DFT Study on the Structure and Racemization Mechanism of 1,1'-Binaphthalene-8,8'-diol

Density functional theory (DFT) was applied to study the ground state geometries and isomerization processes of 1,1'-binaphthalene-8,8'-diol. Three isomers, denoted as ISO1, ISO2, and ISO3, were found, distinguished by different orientations of the OH groups, and each OH-orientational isomer has R- and S- enantiomer. The conformational stabilities of these isomers were investigated by tracking the energy change with respect to the ring-to-ring torsion. The inter-conversions between the three OH-orientational S-isomers were found to have quite low barriers owing to the nearly free rotation of OH groups around the O-C single bonds. The S-R enantiomerization of ISO1 and ISO2 can take place through the ring-ring torsion around the C1-C10 single bond, either in the anti-rotation manner or in the syn-rotation manner. The barriers of the anti routes are lower than those of the corresponding syn routes by 87.95 and 75.04 kJ/mol. For the S-R enantiomerization of ISO3, only the anti route was found. The barriers for the anti route enantiomerizations of ISO1, ISO2, and ISO3 are 119.61, 120.43, and 121.59 kJ/mol, respectively. A parallel reaction mechanism via three anti enantiomerization routes was proposed for the racemization of 1,1'-binaphthalene-8,8'-diol.     

Ionization-induced π→ H site switching dynamics in phenol-Ar3

Electronic excitation spectra of the S(1)← S(0) transition obtained by resonance-enhanced two-photon ionization (REMPI) are analysed for phenol-Ar(n) (PhOH-Ar(n)) clusters with n≤ 4. An additivity rule has been established for the S(1) origin shifts upon sequential complexation at various π binding sites, which has allowed for the identification of two less stable isomers not recognized previously, namely the (2/0) isomer for n = 2 and the (2/1) isomer for n = 3. Infrared (IR) spectra of neutral PhOH-Ar(n) and cationic PhOH(+)-Ar(n) clusters are recorded in the vicinity of the OH and CH stretch fundamentals (ν(OH), ν(CH)) in their S(0) and D(0) ground electronic states using IR ion dip spectroscopy. The small monotonic spectral redshifts Δν(OH) of about -1 cm(-1) per Ar atom observed for neutral PhOH-Ar(n) are consistent with π-bonded ligands. In contrast, the IR spectra of the PhOH(+)-Ar(n) cations generated by resonant photoionization of the neutral precursor display the signature of H-bonded isomers, suggesting that ionization triggers an isomerization reaction, in which one of the π-bonded Ar ligands moves to the more attractive OH site. The dynamics of this isomerization reaction is probed for PhOH(+)-Ar(3) by picosecond time-resolved IR spectroscopy. Ionization of the (3/0) isomer of PhOH(+)-Ar(3)(3π) with three π-bonded Ar ligands on the same side of the aromatic ring induces a π→ H switching reaction toward the PhOH(+)-Ar(3)(H/2π) isomer with a time constant faster than 3 ps. Fast intracluster vibrational energy redistribution prevents any H →π back reaction.     

Symmetry control of radiative decay in linear polyenes: low barriers for isomerization in the S1 state of hexadecaheptaene

The room temperature absorption and emission spectra of the 4-cis and all-trans isomers of 2,4,6,8,10,12,14-hexadecaheptaene are almost identical, exhibiting the characteristic dual emissions S1-->S0 (21Ag- --> 11Ag-) and S2-->S0 (11Bu+ --> 11Ag-) noted in previous studies of intermediate length polyenes and carotenoids. The ratio of the S1-->S0 and S2-->S0 emission yields for the cis isomer increases by a factor of approximately 15 upon cooling to 77 K in n-pentadecane. In contrast, for the trans isomer this ratio shows a 2-fold decrease with decreasing temperature. These results suggest a low barrier for conversion between the 4-cis and all-trans isomers in the S1 state. At 77 K, the cis isomer cannot convert to the more stable all-trans isomer in the 21Ag- state, resulting in the striking increase in its S1-->S0 fluorescence. These experiments imply that the S1 states of longer polyenes have local energy minima, corresponding to a range of conformations and isomers, separated by relatively low (2-4 kcal) barriers. Steady state and time-resolved optical measurements on the S1 states in solution thus may sample a distribution of conformers and geometric isomers, even for samples represented by a single, dominant ground state structure. Complex S1 potential energy surfaces may help explain the complicated S2-->S1 relaxation kinetics of many carotenoids. The finding that fluorescence from linear polyenes is so strongly dependent on molecular symmetry requires a reevaluation of the literature on the radiative properties of all-trans polyenes and carotenoids.     

