Stereospecific elimination of acetic acid in 3- and 4-arylcyclohexyl acetates under electron impact

The elimination of CH₃COOH from the molecular ions of trans-3- and 4-arylcyclohexyl acetates takes place to a greater extent than in the cis isomers. Deuterium labelling shows that the elimination involves mainly the benzylic hydrogen in the trans-acetates, but not in the cis isomers. This behaviour is similar qualitatively to that of the corresponding alcohols and methyl ethers, but entirely different from that of t-butylcyclohexyl acetates, which do not exhibit any stereospecificity. Substituent effects on the elimination for both cis and trans isomers are discussed.     

Field ionization mass spectrometry of higher n-alkenes

In a strong electric field the molecular ions of n-alkenes ≤C-12 decompose via cleavage of the C? C bond β to the double bond to form the characteristic alkenyl ions that may be used for the identification of positional isomers. For 3-alkenes (>C-10), 4-, 5- and 6-alkenes the formation of the ions with m/e 54 via double β-cleavage is typical. The field ionization mass spectra of the cis and trans isomers are indistinguishable.     

Der massenspektrometrische Zerfall isomerer Diacetamido-cyclo-hexane,ihrer N-Phenäthyl-Derivate und Bis (acetamidomethyl)cyclohexane. 32. Mitteilung über massenspektrometrische Untersuchungen,,

The Mass Spectral Decomposition of Isomeric Diacetamido-cyclohexanes, their N-Phenethyl-Derivatives and Bis(acetamidomethyl)cyclohexanes In the mass spectra of the six isomeric diacetamidocyclohexanes 2--4 (cis and trans each, Scheme 2) as well as of the six isomeric bis(acetamidomethyl)cyclohexanes 6--8 (cis and trans each, Scheme 5) are clear differences between the constitutional isomers, whereas cis/trans isomers show very similar spectra. The lack of stereospecific fragmentations is explained by loss of configurational integrity of the molecular ion before fragmentation. However, the mass spectral fragmentation of epimeric diamidocyclohexanes becomes very stereospecific by the introduction of a phenethyl group on one of the nitrogen atoms: this group avoids epimerization of the molecular ion prior to fragmentation. In the N-phenethyl derivatives 10, 11, 13 and 14 (Scheme 8) the typical fragmentations of the cis-isomer after loss of ·C₇H₇ from the molecular ion are the elimination of CH₂CO by formation of cyclic ions, and the loss of p-toluenesulfonic acid or benzoic acid, respectively, with subsequent elimination of CH₃CN (Scheme 9). In the trans-isomer the typical fragmentations are the loss of the side chain bearing a tertiary nitrogen atom, and the elimination of the tosyl or benzoyl radical, respectively, with subsequent loss of CH₃CONH₂ (Scheme 10).     

Inverse stereochemical effects in the mass spectra of carboalkoxycyclopropylsilanes

The electron impact induced decomposition of the cis and trans isomeric pairs of five 1-trialkylsilyl-2-alkoxy-carbonylcyclopropanes were investigated. Compared with carbon analogues, the silicon atom inhibited ring opening and dramatically altered the fragmentation. The main primary decomposition routes were alkyl loss from silicon, and trialkylsilyl ion formation as well as trialkylalkoxysilane elimination. An unexpected inverse stereochemical effect was found for this latter reaction which was far less significant for the cis isomers, where the interacting SiQ₃ and OR groups are close to each other, than for the trans isomers. This is explained by assuming a hidden silicon-oxygen bond formation, which is possible only in the cis isomers for geometric reasons. This interaction favours alkyl elimination over other fragmentations, and has a marked effect on the further decomposition of the [M—alkyl]⁺ ions, where observed differences in abundance as great as three orders of magnitude are accounted for.     

