Regioselective Long‐Range Proton Transfer in New Rifamycin Antibiotics: A Process in which Crown Ethers Act as Stronger Brønsted Bases than Amines

Water‐mediated proton transfer in six new derivatives of 3‐formylrifamycin SV that contain crown, aza‐crown, and benzo‐crown ether rings were investigated by FTIR and NMR spectroscopy. ¹H–¹H COSY couplings provide evidence for the formation of zwitterionic structures of the aza‐crown and crown ether derivatives of rifamycin, in which a proton from one of the phenolic groups is transferred to tertiary and secondary nitrogen atoms. The increased intensity of the continuous absorption in the mid‐infrared region together with the NMR data indicate proton transfer from the phenol group of the rifamycin core to the cavity of the benzo‐crown ether ring. This proton transfer is achieved by formation of hydronium (H₃O⁺) or Zundel ions (H₅O₂⁺), which form intermolecular hydrogen bonds with the oxygen atoms of the crown ether. DFT calculations are in agreement with the spectroscopic data and allow visualization of the structures of all new rifamycin derivatives, characterized by different intramolecular protonation sites.     

Affinity and enantioselectivity of Rifamycin SV towards low molecular weight compounds

Association of amino acids and some other low molecular weight compounds with rifamycin SV in water has been studied by ¹H NMR titrations. Rifamycin binds aromatic amino acids with pronounced enantioselectivity in favor of l-enantiomers and forms complexes with heterocyclic compounds but does not interact with simple benzene derivatives. Binding constants correlate with LUMO energies and hydrophobicities (expressed as log P values) of guest molecules indicating contributions to the binding free energy from aromatic stacking interactions with the naphthohydroquinone fragment of rifamycin SV and from hydrophobic interactions. Proposed mode of binding is supported by semiempirical calculations of structures of host–guest complexes.     

Rifamycin antibiotics—new compounds and synthetic methods. Part 3: Study of the reaction of 3-formylrifamycin SV with primary amines and ketones

In the third stage of our study concerning the search for new antibacterial rifamycin antibiotics, the reactions of 3-formylrifamycin SV (1) with a range of primary alkylamines and ketones of general structure R₁–CH₂–CO–R₂ (R₁H or alkyl and R₂alkyl or aryl) has been investigated. A new synthetic method for the preparation of a new group of rifamycin derivatives with an α,β-unsaturated imine substituent at C-3 has been developed.These compounds showed a tendency to reversibly isomerise in organic solvents and, in the presence of water, to rapidly hydrolyse. The structures of four isolated microcrystalline compounds 2, 3, 4, 5 and a reaction's mechanism have been proposed on the basis of mass spectrometry results as well as (1D) and (2D) ¹H and ¹³C NMR analysis. The new synthetic route reported herein is a promising pathway to new reactive rifamycins displaying broader capabilities than the plain 3-formylrifamycin SV.     

Rifamycin antibiotics—new compounds and synthetic methods. Part 1: Study of the reaction of 3-formylrifamycin SV with primary alkylamines or ammonia

Within the study covering the search for new methods of synthesis of rifamycin antibiotics, the reactions of 3-formylrifamycin SV (2) with primary amines or ammonia were studied. In the reaction of 2 with methylamine, unstable 3-methyliminomethylrifamycin SV (8) was formed, which was further oxidised to stable 3-methyliminomethylrifamycin S (9). In the reaction of 2 with ammonia, N,15-didehydro-15-deoxo-pyrimido-(4,5-b)rifamycin SV (10), a new compound with a chromophore system enlarged by a pyrimidine ring, was obtained. The product of its reduction with sodium borohydride—(11)—was also isolated. The structures of the compounds and an explanation of the synthesis mechanism have been proposed on the basis of mass spectrometry results as well as (1D) and (2D) ¹H- and ¹³C NMR analysis. In vitro antituberculous activity of the new compounds have been investigated.     

