Synthesis of Stereopentad Subunits of Zincophorin and Rifamycin-S through Use of Chiral Allenyltin Reagents

Diastereodivergent and Enantioselective Synthesis of Homoallylic Alcohols via Nickel-Catalyzed Borylative Coupling of 1,3-Dienes with Aldehydes

We present the first enantioselective nickel-catalyzed borylative coupling of 1,3-dienes with aldehydes, providing an efficient route to highly valuable homoallylic alcohols in a single step. The reaction involves the 1,4-carboboration of dienes, leading to the formation of C-C and C-B bonds accompanied by the construction of two continuous stereogenic centers. Enabled by a chiral spiro phosphine-oxazoline nickel complex, this transformation yields products with exceptional diastereoselectivity, E-selectivity, and enantioselectivity. The diastereoselectivity of the reaction can be controlled by employing either (Z)-1,3-dienes or (E)-1,3-dienes.

Recent Advances in the Synthesis of Propargyl Derivatives, and Their Application as Synthetic Intermediates and Building Blocks

The propargyl group is a highly versatile moiety whose introduction into small-molecule building blocks opens up new synthetic pathways for further elaboration. The last decade has witnessed remarkable progress in both the synthesis of propargylation agents and their application in the synthesis and functionalization of more elaborate/complex building blocks and intermediates. The goal of this review is to highlight these exciting advances and to underscore their impact.

Epoxide-Based Synthetic Approaches toward Polypropionates and Related Bioactive Natural Products

Polypropionate units are a common structural feature of many of the natural products in polyketides, some of which have shown a broad range of antimicrobial and therapeutic potential. Polypropionates are composed of a carbon skeleton with alternating methyl and hydroxy groups with a specific configuration. Different approaches have been developed for the synthesis of polypropionates and herein we include, for the first time, all of the epoxide-based methodologies that have been reported over the years by several research groups such as Kishi, Katsuki, Marashall, Miyashita, Prieto, Sarabia, Jung, McDonald, etc. Several syntheses of polypropionate fragments and natural products that employed epoxides as key intermediates have been described and summarized in this review. These synthetic approaches involve enatio- and diastereoselective synthesis of epoxides (epoxy-alcohols, epoxy-amides, and epoxy-esters) and their regioselective cleavage with carbon and/or hydride nucleophiles. In addition, we included a description of the isolation and biological activities of the polypropionates and related natural products that have been synthetized using epoxide-based approaches. In conclusion, the epoxide-based methodologies are a non-aldol alternative approach for the construction of polypropionate.

Stereochemical Control via Chirality Pairing: Stereodivergent Syntheses of Enantioenriched Homoallylic Alcohols

We report herein the development of stereodivergent syntheses of enantioenriched homoallylic alcohols using chiral nonracemic α-CH₂ Bpin-substituted crotylboronate. Chiral phosphoric acid (S)-A-catalyzed asymmetric allyl addition with the reagent gave Z-anti-homoallylic alcohols with excellent enantioselectivities and Z-selectivities. When the enantiomeric acid catalyst (R)-A was utilized, the stereoselectivity was completely reversed and E-anti-homoallylic alcohols were obtained with high E-selectivities and excellent enantioselectivities. By pairing the chirality of the boron reagent with the catalyst, two complementary stereoisomers of chiral homoallylic alcohols can be obtained selectively from the same boron reagent. DFT computational studies were conducted to probe the origins of the observed stereoselectivity. These reactions generate highly enantioenriched homoallylic alcohol products that are valuable for rapid construction of polyketide structural frameworks.

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.

Evolution of an Efficient and Scalable Nine-Step (Longest Linear Sequence) Synthesis of Zincophorin Methyl Ester

Because of both their synthetically challenging and stereochemically complex structures and their wide range of often clinically relevant biological activities, nonaromatic polyketide natural products have for decades attracted an enormous amount of attention from synthetic chemists and played an important role in the development of modern asymmetric synthesis. Often, such compounds are not available in quantity from natural sources, rendering analogue synthesis and drug development efforts extremely resource-intensive and time-consuming. In this arena, the quest for ever more step-economical and efficient methods and strategies, useful and important goals in their own right, takes on added importance, and the most useful syntheses will combine high levels of step-economy with efficiency and scalability. The nonaromatic polyketide natural product zincophorin methyl ester has attracted significant attention from synthetic chemists due primarily to the historically synthetically challenging C(8)-C(12) all-anti stereopentad. While great progress has been made in the development of new methodologies to more directly address this problem and as a result in the development of more highly step-economical syntheses, a synthesis that combines high levels of step economy with high levels of efficiency and scalability has remained elusive. To address this problem, we have devised a new synthesis of zincophorin methyl ester that proceeds in just nine steps in the longest linear sequence and proceeds in 10% overall yield. Additionally, the scalability and practicability of the route have been demonstrated by performing all of the steps on a meaningful scale. This synthesis thus represents by a significant margin the most step-economical, efficient, and practicable synthesis of this stereochemically complex natural product reported to date, and is well suited to facilitate the synthesis of analogues and medicinal chemistry development efforts in a time- and resource-efficient manner.

