Isomers 1a Research Articles

Reaction of the a-Benzylic Anion of Spirophosphoranes with N,a-Diphenylnitrone: Stereochemistry and Formation of Hexacoordinate Phosphate Bearing a 1,2,5-Oxazaphospholidine Ring

The benzylic anion (3a-Li) a to the phosphorus atom generated from C-apical O-equatorial (O-cis) spirophosphorane (la) reacted with N,a-diphenylnitrone to give hexacoordinate phosphates (8) bearing a 1,2,5-oxazaphospholidine ring. The phosphates were characterized by 3 1 P NMR and by the structure of the products of protonolysis. Examination of the corresponding reactions from O-apical C-equatorial (O-trans) isomer (2a) revealed that the phosphates (9) derived from 2a were unstable compared with the hexacoordinated 8 derived from la, mainly due to the stabilizing trans influence of the P-O bond in 8.

Stereochemistry of [4 + 2] Cycloadditions of Perfluorinated Selenocarbonyls to 1, 3‐Dienes

AbstractIn completely stereospecific [4+2] cycloadditions, the perfluorinated selenocarbonyls 1 and 2 react both with trans‐trans‐2, 4‐hexadiene and cis‐trans‐2, 4‐hexadiene to yield 3, 6‐dihydro‐cis‐3, 6‐dimethyl‐2H‐selenapyrans 3, 4a and 4b. The observed stereoselectivity leads to the conclusion, that the [4+2] cycloaddition of perfluorinated selenocarbonyls follows a concerted pathway. An identical mixture of isomers was isolated when using the precursor for 2, trimethylstannyl (pentafluoroethyl)selane, which reacts with both 1, 3‐dienes over several weeks to form a mixture of syn‐2‐fluoro‐3, 6‐dihydro‐cis‐3, 6‐dimethyl‐2‐trifluoromethyl‐2H‐selenapyran (4a) and anti‐2‐fluoro‐3, 6‐dihydro‐cis‐3, 6‐dimethyl‐2‐trifluoromethyl‐2H‐selenapyran (4b) in the same ratio as found for 2, thus proving the intermediate formation of Se=C(F)CF3 (2). Complex 2D NMR experiments were used to distinguish the isomers 4a and 4b and to assign the 1H, 13C and 19F NMR data of the selenaheterocycles.

Different isomers of [Ru II(NO +)(hedta)(H 2O)] prepared from Ru(NO)Cl 3 via chelation by hedta 3− than by NO 2 − addition to [Ru(H 2O)(hedta)] −

NO 2 − substitution on [Ru II(H 2O)(hedta)] −, followed by acidification, forms almost exclusively the cis-equatorial [Ru II(NO +)(hedta)] isomer ( 1a) ( υ NO=1846 cm −1). However, the reaction of a pre-formed ruthenium nitrosyl complex, Ru(NO)Cl 3(H 2O) 2, with hedta 3− (pH∼3.0, 1.5 h, ∼80 °C) produced two different products ( 2a, 2b). Complexes 2a and 2b, [Ru(NO)(hedta)X], have structures that are derived from the energetically much less favorable cis-polar [Ru II(NO +)(hedta)] isomer by a displacement of one glycinato donor by X=H 2O or Cl −, either trans ( 2b) or cis ( 2a) to NO +. Both isomer products ( 2a, 2b) have υ NO=1882 cm −1, indicative of rather similar oxygen donors trans to NO +. Complex 2a (66%) has the NO + group cis to in-plane X=H 2O or Cl − and the two N donors of hedta 3−. Complex 2a also has one axial glycinato chelate from the N-hydroxyethyl nitrogen ( R), and one in-plane glycinato chelate from the second N donor ( R). The ( R, R) placement of the pendant ethyl and glycinato units are in an ‘up,down’ arrangement for 2a, minimizing repulsions for the pendant groups. Complex 2b (33%) has two in-plane glycinato chelate rings and the two in-plane en-type N donors, as for cis-polar [Ru(NO)(hedta)], but with the axial glycinato group of 2a moved to the in-plane location, and retaining the ( R, R) ‘up,down’ chiralities. The coordination assignments for the glycinato rings from 1H and 13C NMR data are supported by MMFF94 calculations wherein 2a ( R, R) (‘up,down’) is more stable than ( R, S) (‘up,up’) by approximately 36 kcal mol −1. The calculations are sensitive to the central angles with the Ru center, which were constrained to 90° angles in the calculations. For lesser isomer 2b, it was determined that the stability of the placement of pendant groups follows the order ( R, R) (‘up,down’)>( S, R) (‘down,down’)∼( S, S) (‘down,up’) >( R, S) (‘up,up’). The Ru II(NO +)/Ru II(NO −) reduction potential of 2a and 2b is −0.12 V versus NHE, similar to −0.10 V for 1a. Since the [Ru(NO)(hedta)] coordination is most favored for a cis-equatorial complex than complexes derived from cis-polar, the addition of the hedta 3− ligand forms 2a and 2b under kinetic control wherein substitution of the first and second N donors occur cis to the NO + group.