Synthesis,separation, and resolution of cis- and trans-3-ethylproline

The synthesis, separation, and optical resolution of cis- and trans-3-ethylproline are described. Two different approaches were employed: (1) The Michael addition reaction of 2-pentenal with diethyl-N-carbobenzyloxyaminomalonate gave the intermediate 3-ethyl-5-hydroxy-N-benzyloxypyrrolidine. Hydrogenolysis of this intermediate followed by acid hydrolysis gave a mixture of cis- and trans-3-ethylproline. Separation of the isomers was accomplished by selective saponification of N-(p-toluenesulfonyl)-cis- and trans-3-ethylproline methyl esters using 0.25N methanolic sodium hydroxide. (2) The Michael condensation of diethyl acetamidomalonate with 2-pentenoic acid ethyl ether produced the intermediate 5,5-bis(ethoxycarbonyl)-4-ethylpyrrolidine. Partial saponification followed by decarboxylation afforded a mixture of cis- and trans-isomers of ethyl-3-ethylpyroglutamate. The diastereoisomers were separated using low temperature fractional crystallization. Reduction of these isomers and tosylation in situ afforded the corresponding N-(p-toluenesulfonyl)-cis- and trans-3-ethylprolinols. Chromic acid oxidation gave N-(p-toluenesulfonyl)-cis- and trans-3-ethylproline. Reaction of these tosylates with 30% hydrogen bromide in acetic acid gave cis- and trans-3-ethylproline. Both optically active isomers of D(+)-and L(-)-trans-3-ethylproline were successfully resolved using (+)-dibenzoyl-D -tartaric acid and (-)-dibenzoyl-L -tartaric acid as resolving agents. The absolute configurations of the optically active isomers were determined by circular dichroism spectroscopy.     

Stereochemical studies. Part 67. Saturated heterocycles. Part 52. The mass spectra of some condensed skeleton 1,3-oxazin-4-one derivatives

The mass spectrometric behaviour of twenty saturated heterocyclic compounds with a 1,3-oxazin-4-one moiety fused with cis or trans anellation to a cycloalkane ring (C₅-C₈) was studied. The roles of the C-2 and N-3 substituent(s) were found to be characteristic, while the size of the cycloalkane ring seemed to be unimportant. Some fragmentation processes involving breakdown of the oxazinone ring of the cis or trans isomers displayed significant stereoselectivity. A striking new decomposition process involving significant chlorine elimination from the molecular ion of some 2-p-chlorophenyl derivatives was observed and was studied in some detail.     

The mass spectra of carboxylic acids·IV: Carboxyl-carboxyl interaction in some cycloalkane 1,2 dicarboxylic acids and its relationship with molecular geometry

The interaction of the carboxyl groups in the fragmentation of the molecular ions of trans cyclohexane-1,2-dicarboxylic acid and cis cyclobutane-1,2-dicarboxylic acid serves to distinguish these acids from their cis and trans isomers respectively. This ‘ortho effect’ can be related to the geometrical arrangement of the carboxyl groups in the ground-state molecules.     

Pathways of Liquid-Phase Oxidation of Cyclohexanol

The kinetics of product accumulation in uncatalyzed oxidation of cyclohexanol at 403 K was studied. Along with the compounds originating from oxidation of cyclohexanol at position 1 (cyclohexanone, hydrogen peroxide, 1-hydroxycyclohexyl hydroperoxide), products formed by oxidation of C-H bonds at positions 2-4 were detected: 2-, 3-, and 4-hydroxycyclohexyl hydroperoxides (cis and trans isomers), 1,2-, 1,3-, and 1,4-dihydroxycyclohexanes (cis and trans isomers), 2- and 4-hydroxycyclohexanones, and 2-cyclohexenone.     

Stereochemistry of alkyl 3-substituted 4-halotetrahydro-2-oxo-3-furancarboxylates: 2—1H and 13C NMR spectra

The ¹H NMR parameters of methyl 3-substituted cis-4-halotetrahydro-2-oxo-3-furancarboxylates are reported, with assignments of the ring protons based on solvent-induced changes in the vicinal trans coupling constants, ³J(H-4, H-5). Preferred conformations, ce with a pseudo-equatorial halogen for the cis isomers and ta with a pseudo-axial halogen for the trans isomers, have been suggested on comparison of the magnitudes of J(trans) and J(gem) in both series. The ³J(¹³CH₃, H-4) values measured for methyl cis-4-bromotetrahydro-3-methyl-3-furancarboxylate, methyl trans-4-bromotetrahydro-3-methyl-3-furancarboxylate and trans-3,4-dibromodihydro-3-methyl-2(3H)-furanone have confirmed the stereochemical assignments.     

Synthesis and 13C NMR spectra of cis- and trans-{2-(haloaryl)-2-[(1H-imidazol)-1-yl]methyl]}-1,3-dioxolane-4-methanols

Syntheses and ¹³C nmr spectra of a number of cis and trans 2-(haloaryl)-2-[(1H-imidazol-1-yl)rnethyl]-4-(hydroxymethyl)-1,3-dioxolanes are described. The haloaryl groups are 2,4-dichloro, 2,4-difluoro-, 4-chloro-and 4-bromophenyl. In these series, some of the cis compounds become available through crystalline bromo benzoates 5 . Separations of some trans isomers are achieved through fractional crystallizations of imidazolyl benzoate nitrates 6 . Stereochemical assignments are based primarily on one major ¹³C chemical shift difference, namely that of C-4 of the 1,3-dioxolane ring, the chemical shift of the trans isomers being 1.0-2.5 ppm downfield from that of the cis isomers.     

