Indole alkaloids from Vinca erecta type of sarpagine and ajmaline

Abstract

The single crystal X-ray diffraction method established the absolute configuration of the Vinca erecta indole alkaloids of the akuammidine sarpagine type (3S, 5S, 15R, 16R) and its o-acyl derivative, as well as the type of ajmaline, quebrachidine (2S, 3S, 5S, 7R, 15S, 16R, 17S) and majoridine (2R, 3S, 5S, 7R, 15R, 16S, 17R). Crystal data for C₂₁H₂₄N₂O₃ (1): orthorhombic, space group P2₁2₁2₁ (no. 19), a = 6.3949(5) Å, b = 13.5009(10) Å, c = 22.461(3) Å, Z = 4, 7694 reflections measured (7.64° ≤ 2Θ ≤ 152.294°), 3813 unique (R_int = 0.0798) which were used in all calculations. The final R₁ was 0.0680 (I > 2σ(I)) and wR₂ was 0.1650 (all data). Crystal data for C₂₃H₂₆N₂O₄ (2): orthorhombic, space group P2₁2₁2₁ (no. 19), a = 9.9730(13) Å, b = 10.2090(10) Å, c = 20.409(3) Å, Z = 4, 7959 reflections measured (8.666° ≤ 2Θ ≤ 151.998°), 4212 unique (R_int = 0.0386) which were used in all calculations. The final R₁ was 0.0477 (I > 2σ(I)) and wR₂ was 0.1171 (all data). Crystal data for C₄₂H₄₈N₄O₆ (3): monoclinic, space group P2₁ (no. 4), a = 8.9320(10) Å, b = 21.515(5) Å, c = 9.5420(10) Å, β = 97.103(10)°, Z = 2, 16677 reflections measured (9.34° ≤ 2Θ ≤ 151.836°), 7393 unique (R_int = 0.0278) which were used in all calculations. The final R₁ was 0.0366 (I > 2σ(I)) and wR₂ was 0.1037 (all data). Crystal data for C₂₃H₂₈N₂O₃ (4): orthorhombic, space group P2₁2₁2₁ (no. 19), a = 10.636(2) Å, b = 11.208(12) Å, c = 16.725(13) Å, Z = 4, 1650 reflections measured (9.498° ≤ 2Θ ≤ 119.97°), 1650 unique (R_int = 0.0436) which were used in all calculations. The final R₁ was 0.0608 (I > 2σ(I)) and wR₂ was 0.1720 (all data). In alkaloids such as sarpagine and ajmaline exo, the substituents of alkaloids do not lead to conformational changes of a stable polycyclic framework. In the series of sarpagine, alkaloids form mono-salts in the tetrahedral nitrogen N4, and in indolines of the ajmaline type, the tetrahedral hybridization of the N1 and N4 atoms favors the formation of disols. In V. erecta alkaloids, the exomethylene fragment (C18-C19=C20-C21) of the polycyclic backbone always takes on the E-state.