Total Synthesis of Apoptolidin: Construction of Enantiomerically Pure Fragments

Expeditious Asymmetric Synthesis of Polypropionates Relying on Sulfur Dioxide-Induced C–C Bond Forming Reactions

For a long time, the organic chemistry of sulfur dioxide (SO2) consisted of sulfinates that react with carbon electrophiles to generate sulfones. With alkenes and other unsaturated compounds, SO2 generates polymeric materials such as polysulfones. More recently, H-ene, sila-ene and hetero-Diels–Alder reactions of SO2 have been realized under conditions that avoid polymer formation. Sultines resulting from the hetero-Diels–Alder reactions of conjugated dienes and SO2 are formed more rapidly than the corresponding more stable sulfolenes resulting from the cheletropic additions. In the presence of a protic or Lewis acid catalyst, the sultines derived from 1-alkoxydienes are ionized into zwitterionic intermediates bearing 1-alkoxyallylic cation moieties which react with electro-rich alkenes such as enol silyl ethers and allylsilanes with high stereoselectivity. (C–C-bond formation through Umpolung induced by SO2). This produces silyl sulfinates that react with carbon electrophiles to give sulfones (one-pot four component asymmetric synthesis of sulfones), or with Cl2, generating the corresponding sulfonamides that can be reacted in situ with primary and secondary amines (one-pot four component asymmetric synthesis of sulfonamides). Alternatively, Pd-catalyzed desulfinylation generates enantiomerically pure polypropionate stereotriads in one-pot operations. The chirons so obtained are flanked by an ethyl ketone moiety on one side and by a prop-1-en-1-yl carboxylate group on the other. They are ready for two-directional chain elongations, realizing expeditious synthesis of long-chain polypropionates and polyketides. The stereotriads have also been converted into simpler polypropionates such as the cyclohexanone moiety of baconipyrone A and B, Kishi’s stereoheptad unit of rifamycin S, Nicolaou’s C1–C11-fragment and Koert’s C16–CI fragment of apoptolidin A. This has also permitted the first total synthesis of (-)-dolabriferol.

Enantioselective Total Synthesis of (+)-Heilonine

Chemical transformations that rapidly and efficiently construct a high level of molecular complexity in a single step are perhaps the most valuable in total synthesis. Among such transformations is the transition metal catalyzed [2 + 2 + 2] cycloisomerization reaction, which forges three new C-C bonds and one or more rings in a single synthetic operation. We report here a strategy that leverages this transformation to open de novo access to the Veratrum family of alkaloids. The highly convergent approach described herein includes (i) the enantioselective synthesis of a diyne fragment containing the steroidal A/B rings, (ii) the asymmetric synthesis of a propargyl-substituted piperidinone (F ring) unit, (iii) the high-yielding union of the above fragments, and (iv) the intramolecular [2 + 2 + 2] cycloisomerization reaction of the resulting carbon framework to construct in a single step the remaining three rings (C/D/E) of the hexacyclic cevanine skeleton. Efficient late-stage maneuvers culminated in the first total synthesis of heilonine (1), achieved in 21 steps starting from ethyl vinyl ketone.

Expedient Total Syntheses of Pladienolide-Derived Spliceosome Modulators

Atom and step economical total syntheses of spliceosome modulating natural products pladienolides A and B are described. The strategic functionalization of an unsaturated macrolide precursor enabled the most concise syntheses of these natural products to date and provides convenient, flexible access to stereodefined macrolides to streamline medicinal chemistry explorations. Notably, this synthetic route does not depend on protecting group manipulations that traditionally define synthesis planning for polyhydroxylated natural products of polyketide origin. Its utility is further demonstrated by the enantioselective total synthesis of H3B-8800, a hitherto semisynthetic pladienolide-derived spliceosome modulator undergoing clinical trials for hematological malignancies.

Modular Approaches to Lankacidin Antibiotics

Lankacidins are a class of polyketide natural products isolated from Streptomyces spp. that show promising antimicrobial activity. Owing to their complex molecular architectures and chemical instability, structural assignment and derivatization of lankacidins are challenging tasks. Herein we describe three fully synthetic approaches to lankacidins that enable access to new structural variability within the class. We use these routes to systematically generate stereochemical derivatives of both cyclic and acyclic lankacidins. Additionally, we access a new series of lankacidins bearing a methyl group at the C4 position, a modification intended to increase chemical stability. In the course of this work, we discovered that the reported structures for two natural products of the lankacidin class were incorrect, and we determine the correct structures of 2,18-seco-lankacidinol B and iso-lankacidinol. We also evaluate the ability of several iso- and seco-lankacidins to inhibit the growth of bacteria and to inhibit translation in vitro. This work grants insight into the rich chemical complexity of this class of antibiotics and provides an avenue for further structural derivatization.