Effects of the Interaction of Rifamycin SV with Serum Albumins on the Resonance Rayleigh Scattering Spectra and Their Analytical Application

In pH 4.5–4.8 Britton‐Robinson buffer solution, rifamycin SV (i.e. rifamycin sodium) can react with serum albumin such as human serum albumin (HSA) and bovine serum albumin (BSA) to form macromolecular complexes by electrostatic attraction and hydrophobic force. As a result, the resonance Rayleigh scattering (RRS) of the drug was enhanced remarkably and the RRS peaks were at 374 and 552 nm. The enhancement of RRS (ΔI) is directly proportional to the concentration of HSA or BSA. The linear ranges and the detection limits are 0.03–6.0 µg/mL and 9.0 ng/mL for HSA, and 0.01–8.0 µg/mL and 2.0 ng/mL for BSA, respectively. In this work, a sensitive, selective, simple and fast method for the determination of trace amounts of serum albumin by RRS technique has been developed, which was applied to the determination of serum albumin in the synthesized samples and human urine samples with satisfactory results.     

[(1,3-Dioxolan-2-yliden)methyl]phosphonate und -phosphinate als (einfache) Synthone in der Heterocyclensynthese

[(1,3-Dioxolan-2-ylidene)methyl]phosphonates and -phosphinates as [simple] Synthons in Heterocyclic Synthesis The readily available [(1,3-dioxolane-2-ylidene)methyl]phosphonates and -phosphinates 2a–f (Scheme 1) can be transformed with amines to aliphatic ketene N,O-and N,N-acetales (see Scheme 2, 2a → 3–7 ). Alkanediamines yield with 2a–f the imidazolidines 8a–f and the hexahydropyrimidines 9a–d (Scheme 3). the oxazolidine derivatives 10a–e and the thiazolidine 11 are accessible under special reaction conditions starting from 2a, b (Scheme 4). Hydrazines react with the CN-group-containing ketene O,O-acetals 2a–c to the pyrazoles 12a–g , whereof 12a, d, e can be cyclized to pyrazolo[1,5-a]pyrimidines 13a–d (Scheme 5). Amidines as starting materials transform 2a–c in an analogous way to the pyrimidine derivatives 14a–c (Scheme 6).     

Synthesen,chemische Reaktionen und NMR-spektroskopische Untersuchungen substituierter Phosphonopyruvate

Phosphonopyruvates: Syntheses, NMR Investigations, and Reactions The new 3-(diethoxyphosphoryl)-2-oxopropanoates 5–24 and -propanamides 25–38 with various substituents at C(3) were prepared in moderate-to-good yields (Schemes 2 and 3, Tables 1 and 2). It was shown that they adopt a preferred conformation in which the diethoxyphosphoryl group and the substituent at C(4) are antiperiplanar to each other (see B ). The keto-enol tautomerization of phosphonopyruvates with Ph? C(3) (see 20 ) and MeS? C(3) (see 24 and 33 ) was examined. In CHCl₃, two tautomeric species exist, whereas in dimetylsulfoxide (DMSO), three tautomeric forms are observed. Oxime ethers, an oxime, and a phenylhydrazone of unsubstituted phosphonopyruvates were prepared (see 40–44 ), and quinoxalin-2(1H)-ones could be obtained from the reaction of pyruvates with 4,5-dimethylbenzene-1,2-diamine (see 45–47 ).     

Lipase-Catalysed Regioselective Deacetylation of androstane derivatives

A series of acetoxy derivatives of androstane was deacetylated in organic solvents by several lipases. The most satisfactory results were obtained with lipase from Candida cylindracea (CCL) and Candida antarctica (CAL). In some derivatives, CCL and CAL showed an overwhelming regioselectivity towards the removal of the 3β- or the 17β-acetyl group (see Table 2). Three new steroid derivatives were obtained through this approach. A hypothetical rationale for the behaviour of these enzymes is given.     

