Synthesis of dl-[2-13C]leucine and it use in the preparation of [3-dl-[2-13C]leucine]oxytocin and [8-dl-[2-13C]leucine]oxytocin: Preparative separation of diastereoisomeric peptides by partition chromatography and high pressure liquid chromatography

dl-[2-¹³C]Leucine was prepared by condensing the sodium salt of ethyl acetamido-[2-¹³C]cyanoacetate with isobutylbromide in hexamethylphosphoroustriamide followed by acid hydrolysis. N-Boc-dl-[2-¹³C]Leucine was prepared and incorporated into [8-dl-[2-¹³C]leucine]oxytocin by total synthesis. The ¹³C-labeled hormone derivative [8-[2-¹³C]leucine]oxytocin was separated from its 8-position diastereoisomer by partition chromatography. The specifically ¹³C-labeled peptide hormone diastereoisomeric analog [3-dl-[2-¹³C]leucine]oxytocin also was prepared by solid phase peptide synthesis. No suitable solvent system for partition chromatography separation of the latter diastereoisomeric peptide mixture could be found. However an excellent preparative separation of the diastereoisomers could be obtained by reverse phase high pressure liquid chromatography on a partisil 10 M9 ODS column using the solvent system 0.05 M ammonium acetate (pH 4.0), acetonitrile (81:19, $\text{v}\text{v}$) to give pure [3-(2-¹³C]leucine]oxytocin and [3-D-(2-¹³C]leucine]oxytocin. An excellent separation of [8[2-¹³C]leucine]oxytocin and the corresponding 8-D-leucine diastereoisomer derivative could also be accomplished by high pressure liquid chromatography.     

Mo16O53F2]12-: a new polyoxofluoromolybdate anion

Single crystals of a new beta-octamolybdate salt containing protonated 1,4-diazabicyclo[2.2.2]octane cations were prepared under mild hydrothermal conditions. This compound, [C6H13N2]2[C6H14N2][Mo8O26], was then used as a starting material in the synthesis of [C6H13N2]6[Mo16O53F2].4H2O, which contains previously unreported [Mo16O53F2]12- anions. The structure-directing properties of gamma-[Mo8O26]4-, a likely intermediate in this pH-dependent transformation, are responsible for the site selection of the fluoride incorporation. [Mo16O53F2]12-, the largest reported polyoxofluoromolybdate cluster, expands upon the limited number of such anions in the literature. The structures of both compounds were determined using single-crystal X-ray diffraction.     

13C,15N]2-Acetamido-2-deoxy-D-aldohexoses and their methyl glycosides: synthesis and NMR investigations of J-couplings involving 1H, 13C, and 15N

[reaction: see text] A series of 2-amino-2-deoxy-D-[1-13C]aldohexoses and their methyl glycosides was prepared with use of a simplified cyanohydrin reduction route. Four d-aldopentosylamines (arabino, lyxo, ribo, xylo) were prepared from the corresponding D-aldopentoses by reaction with NH3(g) in MeOH solvent, isolated in solid form, and characterized by 13C and 1H NMR. Hydrolysis of beta-D-xylopyranosylamine was studied using 13C-labeled substrates to establish optimal solution conditions for cyanohydrin formation. Major hydrolytic intermediates were observed and identified by time-lapse 1D and 2D NMR analyses of reaction mixtures. The aldopentosylamines were subsequently employed in cyanohydrin reduction reactions with K13CN to yield C2-epimeric [1-13C]2-aminosugars, which were separated by chromatography on ion-exchange columns. N-Acetylation and methyl glycosidation followed by chromatography gave pure 2-acetamido-2-deoxy-D-[1-13C]aldohexopyranosides. J(CH) and J(CC) spin-spin coupling constants involving the labeled anomeric carbon were measured and compared to those observed previously in methyl D-[1-13C]aldohexopyranosides. In parallel studies, theoretical J-couplings were calculated in model N-acetylated aldopyranosides using density functional theory (DFT) to predict the effect of OH vs NHCOCH(3) substitution at C2 on J(CH) and J(CC) values in aldopyranosyl rings. The synthetic method was also modified to accommodate (15)N- and (13)C-labeling within the N-acetyl side-chain, and some J-couplings involving 1H, 13C, and 15N atoms in 2-[1,2-13C2;15N]acetamido-2-deoxy-D-[1-13C]glucose were measured and interpreted.     