Synthesis, structural characterization and solution dynamics of osmium–rhodium mixed-metal cluster [Os 4Rh(μ-H) 4(CO) 13(η 2-NC 5H 4C 2H 2)

Reaction of [Os 3Rh(μ-H) 3(CO) 12] 1 with an excess amount of 2-vinylpyridine in refluxing CHCl 3 afforded a new cluster, [Os 4Rh(μ-H) 4(CO) 13(η 2-NC 5H 4C 2H 2)] 2, in moderate yield. A pair of isomers 2a and 2b, which differ in configuration of the metallated vinylpyridine exist in the solution of 2. Through the variable temperature EXSY experiment, the activation barrier (ΔG ≠) for the isomerisation of 2 was determined to be 89.8 kJ mol −1 .

Synthesis and properties of triply-bridged syn-carbazolophanes.

The intramolecular [2 + 2] photocycloaddition of alpha,omega-bis(3,6-divinyl-N-carbazolyl)alkanes 3 afforded triply bridged syn-[2.2.n](3,6,9)carbazolophanes 4a-6a (n = 4) and 4b,5b (n = 5) composed of isomers derived from the difference in the direction of cyclobutane rings. In major isomers 4a and 4b, excimer fluorescence was observed, the maximum position remarkably depending on the tether length.

N1 and N3 linkage isomers of neutral and deprotonated cytosine with trans-[(CH3NH2)2PtII.

A series of complexes obtained from the reaction of trans-[(CH3NH2)2PtII] with unsubstituted cytosine (CH) and its anion (C), respectively, has been prepared and isolated or detected in solution: trans-[Pt(CH3NH2)2(CH-N3)Cl]Cl.H2O (1), trans-[Pt(CH3NH2)2(CH-N3)2](ClO4)2 (1a), trans-[Pt(CH3NH2)2(C-N3)2].2H2O (1b), trans-[Pt(CH3NH2)2(CH-N3)2](ClO4)(2).2DMSO (1c), trans-[Pt(CH3NH2)2(CH-N1)2] (NO3)(2).3H2O (2a), trans-[Pt(CH3NH2)2(C-N1)2].2H2O (2b), trans-[Pt(CH3NH2)2(CH-N1)(CH-N3)](ClO4)2 (3a), trans-[Pt(CH3NH2)2(C-N1)(C-N3)] (3b), and trans-[Pt(CH3NH2)2(N1-CN3)(N3-C-N1)Cu(OH)]ClO(4).1.2H2O (4). X-ray crystal structures of all these compounds, except 3a and 3b, are reported. Complex 2a is of particular interest in that it contains the rarer of the two 2-oxo-4-amino tautomer forms of cytosine, namely that with the N3 position protonated. Since the effect of PtII on the geometry of the nucleobase is minimal, bond lengths and angles of CH in 2a reflect, to a first approximation, those of the free rare tautomer. Compared to the preferred 2-oxo-4-amino tautomer (N1 site protonated) of CH, the rare tautomer in 2a differs particularly in internal ring angles (7-11 sigma). Formation of compounds containing the rare CH tautomers on a preparative scale can be achieved by a detour (reaction of PtII with the cytosine anion, followed by cytosine reprotonation) or by linkage isomerization (N3-->N1) under alkaline reaction conditions. Surprisingly, in water and over a wide pH range, N1 linkage isomers (3a, 2a) form in considerably higher amounts than can be expected on the basis of the tautomer equilibrium. This is particularly true for the pH range in which the cytosine is present as a neutral species and implies that complexation of the minor tautomer is considerably promoted. Deprotonation of the rare CH tautomers in 2a occurs with pKa values of 6.07 +/- 0.18 (1 sigma) and 7.09 +/- 0.11 (1 sigma). This value compares with pKa 9.06 +/- 0.09 (1 sigma) (average of both ligands) in 1a.