Adsorptive Stripping Voltammetry of Rifamycins at Unmodified and Surfactant‐Modified Carbon Paste Electrodes

The electrochemical behavior of the antibiotics rifampicin and rifamycin SV is investigated by cyclic voltammetry at carbon paste and in situ surfactant modified carbon paste electrodes. Both antibiotics adsorb on the unmodified electrodes and show a reversible redox process due to the oxidation of the 6,9‐dihydroxynaphthalene moiety to the corresponding naphthoquinone. This process is used as analytical signal for developing adsorptive voltammetric methods for the determination of the antibiotics. Experimental parameters, such as pH of the supporting electrolyte, accumulation potential and time are optimized. After accumulation from acidic solutions (0.1 M KCl pH 2 or HCl 0.2 M) at ?0.1 or 0 V for 3 min, the differential pulse oxidation peak current changes linearly with the antibiotic concentration in the range 3.5×10^?10 M ?5.4×10^?9 M or 5×10^?11 M ?1.0×10^?9 M for rifampicin and rifamycin SV, respectively. Rifamycin SV is not accumulated on carbon paste electrodes modified in situ with the anionic surfactant sodium dodecyl sulfate, whereas rifampicin is readily accumulated on this modified electrodes resulting in a signal enhancement and allowing rifampicin determinations without interference from rifamycin SV. On the other hand, selective determination of rifamycin SV in the presence of rifampicin is achieved by using carbon paste electrodes in situ modified with the cationic surfactant cetyltrimethylammonium chloride.     

Desoxy-nitrozucker. 13. Mitteilung. Herstellung ungeschützter und partiell geschützter 1-Desoxy-1-nitro-D-aldosen sowie Röntgenstrukturanalysen einiger ihrer Vertreter

Preparation of Unprotected and Partially Protected 1-Deoxy-1-nitro-D -aldoses and Some Representative X-Ray Structure Analyses The unprotected and partially protected 1-deoxy-1-nitro derivatives of α-and β-D -glucopyranose (see 15 and 14 ), β-D -mannopyranose (see 16 ), N-acetyl-β-D -glucosamine (see 17 ), β-D -galactofuranose (see 19 ), β-D -ribofuranose (see 20 ), α-D -arabinofuranose (see 21 ), 4,6-O-benzylidene-β-D -glucose (see 40 ), N-acetyl-4,6-O-benzylidene-β-D -glucosamine (see 41 ), and 4,6-O-benzylidene-β-D -galactose (see 42 ) were prepared by ozonolysis of the corresponding nitrones which were obtained from the acid-catalyzed reaction of p-nitrobenzaldehyde with the hydroxylamine 4 , the unprotected oximes 3 and 5–9 and the 4,6-O-benzylidene oximes 35–37 , respectively (Schemes 1–3). The gluco- and manno-nitrones 10 and 12 were isolated, and their ring size and their anomeric and (E/Z) configurations were determined by NMR spectroscopy and by their transformation into their corresponding nitro derivatives. The structure of the deoxynitroaldoses were determined by NMR spectroscopy, polarimetry, and, in the case of 14 , 16 , and 17 , by formation of the 4,6-O-benzylidene ( 14 → 40 ) or 4,6-O-isopropylidene ( 16 → 43 , 17 → 23 ) derivatives (Scheme 3). Acetylation of the nitroglucopyranose 14 , the 2-acetamido-nitroglucopyranose 17 , and the nitrogalactofuranose 19 gave the crystalline peracetylated nitroaldoses 22 , 24 , and 45 , respectively (Scheme 4, Figs. 1 and 3); acetylation of the nitromannopyranose 16 gave the nitro-arabino-glycal 44 (Scheme 4). The structure of the peracetylated nitroglucopyranose 22 , the nitroglucosamine 25 , the nitrogalactofuranose 45 , and the nitroribofuranose 20 were confirmed by X-ray analysis (Figs. 1 4). In all cases, including the β-D -glucopyranose derivative 22 , considerably shortening of the (endocyclic) C(1)-O bond was observed. Base-catalyzed anomerization of the β-D -configurated nitroglucopyranose 14 , the nitromannopyranose 16 , the benzylidene acetal 40 of nitroglucose, and the 2,3,4,6-tetraacetylated glucosamine derivative 24 gave the corresponding nitro-α-D -aldoses 15 , 26 , 47 , and 25 , respectively (Scheme 4).     

