Synthesis of complex nucleoside antibiotics

Herbicidin B and fully prtected tunicaminyluracil, which were undecose nucleoside antibiotics, were synthesized using a samarium diiodide (SmI2) mediated aldol reaction with the use of alpha-phenylthioketone as an enolate. The characteristics of the SmI2-mediated aldol reaction are that the enolate can be regioselectively generated and the aldol reaction proceeds under near neutral condition. This reaction is proved to be a powerful reaction for the synthesis of complex nucleoside antibiotics. The synthesis of caprazol, the core structure of caprazamycins, was conducted by the strategy including beta-selective ribosylation without using a neighboring group participation and the construction of a diazepanone by a modified reductive amination. Our synthetic route would provide a range of key analogues with partial structures to define the pharmacophore, which can be a lead for the development of more effective anti-bacterial agents.     

SYNTHESIS OF COMPLEX NUCLEOSIDE ANTIBIOTICS

Herbicidin B and fully protected tunicaminyluracil, which were undecose nucleoside antibiotics, were synthesized using a samarium diiodide (SmI₂) mediated aldol reaction with the use of α-phenylthioketone as an enolate. The characteristics of the SmI₂-mediated aldol reaction are that the enolate can be regioselectively generated and the aldol reaction proceeds under near neutral condition. This reaction is proved to be a powerful reaction for the synthesis of complex nucleoside antibiotics. The synthesis of caprazol, the core structure of caprazamycins, was conducted by the strategy including β-selective ribosylation without using a neighboring group participation and the construction of a diazepanone by a modified reductive amination. Our synthetic route would provide a range of key analogues with partial structures to define the pharmacophore, which can be a lead for the development of more effective anti-bacterial agents.     

Intramolecular aldol cyclization of C-4-ulopyranosyl-2'-oxoalkanes controlled by steric effects. Asymmetric synthesis of substituted 8-oxabicyclo[3.2.1]octanones and -octenones and cyclopentenones

Whereas C-2- and 4-ulopyranosyl compounds (C-2- and C-4-ulosides) can be converted to cyclopentenones under base conditions through beta-elimination and ring contraction, base-initiated beta-elimination of C-glycosyl 2'-aldehydes and 2'-ketones results in the formation of acyclic alpha,beta-unsaturated aldehydes or ketones. By combining both molecular features we synthesized 1-C-(4-ulopyranosyl)-2-oxoalkanes 6, 13, and 20 and investigated their reactions when they were treated with base. Both alpha- and beta-anomers of C-(4-ulopyranosyl)acetaldehydes 6 and 13 underwent a fast intramolecular aldol reaction between the C-5 enolate and 2'-aldehyde to form optically pure 8-oxabicyclo[3.2.1]octanones, which further transformed to 8-oxabicyclo[3.2.1]octenones 14 and 15 by beta-elimination. However, this aldol reaction did not occur when 1-C-(4-ulopyranosyl)propan-2-one 20 was treated with base because of steric hindrance exerted by the additional methyl group. Instead, an alternate C-3 enolization led to beta-elimination and further electro-ring opening to form an acyclic enol, which was then converted through a disrotatory intramolecular aldol cyclization to a cis-substituted cyclopentenone 21.     

The first synthesis of herbicidin B, a tricyclic-sugar adenine nucleoside antibiotic, using samarium diiodide-promoted aldol-type C-glycosidation reaction as a key-step.

A first total synthesis of the nucleoside antibiotic herbicidin B (1) was achieved in which a novel aldol-type C-glycosidation reaction promoted by samarium diiodide (SmI2) was used as a key step. Construction of the desired stereochemistry of the tricyclic-sugar moiety was successfully achieved by conformational restriction strategy based on repulsion between adjacent bulky protecting groups on the pyranose ring.     

Horseradish peroxidase-catalyzed oxidation of chlorophyll a with hydrogen peroxide: Characterization of the products and mechanism of the reaction