Reactions of N-silylphosphoranimines with alcohols: synthesis and structure of cyclotriphosphazenes with nongeminal methyl and phenyl substituents.

The reactions of Me(3)SiN=P(OR")RR'(R" = Ph, CH(2)CF(3); R, R' = Me, Ph) with alcohols were investigated. With nonequivalent amounts of CF(3)CH(2)OH, the reactions produced high yields of the cyclic phosphazene (Me(2)PN)(3) and both the cis and trans isomers of nongeminally substituted [(Ph)(Me)PN](3). The isomers of this new cyclic phosphazene were separated by column chromatography and characterized by NMR and IR spectroscopy, elemental analysis, and X-ray crystallography. Crystals of the cis isomer 6a have a monoclinic crystal system, while the trans isomer 6b has a triclinic crystal system with two different molecules in an asymmetric unit. The bond lengths and bond angles are very similar to those of the simpler cyclic trimers (Me(2)PN)(3) and (Ph(2)PN)(3.) A likely pathway for the formation of these compounds is discussed.

Synthesis of 17β- N-Substituted 19-Nor-10-azasteroids as Inhibitors of Human 5α-Reductases I and II

The synthesis of 17β-[ N-(phenyl)methyl/phenyl-amido] substituted 10-azasteroids has been accomplished by either the TiCl 4- or TMSOTf-catalysed reaction of carbamates 11 and 12 with Danishefsky's diene. The reaction provided 5α-H isomers 3a– 5a and 5β-H isomers 3b– 5b depending on the reaction conditions. Both epimers of each compound were tested against human 5α-reductase types I and II. Unexpectedly, 5β-H compounds were found more active than their 5α-H counterparts, the best inhibitors being 3b (IC 50=279 and 2000 nM toward isoenzyme I and II, respectively) and 5b (IC 50=913 and 247 nM toward isoenzymes I and II, respectively).

Synthesis of a DMPM and Hydrido-Bridged Diiridium Complex, [(Cp*Ir)2(μ-dmpm)(μ-H)2][OTf]2, and Its Reactivity toward Alkynes and Isocyanides

Reaction of [Cp*IrCl(μ-H)]2 (2) (Cp* = pentamethylcyclopentadienyl) with equimolar amounts of bis(dimethylphosphino)methane (dmpm) in THF followed by 2 equiv of AgOTf gives a new binuclear iridium complex, [(Cp*Ir)2(μ-dmpm)(μ-H)2][OTf]2 (1). Complex 1 reacts with ethyne to give a μ-vinyl complex, [(Cp*Ir)2(μ,η1,η2-CHCH2)(μ-dmpm)(μ-H)][OTf]2 (4), via addition of the iridium hydride to the carbon−carbon triple bond. Reactions of 1 with terminal alkynes give mixtures of α-isomers of a μ-vinyl complex, [(Cp*Ir)2(μ,η1,η2-CRCH2)(μ-dmpm)(μ-H)][OTf]2 [R = Ph (5a), PhCH2 (5b), nBu (5c)], and β-trans isomers [(Cp*Ir)2(μ,η1,η2-CHCHR)(μ-dmpm)(μ-H)][OTf]2 [R = Ph (6a), PhCH2 (6b), nBu (6c)]. The μ-vinyl ligand in 6a−c undergoes a fluxional process. Heating the α-isomers 5a−c at 120 °C results in isomerization to β-trans isomers 6a−c. Complex 1 reacts with tert-butylisocyanide to give the simple adduct [(Cp*Ir)(CNtBu)(μ-dmpm)(μ-H)(Cp*Ir)(H)][OTf]2 (7), while reaction of 1 with phenylisocyanide or cyclohexylisocyanide gives μ-aminocarbyne complexes [(Cp*Ir)2(μ,η1-CN(H)R)(μ-dmpm)(μ-H)][OTf]2 [R = Ph (8a), Cy (8b)]. The structures of 1, 5c, 6a, 7, and 8a have been determined by X-ray diffraction methods.

