Alkaloids of Schelhammera pedunculata (Liliaceae). II. Reactions of schelhammeridine

Abstract

Catalytic hydrogenation of schelhammeridine (I) in acetic acid solution gives demethoxydihydroschelhammeridine (II), 1,2,6α,7- tetrahydroschelhammeridine (III), dihydroschelhammeridine (IV), and a tetrahydro derivative (V), the formation of which involves cleavage of the C5-N9 bond. The major product from heating schelhammeridine with 10% aqueous hydrochloric acid is the alcohol (VIIa) with the configuration at C 3 opposite to that in schelhammeridine. Other products are the alcohol (IX) and two amino alcohols (Xa) and (XIIa) which have a diphenyl ring system formed by the aromatization of ring A. The compounds (Xa) and (XIIa) have been shown to be diastereoisomers with the same absolute configuration for the dissymmetric diphenyl system, but the opposite configuration at C 7, and they have been characterized as N-acetyl derivatives (Xb) and (XIIb) which have been assigned the respective configurations shown in (XX) and (XVIII). On heating with acetic anhydride, schelhammeridine is converted into the N,O-diacetyl compound (XVa) which on hydrolysis affords (XVb), the optical antipode of (Xb). All three stereo-isomeric N-acetylamides (Xb), (XIIb), and (XVb), are oxidized to the same optically inactive ketone (XI). A mechanism for the acetic anhydride reaction that explains the formation of only a single isomer is discussed, and it has been shown that the formation of (XVa), enantiomeric with (Xc), in this reaction necessarily involves inversion of the configuration of the diphenyl system in the first-formed product, which must be the O-acetyl derivative of (XVIII). It is concluded that amide alcohols (XVIII) and (XX) retain the configuration for the diphenyl system that would be predicted from the known absolute configuration of schelhammeridine, but that the initially formed O-acetyl derivative of (XVIII) obtained in the reaction with acetic anhydride is converted quantitatively at the temperature of the reaction mixture into the O-acetyl derivative of (XXI) by inversion of the diphenyl configuration.