Horseradish peroxidase was verified to catalyze, without any phenol, the hydrogen peroxide oxidation of chlorophyll a (Chl a), solubilized with Triton X-100. The 13²(S) and 13²(R) diastereomers of 13²-hydroxyChl a were characterized as major oxidation products (ca. 60%) by TLC on sucrose, UV-vis, ¹H, and ¹³C NMR spectra, as well as fast-atom bombardment MS. A minor amount of the 15²-methyl, 17³-phytyl ester of Mg-unstable chlorin was identified on the basis of its UV-vis spectrum and reactivity with diazomethane, which converted it to the 13¹,15²-dimethyl, 17³-phytyl ester of Mg-purpurin 7. The side products (ca. 10%) were suggested to include the 17³-phytyl ester of Mg-purpurin 18, which is known to form easily from the Mg-unstable chlorin. The side products also included two red components with UV-vis spectral features resembling those of pure Chl a enolate anion. Hence, the two red components were assigned to the enolate anions of Chl a and pheophytin a or, alternatively, two different complexes of the Chl a enolate ion with Triton X-100. All the above products characterized by us are included in our published free-radical allomerization mechanism of Chl a, i.e. oxidation by ground-state dioxygen. The HRP clearly accelerated the allomerization process, but it did not produce bilins, that is, open-chain tetrapyrroles, the formation of which would require oxygenolysis of the chlorin macrocycle. In this regard, our results are in discrepancy with the claim by several researchers that ‘bilirubin-like compounds’ are formed in the HRP-catalyzed oxidation of Chl a. Inspection of the likely reactions that occurred on the distal side of the heme in the active centre of HRP provided a reasonable explanation for the observed catalytic effect of the HRP on the allomerization of Chl. In the active centre of HRP, the imidazole nitrogen of His-42 was considered to play a crucial role in the C-13² deprotonation of Chl a, which resulted in the Chl a enolate ion resonance hybrid. The Chl enolate was then oxidized to the Chl 13²-radical while the HRP Compound I was reduced to Compound II. The same reactive Chl derivatives, i.e. the Chl enolate anion and the Chl 13²-radical, which are produced twice in the HRP reaction cycle, happen to be the crucial intermediates in the initial stages of the Chl allomerization mechanism.     

Asymmetric aldol reaction using boron enolates

The protocol for the preparation of boron enolates and their subsequent reaction with aldehydes is described, providing convenient access to beta-hydroxy ketones in good yields and with high stereoselectivities. The reaction consists of three steps: first, the ketone is rapidly converted to the corresponding boron enolate, by exposure to a chlorodialkylborane and tertiary amine base, which is then reacted in situ with the aldehyde. Finally, oxidative workup of the resultant boron aldolate provides aldol adduct. The reaction procedure requires approximately 28 h to complete over a 2-d period, consisting of 5 h to set up the reaction, whereupon the reaction mixture is left at -20 degrees C overnight (16 h), followed by 7 h for workup and purification.     

Stereoselective Syntheses of (-)-Podorhizol Lignan and its Derivatives: erythro and threo Preferential Aldol Condensation of Potassium Enolate from γ-Butyrolactone with Alkoxybenzaldehyde

(-)-Podorhizol (1) was stereoselectively synthesized by erythro preferential aldol condensation of 3,4,5-trimethoxy- benzaldehyde with potassium enolate from (+)-(R)-3- (3,4-methylenedioxybenzyl)-4-butanolide (2) (erythro:threo=85:15). Erythro selectivity was observed in the aldol condensation of many alkoxybenzaldehydes with potassium enolate from (+)-γ-butyrolactone 2. However, benzaldehydes having methoxy groups at both the 2 and 6 positions gave threo selectivity in the aldol condensation with potassium enolate from (+)-γ-butyrolactone 2.     

Simple Synthesis of Dehydrololiolide

Dehydrololiolide, isolated from tobacco leaves, was synthesized via the intramolecular aldol reaction, Reformatsky reaction, and Wittig reaction of the key intermediate keto esters.     

Stoichiometry of the reaction between horseradish peroxidase and p-cresol.

Over a wide range of pH horseradish peroxidase compound I can be reduced quantitatively via compound II to the native enzyme by only 1 molar equivalent of p-cresol. Since 2 molar equivalents of electrons are required for the single turnover of the enzymatic cycle, p-cresol behaves as a 2-electron reductant. With p-cresol and compound I in a 1:1 ratio compound II and p-methylphenoxy radicals are obtained in the transient state. Compound II is then reduced to the native enzyme. A possible explanation for the facile reduction of compound II involves reaction with the dimerization product of these radicals, 1/2 molar equivalent of 2,2'-dihydroxy-5,5'-dimethylbiphenyl. If only 1/2 molar equivalent of p-cresol is present, than at high pH the reduction stops at compound II. The major steady state peroxidase oxidation product of p-cresol (with p-cresol in large excess compared to the enzyme concentration) is Pummerer's ketone. Pummerer's ketone is only reactive at pH values greater than about 9 where significant amounts of the enol can be formed via the enolate anion. Therefore, in alkaline solution it is reactive with compound I, but not with compound II, which is converted into an unreactive basic form. These results indicate that Pummerer's ketone cannot be the intermediate free radical product responsible for reducing compound II in the single turnover experiments. It is postulated that Pummerer's ketone is formed only in the steady state by the reaction of the p-methylphenoxy radical with excess p-cresol.     