Synthesis and characterisation of derivatives of [HIr 4(CO) 10(μ-PPh 2)] with mono and diphosphines; X-ray molecular structures of [HIr 4(CO) 8(PPh 3) 2(μ-PPh 2)] and [HIr 4(CO) 8{Ph 2P(CH 2) nPPh 2}(μ-PPh 2)] ( n=1 and 4)

The reactions of the cluster compound [HIr 4(CO) 10(μ-PPh 2)] ( 1) with a series of phosphines, PMe 3 and PPh 3, and diphosphines, Ph 2P(CH 2) nPPh 2 ( n=1, dppm, n=2, dppe and n=4, dppb) and cis-PPh 2CHCHPPh 2 (dppen), have been investigated. Reaction of 1 with PMe 3 yielded the mono and di-substituted derivatives [HIr 4(CO) 10− n (PMe 3) n (μ-PPh 2)] ( n=1, 2 and n=2, 3). 1H and 31P-NMR spectroscopy monitoring revealed that the two kinetic isomers 2b and 2c undergo conversion to the thermodynamic isomer 2a through a process, which apparently involves intra-molecular migration of the PMe 3 ligand. In contrast, analogous kinetic isomers were not detected during the synthesis of the PPh 3 derivative [HIr 4(CO) 9(PPh 3)(μ-PPh 2)] ( 4). According to 1H and 31P-NMR data, compounds [HIr 4(CO) 8(L) 2(μ-PPh 2)] (L=PMe 3 3 and PPh 3 5) exist in solution in three isomeric forms 3a– 3c and 5a– 5c, respectively, at −60 °C. Compound 1 also reacts with diphosphines to give CO substitution products, whose composition depends on the size of the methylene chain. Whilst reaction with dppm and dppen yields only the bi-substituted species [HIr 4(CO) 8(dppm)(μ-PPh 2)] ( 6) and [HIr 4(CO) 8(dppen)(μ-PPh 2)] ( 7), respectively, with dppb, both the dimeric compound [{HIr 4(CO) 9(μ-PPh 2)} 2(dppb)] ( 8a) and the monomeric species [HIr 4(CO) 8(dppb)(μ-PPh 2)] ( 8b) are formed, and with dppe, only the dimeric compound [{HIr 4(CO) 9(μ-PPh 2)} 2(dppe)] ( 9) is obtained. The dppm derivative exists in solution in the form of two inter-converting isomers 6a and 6b that differ with respect to the mode of coordination of the dppm and CO ligands. Isomerisation of 6a– 6b is proposed to involve intra-molecular migration of dppm and carbonyl ligands. Structures were proposed for all compounds, on the basis of 1H and 31P[ 1H]-NMR studies and of correlations previously established for the phosphine and diphosphine derivatives of [Ir 4(CO) 12]; the molecular structures of compounds 5c, 6a and 8b were established by X-ray diffraction studies which confirmed the structures proposed based on spectroscopic data.

Electron Transfer versus Proton Transfer in Excited States of Bichromophoric Aniline/Olefin Systems

Photolysis of 2-allylaniline (1a) and trans-2-cinnamylaniline (2a) produced mainly the five- or the six-membered ring products 3a or 9a, respectively. Compound 1b, the N-acetyl derivative of 1a, preferentially underwent photo-Fries rearrangement of the anilide moiety, while − in contrast − the analogous compound 2b, derived from 2a, displayed competition between photocyclisation and double-bond isomerisation. The latter process, characteristic of the styrene chromophore, largely predominated in the case of 2c, the N-trifluoroacetyl derivative of 2a, while the allyl analogue 1c was essentially unreactive. The photochemical behaviour of the cis-cinnamyl compounds 7a and 7b was analogous to that of their trans isomers 2a and 2b, although double bond isomerisation occurred to a smaller extent. Thus, the introduction of electron-withdrawing acyl groups decreased photocyclisation. The nature of the excited states involved in the photochemistry of 1a−c, 2a−c and 7a and 7b was studied by fluorescence measurements. The most remarkable observation was the formation of intramolecular charge-transfer exciplexes in the cases of 1a, 2a, 2b, 7a and 7b. The exciplex bands of the cinnamyl compounds 2a, 2b, 7a and 7b in acetonitrile were considerably red-shifted (maxima at ca. 500 nm). A satisfactory correlation of the photochemical and photophysical data could be achieved by considering that photocyclisation took place when clear exciplex emission was observed. Alltogether, the above data strongly supported the involvement of an excited state electron-transfer mechanism in the photocyclisation of aniline/olefin bichromophoric systems. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Ligand-Induced Conversion of π to σ C60−Metal Cluster Complexes: Full Characterization of the μ3-η1:η2:η1-C60 Bonding Mode