Biosynthesis of chaetochromin A, a bis(naphtho-gamma-pyrone), in Chaetomium spp

The biosynthesis of chaetochromin A, a metabolite of Chaetomium gracile, has been studied using [13CH3]methionine, sodium [1-13C]acetate, sodium [1,2-13C2]acetate, sodium [1-13C,2,2,2-2H3]acetate, and sodium [1-13C,1,1-18O2]acetate as precursors. The folding pattern of the polyketide chain in chaetochromin A, biosynthesized from sodium [1,2-13C2]acetate as the precursor, was determined to be the same as that of rubrofusarin by carbon-13 nuclear magnetic resonance (13C-NMR) analysis. By using [13CH3]methionine as a precursor, the source of 2-CH3 was determined. When sodium [1-13C,2,2,2-2H3]acetate was fed, a beta-isotope-shifted peak was observed only for carbon 2. In the 13C-NMR spectra of chaetochromin A and of its hexamethyl ether derived from sodium [1-13C,1,1-18O2]acetate, isotope-shifted peaks were observed for carbons 4, 5, 6, 8 and 10a, but not for carbon 2. These results showed that oxygen 1 originated from the same unit of acetate as carbon 10a.     

Electrochemical synthesis and structural and physical characterization of one- and two-electron-reduced forms of [SMo12O40]2-

Isolation of a soluble [NHex4]+ salt has allowed a detailed electrochemical study of the anion alpha-[SMo12O40]2- to be undertaken. Four reversible one-electron-reduction processes are observed in CH2Cl2 solution. Controlled potential electrolysis led to isolation of tetraalkylammonium salts of the one-electron-reduced anion alpha-[SMo12O40]3- and the two-electron-reduced anion alpha-[SMo12O40].4- [SMo12O40]3- is stable to disproportionation in dry solvents (Kdis = 10(-7.4). EPR and magnetic susceptibility data indicate that [SMo12O40]3- is a simple paramagnet (S = 1/2) while [SMo12O40]4- is paramagnetic with the mu eff values decreasing at low temperatures. Solutions of the two-electron-reduced species are EPR silent, but microcrystalline powders show very weak signals. The crystal structure of alpha-[NBu4]3[SMo12O40] has been determined (triclinic P1; a = 13.840(3) A; b = 15.587(4) A; c = 19.370(3) A; alpha = 94.82(2) degrees; beta = 93.10(1) degrees; gamma = 91.05(2) degrees; Z = 2). There is disorder around the C2 axis of the central SO4(2-) tetrahedron. In the presence of aqueous HClO4 (0.045 M) in thf/H2O or MeCN/H2O (98/2 v/v), [SMo12O40]2- exhibits five two-electron-reduction processes. Under these conditions, [SMo12O40]3- protonates and disproportionates into [SMo12O40]2- and the (2e-, 2H+)-reduced anion [H2SMo12O40]2-.     

Fragmentation of the deprotonated ions of peptides containing cysteine, cysteine sulfinic acid, cysteine sulfonic acid, aspartic acid, and glutamic acid