Direct Generation of Triketide Stereopolyads via Merged Redox-Construction Events: Total Synthesis of (+)-Zincophorin Methyl Ester

(+)-Zincophorin methyl ester is prepared in 13 steps (longest linear sequence). A bidirectional redox-triggered double anti-crotylation of 2-methyl-1,3-propane diol directly assembles the triketide stereopolyad spanning C4-C12, significantly enhancing step economy and enabling construction of (+)-zincophorin methyl ester in nearly half the steps previously required.

Synthesis of a C1-C11 fragment of Zincophorin using planar chiral, neutral π-allyl iron complexes

A key step in the synthesis of a C1-C11 fragment of the ionophore antibiotic Zincophorin involves the addition of an α-alkoxyalkylcopper(I) reagent to a planar chiral, neutral π-allyl iron complex. The key allylic alkylation reaction is highly regio- and stereoselective with addition taking place at the γ-position anti to the metal centre.

Toward more "ideal" polyketide natural product synthesis: a step-economical synthesis of zincophorin methyl ester

A highly efficient and step-economical synthesis of zincophorin methyl ester has been achieved. The unprecedented step economy of this zincophorin synthesis is principally due to an application of the tandem silylformylation-crotylsilylation/Tamao oxidation-diastereoselective tautomerization reaction, which achieves in a single step what would typically require a significant multistep sequence.

Challenges in the synthesis of a unique mono-carboxylic acid antibiotic, (+)-zincophorin

(+)-Zincophorin, also referred to as M144255 or griseocholin, is a polyoxygenated ionophoric antibiotic that was isolated from Streptomyces griseus in 1984. It possesses strong in vivo activity against Gram-positive bacteria and Clostridium coelchii. Its methyl ester was reported in a patent as having strong inhibitory properties against influenza WSN/virus with reduced toxicity for the host cell. Its ability to strongly bind with Zn2+, which is also present in its X-ray structure, is the basis for its name. Over the last two decades, (+)-zincophorin has attracted an impressive array of synthetic efforts including Danishefsky's first total synthesis, along with two recent elegant total syntheses reported by Cossy and Miyashita as well as our own formal total synthesis. This review provides a comparison of the different synthetic efforts on this novel mono-carboxylic acid antibiotic and documents its interesting isolation, structure determination, and biological activities.

Tandem silylformylation-crotylsilylation/Tamao oxidation of internal alkynes: a remarkable example of generating complexity from simplicity

The rhodium-catalyzed tandem silylformylation-crotylsilylation reaction has been extended to include internal alkynes. Tamao oxidation of the initial product leads to the production of a substituted enol, which undergoes highly diastereoselective tautomerization. The resulting one-pot procedure fashions three new stereocenters, a ketone, and a terminal alkene from a butenyl group, a propynyl group, a silyl hydride, H2O2, and CO.

Use of sultines in the asymmetric synthesis of polypropionate antibiotics

At low temperature and in the presence of an acid catalyst, SO2 adds to 1,3-dienes equilibrating with the corresponding 3,6-dihydro-1,2-oxathiin-2-oxides (sultines). These compounds are unstable above -60 °C and equilibrate with the more stable 2,5-dihydrothiophene 1,1-dioxides (sulfolenes). The hetero-Diels-Alder additions of SO2 are suprafacial and follow the Alder endo rule. The sultines derived from 1-oxy-substituted and 1,3-dioxy-disubstituted 1,3-dienes cannot be observed at -100 °C but are believed to be formed faster than the corresponding sulfolenes. In the presence of acid catalysts, the 6-oxy-substituted sultines equilibrate with zwitterionic species that react with electron-rich alkenes such as enoxysilanes and allylsilanes, generating β,γ-unsaturated silyl sulfinates that can be desilylated and desulfinylated to generate polypropionate fragments containing up to three contiguous stereogenic centers and an (E)-alkene unit. Alternatively, the silyl sulfinates can be reacted with electrophiles to generate polyfunctional sulfones (one-pot, four-component synthesis of sulfones), or oxidized into sulfonyl chlorides and reacted with amines, then realizing a one-pot, four-component synthesis of polyfunctional sulfonamides. Using enantiomerically enriched dienes such as 1-[(R)- or 1-(S)-phenylethyloxy]-2-methyl-(E,E)-penta-1,3-dien-3-yl isobutyrate, derived from inexpensive (R)- or (S)-1-phenylethanol, enantiomerically enriched stereotriads are obtained in one-pot operations. The latter are ready for further chain elongation. This has permitted the development of expeditious total asymmetric syntheses of important natural products of biological interest such as the baconipyrones, rifamycin S, and apoptolidin A.

