Asymmetric, Stereocontrolled Total Synthesis of Paraherquamide A This work was supported by the NIH (Grant CA 70375). The Japanese Society for the Promotion of Science (JSPS) is acknowledged for providing support to H.T. Mass spectra were obtained on instruments supported by the NIH Shared Instrumentation Grant (GM 49631). We also wish to thank Dr. Timothy Blizzard (Merck & Co.) for providing an authentic sample of natural paraherquamide A. We also wish to acknowledge Dr. Byung H. Lee (Pharmacia-Upjohn Co.) for providing NMR spectra and an authentic specimen of 14-oxoparaherquamide B. We also wish to thank Dr. Alfredo Vazquez for assistance with purification and characterization of synthetic paraherquamide A

Total synthesis of complex 2,5-diketopiperazine alkaloids

The 2,5-diketopiperazine (DKP) motif is present in many biologically relevant, complex natural products. The cyclodipeptide substructure offers structural rigidity and stability to proteolysis that makes these compounds promising candidates for medical applications. Due to their fascinating molecular architecture, synthetic organic chemists have focused significant effort on the total synthesis of these compounds. This review covers many such efforts on the total synthesis of DKP containing complex alkaloid natural products.

Structural and stereochemical diversity in prenylated indole alkaloids containing the bicyclo[2.2.2]diazaoctane ring system from marine and terrestrial fungi

Covering: up to February 2017 Various fungi of the genera Aspergillus, Penicillium, and Malbranchea produce prenylated indole alkaloids possessing a bicyclo[2.2.2]diazaoctane ring system. After the discovery of distinct enantiomers of the natural alkaloids stephacidin A and notoamide B, from A. protuberus MF297-2 and A. amoenus NRRL 35660, another fungi, A. taichungensis, was found to produce their diastereomers, 6-epi-stephacidin A and versicolamide B, as major metabolites. Distinct enantiomers of stephacidin A and 6-epi-stephacidin A may be derived from a common precursor, notoamide S, by enzymes that form a bicyclo[2.2.2]diazaoctane core via a putative intramolecular hetero-Diels-Alder cycloaddition. This review provides our current understanding of the structural and stereochemical homologies and disparities of these alkaloids. Through the deployment of biomimetic syntheses, whole-genome sequencing, and biochemical studies, a unified biogenesis of both the dioxopiperazine and the monooxopiperazine families of prenylated indole alkaloids constituted of bicyclo[2.2.2]diazaoctane ring systems is presented.

Oxidative radical cyclizations of diketopiperazines bearing an amidomalonate unit. Heterointermediate reaction sequences toward the asperparalines and stephacidins

A novel approach to the diazabicyclo[2.2.2]octane core of prenylated bridged diketopiperazine alkaloids is described by direct oxidative cyclizations of functionalized diketopiperazines mediated by ferrocenium hexafluorophosphate or the Mn(OAc)₃•2H₂O/Cu(OTf)₂ system. Divergent reaction pathways take place depending on the substitution pattern of the substrates and the oxidation conditions such as temperature or the presence or absence of persistent radical TEMPO. For ester-substituted diketopiperazines, the ester group exerts a significant influence on the reaction outcome and stereochemistry of the radical cyclizations.

A Novel Spirooxindole Derivative Inhibits the Growth of Leishmania donovani Parasites both In Vitro and In Vivo by Targeting Type IB Topoisomerase

Visceral leishmaniasis is a fatal parasitic disease, and there is an emergent need for development of effective drugs against this neglected tropical disease. We report here the development of a novel spirooxindole derivative, N-benzyl-2,2'α-3,3',5',6',7',7α,α'-octahydro-2methoxycarbonyl-spiro[indole-3,3'-pyrrolizidine]-2-one (compound 4c), which inhibits Leishmania donovani topoisomerase IB (LdTopIB) and kills the wild type as well as drug-resistant parasite strains. This compound inhibits catalytic activity of LdTopIB in a competitive manner. Unlike camptothecin (CPT), the compound does not stabilize the DNA-topoisomerase IB cleavage complex; rather, it hinders drug-DNA-enzyme covalent complex formation. Fluorescence studies show that the stoichiometry of this compound binding to LdTopIB is 2:1 (mole/mole), with a dissociation constant of 6.65 μM. Molecular docking with LdTopIB using the stereoisomers of compound 4c produced two probable hits for the binding site, one in the small subunit and the other in the hinge region of the large subunit of LdTopIB. This spirooxindole is highly cytotoxic to promastigotes of L. donovani and also induces apoptosis-like cell death in the parasite. Treatment with compound 4c causes depolarization of mitochondrial membrane potential, formation of reactive oxygen species inside parasites, and ultimately fragmentation of nuclear DNA. Compound 4c also effectively clears amastigote forms of wild-type and drug-resistant parasites from infected mouse peritoneal macrophages but has less of an effect on host macrophages. Moreover, compound 4c showed strong antileishmanial efficacies in the BALB/c mouse model of leishmaniasis. This compound potentially can be used as a lead for developing excellent antileishmanial agents against emerging drug-resistant strains of the parasite.

