Etude RMN 13C d'aziridines non substituées sur l'azote II—Aza-7 bicyclo[4.1.0]heptanes,configurations relatives et conformations privilégiées

The configuration and favoured conformations of 1,2-dialkyl-(or alkyl-phenyl)-7-azabicyclo[4.1.0]heptane diastereoisomers have been determined by means of ¹³C NMR. The substituent in position-2 is ‘pseudo-axial’ in cis isomers and ‘pseudo-equatorial’ in trans isomers.     

Bemerkung zur vorstehenden Notiz [1] von H. REMBOLD

The possible transition state conformations (chair (S), boat (W), and twist (T), respectively cross (K) forms) and methods for their determination in the thermal ortho-CLAISEN rearrangement of allyl aryl ethers are discussed. Crotyl 3,5-dimethylphenyl ether ( 11 ) gives a mixture of 2-(α-methylallyl)-3,5-dimethyl-phenol ( 12 ) and 4-crotyl-3,5-dimethyl-phenol ( 13 ) on heating in N, N-diethylaniline. Values of 3 and 31 were obtained for the ratio of 12 / 13 for trans- 11 and cis- 11 , respectively. It therefore follows that both ethers rearrange steroselectively ( > 90%) by the S or W forms of the activated complex. αMethylallyl 6-alkylphenyl ethers rearrange on heating in various solvents to a mixture of trans-and cis-2-crotyl-6-alkyl-phenols. The amount of the cis-phenols in the rearrangement products decreases with the increasing bulk of the 6-alkyl substituent. This result is only obvious if the chair form of the transition state during the rearrangement of these ethers is highly favoured. trans-Crotyl 2,6-dimethylphenyl ether (trans- 33 ) rearranges highly steroselectively (94%) on heating to trans-4-crotyl-2,6-dimethyl-phenol (trans- 34 ). In the case of the corresponding cis ether 33 , the rapid cis → trans isomerisation of this ether and the cis/trans ratio of the phenol 34 indicate that the reverse rearrangement of the intermediate ortho-dienone to the ether 33 and the further rearrangement to 4-crotyl-2,6-dimethyl-phenol ( 34 ) has little stereoselective character.     

Asymmetric Synthesis of the cis‐ and trans‐3,4‐Dihydro‐2,4,8‐trihydroxynaphthalen‐1(2H)‐ones

A short and efficient protocol for the asymmetric synthesis of cis‐ and trans‐3,4‐dihydro‐2,4,8‐trihydroxynaphthalen‐1(2H)‐one ( 1 and 2 , resp.) is described, with a phthalide annulation as the key step. Introduction of a OH substituent at position 2 was performed by Sharpless dihydroxylation of a silyl enol ether or by means of an N‐sulfonyloxaziridine. The absolute configuration of each isomer was determined via Mosher‐ester derivatives. By comparison with previously recorded CD spectra of our natural sample, we established that the natural trans‐ and cis‐isomers from Ceratocystis fimbriata sp. platani were the (?)‐(2S,4S)‐isomer (?)‐ 2 and the (+)‐(2S,4R)‐isomer (+)‐ 1 , respectively.     

Configuration and conformation of substituted azetidines

Studies of compounds such as 1-cyclohexyl-2-carbomethoxy-4-methylazetidine by H-1 nmr coupling constants, nuclear Overhauser effects, C-13 steric shifts, and N-15 nmr support the revised assignment of configuration for cis/trans isomers in a series of N-alkyl substituted azetidines. The trans isomer displayed spectral characteristics consistent with a planar or nearly planar ring. The cis isomer favors a puckered ring, with major substituents at C-2 and C-4 equatorial. Compounds lacking the C-4 methyl are also puckered. Puckering angles are estimated. In contrast, trans-1-cyclohexyl-2-carboxy-4-methylazetidine (a zwitterion) prefers a puckered ring, possibly implicating the time averaged effect of nitrogen inversion on ring shape. The N-15 spectra show a large difference in chemical shift between cis and trans isomers of the azetidines, although analogous aziridines show an even larger difference.     

