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Armin de Meijere

Publications and source records attributed to Armin de Meijere.

At least 19 recordsLinked to original sources
PubMed2007

Stereoselective preparation of six diastereomeric quatercyclopropanes from bicyclopropylidene and some derivatives.

Diastereomeric meso- and d,l-bis(bicyclopropylidenyl) (5) were obtained upon oxidation with oxygen of a higher-order cuprate generated from lithiobicyclopropylidene (4) in 50 and 31 % yield, respectively. Their perdeuterated analogues meso-[D(14)]- and d,l-[D(14)]-5 were obtained along the same route from perdeuterated bicyclopropylidene [D(8)]-3 (synthesized in six steps in 7.4 % overall yield from [D(8)]-THF) in 20.5 % yield each. Dehalogenative coupling of 1,1-dibromo-2-cyclopropylcyclopropane (6) gave a mixture of all possible stereoisomers of 1,5-dicyclopropylbicyclopropylidene 16 in 69 % yield, from which (Z)-cis-16 was separated by preparative gas chromatography (26 % yield). The crystal structure of meso-5 looks like a superposition of the crystal structures of two outer bicyclopropylidene units (3) and one inner s-trans-bicyclopropyl unit, whereas the two outer cyclopropyl moieties adopt a gauche orientation with respect to the cyclopropane rings at the inner bicyclopropylidene units in (Z)-cis-16. Birch reduction with lithium in liquid ammonia of meso-5 and d,l-5 gave two pairs of diastereomeric quatercyclopropanes trans,trans-(R*,S*,R*, S*)-17/cis,trans-(R*,S*,R*,R*)-18 and trans,trans-(R*,S*,S*,R*)-19/cis,trans-(R*,S*,S*,S*)-20 in 97 and 76 % yield, respectively, in a ratio 9:1 for every pair. The latter diastereomer was also obtained as the sole product by Birch reduction of (Z)-cis-16 in 96 % yield. Under the same conditions, tetradecadeuterio analogues trans,trans-[D(14)]-(R*,S*,R*,S*)-17/cis,trans-[D(14)]-(R*, S*,R*,R*)-18 (8:1) and trans,trans-[D(14)]-(R*,S*,S*,R*)-19/cis,trans-[D(14)]-(R*,S*,S*,S*)-20 (12:1) were prepared from meso-[D(14)]-5 and d,l-[D(14)]-5 in 37 and 63 % yield, respectively. Reduction of meso-5 with diimine gave the cis,cis-quatercyclopropane (S*,S*,R*,R*)-21 as the main product (58 % yield) along with the cis,trans-diastereomer (S*,S*,R*,S*)-18 (29 % yield). Thus, five of the six possible diastereomeric quatercyclopropanes were obtained from meso-5, d,l-5, and (Z)-cis-16. The X-ray crystal structure analyses of trans,trans-(R*,S*,R*,S*)-17 and cis,cis-(S*,S*,R*,R*)-21 revealed for the both an unusual conformation in which the central bicyclopropyl unit adopts an s-trans-(antiperiplanar) orientation with phi=180.0 degrees , and the two terminal bicyclopropyl moieties adopt a synclinal conformation with phi=49.8 and 72.0 degrees , respectively. In solution the vicinal coupling constants (3)J(H,H) in trans,trans-(R*,S*,R*,S*)-[D(14)]-17, trans,trans-(R*,S*,S*,R*)-[D(14)]-19, trans,cis-(R*,S*,R*,R*)-[D(14)]-18 and trans,cis-(R*,S*,S*,S*)-[D(14)]-20 were found to be 4.1, 4.7, 5.9 and 5.9 Hz, respectively. This indicates a predominance of the all-gauche conformer in (R*,S*,R*,S*)-17 and a decreasing fraction of it in this sequence of the other diastereomers.

Computer Simulation↗
PubMed2007

1,3-bicyclo[1.1.1]pentanediyl: the shortest rigid linear connector of phenylated photochromic units and a 1,5-dimethoxy-9,10-di(phenylethynyl)anthracene fluorophore.

