Total Synthesis and Stereochemical Revision of Phacelocarpus 2‐Pyrone A

The first total synthesis of phacelocarpus 2‐pyrone A is reported. The original natural compound was tentatively assigned (by NMR spectroscopy) as containing two cis‐alkenes and a trans‐vinyl ether connected to a 2‐pyrone ring motif. Our computational predictions indicated that a cis‐vinyl ether motif was equally feasible. Attempts to prepare the trans‐vinyl ether were met with no success. The all cis‐target compound was synthesised in nine steps, employing key regio‐ and stereoselective reactions including Au^I‐catalysed vinyl etherification, Wittig alkenylation and end‐game Stille macrocyclisation. Analysis of the NMR data enabled identification and confirmation of the correct structure of phacelocarpus 2‐pyrone A, containing a cis‐vinyl ether. Our studies pave the way for future development of methodologies to these structurally distinct pyrone skipped‐polyenyne natural products.     

Total Synthesis of (+)‐Madangamine D

Madangamines are a group of bioactive marine sponge alkaloids, embodying an unprecedented diazapentacyclic skeletal type. The enantioselective total synthesis of madangamine D has been accomplished, and represents the first total synthesis of an alkaloid of the madangamine group. It involves the stereoselective construction of the diazatricyclic ABC core using a phenylglycinol‐derived lactam as the starting enantiomeric scaffold and the subsequent assembly of the peripheral macrocyclic rings. The synthesis provides, for the first time, a pure sample of madangamine D and confirms the absolute configuration of this alkaloid family.     

Chemoenzymatic Synthesis of Advanced Intermediates for Formal Total Syntheses of Tetrodotoxin

Advanced intermediates for the syntheses of tetrodotoxin reported by the groups of Fukuyama, Alonso, and Sato were prepared. Key steps include the toluene dioxygenase mediated dihydroxylation of either iodobenzene or benzyl acetate. The resulting diene diols were transformed into Fukuyama's intermediate in six steps, into Alonso's intermediate in nine steps, and into Sato's intermediate in ten steps.     

Organocatalytic,Asymmetric Total Synthesis of (−)‐Haliclonin A

The first total synthesis of the alkaloid (?)‐haliclonin A is reported. The asymmetric synthesis relied on a novel organocatalytic asymmetric conjugate addition of nitromethane with 3‐alkenyl cyclohex‐2‐enone to set the stereochemistry of the all‐carbon quaternary stereogenic center. The synthesis also features a Pd‐promoted cyclization to form the 3‐azabicyclo[3,3,1]nonane core, a SmI₂‐mediated intermolecular reductive coupling of enone with aldehyde to form the requisite secondary chiral alcohol, ring‐closing alkene and alkyne metathesis reactions to build the two aza‐macrocyclic ring systems, and an unprecedented direct transformation of enol into enone.     

Evolution of a Short Route to Strychnine by Using the Samarium‐Diiodide‐Induced Cascade Cyclization as a Key Step

This comprehensive report accounts the development of a highly diastereoselective samarium diiodide‐induced cascade reaction of substituted indolyl ketones. The complexity‐generating transformation with SmI₂ allows the diastereoselective generation of three stereogenic centers including one quaternary center in one step. The obtained tetra‐ or pentacyclic dihydroindole derivatives are structural motifs of many monoterpene indole alkaloids, and their subsequent transformations gave way to one of the shortest approaches towards strychnine (14 % overall yield in ten steps, or 10 % overall yield in eight steps). During the course of this report we discuss the influence of substituents on the cyclization step, plausible mechanistic scenarios for the SmI₂‐induced cascade reaction, diastereoselective reductive amination, and regioselective dehydratization protocols towards the pentacyclic core structure of strychnos alkaloids.     

A Valuable Synthetic Route to the Enantiopure Functionalized N-Substituted Aziridines

Chiral butyrolacto[3,4-b]-2(S)-6(R)-1-N-akylaziridines 7 were synthesized in enantiopure form utilizing racemic 5-methoxy-3-bromo-2(5H)-furanone (5) and available amines (6) as key precursors. After highly effective reduction of 7, the functionaiized 2(S),3(R)-dihyroxymethyl-N-alkylaziridines (8) were obtained in good yields with ≥98% ee. This is a simple and pratical method for the preparation of enantiopure aziridines which are important intermediates in the synthesis of biologic active molecules.     