Determination of molecular structure using vibrational circular dichroism spectroscopy: the keto-lactone product of Baeyer-Villiger oxidation of (+)-(1R,5S)-bicyclo[3.3.1]nonane-2,7-dione

[reaction: see text] The Baeyer-Villiger oxidation of (+)-(1R,5S)-bicyclo[3.3.1]nonane-2,7-dione, 1, can lead to four keto-lactone products, 2a-d. A single isomer is obtained experimentally. We have used IR and VCD spectroscopies to identify the structure of this product. DFT calculations of the IR and VCD spectra of 2a-d show unambiguously that the experimental product is (+)-(1R,6R)-2a, and not the expected product 2b. NMR studies, including comparison of DFT and experimental 1H and 13C spectra, support this conclusion. This work provides the first example of the use of VCD spectroscopy to discriminate between structural isomers of a chiral molecule. The specific rotation of (+)-(1R,6R)-2a, predicted using TDDFT methods, is negative demonstrating that absolute configurations determined from TDDFT calculations of specific rotations are not 100% reliable.     

Probing highly efficient photoisomerization of a bridged azobenzene by a combination of CASPT2//CASSCF calculation with semiclassical dynamics simulation

Mechanism of phototriggered isomerization of azobenzene and its derivatives is of broad interest. In this paper, the S(0) and S(1) potential energy surfaces of the ethylene-bridged azobenzene (1) that was recently reported to have highly efficient photoisomerization were determined by ab initio electronic structure calculations at different levels and further investigated by a semiclassical dynamics simulation. Unlike azobenzene, the cis isomer of 1 was found to be more stable than the trans isomer, consistent with the experimental observation. The thermal isomerization between cis and trans isomers proceeds via an inversion mechanism with a high barrier. Interestingly, only one minimum-energy conical intersection was determined between the S(0) and S(1) states (CI) for both cis → trans and trans → cis photoisomerization processes and confirmed to act as the S(1) → S(0) decay funnel. The S(1) state lifetime is ～30 fs for the trans isomer, while that for the cis isomer is much longer, due to a redistribution of the initial excitation energies. The S(1) relaxation dynamics investigated here provides a good account for the higher efficiency observed experimentally for the trans → cis photoisomerization than the reverse process. Once the system decays to the S(0) state via CI, formation of the trans product occurs as the downhill motion on the S(0) surface, while formation of the cis isomer needs to overcome small barriers on the pathways of the azo-moiety isomerization and rotation of the phenyl ring. These features support the larger experimental quantum yield for the cis → trans photoisomerization than the trans → cis process.     

Total synthesis of (-)-crambidine and definition of the relative configuration of its unique tetracyclic guanidinium core

Total syntheses of the 3S,8S,10S,19R,43S isomer 4a and the 3S,8S,10S,19R,43R isomer 4b of the unique crambescidin alkaloid crambidine are reported. These studies confirm the tetracyclic structure proposed by Braekman and co-workers for crambidine, and establish the rel-3R,8R,10R,19S relative configuration for this moiety. Natural crambidine is most likely the 3S,8S,10S,19R,43S isomer 4a. These syntheses were completed in five steps and approximately 14% overall yield from 1-iminohexahydro[1,2-c]pyrimidine carboxylic ester 10, an intermediate in our earlier total synthesis of 13,14,15-isocrambescidin 800 (3). The signature step in the total syntheses of crambidine and several stereoisomers is chemoselective dehydrogenation of the tethered Biginelli adduct 10 or the derived tetracyclic intermediate 17. Additionally, these studies reveal the unprecedented ring-chain isomerization of the crambidine ring system exemplified by the interconversion of isomers 15a and 15b.     

