Structure elucidation of β-carotene isomers by HPLC-NMR coupling using a C30 bonded phase

The separation of cis/trans isomers of β-carotene has been performed with a C₃₀ stationary phase employing ¹H NMR spectroscopy as an on-line detection technique. 1D as well as 2D NMR spectra have been recorded in the stopped-flow mode for the predominant chromatographic peaks. Structural assignment of the five identified isomers was performed via comparison of simulated 1D ¹H NMR spectra on the basis of the structures of β-carotene cis/trans isomers with the experimental data, and also by the analysis of the proton-proton connectivities in the 2D NMR spectra of three isomers with the highest concentration. The chromatographic retention behaviour of the isomers agreed well with previously reported data. The advantage of the applied hyphenated coupling technique compared to conventional off-line techniques lies in the fact that chromatographic separation and NMR detection are performed in a closed system, so that reisomerization of the separated compounds is inhibited. Received: 29 May 1996 / Revised: 1 July 1996 / Accepted: 4 July 1996     

Photolysis of gamma-(alpha-carboxy-2-nitrobenzyl)-L-glutamic acid investigated in the mcrosecond time scale by time-resolved FTIR

The photolytic release of substrates from caged substrates has proven to be an excellent method to generate concentration jumps for kinetic measurements in the microsecond time scale. In this report we use time-resolved FTIR in the step-scan mode to probe the photolysis mechanism of one such caged compound, namely gamma-(alpha-carboxy-2-nitrobenzyl)glutamate, and to obtain a direct measure of the rate of photorelease of the substrate glutamate. The time-resolved difference FTIR spectra exhibit specific signals that can be assigned to the reactant caged glutamate, photolytically released product glutamate, as well as to the aci-nitro intermediate, the key intermediate of the photolysis reaction. Therefore these signals allow the characterization of the kinetics of formation and decay of the intermediate and products. This is the first such report that provides a direct determination of the rate of formation of the photolysis product from a caged compound in the microsecond time scale. Furthermore, the results presented provide a good basis for further time-resolved FTIR studies of molecular reaction mechanisms, such as ligand protein interactions, in the microsecond time scale through the photolytic release of substrates from caged compounds.     

Syntheses of seven-membered rings: ruthenium-catalyzed intramolecular [5+2] cycloadditions

The Ru-catalyzed intramolecular [5+2] cycloaddition of cyclopropylenynes is investigated with respect to the regio- and diastereoselectivity as well as the functional group compatibility of the reaction. Evidence for the mechanism as occurring through a ruthenacyclopentene intermediate is elucidated from 1) the study of the diastereoselectivity of the cycloaddition; 2) the effect of variation of substituents on the regioselectivity of cyclopropyl bond cleavage in 1,2-trans- and 1,2-cis-disubstituted cyclopropanes and 3) examples that clearly do not involve ruthenacyclohexene as intermediates as products still incorporate the cyclopropyl moiety. The scope and limitations of the Ru-catalyzed cycloaddition are discussed and compared with the Rh-catalyzed reaction. The potential power of this methodology towards natural product total synthesis is demonstrated by the formation of several polycyclic systems with the chosen reaction conditions and readily available cyclopropylenyne substrates.     

The Polynomials Associated with a Julia Set

We prove that, with two exceptions, the set of polynomials withJulia set J has the form {pⁿ:nN,} where p is one of these polynomialsand is the symmetry group of J. The exceptions occur when Jis a circle or a straight line segment.     

