Dihydro corynantheine

The shift assignments to corynantheine [364], (2) dihydro-corynantheine [365], and corynantheidine [366] (103) are easily made... 

It is thus apparent that the absorption spectra of rhynchophylline can be explained entirely satisfactorily by the assumption of oxindole and /J-methoxyacrylate chromophores (74). Hence, the alkaloid was provisionally formulated as the oxindole analog of corynantheine or dihydro-corynantheine (56, 74) the resistance to hydrogenation and the Kuhn-Roth determination (0.6 C-Me) are clearly in favor of the latter. 

The hydrogen atoms at C-15 and C-20 in rhynchophylline are thus oriented trans with respect to each other, a conclusion which is amply confirmed by the partial synthesis of rhynchophylline from dihydro-corynantheine (XXXII) (66). Reaction of XXXII with tertiary butyl hypochlorite gave the chloroindolenine derivative XXXIII, which on methanolysis was converted into the imidoether XXXIV. Hydrolysis of XXXIV with refluxing aqueous acetic acid then gave rhynchophylline. 

In the parent series in which C-9 carries a hydrogen atom, dihydro-corynantheine (25) and corynantheidine (lla) belong to the normal and alio series, respectively. The stereoisomer hirsutine must therefore... 

Takano et a]L. have extended their syntheses of corynantheine relatives from ( )-norcamphor, and have completed the synthesis of the 18,19-dihydro-alkaloids with all four stereochemical conflg-... 

The situation with alkaloids containing the dihydro-corynantheane skeleton is similar there are four stereoisomeric types, normal (e.g. corynantheine YAAA, pseudo, alio (e.g. corynantheidine K9.5) and epiallo. Their stereochemistry has been fully discussed [2]. On the basis of this work a corynantheine type alkaloid can be allotted to one of the four classes by spectroscopic measurement, making the same assumption about Cys) configuration [2]. 

The enamine (/dienamine)-iminium cycle-specific cascade catalysis is an important constituent of amine-catalyzed cascade reactions [10]. This strategy has been explored extensively and also applied to natural product synthesis. One such example is the total synthesis of dihydrocorynantheol, which was first isolated from the bark of Aspidosperma marcgravianum in 1967 [29]. This indole alkaloid is a member of the corynantheine and was found to exhibit antiparasitic, antiviral, or analgetic activities, which have attracted considerable attention from the synthetic community. Among those reported total syntheses, Itoh et al. developed a Mannich-Michael cascade reaction catalyzed by L-proline 52 for the total synthesis of ent-dihydrocorynantheol 54 (Scheme 3.8) [30], The cascade reaction of 3-ethyl-3-buten-2-one 51 with dihydro-P-carboline 50 catalyzed by 30mol% of (S)-proline afforded the tetracyclic core structure 53 in 85% yield. Excellent stereoselectivity was achieved in this cascade reaction (99% enantiomeric excess and almost complete diastereomeric control). Therefore, this organocascade reaction could lead expeditiously to construction of the core structure, which enabled the authors to accomplish the total synthesis of enl-dihydrocorynantheol 54 in just five steps. 

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