Thiazo rifamycins—I : Structure and synthesis of rifamycins p,q and verde,novel metabolites from mutants of nocardia mediterranea

Structures 1, 2 and 3 have been assigned to rifamycins P, Q and Verde, novel metabolites isolated from a mutant strain of Nocardia mediterranea both on the basis of spectroscopic evidence (UV, IR, MS, ¹H and ¹³C NMR) in comparison with the model compounds rifamycin S (4), rifampicin (5) and 4-dimethylamino-4-deoxy-rifamycin SV (6), and of an unambiguous synthesis from rifamycin S (4).     

Carbocyclic analogs of nucleosides. Part 2.. Synthesis of 2′,3′-dideoxy-5′-homonucleoside analogs

A set of derivatives of cyclopentaneacetic acid cis-substituted at position 3 by nucleoside bases (both purines and pyrimidines) were prepared and characterized (see 11, 14 , and 23a, b; Schemes 2–4). These molecules are carbocyclic analogs of 2′,3′-dideoxy-5′-homonucleosides. In this synthesis, the skeleton was constructed from norbornanone and a novel method based on Mitsunobu chemistry used for the introduction of nucleoside-base substituents. The scope of this method was further explored via the preparation of a cyclobutyl analog of dideoxyguanosine (see 17 , Scheme 3).     

Nucleotides. Part LV. Synthesis and application of a novel linker for solid-phase synthesis of modified oligonucleotides

Various bifunctional amino-protecting groups such as the phthaloyl, succinyl, and glutaryl group were investigated as potential linker molecules for attachment to solid-support materials. Pentane-1,3,5-tricarboxylic acid 1,3-anhydride ( 16 ) offered the best properties and reacted with the amino groups of differently sugar-protected adenosine (see 20 and 22 ), cytidine (see 29 ), and guanosine derivatives (see 32 ) to the corresponding 2-(2-carboxyethyl)glutaryl derivatives 23 , 24 , 30 , and 33 . The usefulness of the new linker-type molecules was demonstrated by the solid-support synthesis of the potentially antivirally active 3′-deoxyadenylyl-(2′–5′)-2′-adenylic acid 2′-{2-[(adenin-9-yl)methoxy]ethyl} ester ( 38 ) starting from the 2′-end with N⁶,N⁶-[2-(2-carboxyethyl)glutaryl]-9-{{2-[(4,4′-dimethoxytrityl)ethoxy]methyl}adenine ( 12 ).     

Electrofugal Fragmentation of Alkylcobalamin Derivatives Using Cob(I)Alamin and Heptamethyl Cob (I)yrinate as Catalysts

The cob (I)alamin- ( 1(I) ) and the heptamethyl cob(I)ynnate- ( 2(I) ) catalyzed transformation of an epoxide to the corresponding saturated hydrocarbon 3→4→5 is examined (see Schemes 1 and 3–5). Under the reaction conditions, the epoxyalkyl acetate 3 is opened by the catalysts with formation of appropriate (b?-hydroxyalkyl)-corrinoid derivatives ( 13 , 14 , 17 , 18 , see Schemes 12 and 14). Triggered by a transfer of electrons to the Co-corrin-π system, the Co, C-bond of the intermediates is broken, generating the alkenyl acetate 4 (cf. Schemes 12 and 14) following an electrofugal fragmentation (cf. Schemes 2 and 12). The double bond of 4 is also attacked by the catalysts, leading to the corresponding alkylcorrinoids ( 15 , 19 , see Schemes 12 and 14) which in turn are reduced by electrons from metallic zinc, the electron source in the system, inducing a reductive cleavage of the Co, C-bond with production of the saturated monoacetate 5 (see Schemes 2, 5 and 12). In the cascade of steps involved, the transfer of electrons to the intermediate alkylcorrinoids ( 13–15 , 17–19 , see Schemes 12 and 14) is shown to be rate-limiting. Comparing the two catalytic species 1(I) and 2(I) , it is shown that the ribonucleotide loop protects intermediate alkylcobalamins to some extent from an attack by electrons. The protective function of the ribonucleotide side-chain is shown to be present in alkylcobalamins existing in the base-on form (cf. Chap. 4 and see Scheme 14).     