Ab initio static and molecular dynamics study of 4-styrylpyridine.

We report an in‐depth theoretical study of 4‐styrylpyridine in its singlet S₀ ground state. The geometries and the relative stabilities of the trans and cis isomers were investigated within density functional theory (DFT) as well as within Hartree–Fock (HF), second‐order Møller–Plesset (MP2), and coupled cluster (CC) theories. The DFT calculations were performed using the B3LYP and PBE functionals, with basis sets of different qualities, and gave results that are very consistent with each other. The molecular structure is thus predicted to be planar at the energy minimum, which is associated with the trans conformation, and to become markedly twisted at the minimum of higher energy, which is associated with the cis conformation. The results of the calculations performed with the post‐HF methods approach those obtained with the DFT methods, provided that the level of treatment of the electronic correlation is high enough and that sufficiently flexible basis sets are used. Calculations carried out within DFT also allowed the determination of the geometry and the energy of the molecule at the biradicaloid transition state associated with the thermal cis?trans isomerization and at the transition states associated with the enantiomerization of the cis isomer and with the rotations of the pyridinyl and phenyl groups in the trans and cis isomers. Car–Parrinello molecular dynamics simulations were also performed at 50, 150, and 300 K using the PBE functional. The studies allowed us to evidence the highly flexible nature of the molecule in both conformations. In particular, the trans isomer was found to exist mainly in a nonplanar form at finite temperatures, while the rotation of the pyridinyl ring in the cis isomer was incidentally observed to take place within ≈1 ps during the simulation carried out at 150 K on this isomer.     

Photochemical E (trans)-->Z (cis) isomerization in substituted 1-naphthylacrylates.

Substituted naphthylacrylates, 1-3, not showing rotamerism have been synthesized with a view to study photochemical E (trans)-->Z (cis) isomerization. Photostationary state composition of the isomers upon direct excitation, triplet sensitized isomerization, quantum yield of isomerization, and steady state and time-resolved fluorescence behavior have been studied for these naphthylacrylates. The direct excitations of the compounds yield high Z (approximately 80%) isomer composition, whereas the triplet sensitization results in less Z (approximately 20%) isomer composition. This indicates that the singlet pathway is very efficient in converting the E isomer to the Z isomer. The naphthylacrylates 1 and 2 exhibit structured fluorescence at room temperature in hexane and upon changing the solvent to CH3CN; the structure of the fluorescence is lost, indicating that the singlet excited-state develops a polar character in a polar environment. The polar nature of the singlet excited state becomes more clear in the case of 3 from its fluorescence solvatochromism. The naphthylacrylates did not exhibit excitation wavelength-dependent fluorescence at room temperature suggesting that the ground state conformers (rotamers) are not involved. Fluorescence lifetimes measured for these compounds displayed biexponential behavior, which is explained using a two-state model.     

Photoisomerizable bipyridine ligands and macroligands: absorption, photoisomerization properties and theoretical study.