Reactivities of cis- and trans-3-substituted acrylic acids on copolymerization with acrylamide

Several pairs of cis- and trans-3-substituted acrylic acids (3SAA) were copolymerized with acrylamide in order to determine the major factors affecting the relative reactivities of geometrical isomers of 1,2-disubstituted ethylenes (1,2-DE). The results were that the relative reactivity of cis isomer is larger than that of trans isomer when one substituent is electron-withdrawing and the other is electron-donating. The trans isomer is more reactive than the cis isomer when both substituents are electron-withdrawing. A new method of reactivity comparison of cis- and trans-1,2-DE is proposed in regard to the inductive substituent constant.     

Cis‐ and trans‐bis(2‐cyano­ethyl­sulfan­yl)(decane‐1,10‐diyldithio)tetra­thia­fulvalene

The isomeric title compounds, 2,7‐bis(2‐cyanoethylsulfanyl)‐3,6‐(decane‐1,10‐diyldithio)tetrathiafulvalene and 2,6‐bis­(2‐cyanoethylsulfanyl)‐3,7‐(decane‐1,10‐diyldithio)­tetra­thiafulvalene, both C₂₂H₂₈N₂S₈, comprise bis­(2‐cyano­ethyl­sulfan­yl)tetra­thia­fulvalene units tethered by a saturated deca­methyl­enedithio linker attached in either a cis or a trans manner. The tetra­thia­fulvalene (TTF) group is planar in the cis isomer, but distorted significantly from planarity and twisted about its long axis in the trans isomer. In both structures, inter­molecular inter­actions are segregated into regions in which TTF units are brought into close contact and regions where the polymethyl­ene chains are brought into close contact. In the cis isomer, TTF units exhibit π–π stacking inter­actions, while in the trans isomer they do not.     

Ring opening of pyridines: the pseudo‐cis and pseudo‐trans isomers of tetra‐n‐butylammonium 4‐nitro‐5‐oxo‐2‐pentenenitrilate

The reaction of 2‐chloro‐5‐nitropyridine with two equivalents of base produces the title carbanion as an intermediate in a ring‐opening/ring‐closing reaction. The crystal structures of the tetra‐n‐butylammonium salts of the intermediates, C₁₆H₃₆N⁺·C₅H₃N₂O₃⁻, revealed that pseudo‐cis and pseudo‐trans isomers are possible. One crystal structure displayed a mixture of the two isomers with approximately 90% pseudo‐cis geometry and confirms the structure predicted by the S_N(ANRORC) mechanism. The pseudo‐cis intermediate undergoes a slow isomerization over a period of months to the pseudo‐trans isomer, which does not have the appropriate geometry for the subsequent ring‐closing reaction. The structure of the pure pseudo‐trans isomer is also reported. In both isomers, the negative charge is highly delocalized, but relatively small differences in C—C bond distances indicate a system of conjugated double bonds with the nitro group bearing the negative charge. The packing of the two unit cells is very similar and largely determined by the interactions between the planar carbanion and the bulky tetrahedral cation.     

Steric as well as n→π* Interaction Controls the Conformational Preferences of Phenyl Acetate: Gas‐phase Spectroscopy and Quantum Chemical Calculations

Herein, we report that the conformational preference of phenyl acetate is governed by steric effect and n→π* interaction. Conformation‐specific electronic and IR spectroscopy combined with quantum chemistry calculations confirm the presence of only the cis conformer of phenyl acetate in the experiment. The cis conformer of phenyl acetate has n→π* interaction between the lone‐pair electrons on the carbonyl oxygen atom and the π* orbitals of the phenyl group. The n→π* interaction is absent in the trans conformer which has additional steric repulsion between the methyl group and phenyl ring. The trans conformer is higher in energy than the cis conformer by ≈3 kcal mol^?1. We have found the effect of methyl substitution on the strength of the n→π* interaction, steric repulsion, and hyperconjugation in phenyl acetate. The red‐shift observed in the cis conformer of phenyl acetate with respect to the trans conformer is affected due to the influence of the methyl substituent on the strength of the n→π* interaction as well as hyperconjugation. The present result demonstrates that the introduction of a bulkier substituent can induce steric as well as electronic control to reduce conformational heterogeneity of a molecular system. Understanding the effect of bulkier substituents to promote defined conformations having specific non‐covalent interactions may have implication in better perception of the optimum structure and function of biomolecules as well as recognition of drugs by biomolecules.     

