Skytanthines, monoterpene alkaloids

The monoterpene alkaloids constitute a relatively small group of compounds and their occurrence has been restricted to several species of the genus Kopsia, viz., K. pauciflora, K. macrophylla and K. dasyrachis, from which several new monoterpene alkaloids related to skytanthine have been recently isolated. 

The second monoterpene alkaloid kopsone gave a molecular, ion which analysed for C11H19NO. The IR (1720 cm ) and C NMR (8 218) spectral data indicated the presence of a ketone function. Other groups indicated by the NMR spectra were two CHMe groups, an N-methyl, three methylenes (one deshielded at 8 56) and four methines (one deshielded at 8 72). These, as well as a postulated common origin of 51 and 52 from the hypothetical 9-hydroxy-skytanthine precursor 53 (Scheme 1), led to the proposed structure for kopsone. The relative stereochemistry was deduced from analysis of the H NMR spectrum. 

A new monoterpene alkaloid, kinabalurine G (330) has been obtained from the leaf extract of Kopsia dasyrachis in addition to 11 known alkaloids and the novel pentacyclic indole, danuphylline vide infra) [223]. Analysis of the spectral data allowed assignment of the gross structure as well as determination of the stereochemistry at the various centers. Kinabalurine G (330) is the AZ-oxide of 9-hydroxy-S-skytanthine, which is unknown, although a 9-hydroxy-skytanthine of unknown stereochemistry has been previously reported from Tecoma stans [224]. 

The leaves of Tecoma stans (Bignoniaceae) were studied previously (1) because of their reputed antidiabetic properties, which appear to have been traced to the monoterpene alkaloid fraction. Further studies of this plant by Lins and D arc Felicio (42) have now revealed the presence of two new alkaloids, 7-hydroxy-5,6-dehydroskytanthine (58) and 4-hydroxy-tecoman-ine (59), in the fruits of T. stans. In addition, two known alkaloids, 5/3-hydroxy-skytanthine (44) and tecomanine (55) were obtained. H and 13C NMR data for these known alkaloids were reported for the first time. 

Another type of rearrangement involving 1,5-asymmetric induction is the ketene aza-Claisen rearrangement, used in the stereoselective synthesis of intermediate 16, a common precursor for the synthesis of monoterpene alkaloids (-)-a-skytanthine, (-)-jV-demethyl-<5-skytamhine, and (+ )-epidihydrotecomanine 649. 

A number of important alkaloids are derived from monoterpenes. Monoterpenoid alkaloids are comparably simple compounds formed by addition of nitrogen atom to an existing monoterpene, mainly of the secologanin type. Several monoterpenoid alkaloids have been isolated and characterized and classified based upon their structural characteristics. The more common and pharmaceutically important types such as actinidine, skytanthine, and gentianine are discussed herein. 

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