Ostatnie postępy w syntezie alkaloidów tropanowych i pochodnych tropanu

Tropane alkaloids are a long-known class of compounds possessing an 8-azabicyclo[ 3.2.1]octane skeleton. Many tropane alkaloids posses biological activity (anticholinergic, anti-Parkinsonian, hypotensive), and as such had a significant influence on medicine and played a notable role in the development of organic chemistry [1]. The most known representatives of biologically active tropane alkaloids are: cocaine, atropine, scopolamine, ecgonine, and Bao Gong Teng A. A number of natural tropane alkaloids are chiral compounds, whose preparation in optically active forms is still a big challeng [2]. The biological activity of enantiomers often differs depending on their configurations. Alkaloids are a subject of an intensive research: scopus database contains nearly 200 thousand publications with the word „alkaloid”, and almost 4,500 publications with the phrase „tropane alkaloids” (about half of them have appeared in the last ten years). About 55 papers are devoted to stereoselective synthesis of tropane derivatives in 2000-2015. About half of this concernes stereoselective methods. The organic synthesis of alkaloids has a long history and numerous synthetic approaches to the tropane skeleton have been developed, from the classical synthesis of tropinone by Willstätter at the beginning of the XX century, to more recent developments dealing with asymmetric deprotonation of tropinone with chiral lithium amide bases for the enantioselective synthesis of a range of tropanes [3, 4]. Owing to extensiveness of the field, the current review presents the most interesting, from a synthetic point of view, approaches to tropane derivatives and tropane analogues. Most of the methods of synthesis are long (often several steps), time- and recourses-intensive, and often required elaborate and hardly available starting materials. But there are also notable exceptions, based on the asymmetric deprotonation approach; e.g., from the syntheses of cocaine described in this article, the most efficient one was reported by Lee in 2000 [5]. The concise synthesis (6 steps) gave the unnatural enantiomer of cocaine starting from commercially available tropinone in 78% overall yield. This approach allows to obtain both enantiomers and racemate, by changing type of one reactant only. However, most strategies provide only one enantiomer or racemic mixture of an alkaloid. As can be seen, despite of advances in chemicall science, there is no general way to synthesize majority of the representatives of this group of structurally related compounds.