Scandium triflate-catalyzed selective ring opening and rearrangement reaction of spiro-epoxyoxindole and carbonyl compounds

Scandium triflate-catalyzed reactions between spiro-epoxyoxindole and carbonyl compounds have been disclosed. Furthermore, an unprecedented rearrangement takes place to yield alkylidene oxindoles when aromatic aldehydes are used as carbonyl components.

Multicomponent reaction to construct spirocyclic oxindoles with a Michael (triple Michael)/cyclization cascade sequence as the key step

Multicomponent cycloadditions with readily available isocyanides, allenoates, and isatylidene malononitriles are disclosed. This reaction, which does not require the aid of any catalyst, allows the efficient syntheses of spirocyclic oxindoles with excellent regioselectivity. Reactions with ethyl 2,3-butadienoate and various structurally diverse α- and γ-substituted allenoates are also fully explored. Remarkably, we have shown that the usual three-component process can be further developed into an unprecedented four-component cycloaddition in the presence of water, which provides a new strategy to access highly unusual tricyclic oxindoles. From a synthetic point of view, this protocol is very interesting considering the high level of complexity reached in one step. The mechanism is thought to proceed by a triple Michael/cyclization process by using allenoate as a three carbon atom component (3 C). Furthermore, multicomponent reaction with γ-substituted allenoate also results in a very interesting conversion. In such cases, the unusual cleavage of the "C=C" double bond of isatylidene malononitrile and one of the "C=C" double bonds of allenoate is always observed.

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.

C-H functionalization logic in total synthesis

In this critical review, the strategic and economic benefits of C-H functionalization logic will be analyzed through the critical lens of total synthesis. In order to illustrate the dramatically simplifying effects this type of logic can potentially have on synthetic planning, we take the reader through a series of case studies in which it has already been successfully applied. In the first section, a chronological look at key historical syntheses will be examined, leading into modern day examples. In the second section, our own experience with applying and executing synthesis with a C-H functionalization "mindset" will be discussed (114 references).

Epothilone analogues with benzimidazole and quinoline side chains: chemical synthesis, antiproliferative activity, and interactions with tubulin

A series of epothilone B and D analogues bearing isomeric quinoline or functionalized benzimidazole side chains has been prepared by chemical synthesis in a highly convergent manner. All analogues have been found to interact with the tubulin/microtubule system and to inhibit human cancer cell proliferation in vitro, albeit with different potencies (IC(50) values between 1 and 150 nM). The affinity of quinoline-based epothilone B and D analogues for stabilized microtubules clearly depends on the position of the N-atom in the quinoline system, while the induction of tubulin polymerization in vitro appears to be less sensitive to N-positioning. The potent inhibition of human cancer cell growth by epothilone analogues bearing functionalized benzimidazole side chains suggests that these systems might be conjugated with tumor-targeting moieties to form tumor-targeted prodrugs.

Studies on Paraherquamide Biosynthesis: Synthesis of Deuterium-Labeled 7-Hydroxy-Pre-Paraherquamide, a Putative Precursor of Paraherquamides A, E & F

The stereocontrolled, asymmetric synthesis of triply deuterium-labeled 7-hydroxy-pre-paraherquamide (27) was accomplished, employing a diastereoselective intramolecular S(N)2' cyclization strategy. The deuterium-labeled substrate was interrogated in a precursor incorporation experiment in the paraherquamide-producing organism Penicillium fellutanum. The isolated sample of paraherquamide A revealed incorporation of one of the two geminal deuterons of the CD(2)-group at C-12 exclusively. The lack of signals for the second deuteron of the CD(2)-group at C-12 and for the CH(2)D-group (C-22/C-23) suggests that this substrate suffered an unexpectedly selective catabolic degradation and metabolic re-incorporation of deuterium thus casting doubt on the proposed biosynthetic intermediacy of 27. Consideration of alternative biosynthetic pathways, including oxidation of the indole C-6 position prior to hydroxylation at C-7 or oxidative spiro-contraction of pre-paraherquamide prior to construction of the dioxepin is discussed. The synthesis of 27 also provides for a concise, asymmetric stereocontrolled synthesis of an advanced intermediate that will be potentially useful in the synthesis of paraherquamide E & F.

Total syntheses of (+/-)-crinine, (+/-)-crinamine, and (+/-)-6a-epi-crinamine via the regioselective synthesis and Diels-Alder reaction of 3-aryl-5-bromo-2-pyrone

We have devised a new unified synthetic protocol to (+/-)-crinine, (+/-)-crinamine, and (+/-)-6a- epi-crinamine from the Diels-Alder cycloadduct of 3-(3,4-methylenedioxyphenyl)-5-bromo-2-pyrone with TBS vinyl ether. The requisite 3-(3,4-methylenedioxyphenyl)-5-bromo-2-pyrone was prepared from the C3-selective Stille coupling reaction of 3,5-dibromo-2-pyrone. Also noted is that the vinyl bromide can be used as a handle for further derivatization.

