Two methods for the synthesis of N,19-seco norditerpenoid alkaloids were developed. One method prepared new N,19-seco norditerpenoid alkaloids possessing an oxaziridine group from yunaconitine in five steps involving acetylation, imination, quaternization, formation of N,O-mixed ketal, and oxidation in 50% overall yield. Another method provided a series of novel N,19-seco norditerpenoid alkaloids bearing the oximino or nitro groups through oxidation of the imine N-oxides with HIO4 in moderate yields. Two novel aconanetype diterpenes were synthesized from an N,19-seco nitro compound and an imine N-oxide through Nef reaction and HIO4 oxidation, respectively, in moderate yields.