The Allyl Ester as Carboxy-Protecting Group in the Stereoselective Construction of Neuraminic-Acid Glycosides

The application of the allyl-ester moiety as protecting principle for the carboxy group of N-acetylneuraminic acid is described. Peracetylated allyl neuraminate 2 is synthesized by reacting the caesium salt of the acid 1 with allyl bromide. Treatment of 2 with HCl in AcCl or with HF/pyridine gives the corresponding 2-chloro or 2-fluoro derivatives 3 and 4 , respectively (Scheme 1). In the presence of Ag₂CO₃, the 2-chloro carbohydrate 3 reacts with di-O-isopropylidene-protected galactose 5 to give the 2–6 linked disaccharide with the α-D -anomer 6a predominating (α-D /β-D = 6:1; Scheme 2). Upon activation of the 2-fluoro derivative 4 with BF₃ · Et₂O, the β-D -anomer 6b is formed preferentially (α-D /β-D = 1:5). In further glycosylations of 4 with long-chain alcohols, the β-D -anomers are formed exclusively (see 10 and 11 ; Scheme 4). The allyl-ester moiety can be removed selectively and quantitatively from the neuraminyl derivatives and the neuraminyl disaccharides by Pd(0)-catalyzed allyl transfer to morpholine as the accepting nucleophile (see Scheme 5).     

Hydrogenation of the ansa-Chain of Rifamycins. X-ray crystal structure of (16S)-16,17,18,19-tetrahydrorifamycin S

The catalytic hydrogenation of rifamycin S ( 2 ) over Pd/C, followed by oxidation with K₃[Fe(CN)₆], generates a pair of 16,17,18,19-tetrahydrorifamycins S ( 3/4 ), epimeric at C (16). The use of PtO₂ as catalyst leads to the hydrogenation also of the C(28)?C(29) bond giving, after oxidation by K₃[Fe(CN)₆], a mixture of the epimers (16R)- and (16S)-16,17,18,19,28,29-hexahydrorifamycins S ( 5/6 ). Furthermore, we synthesized the (16R)- and (16S)-3-bromo derivatives 7/8 and (16R)- and (16S)-3-(piperidin-1-yl) derivatives 9/10 . The determination of the X-ray crystal structure of the most abundant epimer 4 of the tetrahydrorifamycins allowed the assignment of the absolute configuration at C(16) of all derivative. A Structure-activity relationship study showed that in general the (16R)-epimers are more potent inhibitors of bacterial RNA polymerase than the (16S)-epimers.     

13C and 15N CP/MAS, 1H–15N SCT CP/MAS and FTIR spectroscopy as tools for qualitative detection of the presence of zwitterionic and non‐ionic forms of ansa‐macrolide 3‐formylrifamycin SV and its derivatives in solid state

¹³C, ¹⁵N CP/MAS, including ¹H–¹³C and ¹H–¹⁵N short contact time CP/MAS experiments, and FTIR methods were applied for detailed structural characterization of ansa‐macrolides as 3‐formylrifamycin SV (1) and its derivatives (2–6) in crystal and in powder forms. Although HPLC chromatograms for 2/CH₃OH and 2/CH₃CCl₃ were the same for rifampicin crystals dissolved in respective solvents, the UV–vis data recorded for them were different in 300–375 nm region. Detailed solid state ¹³C and ¹⁵N CP/MAS NMR and FTIR studies revealed that rifampicin (2), in contrast to 3‐formylrifamycin SV (1) and its amino derivatives (3–6), can occur in pure non‐ionic or zwitterionic forms in crystal and in pure these forms or a mixture of them in a powder. Multinuclear CP/MAS and FTIR studies demonstrated also that 3–6 derivatives were present exclusively in pure zwitterionic forms, both in powder and in crystal. On the basis of the solid state NMR and FTIR studies, two conformers of 3‐formylrifamycin SV were detected in powder form due to the different orientations of carbonyl group of amide moiety. The PM6 molecular modeling at the semi‐empirical level of theory, allowed visualization the most energetically favorable non‐ionic and zwitterionic forms of 1–6 antibiotics, strongly stabilized via intramolecular H‐bonds. FTIR studies indicated that the originally adopted forms of these type antibiotics in crystal or in powder are stable in standard laboratory conditions in time. The results presented point to the fact that because of a possible presence of two forms of rifampicin (compound 2), quantification of the content of this antibiotic in relevant pharmaceuticals needs caution. Copyright © 2013 John Wiley & Sons, Ltd.     