A comparison of linear optical properties and redox properties in chalcogenopyrylium dyes bearing ortho-substituted aryl substituents and tert-butyl substituents

A series of thiapyrylium pentamethine dyes (4 and 12-15) bearing 2,2'-di-tert-butyl-6,6'-diphenyl, 2,2'-di-tert-butyl-6,6'-bis(2,6-dimethylphenyl), 2,2'-di-tert-butyl-6,6'-bis(2-methylphenyl), 2,2',6,6'-tetrakis(2,6-dimethylphenyl), and 2,2',6,6'-tetrakis(2-methylphenyl) substituents, respectively, were prepared and their linear optical properties and electrochemical redox properties were measured and compared to thiapyrylium pentamethine dyes 3 and 5. The tert-butyl and 2,6-dimethylphenyl substituents give nearly identical chromophores with respect to values of lambda(max), molar extinction coefficients (epsilon), bandwidths at half-height (nu(1/2)), and lack of absorption in the visible spectrum. The 2-methylphenyl substituent imparts linear optical properties that are intermediate between those of the tert-butyl and phenyl substituents. The 2,6-dimethylphenyl and 2-methylphenyl substituents impart greater oxidative stability based on anodic shifts in oxidation potential.     

Effect of allylic strain on the conformations of diphenyl-substituted tetrahydro-1,3-oxazin-2-ones and hexahydropyrimidin-2-ones

The conformations of the cis and trans isomers of 4,6-diphenyl-, 4,5-diphenyl- and 5,6-diphenyltetrahydro-1,3-oxazin-2-one and 4,5-diphenylhexahydropyrimidin-2-one, and of some of their N-substituted derivatives, have been studied by ¹H NMR. Conformers with 4a, 6e-, 4a, 5e- and 5a, 6e-phenyl groups are preferred in the respective isomers of the N-H oxazinones, confirming a half-chair conformation of the ring. Allylic strain caused by N-substituents shifts strongly the a,e?e, a equilibria in trans-4,6-diphenyl- and cis-4,5-diphenyl-oxazinones, but only moderately the e,e?a,a equilibria in the compounds with trans-vicinal phenyl groups. In the latter, the diaxial conformation is preferred only in the case of bulky N-substituents. The diaxial conformation is more favoured in the trans-4,5-diphenylpyrimidones.     

A convenient synthesis of substituted piperazines via aminomercuration-demercuration of diallylamines

cis and trans-2,6-Bis[bromomercuriomethyl]piperazines II which bear equal or different substituents at each nitrogen are obtained in the reaction of N-substituted diallylamines with mercury(II) acetate and aryl-amines followed by a double decomposition process with potassium bromide. Their reduction with sodium borohydride lead to the corresponding 1,4-disubstituted cis- and trans-2,6-dimethylpiperazines III. Steric factors account for the remarkable stereoselectivity observed in the preparation of compounds IIIi-IIIn in which a 3:1 cis to trans isomer ratio is found.     

A 1H NMR study of tautomerism in some 1-arylamino-3-aryliminopropenes

¹H n.m.r. spectra at ambient temperatures reveal that an equilibrium exists between the ‘all-trans’ and ‘all-cis’ isomers of some of the 1-arylamino-3-aryliminopropenes. The ‘all-cis’ isomer predominates in nonpolar solvents, whereas the ‘all-trans’ isomer is favoured in hydrogen bonding solvents. From a consideration of the magnitudes of the ³J coupling constants, it is reported that the ‘cis-trans’ isomer is the most stable form of the 4-nitrophenyl derivative in dimethyl sulphoxide.     

Solid-state structures of cis- and trans-1-cyclohexyl-2-phenyl-3-(p-toluyl)aziridines

