Alkaloids of Catha edulis. Part 4. Structures of cathedulins E3, E4, E5, E6, and K12. Novel sesquiterpene alkaloids with mono- and bismacrolide bridges

Abstract

Alcoholysis and reduction shows that cathedulin E3, C₅₄H₆₀N₂O₂₃, is made up of a euonyminol core (3)(octaacetate), together with four acetates, a hydroxyisobutyrate, and evoninate (1) and cathate residues. The latter, a novel residue, is shown to be (4). By study of ¹H and ¹³C n.m.r. data for the intact alkaloid and its hydrogenolysis and partial methanolysis produces, E3 is formulated as (12). Cathedulin E4, converted into E3 by acetylation, is (13). Corresponding to both alkaloids is a minor series in which one acetate is replaced by a benzoate residue.

Cathedulin E6, C₅₇H₆₂N₂O₂₂, formulated as (17), is also based on a euonyminol core but gives, on methanolysis, dimethyl evoninate, methyl benzoate, methyl nicotinate, and trimethyl gallate; a 2-acetoxyisobutyrate and a further acetate residue are also present. Alkaloid E6 bears to E5 (16) the same relationship as does E4 to E3. The minor alkaloid cathedulin K12, C₅₄H₆₂N₂O₂₃, has the structure (18), similar to E3 but with the cathate span reductively cleaved : this suggests a biosynthetic connection.