Total synthesis of rifamycins. 2. Total synthesis of racemic rifamycin S

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.

Total Synthesis and Structure Assignment of Saptomycin H

We report herein the first total synthesis of saptomycin H (2), by which the unidentified absolute stereochemistry of the oxiranyl side chain has been determined as 14R,16S. The keys include (1) concise assembly of three units, anthrone, sugar and side chain, and (2) AZADOL-mediated 6-endo-selective pyranone (A-ring) formation.

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.

Study on Dissolution Thermodynamics and Cooling Crystallization of Rifamycin S

The solubility data of rifamycin S were measured in isopropanol, butyl acetate, and their mixed solvents across the temperature range of 283.15-323.15 K by the gravimetric method. The results demonstrate that the solubility of rifamycin S increases with the increasing temperature in the two pure solvents, and in the mixed solvents, it increases first and then decreases with increasing butyl acetate content. The experimental data of rifamycin S in the mixed solvents were better correlated using the modified Apelblat equation and ideal model equation. Furthermore, the relevant thermodynamic parameters of the dissolution process were determined based on the van't Hoff equation. The obtained dissolution enthalpy and Gibbs free energy are positive in all cases, which indicate that the dissolving process of rifamycin S is endothermic and nonspontaneous. The supersolubility data of rifamycin S were measured by the laser and thermal analytic method. The results demonstrate that the width of the metastable zone of rifamycin S becomes larger with decreasing cooling rate and increasing butyl acetate content. Furthermore, the crystallization process of rifamycin S was optimized on the basis of thermodynamic research. The results showed that when V _{butyl acetate}:V _{mixed solvent} was 0.04, the cooling rate was 0.1 K/min, the stirring rate was 150 rpm, the final crystallization temperature was 283.15 K, and the aging time was 8 h, the purity of rifamycin S crystals could reach 98.5%, and the crystalline yield was 89.6%. After crystallization optimization, the size of rifamycin S crystals increased, and the dissolution in water was improved.

Hydroxylamine-Mediated Anthrapyranone Formation, Solving 5-exo/6-endo Issue toward Synthesis of Pluramycin-Class Antibiotics

In our synthetic study on pluramycin-class antibiotics, an unexpected issue arose, i.e., unfavorable regioselectivity of 5-exo rather than 6-endo cyclization to form the pyranone ring. The issue was solved by an addition-elimination process of a phenol-ynone substrate. AZADOL was specifically effective, enabling the first synthesis of saptomycinone H.

Enantioselective Alcohol C-H Functionalization for Polyketide Construction: Unlocking Redox-Economy and Site-Selectivity for Ideal Chemical Synthesis

The development and application of stereoselective and site-selective catalytic methods that directly convert lower alcohols to higher alcohols are described. These processes merge the characteristics of transfer hydrogenation and carbonyl addition, exploiting alcohols and π-unsaturated reactants as redox pairs, which upon hydrogen transfer generate transient carbonyl-organometal pairs en route to products of C-C coupling. Unlike classical carbonyl additions, stoichiometric organometallic reagents and discrete alcohol-to-carbonyl redox reactions are not required. Additionally, due to a kinetic preference for primary alcohol dehydrogenation, the site-selective modification of glycols and higher polyols is possible, streamlining or eliminating use of protecting groups. The total syntheses of several iconic type I polyketide natural products were undertaken using these methods. In each case, the target compounds were prepared in significantly fewer steps than previously achieved.

The evolving role of chemical synthesis in antibacterial drug discovery

The discovery and implementation of antibiotics in the early twentieth century transformed human health and wellbeing. Chemical synthesis enabled the development of the first antibacterial substances, organoarsenicals and sulfa drugs, but these were soon outshone by a host of more powerful and vastly more complex antibiotics from nature: penicillin, streptomycin, tetracycline, and erythromycin, among others. These primary defences are now significantly less effective as an unavoidable consequence of rapid evolution of resistance within pathogenic bacteria, made worse by widespread misuse of antibiotics. For decades medicinal chemists replenished the arsenal of antibiotics by semisynthetic and to a lesser degree fully synthetic routes, but economic factors have led to a subsidence of this effort, which places society on the precipice of a disaster. We believe that the strategic application of modern chemical synthesis to antibacterial drug discovery must play a critical role if a crisis of global proportions is to be averted.