We examined the fragmentation of the electrospray-produced [M-H]- and [M-2H]2- ions of a number of peptides containing two acidic amino acid residues, one being aspartic acid (Asp) or glutamic acid (Glu), and the other being cysteine sulfinic acid [C(SO2H)] or cysteine sulfonic acid [C(SO3H)], on an ion-trap mass spectrometer. We observed facile neutral losses of H2S and H2SO2 from the side chains of cysteine and C(SO2H), respectively, whereas the corresponding elimination of H2SO3 from the side chain of C(SO3H) was undetectable for most peptides that we investigated. In addition, the collisional activation of the [M-H]- ions of the C(SO2H)-containing peptides resulted in the cleavage of the amide bond on the C-terminal side of the C(SO2H) residue. Moreover, collisional activation of the [M-2H]2- ions of the above Asp-containing peptides led to the cleavage of the backbone N-Calpha bond of the Asp residue to give cn and/or its complementary [zn-H2O] ions. Similar cleavage also occurred for the singly deprotonated ions of the otherwise identical peptides with a C-terminal amide functionality, but not for the [M-H]- ions of same peptides with a free C-terminal carboxylic acid. Furthermore, ab initio calculation results for model cleavage reactions are consistent with the selective cleavage of the backbone N-Calpha bond in the Asp residue.     

Synthesis of Chiral Tetrahomodiazacalix[4]-and [6]Arenes Bridged by Cystine Peptide

Bicyclic tetrahomodiazacalix[4]- and [6]arenes bridged by a cystine peptide were easily synthesized from the cyclization reactions of bis(chloromethyl)phenol-formaldehyde oligomers with cystine peptides in DMF in moderate yields.     

Conformational equilibrium isotope effects in glucose by (13)C NMR spectroscopy and computational studies.

Anomeric equilibrium isotope effects for dissolved sugars are required preludes to understanding isotope effects for these molecules bound to enzymes. This paper presents a full molecule study of the alpha- and beta-anomeric forms of D-glucopyranose in water using deuterium conformational equilibrium isotope effects (CEIE). Using 1D (13)C NMR, we have found deuterium isotope effects of 1.043 +/- 0.004, 1.027 +/- 0.005, 1.027 +/- 0.004, 1.001 +/- 0.003, 1.036 +/- 0.004, and 0.998 +/- 0.004 on the equilibrium constant, (H/D)K(beta/alpha), in [1-(2)H]-, [2-(2)H]-, [3-(2)H]-, [4-(2)H]-, [5-(2)H]-, and [6,6'-(2)H(2)]-labeled sugars, respectively. A computational study of the anomeric equilibrium in glucose using semiempirical and ab initio methods yields values that correlate well with experiment. Natural bond orbital (NBO) analysis of glucose and dihedral rotational equilibrium isotope effects in 2-propanol strongly imply a hyperconjugative mechanism for the isotope effects at H1 and H2. We conclude that the isotope effect at H1 is due to n(p) --> sigma* hyperconjugative transfer from O5 to the axial C1--H1 bond in beta-glucose, while this transfer makes no contribution to the isotope effect at H5. The isotope effect at H2 is due to rotational restriction of OH2 at 160 degrees in the alpha form and 60 degrees in the beta-sugar, with concomitant differences in n --> sigma* hyperconjugative transfer from O2 to CH2. The isotope effects on H3 and H5 result primarily from syn-diaxial steric repulsion between these and the axial anomeric hydroxyl oxygen in alpha-glucose. Therefore, intramolecular effects play an important role in isotopic perturbation of the anomeric equilibrium. The possible role of intermolecular effects is discussed in the context of recent molecular dynamics studies on aqueous glucose.     

Biosynthesis of anthecotuloide,an irregular sesquiterpene lactone from Anthemis cotula L. (Asteraceae) via a non-farnesyl diphosphate route

Retrobiosynthetic analysis of the allergenic sesquiterpene lactone, anthecotuloide, suggested that this natural product could be formed either by head to head condensation of geranyl diphosphate with dimethylallyl diphosphate, or from farnesyl diphosphate (FPP), the accepted regular sesquiterpene precursor via the rearrangement of a germacranolide precursor. Isotopic labelling of anthecotuloide has now been achieved by feeding [1-(13)C]-glucose, [U-13C6]-glucose and [6,6-(2)H2]-glucose to aseptically grown plantlets of Anthemis cotula(family Asteraceae). Analysis of labelling patterns and absolute 13C abundances using quantitative 13C NMR spectroscopy showed that the isoprene building blocks of this sesquiterpene are formed exclusively via the MEP terpene biosynthetic pathway. This was supported by results from an experiment using [U-13C6]-glucose. A deuterium labelling experiment using [6,6-(2)H2]-glucose supported the original proposal and showed that anthecotuloide is formed from a non FPP precursor. Isotope ratio mass spectrometry suggested that there were two pathways for sesquiterpene biosynthesis in A. cotula.     