Regioselective Cleavage of 3,4-Epoxy Alcohols with Substituted Alkynylaluminum Reagents: Application to the Stereoselective Synthesis of Polypropionates

The reaction of 3-O-substituted propynyl aluminum reagents with a TIPS-protected 2,3-epoxy alcohol and several diastereomeric 2-methyl-3,4-epoxy alcohols offers a convenient synthetic approach for the subsequent preparation and epoxidation of allylic alcohols. The yields are low to moderate and the regioselectivity (internal vs external attack) similar to that of the standard diethylpropynyl aluminum reagent. The TMS-acetylide alane reagent gives improved yields and good regioselectivity, and constitutes the reagent of choice for the cleavage of the hindered epoxides. This attractive methodology is applied to the elaboration of two advanced polypropionate precursor fragments.

Chiral allylic and allenic metal reagents for organic synthesis

This account traces the evolution of our work on the synthesis of chiral allylic and allenic organometal compounds of tin, silicon, zinc, and indium and their application to natural product synthesis over the past quarter century.

(R)- and (S)-4-TIPS-3-butyn-2-ol. Useful Precursors of Chiral Allenylzinc and Indium Reagents

A convenient route to the enantiomers of 4-TIPS-3-butyn-2-ol of >95% enantiomeric purity by reduction of the ynone precursor 4 with the Noyori N-tosyl-1,2-diphenylethylenediamineruthenium cymene catalyst is described. The mesylate derivative of the (S) enantiomer (1c) is converted in situ to an allenylzinc or indium reagent in the presence of a catalyst derived from Pd(OAc)2 and Ph3P and either Et2Zn or InI. A second in situ addition of these reagents to aldehydes leads to anti homopropargylic alcohol adducts. The additions proceed in generally high (60-90%) yield with modest to excellent diastereoselectivity and high enantioselectivity. Only slight mismatching (<5%) is observed with chiral alpha-methyl and alpha-silyloxy aldehydes. Additions to alpha-substituted enals are highly diastereoselective, while beta,beta-disubstituted enals afford ca. 2:1 mixtures of anti and syn adducts.

Expeditious Asymmetric Synthesis of a Stereoheptad Corresponding to the C(19)-C(27)-Ansa Chain of Rifamycins: Formal Total Synthesis of Rifamycin S

In the presence of sulfur dioxide and an acid promoter, (-)-(1E,3Z)-2-methyl-1-((1S)-1-phenylethoxy)penta-1,3-dien-3-yl isobutyrate reacts with (Z)-3-(trimethylsilyloxy)pent-2-ene giving a silyl sulfinate intermediate that undergoes, in the presence of palladium catalyst, a desilylation and retro-ene elimination of SO(2) with formation of (-)-(1Z,2S,3R,4S)-1-ethylidene-2,4-dimethyl-5-oxo-3-((1S)-1-phenylethoxy)-heptyl isobutyrate as major product. This ethyl ketone undergoes cross-aldol reaction with (2S)-2-methyl-3-[(tert-butyldimethylsilyl)oxy]propanal giving an aldol that is reduced into a stereoheptad corresponding to the C(19)-C(27)-segment of Rifamycins with high diastereoselectivity and enantiomeric excess.

Total Synthesis of Zincophorin and Its Methyl Ester

A total synthesis of the naturally occurring ionophore zincophorin has been realized. The route features an intramolecular oxymercuration of a cyclopropanemethanol and a Carroll-Claisen rearrangement for the respective elaboration of the C1-C12 and C13-C25 subunits, which have been assembled by using a highly diastereoselective titanium-mediated aldol condensation.

Total Synthesis of Zincophorin Methyl Ester

[reaction: see text]. A convergent total synthesis of the methyl ester of zincophorin, an ionophore antibiotic, has been realized relying on a diastereoselective titanium-mediated aldol coupling between the C1-C12 and C13-C25 subunits. The latter fragment was prepared by using a Carroll-Claisen rearrangement.

