An electron impact study of HCN elimination from indolizine by use of 13C labelling

The study of the loss of HCN from the molecular ions of [1-¹³C]-, [2-¹³C]- and [3-¹³C]-indolizine shows that, if the C-3 atom is eliminated predominantly, as may be expected, the C-2 atom, and (a) carbon atom(s) of the hexagonal ring are also involved. The losses of ¹³CCH₃^. and C₂H₃^. from the [M? H¹²CN]^+˙ ions of the three compounds point to the interference of distinct mechanisms of HCN elimination, leading to different structures for the [C₇H₆]^+˙ ions.     

Protonation Sites in Gaseous Pyrrole and Imidazole: A Neutralization–Reionization and ab initio Study

Mild gas-phase acids C₄H₉⁺ and NH₄⁺ protonate pyrrole at C-2 and C-3 but not at the nitrogen atom, as determined by deuterium labeling and neutralization–reionization mass spectrometry. Proton affinities in pyrrole are calculated by MP2/6–311G(2d, p) as 866, 845 and 786 kJ mol^-1 for protonation at C-2, C-3 and N, respectively. Vertical neutralization of protonated pyrrole generates bound radicals that in part dissociate by loss of hydrogen atoms. Unimolecular loss of hydrogen atom from C-2-and C-3-protonated pyrrole cations is preceded by proton migration in the ring. Protonation of gaseous imidazole is predicted to occur exclusively at the N-3 imine nitrogen to yield a stable aromatic cation. Proton affinities in imidazole are calculated as 941, 804, 791, 791 and 724 for the N-3, C-4, C-2, C-5 and N-1 positions, respectively. Radicals derived from protonated imidazole are only weakly bound. Vertical neutralization of N-3-protonated imidazole is accompanied by large Franck–Condon effects which deposit on average 183 kJ mol^-1 vibrational energy in the radicals formed. The radicals dissociate unimolecularly by loss of hydrogen atom, which involves both direct N-H bond cleavage and isomerization to the more stable C-2 H-isomer. Potential energy barriers to isomerizations and dissociations in protonated pyrrole and imidazole isomers and their radicals were investigated by ab initio calculations.     

Mass spectrometric decomposition processes of 2-methyl-1-hexene

Unimolecular decompositions of 2-methyl-1-hexene and several labelled analogues were studied following 70 eV electron impact (normal and metastable spectra) and field ionization (field ionization kinetic measurements). Molecules labelled with ¹³C in the 1-position and the methyl position were found to behave essentially identically. This is attributed to rapid transfer of a hydrogen atom mainly from C-5 to C-1 (γ-hydrogen shift). Loss of ethene, propene or propenyl do not involve loss of the methyl carbon or C-1. All three reactions are better than 90% specific in this respect under all conditions studied. At shorter times, C₃H₆ loss is the dominant reaction, while at longer times C₂H₆ loss accounts for >90% of the ion current. It is proposed that at least two distinct pathways for C₂H₄ loss operate in linear 1-alkenes, one of which (loss of carbons 1 and 2) is blocked by a 2-methyl substituent. The [C₆H₁₁]^+˙ and [C₅H₁₀]^+˙ ions formed from ¹³C labelled 2-methyl-1-hexenes fragment in an essentially statistical fashion.     

Reversed substituent effect for the 13C NMR chemical shifts in the unsubstituted ring of para-substituted benzophenones

The ¹³C NMR chemical shifts of the C-1′ carbon atom of fifteen para-substituted benzophenones correlate with the ? and ? substituent constants in a reversed manner.     

Gas phase glycosidic cleavage of oxynions from alkyl glycosides

The oxyanion [M? H]^? from several methylglycosides were generated by fast atom bombardment and their decomposition was studied by mass-analysed ion kinetic energy spectrometry. The main decomposition pathway is the loss of methanol. The hydroxylic hydrogen arises by proton transfer from the hydroxyl groups of the sugar. In the gluco-series, no anomeric effect is found. The absence of either the hydroxyl groups at C-2 or C-6 does not inhibit the glycosidic cleavage. However, the blocking of both the hydroxyl groups at C-4 and C-6, by a benzylidene group or two methyl groups, inhibit completely the glycosidic cleavage. From these results, it is proposed that the glycosidic cleavage occurs after opening the sugar ring by a vicinal attack of an oxyanion at C-6 or C-4 to the C-5 carbon atom. Then, the ionized hemi-acetals fragment into a methanolate anion and a 5,6- or 4,5-anhydrosugar which exchange another proton before their separation into charged and neutral species.     

