Réarrangements en milieu acide trifluoroacétique de deux alcaloïdes indoliques: la desformocorymine et la dihydrocorymine

The rearrangement in trifluoroacetic acid of two indole alkaloids of the echitamine series, desformocorymine (14) and dihydrocorymine (9) , has been investigated. Desformocorymine (14) was tranformed into a mixture of carbinolamines 17a , b , with the akuammiline skeleton, which were reduced (Et₃SiH, CF₃CO₂H) into an isomer 12 of cathafoline (6). This sequence constitutes the first example of an interconversion of the corymine skeleton into the akuammiline skeleton (Scheme 2). In the case of dihydrocorymine (9) , the rearrangement followed a different pathway owing to the formation of a hemiacetal between the primary alcohol CH₂(17)-OH and a carbonyl formed at C(3). Treatment of this hemiacetal 26 with aqueous base led to its opening with concomitant formation of a lactam. ¹³C-NMR seems to indicate that this lactam exists under a hydrated form 27. This highly unstable intermediate was cleanly transformed (MeONa-MeOH) into a 2-acyl indole 30 (Scheme 4), the structure of which was determined by X-ray crystallography. The formation of this acylindole involves the rupture of the C(7)? C(16) bond; it is the reverse of the reaction generally postulated as occurring in the biogenesis of the pentacyclic alkaloids. The structure of a by-product 34 was established as 17-hydroxymethylvincoridine by X-ray crystallography. The acid-catalyzed rearrangements involve the rupture of the Ph-N? C? N chromophore, with formation of a carbonyl at C(3). The reversibility of these steps is used in an easy correlation of dihydrocorymine and of 3-epidihydrocorymine via their trifluoroacetates.