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Biomedical subjects

Johann Lex

Publications and source records attributed to Johann Lex.

At least 19 recordsLinked to original sources
PubMed2006

Enantiomerically pure isophorone diamine [3-(aminomethyl)-3,5,5-trimethylcyclohexylamine]: a chiral 1,4-diamine building block made available on large scale.

Isophorone diamine [IPDA, 3-(aminomethyl)-3,5,5-trimethylcyclohexylamine] is a chiral non-C2-symmetric 1,4-diamine which is produced industrially on large scale as the mixture of all four stereoisomers (cis/trans ca. 3:1). Starting from this industrial bulk product, the preparation of the bis-tosyl, bis-Fmoc, bis-Boc and bis-Z derivatives of cis-IPDA, the preparation of the pure cis enantiomers by HPLC on chiral stationary phase, and the assignment of absolute configurations to the isolated enantiomers are described. We furthermore report an efficient method for the optical resolution of IPDA by salt formation with dibenzoyl tartaric acid. The latter method conveniently affords enantiomerically pure cis-IPDA in 100 g quantities. A number of salen ligands have been prepared from this enantiomerically pure 1,4-diamine and fully characterized. The nickel complex of one of the salen ligands was prepared and analyzed by X-ray crystallography. The crystal structure of the Ni4L4 complex illustrates the pronounced preference of cis-IPDA for adopting the chair conformation in which both the amino- and the aminomethyl substituents occupy equatorial positions. As a consequence, the two salicylidene imine moieties of one ligand molecule do not converge on one metal ion, but act as bridging ligands between two nickel ions.

Journal Article↗
PubMed2006

Structural optimization of thiourea-based bifunctional organocatalysts for the highly enantioselective dynamic kinetic resolution of azlactones.

This article describes the synthesis of a library of structurally diverse bifunctional organocatalysts bearing both a quasi-Lewis acidic (thio)urea moiety and a Brønsted basic tertiary amine group. Sequential modification of the modular catalyst structure and subsequent screening of the compounds in the alcoholytic dynamic kinetic resolution (DKR) of azlactones revealed valuable structure-activity relationships. In particular, a "hit-structure" was identified which provides e.g.N-benzoyl-tert-leucine allyl ester in an excellent enantiomeric excess of 95%.

Amines↗
PubMed2005

Total synthesis of (-)-colchicine via a Rh-triggered cycloaddition cascade.

[reaction: see text] A synthesis of the antimitotic alkaloids (-)-colchicine and (-)-isocolchicine is reported. Important steps are (a) enantioselective transfer-hydrogenation of an alkynone, (b) iodine/magnesium exchange with subsequent aromatic acylation, (c) Rh-catalyzed transformation of an alpha-diazoketone into an oxatetracyclic key intermediate through intramolecular [3 + 2]-cycloaddition of an in situ generated carbonyl ylide, and (d) regioselective conversion of the cycloadduct into a tropolone derivative. The new synthetic strategy opens an efficient enantioselective access to colchicine and structural analogues.

Colchicine↗
PubMed2004

Transition-metal-mediated synthesis of novel carbocyclic nucleoside analogues with antitumoral activity.

A diversity-oriented, enantioselective synthesis of new (monoprotected) carbocyclic nucleoside analogues (CNAs) with the nucleobase attached to a 3-hydroxymethyl-4-trialkylsilyloxymethylcyclopent-2-en-1-yl scaffold was developed. As a key intermediate, racemic (5SR,8RS)-8-allyloxy-2-trimethylsilyl-7-oxa-bicyclo[3.3.0]-oct-1-en-3-one was prepared from 1,1-diallyloxy-3-trimethylsilyl-2-propyne in a cobalt-mediated Pauson-Khand reaction. The enantiomerically pure material was obtained through efficient kinetic resolution (selectivity factor s >/= 40 at -78 degrees C) by means of an oxazaborolidine-catalyzed borane reduction (CBS reduction) with catecholborane. The absolute configuration of the resolved products was determined by CD spectroscopy, Mosher ester analysis, and chemical correlation. Subsequent steps involve diastereoselective ketone reduction and fully regio- and diastereoselective introduction of the nucleobase through Pd(0)-catalyzed allylic substitution. The generality of the method was demonstrated by preparation of CNAs in both enantiomeric series with all five natural nucleobases, as well as 5-bromouracil, 5-fluorouracil, and 6-chloropurine. Screening of the various compounds in a cytotoxicity assay with BJAB and ALL tumor cell lines revealed that some of the compounds possess pronounced antitumoral properties (LD50 values down to 9 microM, as determined by lactate dehydrogenase release after 48 h). By measuring DNA fragmentation, it could be shown that the activity results from induction of apoptosis.