Chagosensine: A Riddle Wrapped in a Mystery Inside an Enigma

The marine macrolide chagosensine is supposedly distinguished by a (Z,Z)-configured 1,3-chlorodiene contained within a highly strained 16-membered lactone ring, which also incorporates two trans-2,5-disubstituted tetrahydrofuran (THF) rings; this array is unique. After our initial synthesis campaign had shown that the originally proposed structure is incorrect, the published data set was critically revisited to identify potential mis-assignments. The "northern" THF ring and the anti-configured diol in the "southern" sector both seemed to be sites of concern, thus making it plausible that a panel of eight diastereomeric chagosensine-like compounds would allow the puzzle to be solved. To meet the challenge, the preparation of the required building blocks was optimized, and a convergent strategy for their assembly was developed. A key role was played by the cobalt-catalyzed oxidative cyclization of alken-5-ol derivatives ("Mukaiyama cyclization"), which is shown to be exquisitely chemoselective for terminal alkenes, leaving even terminal alkynes (and other sites of unsaturation) untouched. Likewise, a palladium-catalyzed alkyne alkoxycarbonylation reaction with formation of an α-methylene-γ-lactone proved instrumental, which had not found application in natural product synthesis before. Further enabling steps were a nickel-catalyzed "Tamaru-type" homocrotylation, stereodivergent aldehyde homologations, radical hydroindation, and palladium-catalyzed alkyne-1,2-bis-stannation. The different building blocks were assembled in a serial fashion to give the idiosyncratic chlorodienes by an unprecedented site-selective Stille coupling followed by copper-mediated tin/chlorine exchange. The macrolactones were closed under forcing Yamaguchi conditions, and the resulting products were elaborated into the targeted compound library. Yet, only one of the eight diastereomers turned out to be stable in the solvent mixture that had been used to analyze the natural product; all other isomers were prone to ring opening and/or ring expansion. In addition to this stability issue, our self-consistent data set suggests that chagosensine has almost certainly little to do with the structure originally proposed by the isolation team.

A facile synthesis of mesoporous graphitic carbon nitride supported palladium nanoparticles as highly effective and reusable catalysts for Stille coupling reactions under mild conditions

This paper reports a facile one-pot synthesis of mesoporous graphitic carbon nitride (mpg-CN) supported Pd NPs, denoted as mpg-CN/Pd, as highly efficient catalysts for Stille C–C coupling reactions under mild conditions.

Alkynylation of Pentose Derivatives with Stereochemical Fidelity: Implications for the Regioselectivity of Alkynyl Diol Cycloisomerizations to Cyclic Enol Ethers

This work characterizes a previously undetected epimerization in the preparation of alkynyl diols from pentose precursors utilizing the Ohira-Bestmann reagent. Lithium trimethylsilyldiazomethane (Colvin reagent) additions to the d-ribose and d-lyxose-derived benzylidene acetals provide the respective alkynyl diol stereoisomers, without epimerization. Regioselective tungsten-catalyzed cycloisomerizations of the d-ribose- and d-lyxose-derived alkynyl diols yield rigid bicyclic pyranose glycals, confirming the stereochemical fidelity of the Colvin alkynylation process.

A synthetic study toward the core structure of (-)-apicularen A

A concise synthetic strategy towards the core structure of (-)-apicularen A has been described in an 11-step synthetic sequence from a known intermediate. The key steps include tandem isomerization followed by C-O and C-C bond-forming reactions and iodocyclization strategies for the synthesis of a bicyclic ether embedded in the macrolactone ring. The applied reagent-controlled Keck-Maruoka allylation, Lin Pu alkynylation and Ricket-Diels-Alder reactions were used to simplify the synthetic sequence of related natural products. An intramolecular Yamaguchi lactonization constructed the macrolactone core, while the attempt to install the C11 hydroxyl chiral centre either under catalytic hydrogenation conditions or oxidative conditions was not successful.

Synthesis, Structural Reassignment, and Antibacterial Evaluation of 2,18-Seco-Lankacidinol B

Lankacidins are a group of polyketide natural products with activity against several strains of Gram-positive bacteria. We developed a route to stereochemically diverse variants of 2,18-seco-lankacidinol B and found that the stereochemical assignment at C4 requires revision. This has interesting implications for the biosynthesis of natural products of the lankacidin class, all of which possessed uniform stereochemistry prior to this finding. We have evaluated 2,18-seco-lankacidinol B and three stereochemical derivatives against a panel of pathogenic Gram-positive and Gram-negative bacteria.

