Oncinotine-Type Spermidine Alkaloids from Oncinotis tenuiloba. Transformation of N-acetyloncinotin-12-one to N,N′-diacetylinandenin-12-one

From extracts of Oncinotis tenuiloba STAPF , two novel polyamine alkaloids, oncinotin-11-one ( 5 ) and oncinotin-12-one ( 6 ), were isolated. Peracetylation of 6 provided the N-acetyl derivative 11 as well as N,N′-diacetylinandenin-10-en-12-one ( 12 ) due to a β-elimination-type side reaction resulting in ring enlargement of 11 (Scheme 1). Deuteration of 12 yielded 13 , showing the same retention time as N,N′-diacetylinandenin-12-one ( 14 ), when co-HPLC was performed together with different keto-isomeric N,N′-diacetylinandeninones. Structure elucidation was extended by Schmidt degradation of 6 and N,N′-diacetyl(10,11-²H₂)inandenin-12-one ( 13 ); the degradation products were identified by GC and ESI-MS. The structure of 5 was proposed on the basis of spectroscopic means. Comparison of the spectroscopic data of 5 with those obtained from synthetic material as well as co-HPLC of the N-acetyl derivative 20 together with the corresponding synthetic compound revealed the identity of the substances and confirmed the structure of 5 . Additionally, oncinotine ( 2 ) and neooncinotine ( 3 ) were isolated, separated, and identified with authentic samples by co-HPLC of their N-acetyl derivatives 8 and 9 , respectively.