An excess of bis-1,3-(4-iodophenyl)bicyclo[1.1.1]pentane, prepared in 63 % yield by iodination of 1,3-diphenylbicyclo[1.1.1]pentane, was selectively mono-coupled with 9-ethynyl-1,5-dimethoxy-10-phenylethynylanthracene (26), and subsequently with the zinc derivatives of 1-(2-methyl/methoxy-4-methyl-5-phenylthiophen-3-yl)-2-(2-methyl/methoxy-4-methylthiophen-3-yl)perfluorocyclopentenes (38-H-41-H). Regioselective synthesis of the 2-unsubstituted thiophenes 38-H-41-H required intermediate preparation of 2-trimethylsilyl-3,5-dimethyl-4-bromothiophene (37) or 2-trimethylsilyl-5-methoxy-3-methyl-4-bromothiophene (40). Protection of the alpha-position of the thiophene ring with a 2-trimethylsilyl group blocks the rearrangement of the 4-lithio derivatives into the corresponding 2-lithiated thiophenes. With the bicyclo[1.1.1]pentane fragment linking the photochromic units 1-3 and 1,5-dimethoxy-9,10-di(phenylethynyl)anthracene as a fluorescent part, quantitative resonance energy transfer between the excited state of the fluorophore (donor) and the closed form of the photochromic units 1-3 (acceptors) was observed. The closed forms of the methoxy-substituted photochromic units 2 and 3 are less resistant to UV light (313 nm) than the closed form of 1.

Anthracenes↗
PubMed2006

Oligoindenopyrenes: a new class of polycyclic aromatics.

A new class of polycyclic aromatic hydrocarbons--oligoindenopyrenes--has been synthesized featuring a Pd-catalyzed Suzuki-Heck coupling cascade. The oligoindenopyrenes are robust, highly colored substructures of C(70) and have properties that might prove useful in new organic materials or devices. After excitation, the tetraindenopyrene derivative 3d undergoes efficient deactivation (99%) by internal conversion to the ground state. The small fluorescence quantum yield (0.004) is in accordance with the short (0.6 ns) fluorescence decay time.

Fluorescence↗
PubMed2006

An efficient access to novel enantiomerically pure steroidal delta-amino acids.

A highly chemoselective sequence of Stille and Heck couplings on the heterocyclic bromoenol triflates 2 a-c with the bicycloalkenylstannanes cis-3 and trans-3 furnished the intermediate bromobutadienes 4 a-c in good yields ranging from 73-94 %. A modified Heck coupling protocol employing the palladacycle 8 and an additional bidentate ligand such as 1,4-bis(diphenylphosphinyl)butane allowed a significant reduction in catalyst loading while still obtaining the heterocyclic 1,3,5-hexatrienes 5 a-c in good yields (71-94 %). The unsymmetrically substituted 1,3,5-hexatrienes 5 a-c in solution underwent 6pi-electrocyclizations following an optimized microwave-heating protocol to yield the steroidal tetracycles cis-7 a-c and trans-7 b (59-69 %). Tetracycles cis-7 a-c are the products of a subsequent 1,5-hydrogen shift to the thermodynamically more stable, more highly substituted diene units. Removal of the tert-butyl groups provided the novel steroidal delta-amino acid 9 a and the delta-amino acid derivatives 9 b, c in good yields (76-86 %).

Alkenes↗
PubMed2006

Many density functional theory approaches fail to give reliable large hydrocarbon isomer energy differences.

[structure: see text] Several DFT methods were found to be unreliable for computing hydrocarbon isomer energy differences. The errors grow with system size up to 20 kcal mol(-1) for the relative energies of the (CH)12 isomers; octahedrane is the most stable (CH)12 hydrocarbon. While DFT geometries generally are good, problems arise for structures with single bonds only, especially for small rings. We recommend the use of higher level, non-DFT energy single points computed at DFT-optimized structures.

Journal Article↗
PubMed2006

Syntheses and properties of enantiomerically pure higher (n > or = 7) [n-2]triangulanedimethanols and sigma-[n]helicenes.