Elimination of water from the molecular ion of cycloheptanol

Under electron impact cycloheptanol decomposes by four fragmentation paths: (1) α-cleavage with subsequent losses of C¹-C⁵ fragments, (2) elimination of water, (3) loss of the hydrogen atom from C-1 and (4) loss of the hydroxyl group. The mechanism of water elimination was investigated by means of deuterium labelling. 1,4-Elimination of water predominates in cycloheptanol, with the stereospecific cis-1,3-elimination also being operative. The loss of water is preceded by extensive exchange of the hydroxyl hydrogen with those of the ring. This is attributed to a very facile transannular interaction of the hydroxyl group with the C-3 to C-6 positions that are made accessible due to conformational properties of the 7-membered ring. A kinetic model is proposed, describing migrations of the ring hydrogen atoms.     

13C-{1H} NMR spectra of episulphide copolymers

Ethylene sulphide-isobutylene sulphide and propylene sulphide-isobutylene sulphide copolymers have been prepared using anionic catalysts and investigated by ¹³C-{¹H} NMR spectroscopy. The carbon-13 NMR spectra are assigned in terms of diad and triad sequences. There is discussion of the effects of mono- or dimethyl substitution in the α, β, γ or δ positions on the chemical shift of the main chain carbon atoms. It has also been shown that for isobutylene sulphide, as for propylene sulphide, under the influence of an anionic catalyst, there is a normal ring opening only at the primary carbon atom.     

Communication: Double Bond Migration of a 1,5-Anhydro-3-C-p-Tolylsulfonyl-D-Hex-2-Enitol Derivative and the Corresponding 5a-Carba-Dl-Sulfonyl Sugar

Although the rate of proton abstraction (kinetic acidity) frequently plays an essential role in determination of reaction pathways and is of theoretical interest,¹ it is still controversial whether an oxygen atom activates or deactivates the abstraction of an α-hydrogen atom of an ether. For example, it is well known that oxidative elimination of a seleno group gives an allyl ether as the major product, indicating the oxygen atom deactivates the kinetic acidity.² Abstraction of the equatorial hydrogen atom at C-2 of 6-methyl-1,3-oxathiane-3,3-dioxide 1 is slower than that at C-4.³ On the other hand, the bridgehead hydrogen atom (H_b) adjacent to the oxygen atom of piperazinedione (2) is abstracted more readily than that of the alternative one (H_a).⁴     

A New Synthesis of 3-Chloro Butenolides via Rearrangement of 3,3-Dichloro Beta Lactones

4,4-Dialkyl 3,3-dichloro oxetan-2-ones rearrange under Lewis acid catalysis, accompanied by loss of HC1, to afford 4,5-dialkyl 3-chloro butenolides.

We have recently been investigating the reactions of beta lactones under the influence of Lewis acid catalysis.¹ When the lactone ring oxygen is bonded to a secondary carbon atom, a rearrangement occurs in which the beta lactone 1 expands to a butyrolactone 4 with the concommitant migration of a hydrogen or carbon atom into the lactone ring.² If the oxygen is bonded to a tertiary carbon, an ionization/elimination sequence ensues, producing a β, γ-unsaturated carboxylic acid     

Through-space hydrogen-fluorine and carbon-fluorine spin-spin coupling in 5-fluoro-3,3-dimetryl-1,2,3,4-tetrahydrophenanthrene

The ¹R and ¹³C NMR spectra of the title compound ($\text{1}$) reveal through-space couplings between the fluorine and the C-4 methylene group ¹H and ¹³C), as well as coupling between the fluorine and the C-3 methine carbon and the C-2 methylene carbon.     

Fluoro(phenylsufinyl)methyllithium. Note on the 13C-NMR Spectrum of a Flourocarbenoid

The title compound is synthesized with ¹³C- and ⁶Li-labelling on the fluorinated carbon atom. H/Li-Exchange in fluoromethyl phenyl sulfoxide (1→2) causes a Δδ(¹³C) = + 11.4 ppm, a ΔJ(¹³C,¹H) ≈ 0 Hz, and a ΔJ(¹⁹F,¹³C) = + 80.4 Hz. Tentative conclusions bout the struture of the title compound are drawn from these changes.     

Photochemische Reaktionen 42. Mitteilung [1]. Photoisomerisierung von α, β-Epoxyketonen II Der sterische Verlauf der Umlagerung von 3-Oxo-4, 5-oxido-Steroiden

The photorearrangement previously described [3] of saturated and Δ¹-unsaturated 3-oxo-4,5-epoxy-10β-steroids to 3,5-dioxo-10(5 →4)-abeo compounds proceeds most likely via a radical 1,2-alkyl shift (Chart 1). The similar rearrangements of the related 10α-epoxyketone 10 and the 4-methyl-epoxyketones 13 , 15 , 16 , 20 and 21 to the corresponding 3,5-diketones occurred without epimerization at the migrating carbon atom (C-10) and the site of substitution (C-4) (Chart 3). The stereochemical control of the rearrangement is in agreement with the earlier proposed mechanism of a concerted alkyl radical shift in these alicyclic systems.     