Two 2,2'-bipyridines, substituted at the 4,4'-positions by p-dialkylaminophenylazostyryl moieties p-R2N-C6H4-N=N-C6H4-CH=CH-[6 a, R2N=nBu2N; 6 b, R2N=(nBu)(C4H8OTHP)N; 6 c, R2N=(nBu)(C4H8OH)N], were successfully synthesized by using Wadworth-Emmons reactions. The X-ray structure of 6 a has been determined. Esterification of 6 c with 2-bromoisobutyroylbromide afforded 6 d. This ligand was used as an initiator for the living radical polymerization of methylmethacrylate (MMA) and gave rise to macroligand 6 e. Thin films of good optical quality were obtained by the spin-coating technique. Photoisomerization experiments were carried out on 6 a in solution and on 6 e in both solution and film, and the kinetics of photochemical (E/Z) and thermal (Z/E) isomerization were investigated. They were found to show Z-E back isomerization typical of aminoazobenzene-type rather than of push-pull-type molecules. Density functional theoretical (TD-DFT) calculations were performed on model compound 6 a' (R2N=Me2N) to understand the structural and electronic transitions of the corresponding E-E, E-Z and Z-Z isomers. It was found that the E-E isomer is almost planar as observed experimentally by X-ray diffraction, whereas the Z-Z isomer, which is 35.4 kcal mol(-1) less stable than the E-E isomer, is nonplanar. The theoretical studies also reveal that several transitions of pi-pi*, n-pi* and charge-transfer (CT) types, are involved in the long-wavelength transition of 6 a (E-E). The same observations can be made for the (Z-Z) isomer, and the TD-DFT simulated spectrum fits quite nicely to the experimental, reproducing and explaining the apparition of a blue-shifted charge-transfer band at 390 nm.     

Isomer-specific spectroscopy and conformational isomerization energetics of o-, m-, and p-ethynylstyrenes

The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and pES) were studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), UV-UV hole-burning spectroscopy (UVHB), resonant ion-dip infrared spectroscopy (RIDIRS), and rotationally resolved fluorescence excitation spectroscopy. In addition, the newly developed method of stimulated emission pumping-population transfer spectroscopy (SEP-PTS) was used to determine the energy threshold to conformational isomerization in m-ethynylstyrene. The S(1) <-- S(0) origin transitions of oES and pES occur at 32 369 and 33 407 cm(-1), respectively. In mES, the cis and trans conformations are calculated to be close in energy. In the R2PI spectrum of mES, the two most prominent peaks (32672 and 32926 cm(-1)) were confirmed by UVHB spectroscopy to be S(1) <-- S(0) origins of these two conformers. The red-shifted conformer was identified as the cis structure by least-squares fitting of the rotationally resolved fluorescence excitation spectrum of the origin band. There are also two possible conformations in oES, but transitions due to only one were observed experimentally, as confirmed by UVHB spectroscopy. Density functional theory calculations (B3LYP/6-31+G) predict that the cis-ortho conformer, in which the substituents point toward each other, is about 8 kJ/mol higher in energy than the trans-ortho isomer, and should only be about 5% of the room temperature population of oES. Ground-state infrared spectra in the C-H stretch region (3000-3300 cm(-1)) of each isomer were obtained with RIDIRS. In all three structural isomers, the acetylenic C-H stretch fundamental was split by Fermi resonance. Infrared spectra were also recorded in the excited electronic state, using a UV-IR-UV version of RIDIR spectroscopy. In all three isomers the acetylenic C-H stretch fundamental was unshifted from the ground state, but no Fermi resonance was seen. The first observed and last unobserved transitions in the SEP-PT spectrum were used to place lower and upper bounds on the barrier to cis --> trans isomerization in m-ethynylstyrene of 990-1070 cm(-1). Arguments are given for the lack of a kinetic shift in the measurement. The analogous trans --> cis barrier is in the same range (989-1065 cm(-1)), indicating that the relative energies of the zero-point levels of the two isomers are (E(ZPL)(cis) - E(ZPL)(trans))= -75 to +81 cm(-1). Both the barrier heights and relative energies of the minima are close to those determined by DFT (Becke3LYP/6-31+G) calculations.     

Spectral tuning by switching C-H[...]O hydrogen bonds: rotation-induced spectral shifts of 7-hydroxyquinoline HCOOH isomers