Synthesis and evaluation of cis-hexahydropyrrolo[3,2-b]pyrrol-3-one peptidomimetic inhibitors of CAC1 cysteinyl proteinases

A stereoselective synthesis of functionalised cis-hexahydropyrrolo[3,2-b]pyrrol-3-ones has been developed through Fmoc and Cbz-protected intermediates 5 and 6. Building blocks 5 and 6 were prepared via the intramolecular cyclisation of anti-epoxide 17. The intramolecular reaction occurred exclusively through the anti-epoxide to provide the 5,5-cis-fused bicycle, whereas the syn-epoxide, which theoretically would provide the 5,5-trans-fused bicycle, remained unchanged. These experimental observations are consistent with a key design element that we have introduced within this novel bicyclic ketone scaffold. Our bicyclic design strategy provides chiral stability to the bridgehead stereocentre that is situated alpha to the ketone because the cis-fused geometry is both thermodynamically and kinetically stable. Building blocks 5 and 6 have been utilised in both solid phase and solution phase syntheses of peptidomimetics 22, 36-40, which exhibit potent in vitro inhibition against a range of CAC1 cysteinyl proteinases. Compound 22, a potent and selective inhibitor of human cathepsin K exhibited good primary DMPK properties along with promising activity in an in vitro cell-based human osteoclast assay of bone resorption.     

Crystal Structure of Reaction Intermediates in Pyruvate Class II Aldolase: SUBSTRATE CLEAVAGE,ENOLATE STABILIZATION,AND SUBSTRATE SPECIFICITY*

Crystal structures of divalent metal-dependent pyruvate aldolase, HpaI, in complex with substrate and cleavage products were determined to 1.8–2.0 Å resolution. The enzyme·substrate complex with 4-hydroxy-2-ketoheptane-1,7-dioate indicates that water molecule W2 bound to the divalent metal ion initiates C3–C4 bond cleavage. The binding mode of the aldehyde donor delineated a solvent-filled capacious binding locus lined with predominantly hydrophobic residues. The absence of direct interactions with the aldehyde aliphatic carbons accounts for the broad specificity and lack of stereospecific control by the enzyme. Enzymatic complex structures formed with keto acceptors, pyruvate, and 2-ketobutyrate revealed bidentate interaction with the divalent metal ion by C1-carboxyl and C2-carbonyl oxygens and water molecule W4 that is within close contact of the C3 carbon. Arg⁷⁰ assumes a multivalent role through its guanidinium moiety interacting with all active site enzymatic species: C2 oxygen in substrate, pyruvate, and ketobutyrate; substrate C4 hydroxyl; aldehyde C1 oxygen; and W4. The multiple interactions made by Arg⁷⁰ stabilize the negatively charged C4 oxygen following proton abstraction, the aldehyde alignment in aldol condensation, and the pyruvate enolate upon aldol cleavage as well as support proton exchange at C3. This role is corroborated by loss of aldol cleavage ability and pyruvate C3 proton exchange activity and by a 730-fold increase in the dissociation constant toward the pyruvate enolate analog oxalate in the R70A mutant. Based on the crystal structures, a mechanism is proposed involving the two enzyme-bound water molecules, W2 and W4, in acid/base catalysis that facilitates reversible aldol cleavage. The same reaction mechanism promotes decarboxylation of oxaloacetate.     

Synthesis of N-acylated 7-amino-2,6,7-trideoxy-D-erythroheptopyranosides from methyl alpha-D-mannoside

Hexopyranoside methyl alpha-D-mannoside (8) was homologated to yield 7-(acylamino)-2,6,7-trideoxy-heptopyranosides 19-26. A crucial reaction step is the radical cleavage of benzylidene derivative 10 to obtain bromide 11. Since nucleophilic substitution of 11 with KCN provided the bicyclic nitrile 13 instead of nitrile 14, ketone 11 was protected as the dimethyl acetal 15. Nucleophilic substitution of 15 with KCN, subsequent hydrogenation with H2/Raney Ni and acylation with various carboxylic acid derivatives yielded 7-(acylamino)heptopyranosides 19-22.     