Studies Towards Paraherquamides E & F and Related C-labeled Putative Biosynthetic Intermediates: Stereocontrolled Synthesis of the α-Alkyl-β-Methylproline Ring System

A substituted 2R-allyl-3S-methylproline ethyl ester suitable for elaboration to paraherquamides E, F and related (13)C-labelled putative biosynthesis intermediates have been prepared efficiently in six steps and 24% overall yield. The key steps are a 5-exo-trig cyclization of a zinc enolate on an unactivated alkene and a stereocontrolled alkylation of the enolate formed from 3S-methyl-pyrrolidine-1,2R-dicarboxylic acid 1-tert-butyl ester 2-ethyl ester.

C-H Bond Functionalizations with Palladium(II): Intramolecular Oxidative Annulations of Arenes

Oxidative annulations for the synthesis of carbocycles were developed using a catalytic palladium(II) system. Indoles with pendant olefin tethers were oxidatively cyclized to form annulated products. Electron-rich aromatic systems were also investigated, culminating in the synthesis of benzofurans and dihydrobenzofurans by a similar protocol. These reactions were demonstrated to proceed by an initial C-H bond functionalization event, followed by olefin insertion and β-hydride elimination.

Stereoselective Synthesis of Quaternary Proline Analogues

This review describes available methods for the diastereoselective and asymmetric synthesis of quaternary prolines. The focus is on the preparation of alpha-functionalized prolines with the pyrrolidine moiety not embedded in a polycyclic frame. The diverse synthetic approaches are classified according to the bond which is formed to complete the quaternary skeleton.

Cascade Reactions in Total Synthesis

The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Synthesis of the C3−C19 Segment of Phorboxazole B

[reaction: see text]. Three segment-coupling Prins approaches to the C3-C19 segment of phorboxazole B have been developed. One successful strategy utilized a novel TMSBr-mediated cyclization that proceeded with complete axial selectivity. Displacement of bromide with cesium acetate provided the C13 hydroxyl stereocenter of 22. Additionally, treatment of alpha-acetoxy ether 20 with TFA enabled a more concise synthesis of the C3-C19 target 13 by allowing direct access to the equatorial alcohol.

Samarium Diiodide Promoted Generation and Asymmetric Hydroxyalkylation of N,O-Diprotected (3S)-3-Pyrrolidinol 2-Carbanions

The N,O-diprotected chiral nonracemic 2-pyridyl 3-pyrrolidinol-2-yl sulfide 5a undergoes efficient SmI(2) mediated reduction to give the N,O-diprotected 3-pyrrolidinol 2-carbanion intermediate D, which reacted under Barbier-type conditions with ketones and aldehydes to afford the protected N-alpha-hydroxyalkyl-3-pyrrolidines 10b-h with excellent diastereoselectivity at the newly formed chiral center in the pyrrolidine ring. Application of the present method led to the formal asymmetric syntheses of (2R,3S)-2-hydroxymethyl-3-pyrrolidinol (2) and (2S,3S)-3-hydroxyproline (12). [reaction: see text]

Asymmetric Synthesis of the Tetrahydropyran Ring, C32−C38 Fragment, of Phorboxazoles

The asymmetric synthesis of a model aldehyde (2R,6R)-2 and the C32-C38 fragment of phorboxazoles, (2R,4R,6R)-1, is described using a sulfoxide as chiral auxiliary. Key advances include the stereoselective reductions of beta-keto- or beta,gamma-diketosulfoxides, the acid-catalyzed cyclization of enantiopure sulfinyl hydroxy ketone precursors to the tetrahydropyran ring, and the Pummerer reaction on the pendant sulfoxide to create the formyl group.

Sulfur ylide-initiated thio-claisen rearrangements. The synthesis of highly substituted indolines

The coupling of rhodium carbenoids from vinyl diazoacetates with 2-thio-3-alkyl indoles was found to generate C(3) quaternary substituted indolines via a thionium ylide-initiated [3,3]-sigmatropic rearrangement.

Total synthesis and biosynthesis of the paraherquamides: an intriguing story of the biological Diels-Alder construction

The syntheses and biosyntheses of the paraherquamide and brevianamide families of prenylated indole-derived alkaloids are reviewed. It has been proposed that the unique bicyclo[2.2.2]diazaoctan ring system that is common to this family of natural products, arises by a biological intramolecular Diels-Alder cycloaddition reaction. Both synthetic approaches and total syntheses of several members of this family of natural products are reviewed. The biosynthesis of these alkaloids has also constituted an active area of research and the current state of knowledge on the biosynthesis of these natural products are reviewed.