Directed synthesis of noncentrosymmetric molybdates using composition space analysis

A systematic investigation of the factors governing the reaction product composition, hydrogen bonding, and symmetry was conducted in the MoO3/3-aminoquinuclidine/H2O system. Composition space analysis was performed through 36 individual reactions under mild hydrothermal conditions using racemic 3-aminoquinuclidine. Single crystals of three new compounds, [C7H16N2][Mo3O10] x H2O, [C7H16N2]2[Mo8O26] x H2O, and [C7H16N2]2[Mo8O26] x 4 H2O, were grown. The relative phase stabilities for these products are dependent upon the reactant mole fractions in the initial reaction gel. This phase stability information was used to direct the synthesis of two new noncentrosymmetric compounds, using either (S)-(-)-3-aminoquinuclidine dihydrochloride or (R)-(+)-3-aminoquinuclidine dihydrochloride. [(R)-C7H16N2]2[Mo8O26] and [(S)-C7H16N2]2[Mo8O26] both crystallize in the noncentrosymmetric space group P2(1) (No. 4), which has the polar crystal class 2 (C2). The second-harmonic generation activities were measured on sieved powders. The structure-directing properties of the molybdate components in each compound were determined using bond valence sums. The structures of all five compounds were determined using single-crystal X-ray diffraction.     

pH-dependence of the aqueous electrochemistry of the two-electron reduced alpha-[Mo18O54(SO3)] sulfite Dawson-like polyoxometalate anion derived from its triethanolammonium salt

The electrochemistry of the Dawson-like sulfite polyoxometalate anion alpha-[Mo18O54(SO3)2]6-, derived from the TEAH6{alpha-[Mo18O54(SO3)2]} salt (TEAH+ is the triethanolammonium cation; pKa=7.8), has been investigated in aqueous media using cyclic and rotated disk voltammetry at glassy carbon electrodes and bulk electrolysis, with a focus on the pH-dependence for oxidation to alpha-[Mo18O54(SO3)2]4-. In buffered media at pH>or=4, the cyclic voltammetric response for alpha-[Mo18O54(SO3)2]6- reveals two partially resolved one-electron oxidation processes corresponding to the sequential generation of alpha-[Mo18O54(SO3)2]5- and alpha-[Mo18O54(SO3)2]4-. At lower pH, using electrolytes containing sulfuric acid, the two waves coalesce but the individual apparent E0' reversible formal potential values for the two processes can be extracted down to pH 2 by assuming that reversible protonation accompanies fast electron transfer. The results for 2相似文献   

TOF‐SIMS study of cystine and cholesterol stones

Two different human stones, cystine and cholesterol from the kidney and gall bladder, were examined by time‐of‐flight secondary ion mass spectrometry using Ga⁺ primary ions as bombarding particles. The mass spectra of kidney stone were compared with those measured for the standard compounds, cystine and cysteine. Similar spectra were obtained for the stone and cystine. The most important identification was based on the existence of the protonated molecules [M + H]⁺ and deprotonated molecules [M‐H]^‐. The presence of cystine salt was also revealed in the stone through the sodiated cystine [M + Na]⁺ and the associated fragments, which might be due to the patient treatment history. In the gallstone, the deprotonated molecules [M‐H]⁺ of cholesterol along with relatively intense characteristic fragments [M‐OH]⁺ were detected. Copyright © 2012 John Wiley & Sons, Ltd.     