Access to the Enantiopure Axially Chiral Cyclophane Isoplagiochin D through Atropo‐diastereoselective Heck Coupling

Macrocyclization is typically the key step in the syntheses of cyclophane‐type natural products. Considering cyclophanes with axially chiral biaryl moieties, the control of atroposelectivity is essential with biological activity and is synthetically challenging. We report an atroposelective approach involving Heck cyclization, which for the first time enables the total synthesis of an enantiopure macrocyclic bis(bibenzyl), namely isoplagiochin D. An enantiopure sulfinyl auxiliary in the ortho position of a biaryl axis (still flexible) was used to induce an atropo‐diastereoselective Heck coupling (up to 98 % de). The traceless character of the sulfinyl auxiliary enables the introduction of a hydroxy group to give the target molecule with 98 % ee as well.     

Development of a Divergent Synthetic Route to the Erythrina Alkaloids: Asymmetric Syntheses of 8‐Oxo‐erythrinine,Crystamidine, 8‐Oxo‐erythraline,and Erythraline

A general synthetic methodology toward the erythrina alkaloids has been developed. Inspired by a proposed biosynthetic mechanism, the medium‐sized chiral biaryl lactam was asymmetrically transformed into the common core A–D rings by a stereospecific singlet oxygen oxidation of the phenol moiety, followed by a transannular aza‐Michael reaction to the dienone functionality. The late‐stage manipulation of the oxidation and oxygenation states of the functional groups on the peripheral moieties enabled the flexible syntheses of the erythrina alkaloids.     

Enantioselective Synthesis of the 1,3-Dienyl-5-Alkyl-6-Oxy Motif: Method Development and Total Synthesis

The 1,3-dienyl-5-alkyl-6-oxy motif is widely found in various types of bioactive natural products. However, present synthesis is mainly non-asymmetric which relied upon different olefination or transition metal-catalyzed cross-coupling reactions using enantioenriched precursors. Herein, based upon a newly developed enantioselective α-alkylation of conjugated polyenoic acids, a variety of 1,3-dienyl-5-alkyl-6-oxy motif (with E-configured internal olefin) was generated as the corresponding α-adducts in a highly enantioselective and diastereoselective manner. Utilizing 1,3-dienyl-5-alkyl-6-oxy motif as key intermediates, we further demonstrated their synthetic potential by expedient total syntheses of three types of natural products (glutarimide antibiotics, α-pyrone polyketides and Lupin alkaloids) within 4–7 steps.     

Enantio‐ and Diastereoconvergent Cyclocondensation Reactions: Synthesis of Enantiopure cis‐Decahydroquinolines

Up to four stereocenters with a well‐defined configuration are generated in a single synthetic step by the cyclocondensation of (R)‐phenylglycinol or (1S,2R)‐1‐amino‐2‐indanol with stereoisomeric mixtures (racemates, meso forms, diastereoisomers) of cyclohexanone‐based δ‐keto‐acid and δ‐keto‐diacid derivatives in enantio‐ and diastereoconvergent processes that involve dynamic kinetic resolution and/or desymmetrization of enantiotopic groups. A detailed analysis of the stereochemical outcome of this process is presented. This method provides easy access to enantiopure 8‐ and 6,8‐substituted cis‐decahydroquinolines, including alkaloids of the myrioxazine family.     

Total Synthesis of Dragocins A−C through Electrochemical Cyclization

The first total synthesis of dragocins A−C, remarkable natural products containing an unusual C4’ oxidized ribose architecture bridged by a polyhydroxylated pyrrolidine, is presented through a route featuring a number of uncommon maneuvers. Several generations towards the target molecules are presented, including the spectacular failure of a key C−H oxidation on a late-stage intermediate. The final route features rapid, stereocontrolled access to a densely functionalized pyrrolidine and an unprecedented diastereoselective oxidative electrochemical cyclization to forge the hallmark 9-membered ring. Preliminary studies suggest this electrochemical oxidation protocol is generally useful.     

Covalently Interlocked Cyclohexa‐m‐phenylenes and Their Assembly: En Route to Supramolecular 3D Carbon Nanostructures

In our search to cluster as many phenylene units as possible in a given space, we have proceeded to the three‐dimensional world of benzene‐based molecules by employing covalently interlocked cyclohexa‐m ‐phenylenes, as present in the unique paddlewheel‐shaped polyphenylene 10 . A precursor was conceived, in which freely rotating m ‐chlorophenylene units provide sufficient solubility along with the necessary proximity for the final ring closure to give 10 . Monitoring the assembly of solubilized tert ‐butyl derivatives of 10 into supramolecular carbon nanostructures by dynamic light scattering (DLS) and Brillouin light scattering (BLS) revealed the dimensions of the initially formed aggregates as well as the amorphous character of the solid state.