Stereospezifische Fragmentierungen in den Massenspektren von Cyclohexandiaminen und Bis(aminomethyl)cyclohexanen. 33. Mitteilung über massenspektrometrische Untersuchungen,,

Stereospecific Fragmentations in the Mass Spectra of Cyclohexanediamines and Bis(aminomethyl)cyclohexanes The mass spectral behaviour, especially loss of NH₃, of the six isomeric cyclohexanediamines 1--3 (cis and trans each, Scheme 1) as well as of the six isomeric bis(aminomethyl)cyclohexanes 4--6 (cis and trans each, Scheme 6) has been investigated. The cis- and trans-compounds of the 1,2-isomers 1 and 4 show very similar spectra, because of the ease of ring cleavage at C(1)–-C(2) and the similar geometrical relations in all ring conformations. The cis- and trans-compounds of both the 1,3- and 1,4-isomers 2, 3, 5 and 6 show striking differences in their mass spectra due to stereospecific elimination of NH₃ from the molecular ion.     

Molecular and crystal structure of <Emphasis Type="Italic">trans</Emphasis> and <Emphasis Type="Italic">cis</Emphasis> isomers of 3-cyano-4-[2-(4-methoxyphenyl)vinyl]-6,6-dimethyl-5,6-dihydro-2H-pyran

Monocrystals of the trans and cis isomers of 3-cyano-4-[2-(4-methoxyphenyl)vinyl]-6,6-dimethyl-5,6-dihydro-2H-pyran have been obtained and an X-ray structural analysis of them has been carried out. Both compounds have a molecular structure corresponding to symmetry group C₁. The heterocyclic ring is in a distorted envelope conformation. The crystals of the trans isomer are rhombic and have a fir-tree type of packing. The crystal packing of the cis isomer is formed by pairs of molecules, each of which consists of only one enantiomer. *Dedicated to deeply respected Professor Hank van der Plas in connection with his jubilee. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 535-540, April, 2009.     

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.     

Insight into the Thermal Ring‐Opening Polymerization of Phospha[1]ferrocenophanes

A mixture of cis/trans isomers of phospha[1]ferrocenophanes equipped with one iPr group at the α position to the bridging PhP moiety was prepared. Both isomers (cis‐ 4 and trans‐ 4 ) were obtained as racemates and could be separated so that their thermal properties were investigated individually. The molecular structure of cis‐ 4 was determined by single‐crystal X‐ray analysis showing a tilt angle α=26.35(8)°. Interconversion between both isomers occurred in the melt at elevated temperatures and revealed that the trans isomer is thermodynamically more stable. Structural and thermodynamic data was complemented by DFT calculations (B3PW91/6‐311+G(d,p) and B3PW91‐D3(BJ)/6‐311+G(d,p)). Performance of thermal ring‐opening polymerization (ROP) of trans‐ 4 at 230 °C gave polymers and cyclic oligomers. Gel permeation chromatography (GPC) of the sulfurized polymer resulted in a molecular weight of 62.5 kDa (M_w) and a polydispersity index of 1.39 (PDI). Mass spectrometric analysis of the oligomers showed the presence of cyclic species from dimers to heptamers. After sulfurization, preparative thin layer chromatography led to the separation of three isomeric dimers. Structural characterization of these dimers by single‐crystal X‐ray analysis led to the conclusion that the Fe?Cp bond breaks during the thermal ROP process. A mechanism similar to the known mechanism of the photolytic ROP of ferrocenophanes is proposed.     

Stereochemistries of electron impact induced hydrogen abstraction reactions proceeding through 6-membered transition states

The stereochemistries of nine electron-impact induced eliminations proceeding from derivatives of the cis-4-t-butyl system have been determined. The predominant cis elimination observed in every case is consistent with the substantial integrity of the cyclohexyl ring prior to fragmentation, and with a cyclic transition state for hydrogen abstraction. The stereochemistries of electron impact induced eliminations from 11 derivatives of the trans-4-t-butylcyclohexyl system exhibit a dichotomy. The predominatn trans stereochemistry observed in six electron impact induced eliminations, and the nonstereospecific electron impact induced dehydration of trans-4′-t-butylcyclohexyl-ethanol are consistent with nonconcerted elimination from a chair-like cyclohexyl ring. Conversely, the McLafferty rearrangement of trans-4′-t-butyl-cyclohexyl-2-propanone proceeds nonstereospecifically. trans-4-t-Butylcyclohexyl acetaldehyde, 2-methyl-3-(trans-4′-t-butylcyclohexyl)-1-propane and trans-4-t-butylcyclohexyl-S-methyl xanthate exhibit predominant cis McLafferty rearrangement stereochemistry. This result may be due to fragmentation through boat-like conformers in these compounds.     