Differentiation of Boc- alpha,beta- and beta,alpha-peptides and a pair of diastereomeric beta,alpha-dipeptides by positive and negative ion electrospray tandem mass spectrometry (ESI-MS/MS)

Positive and negative ion electrospray ionization (ESI) tandem mass spectral study of a new series of hybrid peptides, viz, BocN-alpha,beta-peptides and BocN-beta,alpha-peptides, synthesized from C-linked carbo-beta3-amino acids [Caa (S)] and L-Ala has been carried out. The alpha,beta-peptides have been differentiated from beta,alpha-peptides by the collision-induced dissociation (CID) of [M + H]+ and [M - H]- ions in positive and negative ion ESI-MS respectively. The fragment ion [M + H - C(CH3)3 + H]+ formed from [M + H]+ ions by the loss of 2-methyl-prop-2-ene in alpha,beta-peptides with L-Ala at the N-terminus is insignificant or totally absent for beta,alpha-peptides which have the Caa (S) at N-terminus. The fragment ion [M - H-C(CH3)3OH - HNCO]- formed from [M - H]- of beta,alpha-peptide acids is totally absent for alpha,beta-peptide acids. This has been attributed to the absence of the beta-methylene group in alpha,beta-peptides, and the participation of the beta-methylene group in the loss of HNCO in beta,alpha-peptide acids is confirmed by the deuteration experiments. The CID of [M + H-Boc + H]+ ions of these peptides also produce characteristic fragmentation. In the CID spectra of alpha,beta-peptides, the b(n)+ ions and the resulting y(n)+ ions occur at a mass difference of 243 and 71 Da corresponding to the successive losses of Caa and L-Ala, whereas a mass difference of 71 and 243 Da is observed for beta,alpha-peptides. In contrast to the CID of protonated peptides, the CID of [M - H]- ions of the alpha,beta- and beta,alpha-peptide acids do not give b(n)- ions and show abundant z(n) (-) ions. Further, a pair of diastereomeric dipeptide esters and acids have been distinguished by the CID of [M + H]+ ions. The loss of 2-methyl-prop-2-ene is more pronounced for Boc-NH-Caa(R)-D-Ala-OCH3 (21) and Boc-NH-Caa(R)-D-Ala-OH (23) with Caa (R) at the N-terminus, whereas it is totally absent for Boc-NH-Caa (S)-D-Ala-OCH3 (22) and Boc-NH-Caa(S)-D-Ala-OH (24) peptides, which have Caa (S) at the N-terminus. Thus, on the basis of our previous and present studies, we propose that the CID of [M + H]+ ions provides a simple and useful method for distinguishing the configuration of Caa (S or R) at the N-terminus of BocN-carbo beta,alpha- and beta,beta-dipeptides.     

Vibrational spectra of succinonitrile and its [1,4-13C2]-,[2,2,3,3,-2H4]- and 1,4-13C-2,2,3,3-2H4]-isotopomers and a force field of succinonitrile

[1,4-13C2]-succinonitrile, [2,2,3,3-2H4]-succinonitrile, [1,4-13C2-2,2,3,3-2H4]-succinonitrile have been synthesized and, for the first time, the infrared and Raman spectra of these succinonitrile isotopomers have been discussed in detail. The spectra were recorded at ambient temperature and at the temperature of liquid nitrogen and assignments of the vibrational bands of all above isotopomers have been made. In addition to this, the force field of succinonitrile in its two configurations, gauche and trans, have been calculated using all experimental frequencies and the ab-initio method. Also, the assignments of the vibrational bands of the 'normal' succinonitrile molecule have been achieved using the results of the force field calculations to support the assignments for the normal succinonitrile, as well as for the isotopomers.     