Stereoselective synthesis of beta-1-C-substituted 1,4-dideoxy-1,4-imino-D-galactitols and evaluation as UDP-galactopyranose mutase inhibitors

The synthesis of 1-C-substituted 1,4-dideoxy-1,4-imino-D-galactitols involving nitrone umpolung is described. The SmI(2)-induced key coupling proved highly stereoselective in favor of the beta-C-substituted products bearing a three-carbon chain at the pseudoanomeric position. Pyrrolidines 9 and 10, as well as the bicyclic compounds 8 and 11, exhibit weak inhibition of the activity of the UDP-galactopyranose mutase from Escherichia coli.     

Novel ketolide antibiotics with a fused five-membered lactone ring--synthesis, physicochemical and antimicrobial properties

In an effort to find novel semisynthetic macrolides with extended antibacterial spectrum and improved activity we prepared a series of compounds based on commercially available clarithromycin, a potent and safe antimicrobial agent of outstanding clinical and commercial interest. According to the literature, improvement of antibacterial activity of erythromycin type antibiotics can be achieved by introduction of fused heterocycles such as cyclic carbonates or carbamates at positions 11 and 12 (such as in telithromycin). In the course of the work presented here, a similar, hitherto unprecedented set of compounds bearing a five-membered lactone ring fused to positions 11 and 12 was prepared based on carbon-carbon bond formation via intramolecular Michael addition of a [(hetero)arylalkylthio]acetic acid ester enolate to an alpha,beta-unsaturated ketone as the key step. Some of the ketolide compounds described in this paper were highly active against a representative set of erythromycin sensitive and erythromycin resistant test strains. The best compound showed a similar antimicrobial spectrum and comparable activity in vitro as well as in vivo as telithromycin. Furthermore, some physicochemical properties of these compounds were determined and are presented here. On the basis of these results, the novel ketolide lactones presented in this paper emerged as valuable lead compounds with comparable properties as the commercial ketolide antibacterial telithromycin (Ketek).     

Inhibitors designed for the active site of dihydroorotase

Four new compounds have been synthesized as potential inhibitors of dihydroorotase from Escherichia coli. NMR spectroscopy was used to show that 4,6-dioxo-piperidine-2(S)-carboxylic acid (3), exists in solution as a mixture of the hydrate (7), enol (8), and enolate (9) tautomeric forms. This compound was found to be a competitive inhibitor versus dihydroorotate and thio-dihydroorotate at pH values of 7-9. The K(i) of 76 microM was lowest at pH7.0 where the ketone and hydrate forms of the inhibitor 3 predominate in solution. Compound 3 was reduced to the two diastereomeric 4-hydroxy derivatives (4 and 5) and then dehydrated to yield the alkene derivative, 1,2,3,6-tetrahydro-6-oxopyridine-2(S)-carboxylic acid (6). Compounds 4-6 were competitive inhibitors versus thio-dihydroorotate at pH 8.0 with K(i) values of 3.0, 1.6, and 2.3 mM. Dihydroorotase was unable to dehydrate the 4-hydroxy derivative 4 or 5 to the alkene 6 or catalyze the reverse reaction.     

A convenient synthetic method of a 5,7-diarylcyclopenteno-[1,2-b]pyridine-6-carboxylate: a key intermediate for potent endothelin receptor antagonists

A convenient method for the synthesis of the title intermediate 4 was described. The key steps of this synthesis involved: (1) regioselective addition reaction of arylzinc reagent to quinolic anhydride in 42% isolated yield, (2) conversion of a ketoacid to an enone, which was achieved in 65% yield by intramolecular Knoevenagel reaction of beta-ketoester generated by condensation of an acid imidazolide with an ester enolate, followed by dehydration assisted with silica gel, and (3) stereoselective reduction of an allyl alcohol in 75% yield with zinc under acidic conditions. This synthesis enabled us to provide hundreds of grams of without chromatographic purification.     