(P)-(+)-Hexaspiro[2.0.0.0. 0.0.2.1.1.1.1.1]pentadecane [(P)-17] as well as (M)-(-)- and (P)-(+)-octaspiro[2.0.0.0.0.0.0.0.2.1.1.1.1.1.1.1]nonadecanes [(M)- and (P)-25]-enantiomerically pure unbranched [7]- and [9]triangulanes-have been prepared starting from racemic THP-protected (methylenecyclopropyl)methanol 6. The relative configurations of all important intermediates as well as the absolute configurations of the key intermediates were established by X-ray crystal structure analyses. This new convergent approach to enantiomerically pure linear [n]triangulanes for n=7, 9 was also tested in two variants towards [15]triangulane. Some of the most prominent and unexpected features of the newly prepared compounds are the remarkable modes of self-assembly of the diols (P)-14, (E)-(3S,3'S,4S,4'S,5R,5'R)-21, (P)-(+)-22, and (E)-31 in the solid state through frameworks of intermolecular hydrogen bonds leading to, depending on the respective structure, nanotube- [(P)-14, (P)-(+)-22, and (E)-31], honeycomb-like structures [(E)-(3S,3'S,4S,4'S,5R,5'R)-21] or a supramolecular double helix [(P)-(+)- and (M)-(-)-22]. Liquid crystalline properties of the esters and ethers of the diols (P)-14, (P)-, and (M)-22 have also been tested. Although all of these [n]triangulanes have no chromophore which would lead to significant absorptions above 200 nm, they exhibit surprisingly high specific rotations even at 589 nm with [alpha](20)(D)=+672.9 (c=0.814 in CHCl(3)) for (P)-(+)-17, +909.9 (c=0.96 in CHCl(3)) for (P)-(+)-25, -890.5 (c=1.01 in CHCl(3)) for (M)-(-)-25, and -1302.5 (c=0.36 in CHCl(3)) for (M)-(-)-39, and the specific rotations increase drastically on going to shorter wavelengths. This outstanding rotatory power is in line with their rather rigid helical arrangement of sigma bonds, and accordingly these helically shaped unbranched [n]triangulanes may be termed "sigma-[n]helicenes", as they represent the sigma-bond analogues of the aromatic pi-[n]helicenes. Density functional theory (DFT) computations at the B3 LYP/6-31+G(d,p) level of theory for the geometry optimization and time-dependent DFT for determining optical rotations with a triplet-zeta basis set (B3 LYP/TZVP) reproduce the optical rotatory dispersions (ORD) very well for the lower members (n=4, 5) of the sigma-[n]helicenes. For the higher ones (n=7, 9, 15) the computed specific rotations turn out increasingly larger than the experimental values. The remarkable increase of the specific rotation with an increasing number of three-membered rings is proportional neither to the molecular weight nor to the number of cyclopropane rings in these sigma-[n]helicenes.

Journal Article↗
PubMed2006

Syntheses and structures of sterically congested linear and branched cobalta[n]triangulanes.

Treatment of {eta(5):eta(1)[2-(di-tert-butylphosphanyl-P)ethyl]cyclopentadienyl}cobalt(I) chloride (5) with methylenecyclopropane (3) or bicyclopropylidene (4), as well as with their spirocyclopropanated analogues methylenespiropentane (7), cyclopropylidenespiropentane (10), or 7,7'-bi(dispiro[2.0.2.1]heptylidene) (15) in the presence of sodium amalgam at -50 degrees C, furnished the stable cobalt complexes 6, 9, 8, 11, and 16, respectively, in 72, 83, 84, 86, and 54 % isolated yield, respectively. The complexes 14 and 16 were also obtained by ligand exchange of the ethene complex {eta(5):eta(1)[2-(di-tert-butylphosphanyl-P)ethyl]cyclopentadienyl}(eta(2)-ethene)cobalt(I) (12) with 13 and 15 in 79 and 52 % yield, respectively. The X-ray crystal-structure analyses of complexes 9, 14, and 16, as well as the NMR-spectroscopic data of all complexes, reveal that they can be regarded as linear and branched cobalta[n]triangulanes. The thermal stability of complexes 6, 8, and 9 up to 109, 145, and 160 degrees C was determined by differential thermal analysis-thermogravimetry (DTA-TG) analysis.

