The occurrence of diterpenoid alkaloids in various plant genera is reviewed. Skeletal types occurring in C19- and C20-diterpenoid alkaloids, as well as characteristic structure features of alkaloids isolated to date, are discussed. The observation that the normal-type oxazolidine ring-containing C20-diterpenoid alkaloids, e.g. atisine, veatchine, and garryfoline, exist as a mixture of C(20)-epimers led to an investigation of the behavior of the carbinolamine ether linkage (N-C-O) in these alkaloids. Piefiuxing ajaconine in methanol afforded a new compound, 7α-hydroxyisoatisine. A mechanism for this unusual rearrangement of ajaconine into 7 ahydroxyisoatisine, via a “disfavored” 5-endo-trig ring closure, is proposed. Veatchine acetate in methanol at room temperature hydrolyzes to veatchine without using any external base. To clarify the nature of this unusual hydrolysis, a rearrangement of 7a-acetoxyatisine acetate in methanol was examined. 7α-Acetoxyatisine acetate in methanol at room temperature yielded a mixture of 7α-hydroxyisoatisine, ajaconine, and their C(15) acetates. A mechanism for this unusual rearrangement is discussed. Treatment of ethylene oxide, glycidol, and oxitane with various inline derivatives of Czo-diterpenoid alkaloids affords 5 and 6-membered cyclic carbinolamine ethers. These results and the earlier mentioned rearrangement of ajaconine are discussed in terms of Baldwin's cyclization rules and several apparent exceptions are noted. The earlier assignments of stereochemistry of the C(16)-methyl group and the oxazolidine ring at C (20) in cuauchichicine have been revised on the basis of 13C nmr spectral analysis. These assignments have been confirmed by a single crystal x-ray analysis of cuauchichicine. Cuauchichicine is the first normal-type oxazolidine ringcontaining C2o-diterpenoid alkaloid which does not exist as a pair of epimers at C (20). The structure previously assigned to (-)-”β”-dihydrokaurene has been confirmed by X-ray crystallography. Consequently, during the correlation of cuauchichicine with (-)-”β”-dihydrokaurene, an unanticipated epimerization must have occurred in the first Wolff-Kishner reduction step. The acid-catalyzed rearrangement of garryfoline to cuauchichicine has been studied by deuterium labeling to establish the mechanism of the reaction. Treatment of isogarryfoline with 10% DC1 in D2O at room temperature yielded a product with a C(16αD)-βCH2D group, a fact which demonstrates that a 15→16 hydride shift mechanism is not involved in this rearrangement. A mechanism involving formation of an enol is suggested forthis acid-catalyzed rearrangement of garryfoline to cuauchichicine. A new one-step method for degradation of the oxazolidine ring of C20-diterpenoid alkaloids to their corresponding imine derivatives is described. A new simple method using active manganese dioxide for converting the N-CH2-CH2OH group-containing alkaloids into their isooxazolidine ring-containing alkaloids is reported. This method has advantages over the old methods in that it is very simple and uses an inexpensive and non-toxic reagent. © 1980, American Chemical Society. All rights reserved.