Stereochemistry of the allylation and crotylation reactions of alpha-methyl-beta-hydroxy aldehydes with allyl- and crotyltrifluorosilanes. Synthesis of anti,anti-dipropionate stereotriads and stereodivergent pathways for the reactions with 2,3-anti- and 2,3-syn-alpha-methyl-beta-hydroxy aldehydes

A new method for the stereoselective synthesis of the anti,anti-dipropionate stereotriad via the reaction of alpha-methyl-beta-hydroxy aldehydes with (Z)-crotyltrifluorosilane (24) is described. These reactions were designed to occur through bicyclic transition states (e.g., 31) in which the silane reagent is covalently bound to the beta-hydroxyl group of the aldehyde and the crotyl group is transferred intramolecularly. This methodology was used to synthesize the C(7)-C(16) segment (58) of zincophorin, which contains a synthetically challenging all-anti stereopentad unit. Surprisingly, 2,3-anti- and 2,3-syn-alpha-methyl-beta-hydroxy aldehydes react in a stereodivergent manner with 24: 2,3-anti-beta-hydroxy aldehydes give the targeted anti,anti-dipropionate adducts with high selectivity, but the reactions of 2,3-syn-beta-hydroxy aldehydes are poorly selective. The stereodivergent behavior of 2,3-syn- vs 2,3-anti-alpha-methyl-beta-hydroxy aldehydes is also exhibited in their reactions with the allyl- (68) and (E)-crotyltrifluorosilanes (27). Competition experiments performed with beta-hydroxy aldehydes 37a (anti) and the corresponding p-methoxybenzyl (PMB) ether 48, and between aldehyde 39 (syn) and the PMB ether 90, established that the 2,3-anti-beta-hydroxy aldehydes react predominantly through bicyclic transition states while the 2,3-syn aldehydes react predominantly through conventional Zimmerman-Traxler transition states. NMR studies established that both the 2,3-syn and the 2,3-anti aldehydes form stable, pentavalent silicate intermediates (98 and 100) with PhSiF(3), but chelated structures 99 and 101 could not be detected. The activation energies for the competing bicyclic and conventional Zimmerman-Traxler transition states were calculated by using semiemperical methods (MNDO/d). These calculations indicate that the stereodivergent behavior of the 2,3-syn-beta-hydroxy aldehydes and the 2,3-anti-beta-hydroxy aldehydes is due to differences in nonbonded interactions in the bicyclic transition states. Specifically, nonbonded interactions in the bicyclic transition states for the allylation/crotylation reactions of the 2,3-syn-beta-hydroxy aldehydes permits the traditional Zimmerman-Traxler transition states to be preferentially utilized.

Efficient strategy for the synthesis of stereopentad subunits of scytophycin, rifamycin S, and discodermolide

[reaction: see text] An efficient, simple method has been developed for the stereocontrolled synthesis of polypropionate stereopentads in high enantio- and diastereomeric purities.

Synthesis of stereopentad analogues of the C14-C22 segment of callystatin A through additions of chiral allenylzinc reagents to stereotriads

The addition of (P)- and (M)-allenylzinc reagents, prepared in situ through Pd-catalyzed metalation of (R)- and (S)-3-butyn-2-ol mesylates, to diastereomeric stereotriad aldehydes 8, 13, 18, and 23 of syn,syn, syn,anti, anti,anti, and anti,syn stereochemistry was examined. Additions to the former two aldehydes afforded the four anti adducts with high diastereoselectivity and negligible mismatching. Significant mismatching was observed with the latter two aldehydes and the (M)-allenylzinc reagent. An evaluation of possible transition states is presented in consideration of steric and dipolar control elements.

Lipase-mediated resolution of 4-TMS-3-butyn-2-ol and use of the mesylate derivatives as a precursor to a highly stereoselective chiral allenylindium reagent

[reaction: see text]. An improved procedure for the resolution of 4-trimethylsilyl-3-butyn-2-ol has been developed. The mesylate derivatives of the resolved alcohols have been found to undergo highly enantio-, regio-, and diastereoselective additions to aldehydes, leading to homopropargylic alcohol adducts.

Cyclofunctionalization and free-radical-based hydrogen-transfer reactions. An iterative reaction sequence applied to the synthesis of the C(7)-C(16) subunit of zincophorin

The strategy considered herein features an iodocyclofunctionalization/hydrogen-transfer reaction sequence for the elaboration of propionate motifs. Proceeding with excellent yield and diastereoselectivity, the synthetic sequence proposed gives access to the anti-anti dipropionate motif when the reduction step is performed under the control of the exocyclic effect. The tandem sequence is applied successfully to the synthesis of the C(7)-C(16) subunit of zincophorin, and iteration of the process gives the desired anti-anti-anti-anti polypropionate stereopentad. Modifications of the reaction sequence--including phenylselenocyclofunctionalization, carbonate hydrolysis, and chelation-controlled radical reduction reactions--lead to the formation of the anti-syn dipropionate motif with remarkable diastereocontrol.