Arto Tolvanen

First total synthesis of (±)-tangutorine

Abstract The first total synthesis of the novel indole alkaloid, tangutorine 1 was performed in seven steps from 7,8-dihydroquinoline-5(6H)-one 2 .

SYNTHESIS OF THE TANGUTORINE SKELETON

The ring system of the novel indole alkaloid tangutorine (1) has been synthesized via the Fry reaction. The final key steps were an oxidation/reduction procedure or alternatively, an epimerization sequence. Conformational and stereochemical aspects of the skeleton are discussed.

Stereoselective synthesis of methyl 3α-ethyl-1,2,3,4,6,7,12,12bβ-octahydroindolo[2,3-a]quinolizine-1α-carboxylate: A key intermediate for the preparation of tacamine-type indole alkaloids

Abstract Methyl 3α-1,2,3,4,6,7,12,12bβ-cotahydroindolo[2,3- a ]quinolizine-1α-carboxylate ( 6 ), a key intermediate for the synthesis of tacamine-type indole alkaloids, was prepared in six simple steps from methyl 5-(1′-hydroxyethyl)nicotinate ( 7 ). The last step was the catalytic hydrogenation of the two ethylidene isomers 14 and 15 , both of which gave the target ester stereoselectively.

1,4-Dihydropyridine equivalents: a novel approach to 2,6-dicyanopiperidine derivatives

For the first time, it has been shown that 2,6-dicyanopiperidines are formed in the Fry reaction via 1,4-dihydropyridine intermediates. A new synthetic approach for building 2,6-dicyanopiperidine derivatives by an extension of the sodium dithionite reduction of pyridinium salts is described. The stereochemistry of 2,6-dicyanopiperidine derivatives formed is discussed.

Acid-catalysed epimerization of bioactive indole alkaloids and their derivatives

Abstract The acid-catalysed epimerization reaction of bioactive indole alkaloids and their derivatives is reviewed. The three mechanisms, whichhave been proposed for the β-carboline-type indole alkaloids, are discussed. Through recent developments, evidence for all three mechanisms has been obtained, which shows the complexity of the epimerization reaction. The epimerization seems to depend on structural features and reaction conditions making it difficult to define one universal mechanism. On the other hand, the isomerization mechanism of oxindole alkaloids has been widely accepted. The acid-catalysed epimerization reaction provides a powerful tool in selectively manipulating the stereochemistry at the epimeric centre and it can also have a marked effect on the pharmacology of any epimerizable compound. Therefore, examples of this reaction in the total synthesis of indole alkaloids are given and pharmacological activities of some C-3 epimeric diastereomers are compared. Finally, literature examples of acid-catalysed epimerization reactions are presented.