The reaction of Os3(CO)9(μ3-η2:η2:η2-C60) (1) with PhCH2NPPh3 in chlorobenzene affords the benzyl isocyanide substituted product Os3(CO)8(CNR)(μ3-η2:η2:η2-C60) (2a, R = CH2Ph) in 76% yield. Photolysis of 1 in the presence of an excess of benzyl isocyanide gives the isocyanide-inserted product Os3(CO)9(μ3-CNR)(μ3-η1:η2:η1-C60) (2b) in 11% yield. Thermal reaction of 2a with an excess of RNC in chlorobenzene at 80 °C produces two isocyanide-inserted, isomeric products, Os3(CO)8(CNR)(μ3-CNR)(μ3-η1:η2:η1-C60) (3a, 22%; 3b, 27%). The two isomers 3a and 3b can be selectively prepared by photolysis of 2a in the presence of excess isocyanide in 49% yield and by decarbonylation of 2b with Me3NO/MeCN and subsequent reaction with isocyanide in 56% yield, respectively. Compounds 2a,b and 3a,b have been characterized by microanalytical and spectroscopic methods. In particular, the sp3 carbons of M−C(C60) σ-interactions in μ3-η1:η2:η1-C60 complexes exhibit unique upfield resonances around 39 ppm in the 13C NMR spectra, compared to the π-bonded sp3 carbon resonances observed in the range of 95−65 ppm. The X-ray crystallographic studies for 2a,b and 3a,b reveal that μ3-η2:η2:η2-C60 ligands in 1 and 2a transform into new σ-type μ3-η1:η2:η1-C60 ligands in 2b and 3a,b by insertion of the isocyanide ligand concomitant with Os−Os bond cleavage, forming a bent triosmium framework. In compounds 2b and 3b, a boat-shaped 1,4-cyclohexadiene-like C6 ring in the C60 molecule is clearly observed and the M−C60 σ-bond distances (average 2.24 Å) are shorter than the π-bond lengths (average 2.41 Å). This study represents a novel ligand-induced conversion of π to σ C60−metal complexes.

1,1,1-Trisborylalkanes as Precursors for Dicarbapentaboranes(5) − Synthesis, Reactivity, and Structures ofcloso-1,5-Bis(neopentyl)-2,3,4-trichloro-1,5-dicarbapentaborane and Its Derivatives

The treatment of 3,3-dimethyl-1-butynyllithium with boron trichloride followed by double hydroboration with HBCl2 afforded 1,1,1-tris(dichloroboryl)-3,3-dimethylbutane (1a), characterized as its catechol derivative 1b. Analogously, three isomers (3a, 4a, and 5a) were obtained with trimethylsilylacetylide and identified as catechol derivatives 3b, 4b, and 5b. On heating, 1a formed 2,3,4-trichloro-1,5-dicarba-closo-pentaborane(5) (6a) with elimination of boron trichloride. Compound 6a reacted with methyllithium and trimethylaluminium, and also with pentamethylsilazane and acetylides, to give the corresponding carborane derivatives 6c, 6d, 6e, and 6f. The compositions of the products followed from analytical data and X-ray structure analyses of 1b, 4b, 5b, 6a, and 6f. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Photolabile precursors of cyclic nucleotides with high aqueous solubility and stability.

Photolabile phosphotriester derivatives of cyclic AMP and cyclic GMP are described, where the additional group on the phosphate is a 2-nitrobenzyl that bears an electron-withdrawing and dianionic substituent. This confers high aqueous solubility and excellent resistance to hydrolysis of the phosphotriester (t(1/2) for hydrolysis at pH 7, 22 degrees C, is >3 months for the axial isomers 3a and 4a and >1 month for the equatorial isomers 3b and 4b). The photolysis quantum yields are in the range 0.15-0.24, and the product release rate upon flash photolysis is 1.7 s(-1) at pH 7.0, 20 degrees C.

Synthesis, characterization, and reactivity of mononuclear O,N-chelated vanadium(IV) and -(III) complexes of methyl 2-Aminocyclopent-1-ene-1-dithiocarboxylate based ligand: reporting an example of conformational isomerism in the solid state.

