Wenkert enamine, synthesis

Eburnane Alkaloids. A short and highly efficient synthesis of a key intermediate Wenkert Enamine (223) for the synthesis of Eburnane alkaloids such as vincamine has been reported (126,127). 

Cyclization of an indolylethyl-tetrahydropyridine derivative is also a key stage in an extremely brief and elegant synthesis of the yohimbine ring system and a number of ajmalicinoid bases by Wenkert and his collaborators.78 Thus, internal nucleophilic attack by enolate anion at the y-position to the nitrogen atom in (122) gave a tetrahydropyridine derivative which readily cyclized to the pentacyclic enamine (123), reduction of which gave (db)-pseudoyohimbone (124) (Scheme 11). 

Aza-annulation of 104 with a variety of acrylate reagents has been utilized in the synthesis of indoloquinoiizidine alkaloid skeletons (eq. 25). Aza-annulation of 104 was affected with acrylic acid (91%), acrylic acid/DPPA (95%), acryloyl chloride/DMAP (64, 63%), and methyl acrylate (37, 52%) to generate the pentacyclic eburnane skeleton 105.42 Carbonyl reduction gave Wenkert s enamine (106), which was carried on in the synthesis of ( )-apovincamine (107) and the clinically active synthetic analog ( )-Cavinton (108).42... 

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