An efficient three-step biomimetic synthesis of the four di-spectively, in 74-86% yield, hydrogenation of which gives astereomeric 18,19-dihydroantirhines 4a-d starting from the mainly 15 and 16 with the skeleton of the vallesiachotamine tetrahydrocarboline aldehydes 5a and 5b is described. Do-indole alkaloids (55-86%). In addition, small amounts of 17 mino reaction of the aldehydes 5a and 5b, respectively, with and 18 with the corynanthe skeleton are also formed Meldrum's acid (6) and the enol ethers 8a and 8b in the pres-(10-12%). Reduction of both 15 and 16 with LiAlH, yields ence of catalytic amounts of ethylenediammonium diacetate the diastereomeric rac-l8,19-dihydroantirhines 4a-d in leads to the strictosidine analogues lOa-d and l l a -h , re-78-86'30 yield.In the biosynthesis['l of the over 2000 monoterpenoid indole alkaloids, strictosidine (1 a), formed from secologanin[*] and tryptamine under catalysis by the enzyme strictosidine synthetase"], is the key intermediate. Enzymatic glycoly~is[~] of strictosidine 1 a provides the highly reactive aglucon lb, which reacts in vivo via its open dialdehyde form either by an N-4/C-17 cyclization to the give indole alkaloids of the vallesiachotamine (2) group, or alternatively by a competitive N-4/C-2 1 cyclization with formation of indole alkaloids of the corynanthe group, e.g. geissoschizine. Interestingly, the in vitro treatment of strictosidine 1 a with emulsine (P-glucosidase) gives almost exclusively vallesiachotamine (2) as the thermodynamically more stable product ['].

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5 / e + l l . Heavy atom method, anisotropic refinement of all atoms (Program system SHELX 76): 4Nb in 4a, 4 8 in 4b, 16 0 in 16h with x=0.0712(3), z=0.1994(4), R=0.025, R,, = 0.02 I (including unobserved reflections), 12 parameters,

Strictosidin, eine Schlüsselverbindung in der Biosynthese von über tausend Alkaloiden, ist bisher noch nicht totalsynthetisch erhalten worden. Durch Tandem‐Knoevenagel/Hetero‐Diels‐Alder‐Reaktion wurden nun die diastereomeren (gut trennbaren) Analoga vom Typ 1 hergestellt (R1 = Benzyloxycarbony, R2 = Benzyl) und zu Indolalkaloiden umgesetzt.

Tandem Knoevenagel/hetero‐Diels‐Alder reaction of the aldehyde (I) ‐ derived from tryptamine and a mixed malondialdehyde acetal in a 4‐step procedure ‐ with the barbituric acid (II) and the butenyl ether (III) yields a mixture of the strictosidine analogues (IV) and (VII).