Temperature and Concentration Dependent Circular Dichroism of Mono- and Di-cis Isomers of (3S,3′S)-Astaxanthin Diacetate

The circular dichroism (CD.) spectra of all-trans-(3S, 3′S) astaxanthin diacetate and its 9-cis, 13-cis, 9,9′-di-cis, 9,13′-di-cis, and 9,13-di-cis isomers conform to the rules previously formulated for optically active carotenoids with a 4-oxo-β-end ring containing an asymmetric C-atom [1]. Thus the CD. bands of the all-trans and the di-cis isomers show the same signs whereas those of the mono-cis isomers have opposite signs. The CD. spectra of all the astaxanthin diacetate isomers invert sign upon cooling to ?180°. The CD. spectra of the 9-mono-cis and 9,9′-di-cis isomers and to a lesser extent also those of the 9, 13′-di-cis and 9, 13-di-cis isomers are concentration dependent at ?180°, with the longest wavelength band giving at the higher concentration a bisignate CD. curve under the main absorption characteristic of aggregation. This phenomenon has been observed only in isomers with a 9-cis linkage. It is suggested that steric hindrance prevents such aggregation taking place in the other isomers.     

The reactive states of ions: Dimethyl-hexadienes and tetramethylcyclobutenes

The principal decomposition routes of molecular ions of cis, cis-3,4-dimethyl-2-4-hexadiene, cis, trans-3,4-dimethyl-2,4-hexadiene, cis-1,2,3,4-tetramethylcyclobutene and trans-1,2,3,4-tetramethylcyclobutene were studied using ion kinetic energy spectroscopy. Evidence indecates that loss of radicals from [M]⁺· appears to proceed via an electronically excited state, while loss of a neutral molecule appears to involve complete equilibration of structure within the system, or may involve both ground state and excited state pathways.     

Saturated heterocycles, 248. Synthesis of 2,4-dioxo and 4-oxo-2-thioxo derivatives of octahydrocyclopenta[d]pyrimidines

Unsubstituted and 1-benzyl-substituted cis-cyclopenta[d]pyrirnidine-2,4-diones and cis-2-thioxo-cyclopenta[d]pyrimidin-4-ones 9a,b and 10a,b were prepared from the corresponding cis-2-amino-1-cyclopentanecarboxylates 3 and 5 with potassium cyanate and thiocyanate. It was found that the cis derivatives 7a-h readily underwent ring closure, resulting in 3-substituted cis-2,4-cyclopenta[d]pyrimidinediones and cis-2-thioxocyclopenta[d]pyrimidin-4-ones 11a-d and 12a-d , whereas the trans counterparts 8a-d failed to cyclize, but gave hydrolysed amino acid derivatives 13a,b and 14 . This difference in the reactivities of the cis and trans isomers is a further example of the difficulty of preparing cyclopentane trans-fused six-membered 1,3-heterocycles by ring closure.     

Isomérisation cis-trans en Série Alkyl-4-vinyl-3-pipéridine par Sigmatropie [3.3]. Synthèse des Epivinyl Isomères des Alcaloïdes du Quinquina

cis, trans Isomerization of 4-Alkyl-3-vinylpiperidines by [3.3] Sigmatropic Rearrangement. Synthesis of Epivinyl Isomers of Cinchona Alkaloids Isomerization of 4-alkyl-3-vinylpiperidines induced by formaldehyde has been studied for ethyl meroquinate and quinotoxine. The equilibration of cis- and trans-isomers occurs by involvement of a [3.3] sigmatropic rearrangement. This process permits an easy access to the trans-isomers from natural products of cis-configuration and has been used to obtain the epivinyl isomers of cinchona alkaloids.     

Effects of annulation on the mass spectral behaviour of bicyclo[4.3.0]-3-nonene derivatives: Alkoxy esters

Fragmentation of ethyl(18-alkoxybicyclo[4.3.0]-3-nonene-7-carboxylates) was found to depend on the annulation of the rings. Following ionization, the cis-annulated isomers lose alcohol and benzene, while the trans-isomers fragment by other reaction paths. The regiospecificity of alcohol elimination from both the ester and the ether groups was determined by using deuterium-labelled analogues. A mechanism is discussed which rationalizes the differences observed in the spectra of the annulation isomers.