Microwave spectra of cis and trans 2-furylisocyanate conformers

The microwave spectrum of 2-furylisocyanate has been obtained in the frequency region from 8 to 40 GHz. This spectrum is attributed to the ground state of the cis and trans configurations. The rotational constants for both ground vibrational states have been determined. Two sets of vibrational satellites are observed and assigned to the modes of C-N torsion and CNC bending. The microwave results show and MINDO/3 calculations confirm that the barrier between the cis and trans conformers is high and that the cis conformer is more stable than the trans by 3 kcal mol^?1.     

Synthesis and structural characterization of enantiopure exo and endo six-membered oxazoline-derived palladacycles

Direct palladation of (S)-4-benzyl-2-methyl-2-oxazoline (1) and (S)-2-benzyl-4-tert-butyl-2-oxazoline (2) using Pd(OAc)₂ in MeCN afforded the corresponding μ-acetato-dimeric complexes with six-membered exo and endo palladacycles, respectively. The same complexes were obtained by reacting coordination complexes Pd(1)₂(OAc)₂ and Pd(2)₂(OAc)₂ with Pd(OAc)₂ in MeCN. Metalation of (S)-2,4-dibenzyl-2-oxazoline (3) with Pd(OAc)₂ in AcOH, MeCN or CH₂Cl₂ resulted in the regiospecific formation of the six-membered endo palladacycle. The obtained μ-acetato-dimeric complexes were converted to the corresponding μ-chloro-dimeric derivatives 7, 11 and 13 by treatment with LiCl in acetone. The mononuclear PPh₃ adducts 8, 12 and 14 were obtained by reacting dimers 7, 11 and 13 with PPh₃ in benzene. NMR spectroscopy data supported the proposed structures of all complexes and suggested that exo and endo palladacycles in 8 and 12 have rigid boat conformations in CHCl₃. The X-ray crystal structures of the μ-acetato dimer 6 with the exo palladacycle and the PPh₃ adduct 14 with the endo metalacycle revealed boat conformation of both palladacycles and chiral twisted conformations δ(S) and λ(S), respectively, of the oxazoline rings in the solid state.     

Dynamic solvation effects on the endo/exo selectivity of the Diels-Alder reaction

The endo/exo stereoselectivity of the Diels-Alder cycloaddition between cyclopentadiene and 3-crotonoyl-2-oxazolidinone catalysed by Et₂AlCl is temperature- and solvent-dependent. Eyring plots of ln(endo/exo) versus 1/T show different inversion temperatures (T_inv) depending on the reaction solvent. Using toluene-d₈ we found a break temperature (T_NMR) in a variable temperature ¹³C NMR experiment of the starting oxazolidinone, and this T_NMR corresponded to the T_inv. In several cases the differential activation entropy manages the observed stereoselectivity.     

Factors affecting the endo : exo ratio in diels-alder reactions of cyclopentadiene

The ratio of endo-CHO:exo-CHO in the Diels-Alder addition of a trans-αβ-unsaturated aldehyde to cyclopentadiene can be changed from 1:2 to 8:1, depending on temperature and BF₃-catalysis.     

First highly stereocontrolled synthesis of the unusual 1-hydroxy endo,endo-2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane lignan skeleton

This Letter describes a highly diastereoselective synthesis of the unique 1-hydroxy endo,endo-2,6-diaryl-3,7-dioxabicyclo[3.3.0]octane skeleton starting from a stereodefined 4-arylidenetetrahydrofuran obtained by a multicomponent reaction. The key step of this synthesis is a dealkylative cyclization reaction performed on the corresponding epoxide, generating four contiguous stereogenic centers with total stereocontrol.     

