Diastereodivergente inverse Prenylierung von Indol‐ und Tryptophan‐Derivaten: Totalsynthese von Amauromin, Novoamauromin und epi ‐Amauromin

Regioselective Biocatalytic C4-Prenylation of Unprotected Tryptophan Derivatives

Regioselective carbon−carbon bond formation belongs to the challenging tasks in organic synthesis. In this context, C−C bond formation catalyzed by 4‐dimethylallyltryptophan synthases (4‐DMATSs) represents a possible tool to regioselectively synthesize C4‐prenylated indole derivatives without site‐specific preactivation and circumventing the need of protection groups as used in chemical synthetic approaches. In this study, a toolbox of 4‐DMATSs to produce a set of 4‐dimethylallyl tryptophan and indole derivatives was identified. Using three wild‐type enzymes as well as variants, various C5‐substituted tryptophan derivatives as well as N‐methyl tryptophan were successfully prenylated with conversions up to 90 %. Even truncated tryptophan derivatives like tryptamine and 3‐indole propanoic acid were regioselectively prenylated in position C4. The acceptance of C5‐substituted tryptophan derivatives was improved up to 5‐fold by generating variants (e. g. T108S). The feasibility of semi‐preparative prenylation of selected tryptophan derivatives was successfully demonstrated on 100 mg scale at 15 mM substrate concentration, allowing to reduce the previously published multistep chemical synthetic sequence to just a single step.

Biocatalytic C3-Indole Methylation-A Useful Tool for the Natural-Product-Inspired Stereoselective Synthesis of Pyrroloindoles

Enantioselective synthesis of bioactive compounds bearing a pyrroloindole framework is often laborious. In contrast, there are several S-adenosyl methionine (SAM)-dependent methyl transferases known for stereo- and regioselective methylation at the C3 position of various indoles, directly leading to the formation of the desired pyrroloindole moiety. Herein, the SAM-dependent methyl transferase PsmD from Streptomyces griseofuscus, a key enzyme in the biosynthesis of physostigmine, is characterized in detail. The biochemical properties of PsmD and its substrate scope were demonstrated. Preparative scale enzymatic methylation including SAM regeneration was achieved for three selected substrates after a design-of-experiment optimization.

Rhodium-Catalyzed Chemodivergent Regio- and Enantioselective Allylic Alkylation of Indoles

The control of C3/N1 chemoselectivity in indole alkylation with the same electrophiles is still challenging. An Rh/bisoxazolinephosphane-catalyzed chemodivergent regio- and enantioselective allylic alkylation of indoles was developed. Chiral C3- and N1-allylindoles can be selectively obtained with high branched/linear ratio and up to 99 % ee by changing the counteranion of Rh, the allylic carbonate, the reaction temperature, and the ligand.

Iridium-Catalyzed Enantioselective Intermolecular Indole C2-Allylation

The enantioselective intermolecular C2-allylation of 3-substituted indoles is reported for the first time. This directing group-free approach relies on a chiral Ir-(P, olefin) complex and Mg(ClO4 )2 Lewis acid catalyst system to promote allylic substitution, providing the C2-allylated products in typically high yields (40-99 %) and enantioselectivities (83-99 % ee) with excellent regiocontrol. Experimental studies and DFT calculations suggest that the reaction proceeds via direct C2-allylation, rather than C3-allylation followed by in situ migration. Steric congestion at the indole-C3 position and improved π-π stacking interactions have been identified as major contributors to the C2-selectivity.

Enantioselective Synthesis of Vicinal All-Carbon Quaternary Centers via Iridium-Catalyzed Allylic Alkylation

The development of the first enantioselective transition-metal-catalyzed allylic alkylation providing access to acyclic products bearing vicinal all-carbon quaternary centers is disclosed. The iridium-catalyzed allylic alkylation reaction proceeds with excellent yields and selectivities for a range of malononitrile-derived nucleophiles and trisubstituted allylic electrophiles. The utility of these sterically congested products is explored through a series of diverse chemo- and diastereoselective product transformations to afford a number of highly valuable, densely functionalized building blocks, including those containing vicinal all-carbon quaternary stereocenters.

Development of the Regiodivergent Asymmetric Prenylation of 3-Substituted Oxindoles

This paper describes our efforts to design a Pd-catalyzed asymmetric prenylation of 3-substituted oxindoles that affords access to both the linear and reverse-prenylated products. Both 3-alkyl- and 3-aryloxindoles performed well under our optimized reaction conditions. The regiodivergent alkylation of monoterpene-derived electrophiles using this methodology was also investigated. The utility of this methodology in natural product synthesis was demonstrated through the efficient total syntheses of four Flustra alkaloids, which also allowed the absolute stereochemistry of the prenylated oxindole products to be assigned. Surprisingly, the same enantiomer of ligand produced linear and branched regioisomers of opposite chirality.