Stereocontrolled Total Synthesis of Nonenolide

Nonenolide (1) was first isolated from the entomopathogenic fungus Cordyceps militaries BCC2816 and exhibited good antimalarial activity against Plasmodium falciparum K1. Structurally, it features a decanolide with a trans-double bond attached to two chiral hydroxy groups, making the total synthesis of the exclusive isomer of 1 more difficult. Herein, we report the successful synthesis of 1 by employing a MacMillan α-hydroxylation to generate three chiral centers in both the key fragments, starting from 1,6-hexanediol and 1,4-butanediol, followed by Steglich esterification of compounds 2 and 3. The exclusive E-isomer was obtained via a ring-closing metathesis of the mono-PMB-protected diene 19. Deprotection provided the required natural product 1.

Total Synthesis, Stereochemical Revision, and Biological Reassessment of Mandelalide A: Chemical Mimicry of Intrafamily Relationships

Mandelalide A and three congeners had recently been isolated as the supposedly highly cytotoxic principles of an ascidian collected off the South African coastline. Since these compounds are hardly available from the natural source, a concise synthesis route was developed, targeting structure 1 as the purported representation of mandelalide A. The sequence involves an iridium-catalyzed two-directional Krische allylation and a cobalt-catalyzed carbonylative epoxide opening as entry points for the preparation of the major building blocks. The final stages feature the first implementation of terminal acetylene metathesis into natural product total synthesis, which is remarkable in that this class of substrates had been beyond the reach of alkyne metathesis for decades. Synthetic 1, however, proved not to be identical with the natural product. In an attempt to clarify this issue, NMR spectra were simulated for 20 conceivable diastereomers by using DFT followed by DP4 analysis; however, this did not provide a reliable assignment either. The puzzle was ultimately solved by the preparation of three diastereomers, of which compound 6 proved identical with mandelalide A in all analytical and spectroscopic regards. As the entire "northern sector" about the tetrahydrofuran ring in 6 shows the opposite configuration of what had originally been assigned, it is highly likely that the stereostructures of the sister compounds mandelalides B-D must be corrected analogously; we propose that these natural products are accurately represented by structures 68-70. In an attempt to prove this reassignment, an entry into mandelalides C and D was sought by subjecting an advanced intermediate of the synthesis of 6 to a largely unprecedented intramolecular Morita-Baylis-Hillman reaction, which furnished the γ-lactone derivative 74 as a mixture of diastereomers. Whereas (24R)-74 was amenable to a hydroxyl-directed dihydroxylation by using OsO4 /TMEDA as the reagent, the sister compound (24S)-74 did not follow a directed path but simply obeyed Kishi's rule; only this unexpected escape precluded the preparation of mandelalides C and D by this route. A combined spectroscopic and computational (DFT) study showed that the reasons for this strikingly different behavior of the two diastereomers of 74 are rooted in their conformational peculiarities. This aspect apart, our results show that the OsO4 /TMEDA complex reacts preferentially with electron deficient double bonds even if other alkenes are present that are more electron rich and less encumbered. Finally, in a brief biological survey authentic mandelalide A (6) was found to exhibit appreciable cytotoxicity only against one out of three tested human cancer cell lines and all synthetic congeners were hardly active. No significant fungicidal properties were observed.

Total synthesis of periploside A, a unique pregnane hexasaccharide with potent immunosuppressive effects

Periploside A is a pregnane hexasaccharide identified from the Chinese medicinal plant Periploca sepium, which features a unique seven-membered formyl acetal bridged orthoester (FABO) motif and potent immunosuppressive activities. Here, we show the synthesis of this molecule in a total of 76 steps with the longest linear sequence of 29 steps and 9.2% overall yield. The FABO motif is constructed via a combination of Sinaÿ's and Crich's protocol for the formation of orthoester and acetal glycosides, respectively. The 2-deoxy-β-glycosidic linkages are assembled stereoselectively with judicious choice of the glycosylation methods. The epimer at the spiro-quaternary carbon in the FABO motif has also been elaborated in a stereo-controlled manner. This epimer, as well as the synthetic analogues bearing the FABO motif, retain largely the inhibitory activities of periploside A against the proliferation of T-lymphocyte, indicating the importance of the chemical connection of the FABO motif to their immunosuppressive activity.