Synthesis of heterocyclic compounds from 2-phenyl-1,2,3-triazole-4-formylhydrazine

2-Phenyl-1, 2, 3-triazole-4-formylhydrazine (2) was prepared by hydrazinolysis of the corresponding ester 1. Reaction of 2 with CS₂/KOH gave the oxadiazole derivatives (3) which via Mannich reaction with different dialkyl amines furnished 3-N, N-dialkyl derivatives (4a–c). Also, condensation of 2 with appropriate aromatic acid in POCI₃ yielded oxadiazole derivatives (5a–c), or with aldehydes and ketones afforded hydrazones (6a–c). Cyclization of (6a–c) with acetic anhydride gave the desired dihydroxadiazole derivatives (7a–c). On the other hand, reaction of dithiocarbazate (8) with hydrazine hydrate gave the corresponding triazole derivative (9) which on treatment with carboxylic acids in refluxing POCI₃ yielded s-triazole [3, 4–b]-1, 3, 4-thiadiazole derivatives (10a–b). The structures of all the above compounds were confirmed by means of IR, ¹H NMR, MS and elemental analysis.     

Diastereoselektive Alkylierung von 3-Aminobutansäure in der 2-Stellung

Diastereoselective Alkylation 3-Aminobutanoic Acid in the 2-Position The enantiomerically pure 3-aminobutanoic acids (R)- and (S)- 6 are readily available by preparative HPLC separation of the two diastereoisomers 5 obtained from addition of (S)-phenethylamine to methyl crotonate and subsequent hydrogenolysis (Scheme 2). (S)-Methyl 3-(benzoylamino) butanoate ((S)- 3 ) is also available by enzymatic kinetic resolution with pig-liver esterase. The N-benzyl- and N- benzyloxycarbonyl derivatives rac 3 , 8 , and 9 of 3-aminobutanoates are doubly deprotonated with LDA and alkylated or aminated in high selectivity (17 examples, relative topicity like; see Tables 1 and 2). The configuration of three of the products is assigned (Schemes 4–6), and in four cases, the free α-substituted β-amino acid is prepared by acidic hydrolysis (see Table 3). It is shown that the doubly lithiated β-amino-acid derivative is solubilized, and its reactivity may be strongly influenced by the presence of 3 equiv. of LiCl.     

Herstellung von Hilfsstoffen für die asymmetrische Synthese aus Weinsäure. Addition von Butyllithium an Aldehyde in chiralem Medium

Preparation of Auxiliaries for Asymmetric Syntheses from Tartaric Acid. Additions of Butyllithium to Aldehydes in Chiral Media. Chiral derivatives of the complexing 1,2-diheterosubstituted ethanes A–D are prepared from tartaric acid. The key starting materials are the succinic acid derivative 1 , the dioxolane 2a , and the diamide 3a . These are converted to the ethers, alkoxyamines, and alkylthio-amines listed in the first column of Table 2 which also contains the derivatives 21c, 22d , and 23d made from lactic acid, malic acid, and proline, respectively. It is shown that the highest optical yields (up to 40%) in reactions of butyllithium with aldehydes are obtained when mixtures of (?)-1,2,3,4-tetramethoxy-butane ( 4b ), (+)-2,3-dimethoxy-N,N,N′,N′-tetramethyl-1,4-butanediamine ( 17a ), and (?)-1,4-dimethoxy-N,N,N′,N′-tetramethyl-2,3-butanediamine ( 14c ) with pentane are used at temperatures down to ?150° and ratios of auxiliary/butyllithium of up to 10:1 (see equation (1), Tables 2–4).