Vanadium(IV) and -(III) complexes of a tetradentate N(2)OS Schiff base ligand H(2)L [derived from methyl 2-((beta-aminoethyl)amino)cyclopent-1-ene-1-dithiocarboxylate and salicylaldehyde] are reported. In all the complexes, the ligand acts in a bidentate (N,O) fashion leaving a part containing the N,S donor set uncoordinated. The oxovanadium(IV) complex [VO(HL)(2)] (1) is obtained by the reaction between [VO(acac)(2)] and H(2)L. In the solid state, compound 1 has two conformational isomers 1a and 1b; both have been characterized by X-ray crystallography. Compound 1a has the syn conformation that enforces the donor atoms around the metal center to adopt a distorted tbp structure (tau = 0.55). Isomer 1b on the other hand has an anti conformation with almost a regular square pyramidal geometry (tau = 0.06) around vanadium. In solution, however, 1 prefers to be in the square pyramidal form. A second variety of vanadyl complex [VO(L(cyclic))(2)](I(3))(2) (2) with a new bidentate O,N donor ligand involving isothiazolium moiety has been obtained by a ligand-based oxidation of the precursor complex 1 with iodine. Preliminary X-ray and FAB mass spectroscopic data of 2 have supported the formation of a heterocyclic moiety by a ring closure reaction involving a N-S bond. Vanadium(III) complex [V(acac)(HL)(2)] (3) has been obtained through partial ligand displacement of [V(acac)(3)] with H(2)L. Compound 3 has almost a regular octahedral structure completed by two bidentate HL ligands along with an acetylacetonate molecule. Electronic spectra, magnetism, EPR, and redox properties of these compounds are reported.

Cyclopropanation of alkenes mediated by novel chiral Fischer carbene complexes

The cyclopropanation reaction using chiral-at-metal Fischer carbene complexes is examined. Racemic molybdenum and chromium carbene complexes fac- and mer-1 and mer-2, respectively, undergo cycloaddition to acrylonitrile, methyl acrylate and 1-hexene to give cylopropanes 3 and 4 with trans/cis ratio up to 4.7:1. The cyclopropanation of acrylonitrile with the enantiopure carbene complex (+)-mer-1 resulted in a modest asymmetric induction of 2:1 for the trans- isomer 3a.

Variable-temperature nuclear magnetic resonance spectroscopy allows direct observation of carboxylate shift in zinc carboxylate complexes.

Tetranuclear complexes [Zn(4)(bdmap)(2)(OOCR)(6)] 1 (R = Me) and 2 (R = Et), where Hbdmap = 1,3-bis(dimethylamino)-2-propanol, were prepared from zinc carboxylates and Hbdmap in tetrahydrofuran (THF). The solid-state structures of isomers 1a and 2a consist of two pairs of zinc atoms, each bridged by two mu-1,2 and one mu-1,1 carboxylate ligands. Two pairs are connected by two tridentate bdmap ligands with oxygen acting as a bridging donating atom. The complexes retain the tetranuclear structure in solution and two dynamic processes are observed from variable-temperature (1)H and (13)C NMR spectra. A low-temperature process (LT dynamics) observed already below 200 K is a coalescence of the mu-1,2 and the mu-1,1 resonances to a single resonance. An additional dynamic process (HT dynamics) is observed above 247 K (1) and 263 K (2), leading to a coalescence of two dimethylamino resonances. Both dynamic processes are rationalized by a mechanism involving changes in the carboxylate coordination mode termed as carboxylate shift. The LT dynamics is ascribed to interconversions of a single mu-1,2 and a single mu-1,1 carboxylate ligation by rotations of 60 degrees. The interconversions involve all carboxylate ligands in 1 and 2. The HT dynamics is ascribed to the exchange of the coordinating geometries of two carboxylate-bridged zinc atoms. We propose a mechanism that starts with a cleavage of the Zn-N coordination bond. The resulting coordinatively unsaturated zinc atom acquires an additional oxygen donor atom by carboxylate shift of mu-1,2 carboxylate to mu-1,1 mode. The activation parameters (DeltaH values in kilocalories per mole, DeltaS values in calories per mole per kelvin) were determined by line-shape analysis of VT NMR spectra: for 1 in THF-d(8), DeltaH(LT) = 8.1(3), DeltaS(LT) = -12(2), DeltaH(HT) = 17.9(2), DeltaS(HT) = 14(1); for 1 in CDCl(3), DeltaH(HT) = 13.6(5), DeltaS(HT) = 3(3); for 1 in CD(2)Cl(2), DeltaH(HT) = 9.9(3), DeltaS(HT) = -8(2); for 2 in THF-d(8), DeltaH(LT) = 11(1), DeltaS(LT) = -5(3), DeltaH(HT) = 19.6(5), DeltaS(HT) = 18(3). Polymeric [Zn(4)(bdmap)(2)(OOCMe)(6)](n) 1-catena crystallizes from a dichloromethane solution of 1. In 1-catena, the zinc atoms are linked into a chain through mu-1,2 and mu-1,1 acetate alternated by mu-1,2 acetate and bdmap.

