Synthetic Studies on Palau'amine. Construction of the Cyclopentane Core via an Asymmetric 1,3-Dipolar Cycloaddition

Efficient construction of the hexacyclic ring core of palau'amine: the pK a concept for proceeding with unfavorable equilibrium reactions

Palau'amine has received a great deal of attention as an attractive synthetic target due to its intriguing molecular architecture and significant immunosuppressive activity, and we achieved its total synthesis in 2015. However, the synthesized palau'amine has not been readily applicable to the mechanistic study of immunosuppressive activity, because it requires 45 longest linear steps from a commercially available compound. Here, we report the short-step construction of the ABCDEF hexacyclic ring core of palau'amine. The construction of the CDE tricyclic ring core in a single step is achieved by our pK_a concept for proceeding with unfavorable equilibrium reactions, and a palau'amine analog without the aminomethyl and chloride groups is synthesized in 20 longest linear steps from the same starting material. The palau'amine analog is confirmed to retain the immunosuppressive activity. The present synthetic approach for a palau'amine analog has the potential for use in the development of palau'amine probes for mechanistic elucidation.

A palau'amine analog (2) was synthesized from 2-cyclopentenone in 20 steps. The construction of the CDE tricyclic ring core in a single step is achieved by our pK_a concept for proceeding with the unfavorable equilibrium reactions.

Construction of the tetracyclic core of the Lycopodium alkaloid annotinolide C

A concise approach to the tetracyclic core of annotinolide C has been developed which contains two key reactions epoxidation/1,2-migration to construct an aza [6.5] spiro ring (A and B) and semireduction/cyclization to construct lactone ring D.

Substrate-controlled selectivity switch in a three-component reaction: sequential synthesis of spiro-oxazolidinedione-cyclopentenones and hydroxy enaminobarbiturates in water

An efficient eco-friendly catalyst-free three-component domino multicyclization for the synthesis of new spirobicyclic oxazolidinedione containing cyclopentenone moieties has been established by mixing amines, β-dicarbonyl compounds and N,N'-dimethylalloxan in water at room temperature. This domino process involves multiple reactions such as enamination/aldol-like reaction/Stork enamine annulation/intramolecular cyclization under mild conditions.

A Nitrone Dipolar Cycloaddition Strategy toward an Enantioselective Synthesis of Massadine

An enantioselective route to the C,D-bicycle of massadine is reported. Enantiopure intermediates were generated by a single stereoselective reduction using the Corey-Bakshi-Shibata reagent. This initial stereoinduction was translated into the five contiguous stereocenters of the massadine D-ring by a synthetic route that features a diastereoselective and stereospecific Ireland-Claisen rearrangement of a trianionic enolate followed by a diastereoselective nitrone dipolar cycloaddition of a highly electron-poor oxime.

Stereocontrolled synthesis of polyhydroxylated bicyclic azetidines as a new class of iminosugars

We report herein the development of a stereodivergent route towards polyhydroxylated bicyclic azetidine scaffolds, namely 6-azabicyclo[3.2.0]heptane derivatives. The strategy hinges on a common bicyclic β-lactam precursor, which is forged by way of a rare example of a cationic Dieckmann-type reaction, followed by IBX-mediated desaturation. Substrate-controlled diastereoselective oxidations then allow the divergent preparation of novel iminosugar mimics.

Asymmetric Synthesis of Axinellamines A and B

Axinellamines A and B are broad-spectrum antibacterial pyrrole-imidazole alkaloids that have a complex polycyclic skeleton. A new asymmetric synthesis of these marine sponge metabolites is described herein, featuring an oxidative rearrangement and an anchimeric chlorination reaction.

Total synthesis of palau'amine

Palau'amine has received a great deal of attention in the past two decades as an attractive synthetic target by virtue of its intriguing molecular architecture and significant immunosuppressive activity. Here we report the total synthesis of palau'amine characterized by the construction of an ABDE tetracyclic ring core including a trans-bicylo[3.3.0]octane skeleton at a middle stage of total synthesis. The ABDE tetracyclic ring core is constructed by a cascade reaction of a cleavage of the N-N bond, including simultaneous formation of imine, the addition of amide anion to the resulting imine (D-ring formation) and the condensation of pyrrole with methyl ester (B-ring formation) in a single step. The synthetic palau'amine is confirmed to exhibit excellent immunosuppressive activity. The present synthetic route has the potential to help elucidate a pharmacophore as well as the mechanistic details of immunosuppressive activity.

Synthesis of the Carbocyclic Core of Massadine

The carbocyclic core of massadine has been synthesized relying on a stereoselective formal [3 + 2] cycloaddition of lithiumtrimethylsilyldiazomethane with α,β-unsaturated esters to form a Δ(2)-pyrazoline moiety followed by facile N-N bond cleavage. A unique feature of the current approach is the direct installation of the tertiary α-amino center and a β-cyano group in a cis arrangement on the resulting cyclopentane framework via a previously developed formal aminocyanation protocol.

Syntheses of Cyclic Guanidine-Containing Natural Products

Naturally occurring guanidine derivatives frequently display medicinally useful properties. Among them, the higher order pyrrole-imidazole alkaloids, the dragmacidins, the crambescidins/batzelladines, and the saxitoxins/tetradotoxins have stimulated the development of many new synthetic methods over the past decades. We provide here an overview of the syntheses of these cyclic guanidine-containing natural products.

Dimeric pyrrole-imidazole alkaloids: synthetic approaches and biosynthetic hypotheses

The pyrrole-imidazole alkaloids are a group of structurally unique and biologically interesting marine sponge metabolites. Among them, the cyclic dimers have caught synthetic chemists' attention particularly. Numerous synthetic strategies have been developed and various biosynthetic hypotheses have been proposed for these fascinating natural products. We discuss herein the synthetic approaches and the biosynthetic insights obtained from these studies.