Preparation et etude d'un oxyphosphate Fe4(PO4)2O

This compound is obtained in several ways, at 900°C, from the components of the FePO system when the oxygen pressure is made suitable, or from Fe₃(PO₄)₂ + Fe + Fe₂O₃ in a sealed tube under vacuum. It crystallizes under these latter conditions with a trace of FeCl₂. The cell is monoclinic; a = 6.564(1), b = 11.271(2), c = 9.383(2) Å, β = 103.95 (2)°, with Z = 4, group $\text{P2}{}_1\text{c}$. The structure is determined thanks to the use of a direct method and Fourier synthesis and is refined to R = 0.033. The PO₄ tetrahedra are isolated; the iron fills four crystallographic sites: three are more or less distorted octahedra, the fourth is a trigonal bipyramid. The oxyphosphate character is ascertained by the presence of some oxygen atoms connected to iron only, with, moreover, a low site potential. This compound is paramagnetic above 90°K. Its Mössbauer spectrum exhibits four doublets in good agreement with the structure; in order to identify which one corresponds to the hexahedral site, the phase Fe₃Zn(PO₄)₂O has been prepared, but its Mössbauer spectrum, in spite of the zinc affinity for the V coordination, shows that two sites are modified, which does not allow conclusions to be made.     

Photochemical cleavage and release of carboxylic acids from alpha-keto amides

In aqueous media, alpha-keto amides LGCH(2)COCON(R)CH(R')CH(3) (1a, R = Et, R' = H; 1b, R = (i)()Pr, R' = Me; 1c, R = Ph, R' = H) with various carboxylate leaving groups (LG) at the C-3 position undergo photocleavage and release of carboxylic acids with formation of diastereomeric 5-hydroxyoxazolidin-4-ones 2a,c in the cases of 1a,c or 5-methyleneoxazolidin-4-ones 3b in the case of 1b. For 1a,b, Phi(photocleavage) = 0.24-0.38, whereas Phi(photocleavage) = ca. 0.05 for 1c. The proposed mechanism involves transfer of hydrogen from an N-alkyl group to the keto oxygen to produce zwitterionic intermediates 4a-c that eliminate carboxylate anions. The resultant imminium ions, H(2)C=C(OH)CON(+)(R)=C(R')CH(3) 5a-c, cyclize intramolecularly to 3b or undergo intermolecular addition of water followed by tautomerization and cyclization to give 2a,c. These inter- or intramolecular trapping reactions of 5 release protons that decrease the pH and cause bleaching of the 620 nm band of the pH indicator, bromocresol green. Determination of the bleaching kinetics by laser flash photolysis methods in the case of 1a gives time constants of 18-137 mus, depending on the leaving group ability of the carboxylate anion, whereas amides 1b show only a small leaving group effect. For 1a, the large leaving group effect is consistent with rate-limiting carboxylate elimination from 4a, whereas the proton release step would be largely rate determining for 1b. Photolyses of 1a (LG = CH(3)CO(2)(-), PhCH(2)CO(2)(-)) in neat CH(3)CN results in carboxylate elimination to form imminium ion 5a, followed by internal return to give aminals.     

Ring-closing metathesis of olefinic peptides: design, synthesis, and structural characterization of macrocyclic helical peptides

Heptapeptides containing residues with terminal olefin-derivatized side chains (3 and 4) have been treated with ruthenium alkylidene 1 and undergone facile ring-closing olefin metathesis (RCM) to give 21- and 23-membered macrocyclic peptides (5 and 6). The primary structures of peptides 3 and 4 were based upon a previously studied heptapeptide (2), which was shown to adopt a predominantly 3(10)-helical conformation in CDCl(3) solution and an alpha-helical conformation in the solid state. Circular dichroism, IR, and solution-phase (1)H NMR studies strongly suggested that acyclic precursors 3 and 4 and the fully saturated macrocyclic products 7 and 8 also adopted helical conformations in apolar organic solvents. Single-crystal X-ray diffraction of cyclic peptide 8 showed it to exist as a right-handed 3(10)-helix up to the fifth residue. Solution-phase NMR structures of both acyclic peptide 4 and cyclic peptide 8 in CD(2)Cl(2) indicated that the acyclic diene assumes a loosely 3(10)-helical conformation, which is considerably rigidified upon macrocyclization. The relative ease of introducing carbon-carbon bonds into peptide secondary structures by RCM and the predicted metabolic stability of these bonds renders olefin metathesis an exceptional methodology for the synthesis of rigidified peptide architectures.