Spectral tuning effects on visible chromophores by hydrogen bonds are central to the chemistry of vision and of photosynthesis. A model for large spectral tuning effects by hydrogen bond switching is provided by the 7-hydroxyquinoline x HCOOH complex, which forms two isomers, CTN1 and CTN2, both with an HCOOH[...]N hydrogen bond but with different (quinoline)C-H[...]O=C hydrogen bonds. A 180 degrees rotation of the HCOOH moiety around the O-H[...]N hydrogen bond exchanges the C-H[...]O hydrogen bonds, rotates the dipole moment of HCOOH, and leads to an approximately 850 cm(-1) shift of the electronic spectrum. Mass-selected S1<--S0 resonant two-photon ionization, UV-UV holeburning, S1-->S0 fluorescence spectra, and photoionization efficiency curves of the two 7-hydroxyquinoline x HCOOH isomers were measured in supersonic expansions. Comparison to ab initio calculations allow us to determine the H-bond connectivity and structure of the two isomers and to assign their inter- and intramolecular vibrations. The Franck-Condon factors of the intermolecular shear vibration chi in the S1<--S0 spectra indicate that the weak C-H[...]O hydrogen bond contracts markedly in the CTN1 isomer but expands in the CTN2 isomer. These changes of H-bond lengths agree with the spectral shifts. In contrast, the strong O-H[...]N hydrogen bond undergoes little change upon S1[...]S0 excitation.     

Stereochemical Nonrigidity and Geometrical Isomerism in Six-Coordinate Trigonal Prismatic Complexes: The Case of Asymmetric Molybdenum and Tungsten Tris(dithiolenes)

The isomerization of nine asymmetric tris(dithiolenes) of tungsten and molybdenum, of the general formula (R(1)R(2)C(2)S(2))(3)M is studied with NMR methods. In the complexes investigated, R(1) = H, R(2) = p-CH(3)OPh, p-CH(3)Ph, or Ph, and M = W or Mo, or R(1) = H, R(2) = p-ClPh or p-BrPh, and M = W, or R(1) = Ph, R(2) = p-CH(3)OPh, and M = W, as shown in formula I. The complexes are proved to be trigonal prismatic in solution and stereochemically nonrigid at room temperature. An equilibrium favoring the trans isomer (formula III) is established, with the concentration of this isomer being three times that of the cis due to entropy reasons. The kinetics and mechanism of the isomerization is investigated and a scheme is proposed involving the rotation of only one ligand around an axis lying on the dithiolenic ring, passing from the metal to the center of the carbon-carbon bond. This mechanism satisfies energy criteria and is allowed by symmetry selection rules, as theoretical EHMO calculations indicate.     

Theoretical study of the isomerization mechanism of azobenzene and disubstituted azobenzene derivatives

A series of azobenzenes was studied using ab initio methods to determine the substituent effects on the isomerization pathways. Energy barriers were determined from three-dimensional potential energy surfaces of the ground and electronically excited states. In the ground state (S(0)), the inversion pathway was found to be preferred. Our results show that electron donating substituents increase the isomerization barrier along the inversion pathway, whereas electron withdrawing substituents decrease it. The inversion pathway of the first excited state (S(1)) showed trans --> cis barriers with no curve crossing between S(0) and S(1). In contrast, a conical intersection was found between the ground and first excited states along the rotation pathway for each of the azobenzenes studied. No barriers were found in this pathway, and we therefore postulate that after n --> pi (S(1) <-- S(0)) excitation, the rotation mechanism dominates. Upon pi --> pi (S(2) <-- S(0)) excitation, there may be sufficient energy to open an additional pathway (concerted-inversion) as proposed by Diau. Our potential energy surface explains the experimentally observed difference in trans-to-cis quantum yields between S(1) and S(2) excitations. The concerted inversion channel is not available to the remaining azobenzenes, and so they must employ the rotation pathway for both n --> pi and pi --> pi excitations.     

Synthesis, conformation, and complexation of novel 25,26,27,28- tetrahydroxy-5,11:17,23-bis[[2,2'-thioxydi(o-phenylene)dithioxy]- diphenylthio]calix[4]arene