Stereoselective aldol addition reactions of Fischer carbene complexes via electronic tuning of the metal center for enolate reactivity

The aldol reactions of tetracarbonyl(phosphine)methyl(methoxy)methylene chromium complexes and pentacarbonylmethyl (dialkylamino)methylene chromium complexes with aldehydes and ketones were examined. The reactions of the phosphine complexes give only aldol condensation products, but the desired aldol addition products can be isolated from the reactions of amino carbene complexes. This was attributed to the greater reactivity of the enolates of amino carbene complexes which is supported by a determination of the thermodynamic acidity of the dimethylamino complex 13 (pK_a=20.4). The aldol reactions of amino complexes with -chiral aldehydes occur with very high facial selectivities rivaling the best methods that have been developed for facial selectivity in the aldol reaction. The aldol reactions of amino complexes can be considered as direct synthons for amides since amide functions can be obtained in the oxidative cleavage of the aldol adducts of these complexes. As illustrative of the versatility of carbene complexes, it is also demonstrated in a photo-induced carbon-homologative demetallation, that in combination with the aldol addition reaction the unique reactions of carbene complexes provide powerful and novel overall transformations.     

Reaction of astaxanthin with peroxynitrite

The in vitro reactivities of astaxanthin toward peroxynitrite were investigated and the reaction products after scavenging with peroxynitrite were analyzed in order to determine the complete mechanism of this reaction. A series of carotenoids, 13-apo-astaxanthinone (1), 12'-apo-15'-nitroastaxanthinal (2), 12'-apo-astaxanthinal (3), 10'-apo-astaxanthinal (4), 9-cis-14'-s-cis-15'-nitroastaxanthin (5), 14'-s-cis-15'-nitroastaxanthin (6), 13-cis-14'-s-cis-15'-nitroastaxanthin (7), 10'-s-cis-11'-cis-11'-nitroastaxanthin (8), 13,15,13'-tri-cis-15'-nitroastaxanthin (9), 9-cis-astaxanthin (10), and 13-cis-astaxanthin (11), were isolated from the reaction products of carotenoids with peroxynitrite. Our previous studies achieved for the first time the isolation of nitro derivatives from the reaction of astaxanthin with peroxynitrite. Here we identify the major remaining reaction products of this reaction and investigate the stabilities of the nitro astaxanthins.     

Structural basis of leukotriene B4 12-hydroxydehydrogenase/15-Oxo-prostaglandin 13-reductase catalytic mechanism and a possible Src homology 3 domain binding loop

The bifunctional leukotriene B(4) 12-hydroxydehydrogenase/15-oxo-prostaglandin 13-reductase (LTB(4) 12-HD/PGR) is an essential enzyme for eicosanoid inactivation. It is involved in the metabolism of the E and F series of 15-oxo-prostaglandins (15-oxo-PGs), leukotriene B(4) (LTB(4)), and 15-oxo-lipoxin A(4) (15-oxo-LXA(4)). Some nonsteroidal anti-inflammatory drugs (NSAIDs), which primarily act as cyclooxygenase inhibitors also inhibit LTB(4) 12-HD/PGR activity. Here we report the crystal structure of the LTB(4) 12-HD/PGR, the binary complex structure with NADP(+), and the ternary complex structure with NADP(+) and 15-oxo-PGE(2). In the ternary complex, both in the crystalline form and in solution, the enolate anion intermediate accumulates as a brown chromophore. PGE(2) contains two chains, but only the omega-chain of 15-oxo-PGE(2) was defined in the electron density map in the ternary complex structure. The omega-chain was identified at the hydrophobic pore on the dimer interface. The structure showed that the 15-oxo group forms hydrogen bonds with the 2'-hydroxyl group of nicotine amide ribose of NADP(+) and a bound water molecule to stabilize the enolate intermediate during the reductase reaction. The electron-deficient C13 atom of the conjugated enolate may be directly attacked by a hydride from the NADPH nicotine amide in a stereospecific manner. The moderate recognition of 15-oxo-PGE(2) is consistent with a broad substrate specificity of LTB(4) 12-HD/PGR. The structure also implies that a Src homology domain 3 may interact with the left-handed proline-rich helix at the dimer interface and regulate LTB(4) 12-HD/PGR activity by disruption of the substrate binding pore to accommodate the omega-chain.     

Asymmetric synthesis of bicyclic piperidines via L-proline catalyzed aldol reaction of 3-phthalimidopropanal

A highly enantioselective approach for preparing optically active bicyclic piperidines is described. The key step for introducing chiral centers was a L-proline catalyzed direct enantioselective aldol reaction of 3-phthalimidopropanal with aliphatic ketones. In the reactions with alicyclic ketones, a highly enantioselective formation of anti-2-(3-phthalimido-1-hydroxypropyl)cycloketones 1a-1b (>99% ee) was observed. The aldol products 1 could be subsequently converted into bicyclic piperidines 2 via a consecutive reductive deprotection, acylation, ring closure, and hydrolysis.