Pt(II)-mediated nitrile-tetramethylguanidine coupling as a key step for a novel synthesis of 1,6-dihydro-1,3,5-dihydrotriazines

The coupling between tetramethylguanidine, HN=C(NMe2)2, and coordinated organonitriles in the platinum(II) complexes cis/trans-[PtCl2(RCN)2] (R = Me, Et, Ph) proceeds rapidly under mild conditions to afford the diimino compounds containing two N-bound monodentate 1,3-diaza-1,3-diene ligands [PtCl2{NH=C(R)N=C(NMe2)2}2] (R = Et, trans-1; R = Ph, trans-2; R = Me, cis-3; R = Et, cis-4), and this reaction is the first observation of metal-mediated nucleophilic addition of a guanidine to ligated nitrile. Complexes 1-4 were characterized by elemental analyses (C, H, N), X-ray diffraction, FAB mass spectrometry, IR, and 1H and 13C{1H} NMR spectroscopies; assignment of signals from E/Z-forms of 1,3-diaza-1,3-diene ligands and verification of routes for their Z right harpoon over left harpoon E isomerization in solution were performed using 2D 1H,1H-COSY, 1H,13C-HETCOR, and 1D NOE NMR experiments. The newly formed and previously unknown 1,3-diaza-1,3-dienes NH=C(R)N=C(NMe2)2 were liberated from the platinum(II) complexes [PtCl2{NH=C(R)N=C(NMe2)2}2] (1-3) by substitution with 2 equiv of 1,2-bis-(diphenylphosphino)ethane (dppe) to give the uncomplexed HN=C(R)N=C(NMe2)2 species (5-7) in solution and the solid [Pt(dppe)2](Cl)2. The former were utilized in situ, after filtration of the latter, in the reaction with 1,3-di-p-tolylcarbodiimide, (p-tol)N=C=N(tol-p), in CDCl3 to generate (6E)-N,N-dimethyl-1-(4-methylphenyl)-6-[(4-methylphenyl)imino]-1,6-dihydro-1,3,5-triazin-2-amines) (8-10) due to the [4 + 2]-cycloaddition accompanying elimination of HNMe2. The formulation of 8-10 is based on ESI-MS, 1H, 13C{1H} NMR, and X-ray crystal structures determined for 9 and 10. The reaction of 1,3-diaza-1,3-dienes with 1,3-di-p-tolylcarbodiimide, described in this article, constitutes a novel synthetic approach to a useful class of heterocyclic species like 1,6-dihydro-1,3,5-triazines.     

Radical-mediated carboxylation of alkyl iodides with [11C]carbon monoxide in solvent mixtures

[reaction: see text] [carboxyl-(11)C]Carboxylic acids were prepared from alkyl iodides via photoinitiated radical reactions using 10(-)(8) mol of [(11)C]carbon monoxide in binary and ternary homogeneous solvent mixtures. Short- (isobutyric), medium-, and long-chain saturated fatty acids (heptadecanoic) were labeled with isolated decay-corrected radiochemical yields ranging from 55% to 70% in 5-7-min reactions. The conversion of [(11)C]carbon monoxide to products reached 80-90%. To obtain good yields in the reactions performed in water-acetonitrile and water-THF mixtures, the addition of tetrabutylammonium hydroxide or potassium hydroxide was essential. The carboxylation was efficient for primary and secondary alkyl iodides. The carboxylation of tertiary iodides was feasible for 1-iodoadamantane but not for tert-butyl iodide. The dependence of the radiochemical yields on reaction time, photoirradiation conditions, and organic and inorganic additives was studied. The method provides a one-step route to [carboxyl-(11)C]carboxylic acids; traditional methods, in contrast, would require several steps. For example, using the devised reaction conditions, 3.19 GBq of purified [1-(11)C]1,10-decanedicarboxylic acid (specific radioactivity 188 GBq/mumol) was obtained within 35 min of the end of 10 muAh bombardment. (1-(13)C)4-Phenylbutyric acid was synthesized using ((13)C)carbon monoxide for identifying the labeling position with (1)H and (13)C NMR.     