Journal Article↗
PubMed2006

Structural and conformational properties and intramolecular hydrogen bonding of (methylenecyclopropyl)methanol, as studied by microwave spectroscopy and quantum chemical calculations.

The microwave spectra of (methylenecyclopropyl)methanol (H(2)C=C(3)H(3)CH(2)OH) and one deuterated species (H(2)C=C(3)H(3)CH(2)OD) have been investigated in the 20-80 GHz spectral range. Accurate spectral measurements have been performed in the 40-80 GHz spectral interval. The spectra of two rotameric forms, denoted conformer I and conformer IX, have been assigned. Both these rotamers are stabilized by intramolecular hydrogen bonds formed between the hydrogen atom of the hydroxyl group and the pseudo-pi electrons on the outside of the cyclopropyl ring, the so-called "banana bonds". The carbon-carbon bond lengths in the ring are rather different. The bonds adjacent to the methylene group (H(2)C=) are approximately 7 pm shorter that the carbon-carbon bond opposite to this group. It is found from relative intensity measurements of microwave transitions that conformer IX, in which the hydrogen bond is formed with the banana bonds of the long carbon-carbon bond, is 0.4(3) kJ/mol more stable than conformer I, where the hydrogen bond is formed with the pseudo-pi electrons belonging to the shortest carbon-carbon bond of the ring. The microwave study has been augmented by quantum chemical calculations at the MP2/6-311++G, G3 and B3LYP/6-311++G levels of theory.

Journal Article↗
PubMed2006

Helical structure of tercyclopropanedimethanol in solution.

Oligocyclopropanes with repetitive stereochemistry occur in two unusual natural products with interesting bioactivity. X-ray crystal structures are available for these compounds but with partially contradicting results. Because the 1H and 13C NMR spectra of oligocyclopropanes are far from trivial to be assigned even at highest magnetic fields, we have prepared a specifically deuterated sample and have applied high field NMR spectroscopy and DFT calculations to determine its conformation. The helix with equal handedness shown in the stereopicture was found for tercyclopropanedimethanol. A dihedral angle of around +40 degrees is the best representation of the experimental data and characterizes, therefore, the dominating helical conformation of tercyclopropanedimethanol with a single repetitive (+)-gauche interunit dihedral angle. This is in full agreement with the crystal structure of the all syn,trans-quinquecyclopropanedimethanol with an R configuration at the termini that also adopted an all (+)-gauche conformation. However, the crystal structure of the title compound and the solution structure are different.

Crystallography, X-Ray↗
PubMed2006

Synthesis of enantiopure indolizinones by cascade ring enlargements of 4'-chlorospiro[cyclopropane-1,5'-isoxazolidines].

2-Chloro-2-cyclopropylideneacetates (1-Me and 1-Et) and their spiropentane analogues 2 cycloadd enantiopure five-membered cyclic nitrones to give the corresponding adducts (quantitatively, four examples), which undergo cascade ring enlargements to yield indolizinone derivatives (53-70%, four examples). The ring enlargement process is triggered by the abstraction of a bridgehead proton induced by a base and can be suppressed by the presence of a bulky substituent nearby, such as a (triisopropylsilyl)oxy group.

Cyclization↗
PubMed2006

Inhibitor-binding mode of homobelactosin C to proteasomes: new insights into class I MHC ligand generation.

Most class I MHC ligands are generated from the vast majority of cellular proteins by proteolysis within the ubiquitin-proteasome pathway and are presented on the cell surface by MHC class I molecules. Here, we present the crystallographic analysis of yeast 20S proteasome in complex with the inhibitor homobelactosin C. The structure reveals a unique inhibitor-binding mode and provides information about the composition of proteasomal primed substrate-binding sites. IFN-gamma inducible substitution of proteasomal constitutive subunits by immunosubunits modulates characteristics of generated peptides, thus producing fragments with higher preference for binding to MHC class I molecules. The structural data for the proteasome:homobelactosin C complex provide an explanation for involvement of immunosubunits in antigen generation and open perspectives for rational design of ligands, inhibiting exclusively constitutive proteasomes or immunoproteasomes.