Polyketide construction via hydrohydroxyalkylation and related alcohol C-H functionalizations: reinventing the chemistry of carbonyl addition

Despite the longstanding importance of polyketide natural products in human medicine, nearly all commercial polyketide-based drugs are prepared through fermentation or semi-synthesis. The paucity of manufacturing routes involving de novo chemical synthesis reflects the inability of current methods to concisely address the preparation of these complex structures. Direct alcohol C-H bond functionalization via"C-C bond forming transfer hydrogenation" provides a powerful, new means of constructing type I polyketides that bypasses stoichiometric use of chiral auxiliaries, premetallated C-nucleophiles, and discrete alcohol-to-aldehyde redox reactions. Using this emergent technology, total syntheses of 6-deoxyerythronolide B, bryostatin 7, trienomycins A and F, cyanolide A, roxaticin, and formal syntheses of rifamycin S and scytophycin C, were accomplished. These syntheses represent the most concise routes reported to any member of the respective natural product families.

Design and synthesis of novel antimicrobials with activity against Gram-positive bacteria and mycobacterial species, including M. tuberculosis

The alarming increase in bacterial resistance over the last decade along with a dramatic decrease in new treatments for infections has led to problems in the healthcare industry. Tuberculosis (TB) is caused mainly by Mycobacterium tuberculosis which is responsible for 1.4 million deaths per year. A world-wide threat with HIV co-infected with multi and extensively drug-resistant strains of TB has emerged. In this regard, herein, novel acrylic acid ethyl ester derivatives were synthesized in simple, efficient routes and evaluated as potential agents against several Mycobacterium species. These were synthesized via a stereospecific process for structure activity relationship (SAR) studies. Minimum inhibitory concentration (MIC) assays indicated that esters 12, 13, and 20 exhibited greater in vitro activity against Mycobacterium smegmatis than rifampin, one of the current, first-line anti-mycobacterial chemotherapeutic agents. Based on these studies the acrylic ester 20 has been developed as a potential lead compound which was found to have an MIC value of 0.4 μg/mL against Mycobacterium tuberculosis. The SAR and biological activity of this series is presented; a Michael-acceptor mechanism appears to be important for potent activity of this series of analogs.

Crotylboron-based synthesis of the polypropionate units of chaxamycins A/D, salinisporamycin, and rifamycin S

Syntheses of the C(15)-C(27) fragments of chaxamycins A/D, rifamycin S, and the C(12)-C(24) fragment of salinisporamycin have been accomplished in 10 steps from commercially available starting materials. Three crotylboron reagents were utilized to construct the seven contiguous stereocenters in these fragments with excellent stereoselectivity.

Unlocking Hydrogenation for C-C Bond Formation: A Brief Overview of Enantioselective Methods

Hydrogenation of π-unsaturated reactants in the presence of carbonyl compounds or imines promotes reductive C-C coupling, providing a byproduct-free alternative to stoichiometric organometallic reagents in an ever-increasing range of C=X (X = O, NR) additions. Under transfer hydrogenation conditions, hydrogen exchange between alcohols and π-unsaturated reactants triggers generation of electrophile-nucleophile pairs, enabling carbonyl addition directly from the alcohol oxidation level, bypassing discrete alcohol oxidation and generation of stoichiometric byproducts.

Direct generation of acyclic polypropionate stereopolyads via double diastereo- and enantioselective iridium-catalyzed crotylation of 1,3-diols: beyond stepwise carbonyl addition in polyketide construction

Under the conditions of transfer hydrogenation employing the cyclometalated iridium catalyst (R)-I derived from [Ir(cod)Cl](2), allyl acetate, 4-cyano-3-nitrobenzoic acid, and the chiral phosphine ligand (R)-SEGPHOS, α-methylallyl acetate engages 1,3-propanediol (1a) and 2-methyl-1,3-propanediol (1b) in double carbonyl crotylation from the alcohol oxidation level to deliver the C(2)-symmetric and pseudo-C(2)-symmetric stereopolyads 2a and 3a, respectively, with exceptional control of anti-diastereoselectivity and enantioselectivity. Notably, the polypropionate stereopentad 3a is formed predominantly as 1 of 16 possible stereoisomers. Desymmetrization of 3a is readily achieved upon iodoetherification to form pyran 4. The direct generation of 3a enables a dramatically simplified approach to previously prepared polypropionate substructures, as demonstrated by the synthesis of C19-C27 of rifamycin S (eight steps, originally prepared in 26 steps) and C19-C25 of scytophycin C (eight steps, originally prepared in 15 steps). The present transfer hydrogenation protocol represents an alternative to chiral auxiliaries, chiral reagents, and premetalated nucleophiles in polyketide construction.

