| Summary: | The natural tetrahydroisoquinoline phylline (4) was synthesized in a specifically [1,1′-13C2]-labeled form, and fed to callus cultures of Triphyophyllum peltatum. Its incorporation into naphthylisoquinoline alkaloids, among them habropetaline A (8) (as evidenced by1H,13C NMR, and 2D INADEQUATE experiments), proved 4 to be the authentic coupling substrate for the enzyme-mediated phenol-oxidative coupling with a naphthalene portion. During the feeding experiments, a new alkaloid was discovered, 5′-O-methyldioncophylline D (5). It is the first 7,8′-linked (d-type) naphthylisoquinoline isolated from a Dioncophyllaceae plant. The new alkaloid consists of two closely eluting, slowly interconverting atropo-diastereomers, (P)-5 and (M)-5. Their full absolute stereostructures were assigned, i.e., by spectroscopic and online HPLC-CD investigations.
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