Peter P. Lankhorst

Tacamine, the first example of a new class of indole alkaloids

From the leaves of Tabernaemontana eglandulosa a new alkaloid was isolated, which was assigned structure 1 on the basis of speetroscopic evidence.

Confirmation of the structure of kessane by NMR spectroscopy.

ABSTRACT In order to prove the correct relative and absolute stereochemistry of the sesquiterpene ether (−)-kessane that was recently synthesized by us, its chromatographic retention, mass and 1H-NMR spectrum and optical rotation were compared with that of natural kessane. Natural kessane was isolated by means of fractionated vacuum distillation and repeated medium-pressure liquid chromatography from the essential oil of Valeriana officinalis. The recorded data confirm that the two compounds are identical. Further, by means of one- and two-dimensional 1H- and 13C-NMR spectroscopy at 600 MHz, it was possible to assign all protons and determine all coupling constants in the 1H-NMR spectrum. The NMR data are in full agreement with the relative stereochemistry proposed in the original work on kessane.

Confirmation of the C-1 stereochemistry of ginkgolides by NMR

Abstract Unambiguous evidence was obtained for the assignment of the relative stereochemistry at C-1 in ginkgolide B and C by means of ROESY NMR spectroscopy carried out at 600 MHz, chemical shift data and a conformational analysis. The 1-hydroxyl group was found to possess the α-configuration which is in accordance with earlier X-ray crystallographic and synthetic studies but in conflict with a previous detailed NMR study. In the process the 1 H-NMR assignments of H-1α and β in ginkgolide A were reversed and some 13 C-NMR assignments for ginkgolide A were corrected.

Discrimination of the two diastereoisomeric glycosides heterodendrin and epi-heterodendrin by the combined use of NOE and molecular mechanics

Abstract The two glycosides ( S )-heterodendrin and ( R )-epi-heterodendrin were synthesized in a novel, one-step enzymatic synthesis, and separated by means of column chromatography. The 1 H NMR spectra of the two diastereoisomers differ mainly in the chemical shift of H-2′ of the side chain. At first sight the 1 H NMR spectra do not allow a stereospecific assignment. It was found, however, that the NOE between the anomeric proton H-1 and H-2′ of the side chain is considerably larger in epi-heterodendrin than in heterodendrin, which indicates on a time-averaged basis a smaller distance between these two protons in epi-heterodendrin. This difference in conformational behaviour is correctly reproduced by molecular mechanics calculations, thereby offering a method for the discrimination of these two glycosides.

Monogagaine, a Novel Dimeric Indole Alkaloid from Tabernaemontana chippii and Tabernaemontana dichotoma [1]

Abstract From the rootbark of Tabernaemontana chippii and the stembark of T. dichotoma a novel dimeric indole alkaloid was isolated which was named monogagaine. It was assigned structure 1 on the basis of spectral data. It showed antimicrobial activity against Bacillus subtilis.