Platelet antiaggregatory effects and haemodynamic activity of two terfuroxan isomer pairs

A couple of terfuroxan isomer pairs 1a, b and 2a, b were studied for their in vivo vasodilating activity and in vitro antiplatelet action. The haemodynamic profiles of the products resemble that of other NO-donors. Their in vitro antiaggregating activity is influenced both by terfuroxan system and by the substituents.

Synthesis of the Kopsia alkaloids (±)-pauciflorine B, (±)-lahadinine B, (±)-kopsidasine, (±)-kopsidasine- N-oxide, (±)-kopsijasminilam and (±)-11-methoxykopsilongine

Pictet–Spengler condensation of 13 with tryptamine gave 14 , which was converted into 17 . Treatment of 17 with phenyl chloroformate resulted in 18 , which underwent transannular cyclization to give 19 . The more stable cyano-analog 22 was made by treating 18 with Tf 2O/DMAP to generate 18f , and quenching the reaction with trimethylsilyl cyanide. Treatment of 22 with acryloyl chloride (excess) at 75°C gave 23 , which was directly treated with N-hydroxy-2-thiopyridone/Et 3N to give 24 . Irradiation of 24 in the presence of t-BuSH resulted in reductive decarboxylation to give 26 and a small amount of the 2-thiopyridyl ether 25 . Protection of the aniline nitrogen in 26 required the use of triphosgene/pyridine followed by methanol. The final step for the conversion of 27 into 28 required conjugate reduction of the α,β-unsaturated ester followed by α-hydroxylation and gave 28 (11,12-demethoxy lahadinine B). Exposure of 26 to PhI(OAc) 2/MeOH cleanly gave 26a , which was directly reduced with Zn/AcOH to 29 . Conversion of 29 into 30 proceeded as before, and when 30 was treated with AgBF 4/THF followed by aqueous NaHCO 3 it was converted into (±)-kopsidasine 2 , completely characterized as its derived N-oxide 2a . Treatment of 26 with AgBF 4/THF followed by aqueous NaHCO 3, gave 31 . Oxidation of 31 with m-chloroperoxybenzoic acid resulted in the N-oxide 32 which underwent fragmentation to give 33 when exposed to trifluoroacetic anhydride. When the diene 33 was treated with Mn(dpm) 3 (cat)/PhSiH 3/O 2 in isopropyl alcohol at 0°C, it was converted into kopsijasminilam 4 . Peracetic acid in EtOAc (10%) was used to quench the AgBF 4/THF conversion of 28 into 37 , and resulted in 42 / 42a (4:1, 65%) along with small amounts of 38 and 41c . Application of these procedures, with some modifications, to the 11,12-dimethoxy substituted systems gave (±)-lahadinine B 64 . Treatment of 64 with triethylsilane in the presence of trifluoroacetic acid cleanly converted it into 11-methoxykopsilongine 65 (93%). Treatment of (±)-lahadinine B 64 with AgBF 4/THF followed by work-up with EtOAc/MeCO 3H (10%) gave (±)-pauciflorine 6 and the double bond isomer 6a .

Naphthoquinolizinium derivatives as a novel platform for DNA-binding and DNA-photodamaging chromophores

The association of the naphtho[1,2-b]quinolizinium bromide (5a) and naphtho[2,1-b]quinolizinium bromide (5b) with DNA and the propensity of these cationic arenes to damage DNA after UV-A irradiation have been studied. Spectrophotometric and fluorimetric titrations show that the two isomers 5a and 5b bind to DNA (K approximately 10(5) M(-1)). The highest affinity was observed for GC base pairs. The mode of binding was investigated by CD and LD spectroscopy. Whereas quinolizinium 5a exclusively intercalates in DNA, the isomer 5b exhibits a deviation from perfect intercalation into the double helix. Moreover, efficient DNA damage was observed on UV-A irradiation in the presence of the quinolizinium salts. Primer extension analysis indicates that the photocleavage takes place preferentially at guanine-rich regions.