Calix[4]arenes 1a,b having an electron-donating group, i.e., OH and OMe, at the lower rim reacted with thianthrene cation radical perchlorate in CH3CN at room temperature to give the corresponding thianthrenium perchlorates 3a,b in excellent yields. Treatment of 3a,b comprising a mixture of conformational isomers with NaSH.xH2O in DMF at reflux afforded the sulfur-containing cyclized compounds 4a,b, respectively. Compound 4a having a cone conformation consisted of two conformational isomers in a ratio of 0.077:1. Temperature-dependent 1H NMR study in DMF showed that conformational isomerization between the two isomers occurred with energy barriers of 14.97 and 14.10 kcal/mol at 100 and 110 degrees C, respectively. The Jobs plot of 4a against Ag+ picrate indicated that compound 4a strongly produced a 1:2 complex with Ag+ ions. Molecular mechanics calculations indicated that the conformation of the energy-minimized 4a-2Ag+ picrate complex had two crushed pyramidal geometries made up of Ag+ ion and four sulfur atoms, i.e., S1, S2, S4, S5 and S6, S7, S9, S10, respectively.     

A CASSCF/MR-CI study toward the understanding of wavelength-dependent and geometrically memorized photodissociation of formic acid

The S(0), T(1), and S(1) potential energy surfaces for the HCOOH dissociation and isomerization processes have been mapped with different ab initio methods. The wavelength-dependent mechanism for the HCOOH dissociation was elucidated through the computed potential energy surfaces and the surface crossing points. The HCOOH molecules in S(1) by excitation at 248 nm mainly decay to the ground state via the S(0) and S(1) vibronic interaction, followed by molecular eliminations in the ground state. The S(1) direct dissociation to HCO((2)A') + OH((2)Pi) is the dominant pathway upon photoexcitation at 240-210 nm. Meanwhile, there is a slight probability that the system relaxes to the ground state via the S(0) and S(1) vibronic interaction at these wavelengths. After irradiation of HCOOH at 193 nm, the S(1) direct dissociation into HCO((2)A') + OH((2)Pi) is energetically the most favorable pathway. In view of high IC efficiency at the S(0)/S(1) conical crossing, the S(1) --> S(0) internal conversion via the S(0)/S(1) point can occur with considerable efficiency. In addition, the S(1) isomerization probably plays a dominant role in the partially conformational memory of the HCOOH photodissociation, which has been discussed in detail.     

Synthesis and stereodynamics of highly constrained 1,8-bis(2,2'-dialkyl-4,4'-diquinolyl)naphthalenes

The syn and anti isomers of axially chiral 1,8-diquinolylnaphthalenes have been synthesized via Pd-catalyzed Stille coupling of 1,8-dibromonaphthalene and 2-alkyl-4-trimethylstannylquinolines. Optimization of the cross-coupling reaction allowed the preparation of highly constrained 1,8-bis(2,2'-dimethyl-4,4'-diquinolyl)naphthalene, 2, and 1,8-bis(2,2'-diisopropyl-4,4'-diquinolyl)naphthalene, 3, in 42% and 41% yield, respectively. Employing Pd(PPh(3))(4) and CuO as the cocatalysts in the coupling reaction of 1,8-dibromonaphthalene and 2-alkyl-4-trimethylstannylquinolines proved to be superior over other catalysts such as PdCl(2)(dppf), Pd(2)(dba)(3)/P(t-Bu)(3), and POPd. The C(2)-symmetric anti isomers of 2 and 3 were found to be more stable than the corresponding meso syn isomer. The ratio of the two enantiomeric anti conformers to the syn conformer was determined as 7.9:1 for 2 and 8.6:1 for 3 by NMR and HPLC analysis. The atropisomers of 2 and 3 were found to be stable to rotation about the chiral axis at room temperature and all three stereoisomers of 2 were isolated by semipreparative HPLC on a Chiralpak AD column. The diastereoisomers of 3 were separated via preferential crystallization of the anti isomers from diethyl ether. Slow syn/anti interconversion was observed for both atropisomers at enhanced temperature, and the diastereomerization and enantiomerization processes were monitored by NMR and HPLC. The Gibbs activation energy, DeltaG++, for the isomerization of 2 was determined as 116.0 (112.1) kJ/mol for the conversion of the anti (syn) to the syn (anti) isomer at 71.0 degrees C. The rotational energy barrier of 3 was determined as 115.2 (111.1) kJ/mol for the conversion of the anti (syn) to the syn (anti) isomer at 66.2 degrees C.     