Solid-state structures have been determined for cis- and trans-1-cyclohexyl-2-phenyl-3-(p-toluyl)aziridines using single-crystal X-ray diffraction techniques. The cis isomer crystallizes in the centrosymmetric monoclinic space group P2₁/c (No. 14), with a = 18.669(3)Å, b = 5.709(1)Å, c = 17.412(2)Å, β = 96.29(1)° and Z = 4; the trans isomer crystallizes in the noncentrosymmetric orthorhombic space group Pna2₁ (No. 33), with a = 17.089(2)Å, b = 18.729(3)Å, c = 5.749(1)Å and Z = 4. Full-matrix least-squares refinement of the structural parameters led to the following final agreement factors: R₁ (unweighted, based on F) = 0.040 and R₂ (weighted, based on F) = 0.054 for the 2592 independent reflections of the cis isomer having 2θ_MoK¯α <55° and I>3σ₁, and R₁ = 0.033 and R₂ = 0.031 for the 1504 independent reflections of the trans isomer having 2θ_MoK¯α <55° and I>3σ₁. The statistically significant differences that exist between the two isomers for two bond lengths and ten bond angles (p < 0.05) appear to be the direct result of the p-toluyl group orientation with respect to the cyclohexyl and phenyl substituents. In the cis isomer it is anti with respect to the N-cyclohexyl group and cis with respect to the phenyl group, whereas in the trans isomer it is syn with respect to the N-cyclohexyl and trans with respect to the phenyl group. Three-ring to carbonyl hyperconjugation is correlated with stereoelectronic interactions in the trans isomer. Bonding, determined by X-ray and nmr studies, is discussed for the three-membered aziridine ring proper; while bonding, determined by X-ray studies, is discussed for substituents of the aziridine ring. These aziridinyl ketone compounds are of importance as potential mammalian DNA alkylating anti-tumor agents in solid-state solid-state systems. To date only a trans isomer has demonstrated this biological activity in tumor-bearing rats.     

Synthesis,characterization, and solution properties of 5-ethyl-substituted poly(L-prolines). Synthesis of cis- and trans-5-ethyl-L-proline and optically active poly(cis-5-ethyl-D-proline)

This article reports a continuation of our work on the substituent effects on the preferred helical conformations and the mutarotation of substituted poly(L -prolines). The size of the substituent has been increased from a methyl group to an ethyl group in the 5 position. The purpose is twofold: (i) according to our theoretical conformational energy calculations, an ethyl group in the 5 position can exert a greater steric effect than can a methyl group; and (ii) the rotation-isomerization of the ethyl group introduces a new intriguing fact to the problem. The cis isomer of 5-ethylproline was synthesized by catalytic hydrogenation of Δ′-2-ethylpyrroline-5-carboxylic acid, whereas for trans-5-ethylproline, a chemical separation method involving p-toluenesulfonyl chloride was used. The resolution of cis-5-ethylproline and the assignment of absolute configurations have been carried out by fractional crystallization and circular dichroism spectroscopic techniques, respectively. Poly(cis-5-ethyl-D -proline) was obtained from its corresponding N-carboxyanhydrides via a ring opening polymerization.     

Etude Stéréochimique par RMN 1H de Tetrahydropyrannes β-Chlorés α-Substitués

The ¹H NMR study of 2-alkyl-3-chlorotetrahydropyrans, obtained by reaction of Grignard reagents with a mixture of cis/trans-2,3-dichlorotetrahydropyrans, shows cis/trans configuration of two isomers in which the alkyl substituents are exclusively in the equatorial position. 3-Chloro-2-phenyltetrahydropyran exists in trans (eq-eq) configuration only. The ¹H NMR study of cis/trans 2-alkoxy (or aryloxy)-3-chlorotetrahydropyrans, obtained by reaction of alcohols or phenol with 2,3-dichlorotetrahydropyrans, shows the axial position of the alkoxy (or aryloxy) substituent.     

Co‐crystallized cis and trans isomers of dichloro­bis(2‐picolylamine)iron(II)

The octa­hedral cis and trans isomers of dichloro­bis(2‐picolyl­amine)iron(II), [FeCl₂(C₆H₈N₂)₂], co‐crystallize in a 1:1 ratio. The cis isomer lies on a twofold axis, whereas the trans isomer lies on an inversion centre. The structure is fully ordered, with both Fe atoms in a pure high‐spin state. The Fe, Cl and N(H₂) atoms of both isomers lie in the same plane, allowing all Cl and amine H atoms to be engaged in extensive two‐dimensional hydrogen bonding. The hydrogen‐bonded layers are inter­connected through π–π inter­actions between the pyridine rings. Searches in the Cambridge Structural Database uncover very few examples of such isomer co‐existence.     