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.

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.

Application of conformation design in acyclic stereoselection: total synthesis of borrelidin as the crystalline benzene solvate

The total synthesis of (-)-borrelidin (treponemycin), a structurally distinct 18-membered macrolide antibiotic, has been achieved. It was isolated as the crystalline benzene solvate, and its structure was confirmed by a single-crystal X-ray analysis. The deoxypropionate subunit consisting of four alternating C-methyl groups with a C(4)-C(10) syn/syn/anti orientation was elaborated by a new method of iterative cuprate additions to acyclic alpha,beta-unsaturated esters relying on two consecutive 1,3-inductions and starting with d-glyceraldehyde as the chiral progenitor. The unique Z/E cyanodiene unit was obtained as a single isomer by application of the Still-Gennari olefination protocol. The gamma-hydroxycyclopentane carboxylic acid subunit was prepared from L-malic acid utilizing a sequential introduction of C-vinyl and C-allyl groups, capitalizing on 1,2-induction in an acyclic alpha,beta-unsaturated ester and carbocyclization by a Grubbs ring closure metathesis reaction. The prevalence of 1,3-syn-disposed deoxypropionate triads in the cuprate additions is rationalized on the basis of minimized syn-pentane interactions in the transition states. A virtual diamond lattice was used as a visual tool to portray the low-energy conformations of the acyclic substrates, and corroborated by (1)H NMR homodecoupling studies.

Inter- and intramolecular differentiation of enantiotopic dioxane acetals through oxazaborolidinone-mediated enantioselective ring-cleavage reaction: kinetic resolution of racemic 1,3-alkanediols and asymmetric desymmetrization of meso-1,3-polyols

Acetals derived from racemic 1,3-alkanediols undergo kinetic resolution in chiral oxazaborolidinone-mediated ring-cleavage reaction with nucleophiles such as enol silanes and allylic silanes. Enantioselectivity of the reaction is affected by nucleophiles, the N-sulfonyl groups of oxazaborolidinones, and the substituents attached to the acetal carbon. For disubstituted acetals rac-1 and for trisubstituted acetal rac-2, derived from syn-2,4-dimethyl-1,3-pentanediol, satisfactory enantioselectivity is obtained by using methallylsilane 7b,c as a nucleophile in combination with N-mesyloxazaborolidinone 4a. On the other hand, enantioselective reaction of trisubstituted acetal rac-3b, derived from anti-2,4-dimethyl-1,3-pentanediol, is realized by using silyl ketene acetal 5e in combination with N-tosyloxazaborolidinone 4b. The reaction conditions optimized for the kinetic resolution, or enantiomer differentiating reaction, of the racemic acetals are successfully applied to asymmetric desymmetrization of meso-1,3-polyols through intramolecular differentiation of the enantiotopic acetal moieties of the bis-acetal derivatives. The utility of the ring-cleavage reaction as a method for enantioselective terminal differentiation of prochiral polyols is demonstrated in convergent asymmetric synthesis of pentol derivative 35 corresponding to the C(19)[bond]C(27) ansa-chain of rifamycin S.

Synthesis of propionate motifs: diastereoselective tandem reactions involving anionic and free radical based processes

Reported herein is a strategy employing a Mukaiyama reaction in tandem with a hydrogen transfer reaction for the elaboration of propionate motifs. The nature of the protecting groups on the chiral beta-alkoxy aldehyde and the type of Lewis acid used are varied to modulate the stereochemical outcome of the tandem reactions. The mode of complexation is thus controlled (monodentate or chelate) for the Mukaiyama reaction to give access to either syn or anti aldol products, precursors of the free radical reduction reaction. The endocyclic effect is subsequently capitalized upon to control the hydrogen transfer step so that the syn-reduced product may be achieved. Proceeding with excellent yield and diastereoselectivity, the synthetic sequence proposed gives access to syn-syn and syn-anti propionate motifs. Also considered is a complementary approach using a chelation-controlled Mukaiyama reaction in tandem with a free radical allylation reaction under the control of the endocyclic effect that leads to the anti-anti product.