N-methylmonothiocarbamatopentamminecobalt(III): restricted C-N bond rotation and the acid-catalyzed O- to S-bonded rearrangement

High-resolution (1)H and (13)C NMR studies on the linkage isomers [(NH(3))(5)CoOC(S)NHCH(3)](2+) and [(NH(3))(5)CoSC(O)NHCH(3)](2+) reveal that the O-bonded form exists as a 5:1 mixture of Z and E isomers arising from restricted rotation about the C-N bond. Similarly, restricted rotation is observed (at 20 degrees C) for the S-bonded isomer (Z/E ca. 18:1), but not for the isoelectronic carbamate ion [(NH(3))(5)CoOC(O)NHCH(3)](2+), nor for the unsubstituted carbamato complex [(NH(3))(5)CoOC(O)NH(2)](2+). An analysis of the variable-temperature NMR data for the O-bonded carbamato and urea complexes has provided quantitative data on the rotational barriers, and these ions involve much faster C-N bond rotations than the thiocarbamato complexes. The acid-catalyzed reaction of [(NH(3))(5)CoOC(S)NHCH(3)](2+) is confirmed, but there is much less parallel hydrolysis (ca. 2%) than previously reported (40 +/- 10%) for 0.1 M HClO(4). In 1 M HClO(4), [(NH(3))(5)CoSC(O)NHCH(3)](2+) and [(NH(3))(5)CoOH(2)](3+) are formed in parallel as an 83:17 mixture. The kinetic data suggest that the protonated form is at least 20-fold more reactive than the free ion and that the linkage isomerization and hydrolysis pathways are both acid-catalyzed, the latter clearly more so than the rearrangement.     

Synthesis of the spiroacetal-containing anti-Helicobacter pylori agents CJ-12,954 and CJ-13,014

The first synthesis of the spiroacetal-containing anti-Helicobacter pylori agents ent-CJ-12,954 and ent-CJ-13,014 is reported based on the union of a heterocycle-activated spiroacetal-containing sulfone fragment with a phthalide-containing aldehyde fragment; comparison of the 1H and 13C NMR data, optical rotations and HPLC retention times of the synthetic compounds (3S,2"S,5"S,7"S)-(1a) and (3S,2"S,5"R,7"S)-(2a) and the (3R)-diastereomers (3R,2"S,5"S,7"S)-(1b) and (3R,2"S,5"R,7"S)- (2b) with the naturally occurring compounds established that the synthetic isomers (1a) and (2a) were in fact enantiomeric to the natural products CJ-12,954 and CJ-13,014.     

Structural properties of cyclopentanone-bridged bis-macrocyclic ligand dicopper(II) complexes in the solid and in solution: a successful test of the MM-EPR method

The copper(II)-assisted condensation of 2,3,2-tet (3,7-diazanonane-1,9-diamine) with formaldehyde and cyclopentanone yields the mono- and bis-macrocyclic Mannich condensation products L(1) and L(2), as well as the Schiff-base product L(3), all with cyclam-type tetraaza macrocycles, coordinated to copper(II). The combination of molecular mechanics and EPR spectroscopy (MM-EPR) reveals that all three isomers of [Cu(2)(L(2))(OH(2))(n)](2+) (n = 0-4), with the expected trans-III (R,R,S,S) configuration of the 14-membered tetraaza macrocycles, are of similar stability, and that the isomer whose structure is solved by X-ray crystallography has a different structure in solution.     

Synthesis and configuration of the nonadecenetriol isolated from seeds of Persea americana

In an effort to establish the relative as well as absolute configuration of the trypanocidally active natural nonadec-6-en-1,2,4-triol isolated from Persea americana, the (2S,4R), (2S,4S), and (2R,4R) isomers were synthesized. The stereogenic centers taken from enantiopure chiral epoxy building blocks derived from inexpensive and readily available D-glucolactone. The (2R,4R) isomer gave (1)H and (13)C NMR as well as specific rotation in excellent consistence with those reported for the natural triol.