Cross-Metathesis Approach to the Tricyclic Marine Alkaloids (−)-Fasicularin and (−)-Lepadiformine A

Total Synthesis of (-)-Fasicularin

Asymmetric total synthesis of (-)-fasicularin was achieved in nine steps from a commercially available inexpensive material, by leveraging (1) an aryl radical-mediated, copper-catalyzed Sonogashira-type cross-coupling, (2) a Au-catalyzed tandem intramolecular alkyne hydroamination/iminium formation/intramolecular allylation, and (3) a tandem hydrogenation/hydrogenolysis/intramolecular reductive amination as key transformations.

Late-Stage Formal Double C-H Oxidation of Prenylated Molecules to Alkylidene Oxetanes and Azetidines by Strain-Enabled Cross-Metathesis

Prenylation is a ubiquitous late-stage modification in nature that often confers significantly improved bioactivity for secondary metabolites. While this lipophilic modification renders enhanced potency, the lipophilic tag(s) can diminish bioavailability and adversely alter drug transportation and metabolism. Thus, a functional-group-tolerant, mild, and selective late-stage C-H functionalization of prenyl tags would present a great potential in drug discovery programs but could also impact other fields, such as agrochemistry and chemical biology. Herein we report an exocyclic-strain-driven cross-metathesis reaction of prenyl tags, a formal double C-H oxidation protocol, that can be used for the selective late-stage derivatization of prenylated compounds and natural products. This methodology avoids the need for prefunctionalization of target molecules and affords ready access to an unprecedented library of oxo- and aza-prenylated complex molecules. Thus, in a broader context, this methodology extends late-stage functionalization beyond that available to nature.

Total Synthesis of (±)-Fasicularin through Double Consecutive Epimerizations

The total synthesis of tricyclic marine alkaloid fasicularin (1b) has been accomplished from a novel sterically well-defined α-aminonitrile 7, featuring a novel double consecutive epimerization process and Ir-catalyzed reductive functionalization of a tertiary γ-lactam. The required configuration is obtained through the thermodynamically stereoselectively driven isomerization of a readily available 8a-cyanodecahydroquinoline framework. The strategy allows us to achieve the tricyclic core structures efficiently from affordable starting materials through simple operations.

Stereoselective Tandem Synthesis of Pyrrolidine Derivatives under Gold Catalysis: An Asymmetric Synthesis of (-)-Lepadiformine A

A Au-catalyzed tandem alkyne hydroamination/iminium ion formation/allylation reaction was developed for expedient access to pyrrolidine derivatives bearing a tetrasubstituted carbon stereocenter. The tandem reaction was successfully applied to a 12-step asymmetric synthesis of (-)-lepadiformine A, a marine cytotoxic tricyclic alkaloid.

Total Synthesis of (-)-Lepadiformine A via Radical Translocation-Cyclization Reaction

Total synthesis of (-)-lepadiformine A featuring construction of the 1-azaspiro[4.5]decane skeleton by a highly diastereoselective radical translocation-cyclization reaction of a γ-lactam derivative bearing a chiral butenolide moiety is described. The enantioselective construction of butenolide is conducted via Krische's catalytic asymmetric allylation protocol. After the radical translocation-cyclization reaction, a hydroxymethyl group at the C-13 position was stereoselectively introduced by a one-pot partial reduction-allylation protocol of the unprotected lactam derivative. Finally, the total synthesis is completed by formation of a C ring.

Recent applications of the Wittig reaction in alkaloid synthesis

The Wittig reaction is the chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide (the Wittig reagent) to afford an alkene and triphenylphosphine oxide. Noteworthy, this reaction results in the synthesis of alkenes in a selective and predictable fashion. Thus, it became as one of the keystone of synthetic organic chemistry, especially in the total synthesis of natural products, where the selectivity of a reaction is paramount of importance. A literature survey disclosed the existence of vast numbers of related reports and comprehensive reviews on the applications of this important name reaction in the total synthesis of natural products. However, the aim of this chapter is to underscore, the applications of the Wittig reaction in the total synthesis of one the most important and prevalent classes of natural products, the alkaloids, especially those showing important and diverse biological activities.

Bimolecular Cross-Metathesis of a Tetrasubstituted Alkene with Allylic Sulfones

Exquisite control of catalytic metathesis reactivity is possible through ligand‐based variation of ruthenium carbene complexes. Sterically hindered alkenes, however, remain a generally recalcitrant class of substrates for intermolecular cross‐metathesis. Allylic chalcogenides (sulfides and selenides) have emerged as “privileged” substrates that exhibit enhanced turnover rates with the commercially available second‐generation ruthenium catalyst. Increased turnover rates are advantageous when competing catalyst degradation is limiting, although specific mechanisms have not been defined. Herein, we describe facile cross‐metathesis of allylic sulfone reagents with sterically hindered isoprenoid alkene substrates. Furthermore, we demonstrate the first example of intermolecular cross‐metathesis of ruthenium carbenes with a tetrasubstituted alkene. Computational analysis by combined coupled cluster/DFT calculations exposes a favorable energetic profile for metallacyclobutane formation from chelating ruthenium β‐chalcogenide carbene intermediates. These results establish allylic sulfones as privileged reagents for a substrate‐based strategy of cross‐metathesis derivatization.

Synthesis of 8',11'-dihydrospiro[cyclohexane-1,2'-oxepino[2,3-h] chromen]-4'(3'H)-ones with ring closing metathesis as a key step

A series of novel hybrid molecular entities incorporating various spiro chromanone scaffolds onto the benzannulated oxepine core moiety were synthesised using allylation, Claisen rearrangement, Kabbe condensation and Ring Closing Metathesis (RCM) as a key step. During the synthesis we found that the nitrogen functionality in the substrate influences significantly the catalyst load due to electronic effects. Several iterations have been carried out to achieve complete conversion to products 6a-6e.