Biosynthetic precursors of the lipase inhibitor lipstatin

Three putative intermediates in the biosynthesis of the lipase inhibitor lipstatin were synthesized in stable isotope-labeled form and were added to fermentation cultures of Streptomyces toxytricini. Biosynthetic lipstatin was isolated and analyzed by NMR spectroscopy. [3,10,11,12-(2)H]-(3S,5Z,8Z)-3-hydroxytetradeca-5,8-dienoic acid (9) was shown to serve as a direct biosynthetic precursor of lipstatin. [7,8-(2)H(2)]Hexylmalonate (11) was also incorporated into lipstatin, albeit at a relatively low rate. The leucine moiety of [(13)C-formyl,(15)N]-N-formylleucine (10) was diverted to lipstatin under loss of the (13)C-labeled formyl residue.     

PtII-mediated 1,3-dipolar cycloaddition of oxazoline N-oxides to nitriles as a key step to dihydrooxazolo-1,2,4-oxadiazoles

A novel type of heterocycle, viz., 2,3a-disubstituted 5,6-dihydro-3aH-[1,3]oxazolo[3,2-b][1,2,4]oxadiazoles, was generated by an intermolecular PtII-mediated 1,3-dipolar cycloaddition (1,3-DCA) between the oxazoline N-oxide C(Me)2CH2OC(R)=N+(O-) (R = Me, Et) and coordinated nitriles in the complexes trans/cis-[PtCl2(R'CN)2] [R' = Me, Et, CH2Ph, Ph, N(C5H10)]. The reaction is unknown for free RCN and oxazoline N-oxides, but under PtII-mediated conditions, it proceeds smoothly (CH2Cl2, 20-25 degrees C, 18-20 h) and gives pure complexes [PtCl2{N=C(R')ONC(R)OCH2CMe2}2] [R/R' = Me/Me, 1; Me/Et, 2; Me/CH2Ph, 3; Me/Ph, 4; Me/N(C5H10), 5; Et/Me, 6; Et/Et, 7; Et/CH2Ph, 8; Et/Ph, 9; Et/N(C5H10), 10] in 42-84% yields after column chromatography. Compounds 1-10 were characterized by elemental analyses (C, H, N), FAB+-MS, IR, and 1H and 13C{1H} NMR spectroscopies, and X-ray diffraction (for 1, 2, 5, and 9). With the exception of benzonitrile complexes, 1,3-DCA of oxazoline N-oxides to the PtII-ligated nitriles occurred diastereoselectively and afforded mixtures of enantiomers. Depending on the substituents on nitriles, asymmetric atoms in both of the formed heterocyclic ligands have the same (SS/RR) or different (SR/RS) configurations. The heterocyclic ligands were liberated from 1-4 and 6-9 by treatment with excess ethane-1,2-diamine (en) in CH2Cl2 for 1 day at 20-25 degrees C (for R' = Me, Et, CH2Ph) and at 50 degrees C (for R' = Ph) to achieve the free organic species and the well-known [Pt(en)2](Cl)2; the products were separated, and 2,3a-disubstituted 5,6-dihydro-3aH-[1,3]oxazolo[3,2-b][1,2,4]oxadiazoles (11-18) were characterized by ESI+-MS and 1H and 13C{1H} NMR spectroscopies.     

Synthesis of oxytocin analogues with replacement of sulfur by carbon gives potent antagonists with increased stability