Enantioselective synthesis of angularly substituted 1-azabicylic rings: coupled dynamic kinetic epimerization and chirality transfer

A new strategy for enantioselective synthesis of azacyclic molecules in which dynamic kinetic equilibration of diastereomeric iminium ions precedes a stereochemistry-determining sigmatropic rearrangement is reported. The method is illustrated by the synthesis, in high enantiomeric purity (generally 95-99% ee), of a variety of 1-azabicyclic molecules containing angular allyl or 3-substituted 2-propenyl side chains adjacent to nitrogen and up to three stereogenic centers. In these products, the size of the carbocyclic ring is varied widely (5-12 membered); however, useful yields are obtained in forming 1-azabicyclic products containing only fused pyrrolidine and piperidine rings. Chirality transfer from substituents at carbons 1 and 2 of the 3-butenylamine fragment of the starting material is investigated, with methyl and phenyl substituents at the allylic position shown to provide exquisite stereocontrol (generally 98-99% chirality transfer). An attractive feature of the method is the ability to carry out the key transformation in the absence of solvent. Illustrated also is the high yielding conversion of four such products to a new family of bicyclic β-amino acids of high enantiomeric purity.

A biomimetic route for construction of the [4+2] and [3+2] core skeletons of dimeric pyrrole-imidazole alkaloids and asymmetric synthesis of ageliferins

The pyrrole-imidazole alkaloids have fascinated chemists for decades because of their unique structures. The high nitrogen and halogen contents and the densely functionalized skeletons make their laboratory synthesis challenging. We describe herein an oxidative method for accessing the core skeletons of two classes of pyrrole-imidazole dimers. This synthetic strategy was inspired by the putative biosynthesis pathways and its development was facilitated by computational studies. Using this method, we have successfully prepared ageliferin, bromoageliferin, and dibromoageliferin in their natural enantiomeric form.

Enantioselective total syntheses of (-)-palau'amine, (-)-axinellamines, and (-)-massadines

Dimeric pyrrole-imidazole alkaloids represent a rich and topologically unique class of marine natural products. This full account will follow the progression of efforts that culminated in the enantioselective total syntheses of the most structurally ornate members of this family: the axinellamines, the massadines, and palau'amine. A bio-inspired approach capitalizing on the pseudo-symmetry of the members of this class is recounted, delivering a deschloro derivative of the natural product core. Next, the enantioselective synthesis of the chlorocyclopentane core featuring a scalable, catalytic, enantioselective Diels-Alder reaction of a 1-siloxydiene is outlined in detail. Finally, the successful divergent conversion of this core to each of the aforementioned natural products, and the ensuing methodological developments, are described.

Asymmetric synthesis of ageliferin

We describe herein an asymmetric synthesis of ageliferin. A Mn(III)-mediated oxidative radical cyclization reaction was used as the key step to construct the core skeleton of this pyrrole-imidazole dimer. This approach resembles the biogenic [4 + 2] dimerization in an intramolecular fashion.

Scalable, stereocontrolled total syntheses of (±)-axinellamines A and B

The development of a simple, efficient, scalable, and stereocontrolled synthesis of a common intermediate en route to the axinellamines, massadines, and palau'amine is reported. This completely new route was utilized to prepare the axinellamines on a gram scale. In a more general sense, three distinct and enabling methodological advances were made during these studies: (1) an ethylene glycol-assisted Pauson-Khand cycloaddition reaction, (2) a Zn/In-mediated Barbier-type reaction, and (3) a TfNH(2)-assisted chlorination-spirocyclization.

Biosynthesis, asymmetric synthesis, and pharmacology, including cellular targets, of the pyrrole-2-aminoimidazole marine alkaloids

The pyrrole-2-aminoimidazole (P-2-AI) alkaloids are a growing family of marine alkaloids, now numbering well over 150 members, with high topographical and biological information content. Their intriguing structural complexity, rich and compact stereochemical content, high N to C ratio (~1 : 2), and increasingly studied biological activities are attracting a growing number of researchers from numerous disciplines world-wide. This review surveys advances in this area with a focus on the structural diversity, biosynthetic hypotheses with increasing, but still rare, verifying experimental studies, asymmetric syntheses, and biological studies, including cellular target receptor isolation studies, of this stimulating and exciting alkaloid family.

Natural products synthesis: enabling tools to penetrate Nature's secrets of biogenesis and biomechanism

Selected examples from our laboratory of how synthetic technology platforms developed for the total synthesis of several disparate families of natural products was harnessed to penetrate biomechanistic and/or biosynthetic queries is discussed. Unexpected discoveries of biomechanistic reactivity and/or penetrating the biogenesis of naturally occurring substances were made possible through access to substances available only through chemical synthesis. Hypothesis-driven total synthesis programs are emerging as very useful conceptual templates for penetrating and exploiting the inherent reactivity of biologically active natural substances. In many instances, new enabling synthetic technologies were required to be developed. The examples demonstrate the often untapped richness of complex molecule synthesis to provide powerful tools to understand, manipulate and exploit Nature's vast and creative palette of secondary metabolites.

Diversity-oriented approach to pyrrole-imidazole alkaloid frameworks

An exploration of imidazolylpropargyl amides as linchpin synthons for the construction of a diverse array of heterocyclic frameworks, many of which are related to those found in the oroidin derived alkaloids, is described. One such intermediate has been used in a formal total synthesis of cyclooroidin.