Fluorescent probes of the apoptolidins and their utility in cellular localization studies

Apoptolidin A has been described among the top 0.1% most-cell-selective cytotoxic agents to be evaluated in the NCI 60 cell line panel. The molecular structure of apoptolidin A consists of a 20-membered macrolide with mono- and disaccharide moieties. In contrast to apoptolidin A, the aglycone (apoptolidinone) shows no cytotoxicity (>10 μM) when evaluated against several tumor cell lines. Apoptolidin H, the C27 deglycosylated analogue of apoptolidin A, displayed sub-micromolar activity against H292 lung carcinoma cells. Selective esterification of apoptolidins A and H with 5-azidopentanoic acid afforded azido-functionalized derivatives of potency equal to that of the parent macrolide. They also underwent strain-promoted alkyne-azido cycloaddition reactions to provide access to fluorescent and biotin-functionalized probes. Microscopy studies demonstrate apoptolidins A and H localize in the mitochondria of H292 human lung carcinoma cells.

Constructing molecular complexity and diversity: total synthesis of natural products of biological and medicinal importance

The advent of organic synthesis and the understanding of the molecule as they occurred in the nineteenth century and were refined in the twentieth century constitute two of the most profound scientific developments of all time. These discoveries set in motion a revolution that shaped the landscape of the molecular sciences and changed the world. Organic synthesis played a major role in this revolution through its ability to construct the molecules of the living world and others like them whose primary element is carbon. Although the early beginnings of organic synthesis came about serendipitously, organic chemists quickly recognized its potential and moved decisively to advance and exploit it in myriad ways for the benefit of mankind. Indeed, from the early days of the synthesis of urea and the construction of the first carbon-carbon bond, the art of organic synthesis improved to impressively high levels of sophistication. Through its practice, today chemists can synthesize organic molecules--natural and designed--of all types of structural motifs and for all intents and purposes. The endeavor of constructing natural products--the organic molecules of nature--is justly called both a creative art and an exact science. Often called simply total synthesis, the replication of nature's molecules in the laboratory reflects and symbolizes the state of the art of synthesis in general. In the last few decades a surge in total synthesis endeavors around the world led to a remarkable collection of achievements that covers a wide ranging landscape of molecular complexity and diversity. In this article, we present highlights of some of our contributions in the field of total synthesis of natural products of biological and medicinal importance. For perspective, we also provide a listing of selected examples of additional natural products synthesized in other laboratories around the world over the last few years.

A more comprehensive and highly practical solution to enantioselective aldehyde crotylation

The enantioselective crotylation of aldehydes with 1,2-diaminochlorocrotylsilane reagents is effectively catalyzed by Sc(OTf)(3). The one significant limitation on the utility of these reagents--substrate scope--has thus been addressed. The net result is the most comprehensive and highly practical method for enantioselective aldehyde crotylation yet advanced.

Combined chemical and biosynthetic route to access a new apoptolidin congener

Glycosylation of a synthetic aglycone using precursor-directed biosynthesis is facilitated by a chemical ketosynthase "knockdown" of the apoptolidin producer Nocardiopsis sp. This synthetic approach facilitated the preparation of an unnatural disaccharide derivative of apoptolidin D that substantially restores cytotoxicity against H292 cells and deconvolutes the role of the decorating sugars in apoptolidin bioactivity.

Versatile synthesis of the C3-C14 domain of 7-deoxyokadaic acid

Described is a flexible synthesis of the C3-C14 domain of 7-deoxyokadaic acid that is amenable to facile structural diversification. Treatment of ynone 10 under acidic conditions led to net bis-conjugate addition of the latent diol to give the thermodynamically favored spiroketal 27, a versatile intermediate, en route to 7-deoxyokadaic acid and analogs.

Total synthesis of apoptolidin A

A highly convergent, enantioselective total synthesis of the potent antitumor agent apoptolidin A has been completed. The key transformations include highly selective glycosylations to attach the C27 disaccharide and the C9 6'-deoxy-l-glucose, a cross-metathesis to incorporate the C1-C10 trienoate unit, and a Yamaguchi macrolactonization to complete the macrocycle. Twelve stereocenters in the polypropionate segments and sugar units were established through diastereoselective chlorotitanium enolate aldol reactions.

Synthesis and evaluation of the cytotoxicity of apoptolidinones A and D

Apoptolidins A-D are microbial secondary metabolites shown to be selectively cytotoxic against several cancer cell lines and noncytotoxic against normal cells. Total syntheses of apoptolidinones A and D are reported. The efficient synthetic strategy leading to the apoptolidinones features construction of the common 20-membered macrolactone by an intramolecular Suzuki reaction and stereocontrolled aldol reactions establishing the C19/C20 and C22/C23 stereocenters. In contrast to apoptolidin A, the aglycones apoptolidinone A and D were shown to be noncytotoxic when evaluated against human lung cancer cells (H292).