[Chemical reaction: See text] The neuropeptide oxytocin 1 controls mammary and uterine smooth muscle contraction. Atosiban 2, an oxytocin antagonist, is used for prevention of preterm labor and premature birth. However, the metabolic lifetimes of such peptide drugs are short because of in vivo degradation. Facile production of oxytocin analogues with varying ring sizes wherein sulfur is replaced by carbon (methylene or methine) could be achieved by standard solid-phase peptide synthesis using olefin-bearing amino acids followed by on-resin ring-closing metathesis (RCM). These were tested for agonistic and antagonistic uteronic activity using myometrial strips taken from nonpregnant female rats. Peptide 8 showed agonistic activity in vitro (EC50= 1.4 x 10(3) +/- 4.4 x 10(2) nM) as compared to 1 (EC50= 7.0 +/- 2.1 nM). Atosiban analogues 17 (pA2= 7.8 +/- 0.1) and 18 (pA2= 8.0 +/- 0.1) showed substantial activity compared to the parent oxytocin antagonist 2 (pA2= 9.9 +/- 0.3). Carba analogue 35 (pA2= 6.1 +/- 0.1) had an agonistic activity over 2 orders of magnitude less than its parent 3 (8.8 +/- 0.5). A comparison of biological stabilities of 1,6-carba analogues of both an agonist 8 and antagonist 18 versus parent peptides 1 and 2 was conducted. The half-lives of peptides 8 and 18 in rat placental tissue were shown (Table 2) to be greatly improved versus their parents oxytocin 1 and atosiban 2, respectively. These results suggest that peptides 8 and 18 and analogues thereof may be important leads into the development of a long-lasting, commercially available therapeutic for initiation of parturition and treatment of preterm labor.     

Iminoacylation. 3. Formation of platinum(IV)-based metallaligands due to facile one-end addition of vic-dioximes to coordinated organonitriles

The reaction of vic-dioximes with the organonitrile platinum(IV) complexes trans-[PtCl4(RCN)2] (R = Me, CH2Ph, Ph, vic-dioxime = dimethylglyoxime; R = Me, vic-dioxime = cyclohexa-, cyclohepta-, and cyclooctanedione dioximes) proceeds rapidly under relatively mild conditions and affords products of one-end addition of the dioximes to the nitrile carbon, i.e. [PtC4(NH=C(R)ON=[spacer]=NOH)2] (1-6) (R = Me, CH2Ph, Ph, spacer = C(Me)C-(Me) for dimethylglyoxime; R = Me, spacer = C[C4H8]C, C[C5H10]C, C[C6H12]C for the other dioximes), giving a novel type of metallaligand. All addition compounds were characterized by elemental analyses (C, H, N, C1, Pt), FAB mass spectrometry, and IR and 1H, 13C[1H], and 195Pt NMR spectroscopy. X-ray structure determination of the dimethylformamide bis-solvate [PtCl4(NH=C(Me)ON=C(Me)C(Me)=NOH)2] x 2DMF (la) disclosed its overall trans geometry with the dimethylglyoxime part in anti configuration and the amidine one-end (rather than N,N-bidentate) coordination mode of the N-donor ligands. When a mixture of cis- and trans-[PtC4(MeCN)2] in MeCN was treated with dimethylglyoxime, the formation of, correspondingly, cis- and trans-[PtCl4(NH=C(Me)ON=C(Me)C(Me)=NOH)2] (1) was observed and cis-to-trans isomerization in DMSO-d6 solution was monitored by 1H, 2D [1H,15N] HMQC, and 195Pt NMR spectroscopies. Although performed ab initio calculations give evidence that the trans geometry is the favorable one for the iminoacylated species [PtCl4-(ligand)2], the platinum(IV) complex [PtCl4(NH=C(Me)ON=C[C4Hs]C=NOH)2] (4) was isolated exclusively in cis configuration with the two metallaligand "arms" held together by intramolecular hydrogen bonding between the two peripheral OH groups, as it was proved by single-crystal X-ray diffractometry. The classic substitution products, e.g. [PtC12(N,N-dioximato)2] (12-15), are formed in the addition reaction as only byproducts in minor yield; two of them, [PtCl2(C7H11N2O2)2] (14) and [PtCl2(C8H13N2O2)2] (15), were structurally characterized. Complexes (12-15) were also prepared by reaction of the vic-dioximes with [PtCl4L(Me2SO)] (L = Me2SO, MeCN), but monoximes (Me2C=NOH, [C4H8]C=NOH, [C5H10]C=NOH, PhC(H)=NOH, (OH)C6H4C(H)= NOH) react differently adding to [PtCl4(MeCN)(Me2SO)] to give the corresponding iminoacylated products [PtCl4(NH=C(Me)ON=CRR')(Me2SO)](7-11).