Antineoplastic Agents↗
PubMed2004

Enantioselective synthesis of DIANANE, a novel C2-symmetric chiral diamine for asymmetric catalysis.

DIANANE (endo,endo-2,5-diaminonorbornane) is a novel chiral C(2)-symmetric diamine, based on the rigid bicyclo[2.2.1]heptane scaffold. Schiff-base ligands derived from DIANANE have already found use in asymmetric catalysis, e.g., in the highly enantioselective Nozaki-Hiyama-Kishi reaction. We herein describe a practical synthesis of enantiomerically pure DIANANE, starting from norbornadiene in four steps: (i) Pd-MOP catalyzed Hayashi-hydrosilylation/Tamao-Fleming oxidation, (ii) oxidation to norbornane-2,5-dione, (iii) endo-selective reductive amination with benzylamine, and (iv) hydrogenolytic debenzylation. None of the steps involves chromatographic purification. For the Tamao-Fleming oxidation, the use of hydrogen peroxide in the form of its urea clathrate instead of aqueous solution proved beneficial. By the above sequence, enantiomerically pure (ee >or=99%) DIANANE was obtained from norbornadiene in 40-50% overall yield. The relative and absolute configuration of DIANANE was confirmed by X-ray crystallography of the DIANANE bis-tosylamide, and of its bis-camphorsulfonamide. Furthermore, the synthesis and X-ray crystal structure of the Schiff-base ligand derived from DIANANE and 3,5-di-tert-butyl salicylic aldehyde are reported.

Journal Article↗
PubMed2004

Stereoselective generation of vicinal stereogenic quaternary centers by photocycloaddition of 5-methoxy oxazoles to alpha-keto esters: synthesis of erythro beta-hydroxy dimethyl aspartates.

The photocycloaddition of methyl pyruvate and methyl phenylglyoxylate, respectively, to 5-methoxy oxazoles bearing additional substituents at C-2 and C-4 leads to bicyclic oxetanes 2 and 3 with high to moderate (exo) diastereoselectivity that can be easily ring-opened to give bis-quaternary aspartic acid diester derivatives 4 and 5.

Journal Article↗
PubMed2003

Synthesis of erythro-alpha-amino beta-hydroxy carboxylic acid esters by diastereoselective photocycloaddition of 5-methoxyoxazoles with aldehydes.

A new photoaldol route to alpha-amino-beta-hydroxy carboxylic acid esters is initiated by the photocycloaddition of aromatic or aliphatic aldehydes to 5-methoxyoxazoles. The 4-unsubstituted 5-methyloxazole 1 gave the cycloadducts 8a-f in high yields and excellent exo-diastereoselectivities. Hydrolysis of 8a-f gives the N-acetyl alpha-amino-beta-hydroxy esters 9a-f, which could be subsequently converted into the corresponding Z-didehydro alpha-amino acids 10a-f. Quartenary alpha-amino-beta-hydroxy esters 12, 14, 16, 18, and 20, which are stable against dehydration, were obtained from the 4-alkylated 5-methoxyoxazoles 2-6, in most cases highly erythro-selective due to the high degree of stereocontrol (exo) at the photocycloaddition (to give 11, 13, 15, 17, and 19) level. The relative configurations of the N-acetyl alpha-amino-beta-hydroxy esters were determined by NMR spectroscopy and comparison with chiral pool-derived compounds as well as by X-ray structure determination of the ester 23, formed by hydrolysis of the cycloadduct 22, derived from photocycloaddition of propionaldehyde to the isoleucine-derived oxazole 21.

Aldehydes↗
PubMed2003

Spiroconjugation in spirodicorrolato-dinickel(II).

In the present work the effect of spiroconjugation on the electronic spectrum of the recently synthesized metal complex hexadecaethylspirodicorrolato-dinickel(II) (5) (the corrole units in 5 are isoforms) is investigated. To have a suitable reference compound at our disposal, tetraethylhexamethylisocorrolato nickel(II) (7) has been prepared. On comparing the electronic spectra of this reference compound and the spiro-complex, bathochromic shifts of all absorption bands in the NIR/Vis-region are observed for the latter as well as marked changes in the spectral intensities. A detailed analysis of the spectra supported by semiempirical calculations reveals that at least part of the observed changes can be unambiguously attributed to the spiro effect. This effect is further affirmed by electrochemically measured redox potentials.