Chromatographic determination of position and configuration isomers of methyl oleate hydroxides from corresponding hydroperoxides

Summary Studies of lipid oxidation usually employ such model systems as purified fatty-acid methyl ester. While methyl oleate hydroperoxides (MOPHs) can only be readily separated from the matrix by HPLC, because of their heat-susceptibility and relative instability, these same techniques are unable to separatecis MOHP fromtrans isomers. The present study reports an enhanced, rapid separation method forcis andtrans isomers of methyl oleate hydroxides, as well as HPLC determination of positional isomers per fraction of configuration isomer and isomer identification by gas chromatography-mass spectrometry.     

A Conformational Study of Cyclohexane‐1,3,5‐tricarbonitrile Derivatives

Cyclohexane‐1,3,5‐tricarbonitrile reached equilibrium having 1,3‐cis‐1,5‐cis and 1,3‐cis‐1,5‐trans isomers in a ratio of 3:7. The cis, cis‐isomer preferred the conformation with three equatorial cyano groups, where as the cis, trans‐isomer displayed two cyano groups on equatorial positions and another cyano group on axial position. Condensation of cis, cis‐cyclohexane‐1,3,5‐tricarbonitrile with L‐(S)‐valinol by the catalysis of ZnCl₂ in refluxing 1,2‐dichlorobenzene afforded two isomeric cyclohexane‐1,3,5‐trioxazolines in favor of the 1,3‐cis‐1,5‐trans isomer. Metalation of cis, cis‐cyclohexane‐1,3,5‐tricarbonitrile, followed by alkylations with dimethyl sulfate, benzyl bromide or allyl bromide, gave the cor responding trialkylation products with predominance of 1,3‐cis‐1,5‐trans isomers. The cis, trans‐isomer showed two cyano groups on axial positions and another cyano group on equatorial position, where as the cis, cis‐isomer exhibited three axial cyano groups. Treatment of trimethyl cis, cis‐cyclohexane‐1,3,5‐tricarboxylate with lithium diisopropylamide and dimethyl sulfate afforded mainly the trimethyl ester of Kemp's triacid, which showed three axial carboxylate groups. Two competitive factors, i.e. the steric effect of in coming electrophiles and the dipole‐dipole inter actions of the cyano or carboxylate groups, might inter play to give different stereoselectivities in these reaction systems.     

Facile addition of methyl lithium to a 4-vinylpyridine system

The rapid addition of methyl lithium to the 4-vinylpyridine system present in 4-{2,6-dihydroxy-4-(3-methyl-2-octyl)phenyl}-2-methyl-4-(4-pyridyl)but-3-en-2-ol ( 2 ) is reported. The α and β-4-{2,6-dihydroxy-4-(3-methyl-2-octyl)phenyl}-2,3-dimethyl-4-(4-pyridyl)butan-2-ols 4 and 5 formed, are cyclised by heating with 5N hydrochloric acid to trans and cis-3,4-dihydro-5-hydroxy-7-(3-methyl-2-octyl)-4-(4-pyridyl)-2,2,3-trimethyl-2H-1-benzopyran 6 and 7 respectively.     

The keto→enol photoisomerization of N‐salicilydenemethylfurylamine: Nonadiabatic ab initio dynamics simulation

The photoinduced isomerization of cis‐keto and trans‐keto isomers in N‐salicilydenemethylfurylamine has been studied using the surface‐hopping approach at the CASSCF level of theory. After the cis‐keto or trans‐keto isomer is excited to S₁ state, the molecule initially moves to a excited‐state local minimum. The torsional motion around relative bonds in the chain drives the molecule to approach a keto‐form conical intersection and then nonadiabatic transition occurs. According to our full‐dimensional dynamics simulations, the trans‐keto and enol photoproducts are responsible for the photochromic effect of cis‐keto isomer excited to S₁ state, while no enol isomer was obtained in the photoisomerization of trans keto on excitation. The cis keto to enol and cis keto to trans keto isomerizations are reversible photochemical reactions. It is confirmed that this aromatic Schiff base is a potential molecular switch. Furthermore, the torsion of C N bond occurs in the radiationless decay of trans‐keto isomer, while it is completely suppressed by an intramolecular hydrogen bonding interaction in the dynamics of cis‐keto form. Moreover, the excited‐state lifetime of cis keto is longer than that of trans‐keto form due to the O···H N hydrogen bond.