C20 Diterpenoid Alkaloids

C-Chemical Shifts of Various Functional Groups of C20-Diterpenoid Alkaloids... 

Alkaloids of Delphinium dictyocarpum DC.—Dictysine [C21H33N03 m.pt 184—186°C] has been assigned structure (37) on the basis of chemical and spectroscopic studies.25" An unpublished X-ray analysis256 demonstrates that (37) is incorrect and that the structure should be as shown in (37a). Derivatives of dictysine (38)—(42) are shown here as the corrected structures. This alkaloid was isolated from the epigeal parts of Delphinium dictyocarpum DC.26 On acetylation of dictysine with acetyl chloride, the triacetate (38) and the two diacetates, (39) and (40), were obtained. The reaction of dictysine with one molar equivalent of periodic acid for three hours gave the a-hydroxy-ketone (41), while treatment with excess periodic acid for three days yielded the aldehyde carboxylic acid (42). These structures were supported by mass-spectral, i.r., and XH and 13C n.m.r. analyses. This is the first example of a C20 diterpenoid alkaloid which contains hydroxyl groups at C-16 and C-17. 

Conversions of the Oxazolidine Rings in C2o Diterpenoid Alkaloids.—Several studies of the formation and opening of the oxazolidine ring system in the C20 diterpenoid alkaloids have been reported.33-35... 

The earlier work on the chemistry of C20-diterpenoid alkaloids has been reviewed in Volumes IV(4), VII(5), and Xll(6) of this treatise, in books on... 

Included in this chapter is a catalog of all known C20-diterpenoid alkaloids showing the correct structures, physical properties, plant sources, and key references. Previously published books (19 21) and recent reviews (15-16) have reported incorrect structures for several well-known C20-diterpenoid alkaloids. This catalog should be very useful for it presents in a single place important structural information on the C20-diterpenoid alkaloids that has been scattered through hundreds of papers and dozens of review articles. 

It is noteworthy that denudatine is the first C20-diterpenoid alkaloid with an atisine skeleton possessing both an V-ethyl and a C-7 -C-20 bridge. The biosynthetic implications of this alkaloid were mentioned by the Canadian chemists (111). 

In 1974, Japanese chemists (134) demonstrated the utility of 13C-NMR spectroscopy in the field of C20-diterpenoid alkaloids. They determined the structures of miyaconine (145) and apomiyaconine (147), two rearrangement products of miyaconitine, by the aid of 13C-NMR spectroscopy. 

The 13C-NMR spectra of these compounds were analyzed to identify and distinguish skeletal features of the atisine and veatchine-type alkaloids for use in the structure elucidation of new C20-diterpenoid alkaloids. 

The structures of two new C20-diterpenoid alkaloids designated as ovatine (6) and lindheimerine (7), isolated from G. ovata var. lindheimeri, have been elucidated (33) with the help of13C-NMR spectroscopy. Similarly, the structure of dihydroajaconine (81) was also established (S3) with this technique. 

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