Recent Syntheses and Strategies toward Polycyclic Gelsemium Alkaloids

This Minireview is focused on an in‐depth discussion of comparative strategies to construct the gelsemine and gelsedine classes of the gelsemium alkaloids. This document highlights the diversity of strategies used to access specific motifs found within these targets: a) the fused “[3.2.1]bicycle” (in gelsemine) and “oxabicycle” (in gelsedine class); b) the “piroxindole” moiety with C7 quaternary center; c) the “N‐heterocycles” and d) the “THP” moiety with C20 quaternary center (in gelsemine).     

6‐Methylenebicyclo[3.2.1]oct‐1‐en‐3‐one: A Twisted Olefin as Diels–Alder Dienophile for Expedited Syntheses of Four Kaurane Diterpenoids

6‐Methylenebicyclo[3.2.1]oct‐1‐en‐3‐one, a twisted and highly reactive enone, was prepared for the first time by elimination of its bromide precursor. Its reactions as a dienophile with several dienes in Diels–Alder reactions proceeded smoothly to provide tricyclic and tetracyclic adducts, which allowed short syntheses (10–11 steps) of four kurane diterpenoids including 11β‐hydroxy‐16‐kaurene, 11α‐hydroxy‐16‐kaurene, liangshanin G, and gesneroidin B.     

Stereoselective Total Synthesis of Eburnane‐Type Alkaloids Enabled by Conformation‐Directed Cyclization and Rearrangement

Controlling the cis C20/C21 relative stereochemistry remains an unsolved issue in the synthesis of eburnane‐type indole alkaloids. Provided herein is a simple solution to this problem by developing a unified and diastereoselective synthesis of four representative members of this class of natural products, namely, eburnamonine, larutensine, terengganensine B, and melokhanine E. The synthesis features the following key steps: a) an α‐iminol rearrangement transforming the 3‐hydroxyindolenine into spiroindolin‐3‐one, b) a highly diastereoselective conformation‐directed cyclization leading to the melokhanine skeleton with the desired C20/C21 cis stereochemistry, and c) either an aza‐pinacol or an unprecedented α‐aminoketone rearrangement converting spiroindolinone back into the indole skeleton.     

Synthesis of (+)‐Darwinolide,a Biofilm‐Penetrating Anti‐MRSA Agent

Darwinolide, a recently identified marine natural product from the Antarctic sponge Dendrilla membranosa, was previously shown to exhibit promising activity against the biofilm phase of methicillin‐resistant Staphylococcus aureus. Its challenging tetracyclic rearranged spongian diterpenoid structure links a trimethylcyclohexyl subunit to a seven‐membered core with two fused tetrahydrofuran units. Herein, we describe the first synthesis of (+)‐darwinolide, which features a convergent aldol fragment coupling, an Ireland–Claisen rearrangement, and an organocatalytic desymmetrization as the key steps. Our results provide a foundation for the development of novel antibiofilm‐specific antibiotics.     

Total Syntheses of (+)‐Sarcophytin, (+)‐Chatancin, (−)‐3‐Oxochatancin,and (−)‐Pavidolide B: A Divergent Approach

A concise and divergent approach for the total syntheses of four cembrane diterpenoids, namely (+)‐sarcophytin, (+)‐chatancin, (?)‐3‐oxochatancin, and (?)‐pavidolide B, has been developed, and it also led to the structural revision of (?)‐isosarcophytin. The key steps of the strategy feature a double Mukaiyama Michael addition/elimination, a Helquist annulation, two substrate‐controlled facial‐selective hydrations, and a pinacol rearrangement. The described syntheses not only achieved these natural products in an efficient manner, but also provided insight into the biosynthetic relationship between the two different skeletons.     

Collective Total Syntheses of Atisane‐Type Diterpenes and Atisine‐Type Diterpenoid Alkaloids: (±)‐Spiramilactone B, (±)‐Spiraminol, (±)‐Dihydroajaconine,and (±)‐Spiramines C and D

The first total syntheses of the architecturally complex atisane‐type diterpenes and biogenetically related atisine‐type diterpenoid alkaloids (±)‐spiramilactone B, (±)‐spiraminol, (±)‐dihydroajaconine, and (±)‐spiramines C and D are reported. Highlights of the synthesis include a late‐stage biomimetic transformation of spiramilactone B, a facile formal lactone migration from the pentacyclic skeleton of spiramilactone E, a highly efficient and diastereoselective 1,7‐enyne cycloisomerization to construct the functionalized tetracyclic atisane skeleton, and a tandem retro‐Diels–Alder/intramolecular Diels–Alder sequence to achieve the tricyclo[6.2.2.0] ring system.