19S-Hydroxytabersonine

The structure of baloxine (236) has been confirmed by partial synthesis from vindolinine,110 which has previously been converted into 19-hydroxytabersonine (237). The tetrahydropyranyl ether (238) of the (19.S)-epimer, on hydroboration-oxidation, gave a mixture of C-14 epimeric alcohols (239) on oxidation and removal of the tetrahydropyranyl ether grouping, these gave baloxine (236) (Scheme 34). Its formulation as (19S)-hydroxy-14-oxovincadifformine is thus confirmed. 

Trichophylline, a novel alkaloid isolated from the roots of Catharanthus trichophyllus, has the structure 131, according to X-ray crystal structure analysis (145). Reduction of trichophylline with sodium borohydride gives an unsaturated lactone, formulated as 132. Oxidative fission of the C/D ring system in vincadifformine derivatives has been observed previously hence, trichophylline may arise by oxidation at C-21 of an appropriate precursor, such as a 19-hydroxytabersonine (88) or 108, to the hydroperoxide 133, followed by fission of the 20,21-bond and simultaneous migration of C-18. 

S-Hydroxytabersonine (108), in another series of transformations, has been used in a partial synthesis of baloxine (352), an alkaloid of Melodinus balansae. Protection of the hydroxyl group in 108 as its tetrahydropyranyl ether 353, followed by regiospecific hydroboration-oxidation, gave a mixture of epimeric C-14 alcohols (354) that, on oxidation and removal of the tetrahydropyranyl group, gave baloxine (352), whose structure as 19S-hydroxy-14-oxovincadifformine is thus confirmed (241) (Scheme 17). 

These structures were confirmed by partial synthesis from the two 19-hydroxytabersonines 96 and 18. Reaction of each of these compounds with lead tetraacetate gave unstable indolenines 97 and 98 which, by reaction with sodium methoxide, gave 94 and 95 in approximately 85% yield. It was demonstrated that compound 94 from 195-hydroxytabersonine (96) was identical with that obtained from natural sources, thereby establishing the stereochemistry at C-19. 

Overexpression of ORCA2 [52] Transcription activator Hairy root Detectable 19-hydroxytabersonine after induction... 

STR strictosidine synthase SGD strictosidine -D-glucosidase 6,7E 6,7-epoxidase 19H 19-hydroxylase MAT minovincinine 19-hydroxy-O-acetyltransferase T16H tabersonine 16-hydroxylase 160MT 16-hydroxytabersonine-O-methyltransferase T30/T3R tabersonine 3-oxygenase... 

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