derivable from formosanine.

1 further study of the structure of riddellic acid has led to rejection of the previously proposed formula IIIa in favor of formula IVa. The evidence for formula IYa is based on the ultraviolet and n m.r. spectra of dimethyl riddellate and the dimethyl ester of a-methylene-a'-ethylideneglutaric acid, the lead tetraacetate oxidation product of riddellic acid. Chemical confirmation for the terminal methylene group is provided by oxidation of a-methylene-a'-ethylideneglutaric acid with osmium tetroxide and sodium metaperiodate and isolation of formaldehyde as its dimedone derivative. The comparison of the n.m.r. spectra of a variety of analogous natural acids and esters with trisubstituted cr,B-olefinic acids and esters revealed that from the position of the olefinic proton peak, the cis or trans configuration of the trisubstituted olefinic acid can be estabished.The structure of riddellic acid, the alkaline hydrolysis product of the alkaloid riddelline, was the subject of a previous paper from this Laboratory.' Convincing evidence was presented for the presence of two double bonds, an a-hydroxy acid and a glycol system in riddellic acid. The close similarity of the infrared and ultraviolet spectra of riddellic acid, seneciphyllic acid and of their retronecine and dimethyl esters indicated that the double bonds in these two acids are similarly situated. The structure of seneciphyllic acid previously had been proposedl as I ; the alternate structure IT for seneciphyllic acid was considered a t that time but structure I was preferred since no formaldehyde was obtained on ozonolysis, Ruhn-Roth determination gave a value 2.2 for C-methyl groups, and the infrared spectrum failed to show the characteristic absorption for a terminal methylene group. By analogy, then, riddellic acid was represented as I11 H3C CHI H:C CHI

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