Exo- and endo-palladacycles derived from (4R)-phenyl-2-oxazolines

Two novel oxazoline-derived palladacycles bearing an endo- or exo-CN bond were prepared by direct ortho-palladation of (R)-2,4-diphenyl- or (R)-2-methyl-4-phenyl-2-oxazolines, respectively. The structures of the palladacycles' dimeric forms and corresponding mononuclear PPh₃-derivatives were confirmed by IR, ¹H, ¹³C and 2D NMR spectroscopy. An X-ray diffraction study of the μ-chloro-dimeric cyclopalladated derivative of (R)-2,4-diphenyl-2-oxazoline proved the endo structure of the palladacycle.     

New exo/endo selectivity observed in monohydrolysis of dialkyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylates

New monohydrolysis reactions of several exo or endo dimethyl or diethyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylates showed higher selectivity toward monohydrolyses of exo-carboalkoxy groups, although the reaction centers are located away from the norbornene rings.     

Investigation of the exo and endo isomers of dioxolane-type benzylidene derivatives of carbohydrates by 13C NMR spectroscopy

The absolute configurations of nine 2,3-O-benzylidene-α-L-rhamno- and α-D-mannopyranoside diasteteomeric pairs were determined and the ¹³C NMR spectra of further thirteen α-L-rhamno- and α-D-mannopyranosides, having various substituents, were completely assigned.Four ¹³C shifts were found suitable for the determination of the absolute configuration of the dioxolane skeleton. (1) The chemical shift of the acetal carbon in the endo isomers is between 103.9 and 104.7 ppm whereas for the exo isomers this region extends from 102.8 to 103.4 ppm; (2) The formation of the dioxolane ring causes a deshielding effect for the bridgehead carbons, in the exo isomers this effect is more pronounced for C-3 whereas in the endo isomers for C-2. For C-4, shielding effect was found in the exo isomers and deshielding effect in the endo ones; (3) The chemical shift of the quaternary carbon of the phenyl group is greater in the exo isomers than in the endo ones; (4) The difference between the shift of the acetal carbon and that of the quaternary carbon of the phenyl group in the exo isomers is greater than 35.4 ppm, in the endo isomers is less than 33.7 ppm.     

The vibrational spectra and conformations of ethylbenzene

Infrared spectra (4000-250 cm-1) of the liquid, amorphous, crystalline solids and solutions in liquid krypton and Raman spectra (2500-20 cm-1) of the liquid as well as the amorphous and crystalline solids of ethylbenzene and its deuterated analogue-ethylbenzene-d(10) have been recorded. The spectra indicate that in the liquid and amorphous solids a small amount of a second conformer is present, whereas only one conformer remains in the crystalline phases. Assignments of the observed band wave numbers are discussed by comparison with normal mode wave numbers and IR and RS intensities calculated from ab initio 6-31G force fields and optimised geometries for both conformers for two species. All of the normal modes of conformers have been assigned.     

Screening of polymeric supports and enzymes for the development of an endo enzyme cleavable linker

Several polymeric supports possessing an ester moiety were prepared and a range of enzymes was investigated to hydrolyse the ester linkage and release a signalling group into solution for applications in immunoassays. Pseudomonas lipases were found to most readily cleave the solution-phase analogue and this observation translated well to the corresponding polymeric supports, where the most effective were PEGA resins and the LPOS support PEG-6000.     

exo and endo Isomerism of subvalent tin and germanium complexes derived from 1,3-diethers of p-tert-butylcalix[4]arene

A series of germanium and tin complexes of calix[4]arene diethers, namely [Calix^t-Bu(O)₂(OR)₂]M, has been prepared by the reaction of [Calix^t-Bu(OH)₂(OR)₂] with M[N(SiMe₃)₂]₂ (M = Ge, Sn; R = Me, CH₂Ph, CH₂C₆H₄Bu^t, SiPr₃ⁱ). X-ray diffraction studies demonstrate that [Calix^t-Bu(O)₂(OR)₂]M exist as exo and endo isomers in which the metal is, respectively, located either outside or inside the calixarene cavity. The exo isomer is considered to be the kinetic product, whereas the endo isomer is thermodynamically more stable. The ability to access endo-[Calix^t-Bu(O)₂(OR)₂]M, however, depends critically on the nature of R and M.     