Amino Acid Sequence↗
PubMed2006

Radical cations from dicyclopropylidenemethane and its octamethyl derivative.

The radical cations of dicyclopropylidenemethane (2) and its octamethyl derivative (2-Me8) are prone to rearrangements into those of (2-methylallylidene)cyclopropane (2a) and its octamethyl derivative (2a-Me8), respectively, by opening one three-membered ring. In contrast to the radical cations of bicyclopropylidene (1) and its octamethyl derivative (1-Me8), 2*+ and 2-Me8*+ are stable to opening of the second ring, because in this case the resulting species would be a non-Kekulé hydrocarbon with a quartet ground state. Similarly to 1, octamethyl substitution in 2 promotes the tendency to rearrangement. Thus, ESR and ENDOR studies indicate that the primary radical cation 2*+, which is formed upon gamma-irradiation of 2 in a CFCl3 matrix at 77 K, does not rearrange up to 150 K. On the other hand, when 2-Me8 is treated in the same way, only the rearranged radical cation 2a-Me8*+ can be observed and characterized by its ESR and ENDOR spectra. Nevertheless, the existence of the two "missing" species, 2a*+ and 2-Me8*+, is revealed by other methods. According to UV and IR studies, X irradiation of 2 in an Ar matrix leads directly to the ring-opened radical cation 2a*+. Moreover, magnetic field effects on the decay of fluorescence, which appears upon recombination of the radical anion of p-terphenyl with a radical cation generated from 2-Me8 in liquid octane, strongly suggest that 2-Me8*+ (and not 2a-Me8*+) is formed initially. From the temperature dependence of the decay, the activation energy of the ring-opening process 2-Me8*+ --> 2a-Me8*+ is estimated. The radical cations 2a*+ and 2a-Me8*+ are formally distonic with the spin residing in the allylic moiety and the charge accommodated on the central carbon atom of the allene pi-system. The intact cyclopropylidenemethylidene moiety assumes a "bisected" conformation, thus favoring an optimal interaction with the positively charged center on the pi-system.

Journal Article↗
PubMed2005

Linear and branched phospha[n]triangulanes.

Novel, highly stable, linear and branched mono- and diphospha[n]triangulanes were synthesized in high yields by the CuCl-catalyzed phosphinidene addition to spirocyclopropanated methylenecyclopropanes and bicyclopropylidenes. The effect of spirofusion on the electronic properties of these esthetically attractive phosphacycles is apparent from X-ray single crystal structure analyses, which reveals a tightening of the phosphirane ring on additional spirocyclopropanation, and from the NMR features that show deshielded chemical shifts for the ring-phosphorus and -carbon atoms. Steric factors play a role in the addition reaction when the substrate alkene carries a second sphere of spirocyclopropane rings and causes the formation of 2-phosphabicyclo[3.2.0]heptenes in small amounts. These by-products most probably result from addition of the [PhP(Cl)W(CO)(5)]-Cu-L (L=alkene or solvent) reagent to the spirocyclopropanated bicyclopropylidene to give an intermediate sigma-complex, which subsequently, facilitated by steric factors, undergoes a cyclopropylcarbinyl to cyclobutyl ring expansion followed by a [1,3]-sigmatropic shift.

Magnetic Resonance Spectroscopy↗
PubMed2005

An access to 3,4-(aminomethano)proline in racemic and enantiomerically pure form.

Protected racemic and enantiomerically pure 3,4-(aminomethano)prolines rac-9 and (2S,2'R,3R,4R)-9 have been prepared applying a titanium-mediated reductive cyclopropanation as a key step. Thus, cyclopropanations of N,N-dibenzylformamide with titanacyclopropanes generated in situ from racemic or enantiomerically pure tert-butyl N-Boc-3,4-dehydroprolinates rac-8 or (S)-8 proceed diastereoselectively, and furnish the protected racemic and enantiomerically pure diamino acid 9. The latter was incorporated into three tripeptides containing glycyl, alanyl and phenylalanyl moieties.

Crystallography, X-Ray↗