Die Struktur desAlstonia-Alkaloides Villalstonin

Bisindole Alkaloids from the Alstonia Species: Recent Isolation, Bioactivity, Biosynthesis, and Synthesis

Bisindoles are structurally complex dimers and are intriguing targets for partial and total synthesis. They exhibit stronger biological activity than their corresponding monomeric units. Alkaloids, including those containing C-19 methyl-substitution in their monomeric units, their synthetic derivatives, and their mismatched pairs can be attractive targets for synthesis and may unlock better drug targets. We herein discuss the isolation of bisindoles from various Alstonia species, their bioactivity, putative biosynthesis, and synthesis. The total synthesis of macralstonidine, macralstonine, O-acetylmacralstonine, and dispegatrine, as well as the partial synthesis of alstonisidine, villalstonine, and macrocarpamine are also discussed in this review. The completion of the total synthesis of pleiocarpamine by Sato et al. completes the formal synthesis of the latter two bisindoles.

The chemistry of mavacurane alkaloids: a rich source of bis-indole alkaloids

Covering: since early reports up to the end of 2020This review presents a complete coverage of the mavacuranes alkaloids since early reports till date. Mavacuranes alkaloids are a restrictive sub-group of monoterpene indole alkaloids (MIAs), which are represented by their two emblematic congeners, namely, C-mavacurine and pleiocarpamine. Their skeleton is defined by a bond between the indolic N1 nitrogen and the C16 carbon of the tetracyclic scaffold of the corynanthe group in MIA. A limited number of congeners is known as this skeleton can be considered as a cul-de-sac in main MIA biosynthetic routes. Thanks to the enhanced enamine-type reactivity, mavacuranes are frequently involved in the formation of multimeric MIA scaffolds. This review covers isolation aspects and synthetic approaches towards the mavacurane core and bisindole assemblies. To access the mavacurane core, only a few strategies are reported and the main synthetic difficulties usually originate from the important rigidity of the pentacyclic system. For the bisindole assemblies, biomimetic routes are privileged and deliver complex structures using smooth conditions.

Stereoselective Access to the Core Structure of Macroline-Type Indole Alkaloids: Total Synthesis of Macroline and Alstomicine

Rapid synthesis of the pentacyclic core structure of macroline-type indole alkaloids, and its application in the total synthesis of macroline and alstomicine is described. The core structure was accomplished in a highly stereocontrolled manner via two key steps, Ireland-Claisen rearrangement and Pictet-Spengler cyclization, commencing from a readily available starting material l-tryptophan, which obviated the need of a particular chiral source as an external catalyst, reagent, or internal auxiliary.

Application of the Asymmetric Pictet-Spengler Reaction in the Total Synthesis of Natural Products and Relevant Biologically Active Compounds

Tetrahydroisoquinolines are the framework of numerous natural products predominantly alkaloids, an important and one of the most wide spread families of naturally occurring compounds in the plant kingdom. Tetrahydroisoquinolines are commonly constructed through an old reaction, the so-called Pictet−Spengler Reaction (PSR). In this reaction, a β-aryl ethylamine undergoes an acid mediated condensation with a suitable aldehyde or ketone, followed by ring closure. In this review, we aim to highlight the applications of the asymmetric variant of this old name reaction in the total synthesis of natural products, chiefly, alkaloids, which exhibit significant biological properties.

Synthesis of Bisindole Alkaloids from the Apocynaceae Which Contain a Macroline or Sarpagine Unit: A Review

Bisindole natural products consist of two monomeric indole alkaloid units as their obligate constituents. Bisindoles are more potent with respect to their biological activity than their corresponding monomeric units. In addition, the synthesis of bisindoles are far more challenging than the synthesis of monomeric indole alkaloids. Herein is reviewed the enantiospecific total and partial synthesis of bisindole alkaloids isolated primarily from the Alstonia genus of the Apocynaceae family. The monomeric units belong to the sarpagine, ajmaline, macroline, vobasine, and pleiocarpamine series. An up-to-date discussion of their isolation, characterization, biological activity as well as approaches to their partial and total synthesis by means of both synthetic and biosynthetic strategies are presented.

Sarpagine and Related Alkaloids

The sarpagine-related macroline and ajmaline alkaloids share a common biosynthetic origin, and bear important structural similarities, as expected. These indole alkaloids are widely dispersed in 25 plant genera, principally in the family Apocynaceae. Very diverse and interesting biological properties have been reported for this group of natural products. Isolation of new sarpagine-related alkaloids and the asymmetric synthesis of these structurally complex molecules are of paramount importance to the synthetic and medicinal chemists. A total of 115 newly isolated sarpagine-related macroline and ajmaline alkaloids, along with their physicochemical properties have been included in this chapter. A general and efficient strategy for the synthesis of these monomeric alkaloids, as well as bisindoles, has been presented, which involves application of the asymmetric Pictet-Spengler reaction (>98% ee) as a key step because of the ease of scale up of the tetracyclic template. Also included in this chapter are the syntheses of the sarpagine-related alkaloids, published since 2000.

Macroline-sarpagine and macroline-pleiocarpamine bisindole alkaloids from Alstonia angustifolia

Nine bisindole alkaloids, comprising four belonging to the macroline-sarpagine group, and five belonging to the macroline-pleiocarpamine group, were isolated from the stem-bark extracts of Alstonia angustifolia (Apocynacea). Their structures were established using NMR and MS analyses.

General Approach to the Total Synthesis of Macroline-Related Sarpagine and Ajmaline Alkaloids1

Described in this review is a general and efficient strategy for the synthesis of macroline-related sarpagine and ajmaline alkaloids. The tetracyclic ketone in the parent system, as well as the alkoxy substituted series served as templates for the synthesis of these complex molecules. The palladium-mediated enolate cross coupling process, regiospecific hydroboration, and Tollens reaction are some of the key transformations that have been employed for further functionalization of these templates. Synthetic routes that have been improved, in order to obtain gram quantities of these alkaloids form a part of this review.

Enantiospecific, stereospecific total synthesis of (+)-majvinine, (+)-10-methoxyaffinisine, and (+)-N(a)-methylsarpagine as well as the total synthesis of the Alstonia bisindole macralstonidine

[structure: see text] The enantiospecific stereospecific total synthesis of majvinine 1a, 10-methoxyaffinisine 1b, and N(a)-methylsarpagine 1c are reported; this method has also resulted in the total synthesis of the Alstonia bisindole macralstonidine 2.