Thermal isomerizations of cis,anti,cis-tricyclo[6.4.0.0]dodec-3-ene to trans- and cis,endo-tricyclo[6.2.2.0]dodec-9-ene: diradical conformations and stereochemical outcomes in [1,3] carbon shifts

The gas-phase thermal isomerizations at 315 °C of cis,anti,cis-tricyclo[6.4.0.0^2,7]dodec-3-ene to trans-tricyclo[6.2.2.0^2,7]dodec-9-ene and to cis,endo-tricyclo[6.2.2.0^2,7]dodec-9-ene favor the former, the more geometrically strained product, by a ratio of 2.4:1. These products correspond to suprafacial inversion (si) and suprafacial retention (sr) stereochemical outcomes. The reaction stereochemistry shown by the 11-carbon homolog, cis,anti,cis-tricyclo[6.3.0.0^2,7]undec-3-ene, is strikingly different: the [1,3] carbon shift takes place to give only the ‘forbidden’ sr product. Two related bicyclic vinylcyclobutanes, 8-deuterio- and 8-exo-methylbicyclo[4.2.0]oct-2-enes, evidence contrasting reaction stereochemical predilections in [1,3] shifts, but the 12-carbon tricyclic system and the 8-exo-methyl bicyclic analog isomerize with the same si:sr ratio! These observations prompt fresh considerations of structural influences on conformational preferences available to the alkyl, allyl diradical reactive intermediates involved.     

Synthesis of both the enantiomers of endo-brevicomin,the aggregation pheromone of Dryocoetes autographus

(1R,5S,7S)-(+)-endo-Brevicomin (7-ethyl-5-methyl-6,8-dioxabicyclo[3.2.1]octane) and its (1S,5R),7(R)-(?)-isomer were synthesised employing the Sharpless asymmetric epoxidation as the key-step.     

Synthesis of 2-substituted endo-hymenialdisine derivatives

The first synthesis of 2-substituted endo-hymenialdisine derivatives 1-4 is described started with 2-substituted pyrroles and 5-substituted pyrrolo-2-carboxylic acids.     

Microwave spectra of nitromethane and D3-nitromethane

The microwave spectra of isotopic nitromethanes are presently being reinvestigated in order to increase our understanding of the interaction between the almost free internal rotation and the vibrations.The presence of relaxation terms depending on cos3p proves that the symmetry of H_eff is D_3h and furthermore makes a determination of the structure of the methyl group possible.     

Substituent dependence of stable conformations in triazolobenzothiazines

From the ¹H and ¹³C NMR spectra of angularly condensed triazolobenzothiazine derivatives it was established that these compounds can be classified into two conformation types, depending on the substituents. The 10-methyl-3-carbethoxy derivatives are cis-anellated, conformationally flexible systems, where one of the aromatic hydrogens (H-9) and one of the methoxy substituents (8-OMe) of the benzothiazine skeleton on the one hand, and the 1-aryl ring on the other, lie close to each other in the preferred conformer. By assuming this conformation, the molecules avoid the steric hindrance of the 10-Me and 1-aryl groups. All compounds unsubstituted on the angular C-10 atom, and the 3-aryl-substituted derivatives of the 10-Me analogues, occur in a different conformation, containing the 1-aryl ring well removed from H-9 and the 8-OMe group.     

Photoelectron spectra of the gauche and trans conformers of 1,2-dibromoethane

The He I photoelectron spectra of the gauche and trans conformers of 1,2-dibromo-ethane have been obtained. Time-averaged spectra of 1,2-dibromoethane recorded at different temperatures have been analyzed by a spectrum-stripping procedure to give resulting spectra which represent each of the two stable conformers. The correlation of the ionization potentials of the two conformers is consistent with data obtained from MO calculations. In comparison with the case of 1,2-dichloroethane, 1,2-dibromoethane shows different non-bonded Br—Br interactions and generally larger resonance splittings of the ethane orbitals.