Journal Article↗
PubMed2003

Electronically tuned chiral ruthenium porphyrins: extremely stable and selective catalysts for asymmetric epoxidation and cyclopropanation.

We report the use of three enantiomerically pure and electronically tuned ruthenium carbonyl porphyrin catalysts for the asymmetric cyclopropanation and epoxidation of a variety of olefinic substrates. The D(4)-symmetric ligands carry a methoxy, a methyl or a trifluoromethyl group at the 10-position of each of the 9-[anti-(1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracene)]-substituents at the meso-positions of the porphyrin. Introduction of a CF(3)-substituent in this remote position resulted in greatly improved catalyst stability, and turnover numbers of up to 7500 were achieved for cyclopropanation, and up to 14,200 for epoxidation, with ee values typically >90 % and approximately equal to 80 %, respectively. In one example, the axial CO ligand at the ruthenium was exchanged for PF(3), resulting in the first chiral ruthenium porphyrin with a PF(3) ligand reported to date. In cyclopropanations with ethyl diazoacetate, the latter catalyst performed exceedingly well, and gave a 95 % ee in the case of 1,1-diphenylethylene as substrate.

Journal Article↗
PubMed2002

An approach to serrulatane diterpenes via endo-selective conjugate nucleophilic addition to arene-Cr(CO)3 complexes.

[formula: see text] Starting from a nonracemic planar-chiral arene tricarbonyl chromium complex, the serrulatane diterpenoid (+)-20-methoxy-serrulat-14-en-7,8-diol was synthesized in a highly stereoselective fashion. The key step of the synthesis is an endo-selective conjugate nucleophilic addition of lithio-methylphenyl sulfone to a 1-ethylidene-tetralin-Cr(CO)3 derivative. By employing different substrates and nucleophiles it was shown that the surprising and rather general endo selectivity must result from a unique complex induced proximity effect under participation of the Cr(CO)3 moiety.

Journal Article↗
PubMed2002

Metal complexes of porphycene, corrphycene, and hemiporphycene: stability and coordination chemistry.

Porphyrin (P), porphycene (Pc), corrphycene (Cn), and hemiporphycene (Hpc) represent a series of well defined "4-N in" constitutional porphyrin isomers. These isomers, in the form of their octaethyl derivatives, represent a congruent set of porphyrinoids whose properties can be compared. In this study we report how variations in electronic structure and nitrogen-core size in the free-base forms of these four systems are reflected in the properties of their corresponding metal complexes. Specifically, the effects that these differences have on the axial ligation properties of the Zn(II), Mg(II), Ni(II), and Co(II) complexes of P, Pc, Cn, and Hpc in toluene using pyridine as the axial ligand are detailed. Also reported are the relative stabilities of these complexes under acidic conditions. It is shown that for the zinc, magnesium, and cobalt complexes, there are distinct differences in the ability to maintain four-, five-, or six-coordinate geometries in the presence of similar concentrations of pyridine. By contrast, no apparent differences in axial ligand binding affinity are seen for the four nickel complexes. Little difference in stability was likewise seen when these same complexes were subject to acid-mediated demetallation, with all four falling into stability class II, according to the accepted porphyrin stability ranking system. High stabilities were also seen in the case of the cobalt complexes, with the Pc and Cn complexes being of stability class III and the P and Hpc derivatives falling into stability class II. The Zn(II) and Mg(II) complexes were all far less stable than the corresponding Ni(II) and Co(II) complexes. In this case, semiquantitative analyses of the rate of acid-induced decomposition revealed the following stability sequence P>Cn>Hpc>Pc for both the Zn(II) and Mg(II) complexes. Single-crystal X-ray diffraction structures were solved for the Zn(II), Mg(II), and Ni(II) complexes of the octaethyl derivatives of Hpc, Cn, and Pc as well as a Co(II) octamethylcorrphycene and are reported as part of this study. These solid-state structures confirm four-coordinate species for the Ni(II) complexes, four- and five-coordinate species for the Mg(II) and Zn(II) complexes, and a six-coordinate species for the lone Co(II) complex.

Binding Sites↗