Long range carbon—carbon coupling in olefins. 4-13C-4-propyl-3-heptene and 1-13C-1-methylcyclohexene

All ¹³C—¹³C splittings involving the C-4 carbon of 4-propyl-3-heptene and the C-1 carbon of 1-methylcyclohexene were determined from the appropriately labeled (> 90%-¹³C) derivatives. The observed trends in coupling constants continue to offer additional means of carbon chemical shift assignments and to provide mechanistic information regarding the nature of long range carbon—carbon coupling.     

An electron impact study of HCN elimination from indole by use of 13C labelling

The study of the loss of HCN from the molecular ions of [2-¹³C]indole and [3-¹³C]indole shows that, to a good approximation, only the two carbon atoms of the pentagonal ring are involved in this fragmentation process, contrary to the behaviour of the H atoms; the C-2 atom is eliminated predominantly, chiefly in the ion source (85–90%) and a little less in the metastable energy range (75–80%). The losses of ¹³CCH₃^˙ and C₂H₃^˙ from the [M? H¹²CN]^+˙ ions of the two compounds suggest the occurrence of different structures, providing evidence for several mechanisms of HCN elimination.     

The mass spectrum of 2-cyclohexenol: Loss of CH3⋅ and H⋅ from the molecular ion

Methyl radical and hydrogen atom losses from the molecular ion of 2-cyclohexenol and deuterium labelled analogues have been studied. For fragmentations occurring in the first field free region, H? loss is a random process, whereas CH₃? loss is highly specific involving the C-1 hydrogen atom and the C-5 methylene group. A mechanism consistent with these results is proposed.     

The Acetoxyfulvene Synthesis of Prostaglandins: A New,Versatile Synthesis of dl-PGF1δ, dl-PGF2α, dl-PGE1 and dl-PGE2 from a Common Intermediate

Abstract

In the Corey¹ synthesis of prostaglandins and in our recently published modifications^2,3 a synthon containing carbon atoms 14 to 20^θ is first added to a bicyclic intermediate (C-6 to C-13) and completion of the prostaglandin skeleton by addition of a second synthon containing carbon atoms 1 to 5 forms a subsequent step. In a modification^4,5 of the Corey synthesis¹ PGF_1α and PGE₁ were made by reversing the order in which these two synthons were added to the cyclopentane ring (C-6 to C-13). The major limitation of this modified route^4,5 is that it is restricted to the preparation of prostaglandins of the 1-series,^? because hydrogenolysis of the benzyl group of the intermediate ester (1) reduces the C-5, C-6 double bond to form the saturated alcohol (2), which cannot be converted into prostaglandins of the 2-series^?.     

An unusual pathway for the elimination of hydrogen cyanide from benzonitrile upon electron impact as revealed by 13C and 15N labelling

It is shown by ¹⁵N and specific ¹³C labelling that ～50% of the molecules of hydrogen cyanide, eliminated within ～10^?6 s upon electron impact of benzonitrile, contains the original cyano carbon atom, whereas the remaining percentage contains one of the phenyl ring carbon atoms at random. This is even more dramatic for the molecular ions of benzonitrile which decompose in the first and second field-free regions of the VG Micromass ZAB-2F high-field mass spectrometer used. Then only 5–7% of the eliminated molecules of hydrogen cyanide contains the original cyano carbon atom. A cycloaddition-cycloreversion process in the molecular ions, leading to ionized 1-cyano-1,3-hexadien-5-yne as an intermediate in the hydrogen cyanide loss, is proposed to explain this.     

13C NMR spectra of biologically active compounds XI. Diastereomeric effects in C-glycosides

The¹³C NMR spectra of epimeric C-D-glucopyranosides with alkyl, aryl, and alkynyl substituents have been studied. The diastereomeric effects of the chemical shifts have been determined and assignments have been made to the 1-and 1- stereochemical series on the basis of HH COSY and CH HET CORR two-dimensional NMR spectra. The diastereomeric effects observed for the C-3 and C-5 carbon atoms are proposed as characteristic parameters for establishing the stereochemistry at the C-1 carbon atom.Institute of Chemistry, Bashir Scientific Center, Urals Branch, Academy of Sciences of the USSR, Ufa. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 368–373, May–June, 1991.     

1H- und 13C-NMR-untersuchungen an dibromoxabicyclo[n.2.1]alkanen

The ¹H n.m.r. spectra of some dibromooxabicyclo[n.2.1]alkanes are discussed for the determination of the configuration and conformation. The ¹³C n.m.r. spectra confirm the observed stereochemistry. With increasing ring size the ¹³C n.m.r. chemical shifts of the hetero atom substituted C-atoms C-1 and C-2 move steadily to lower fields.     

A novel fragmentation of trimethylsilyl ethers of 3β-hydroxy-Δ5-steroids

An ion formed by loss of 56 mass units from the molecular ion is often seen in mass spectra of trimethylsilyl ethers of C₁₉ and C₂₁ steroids having a 3β-hydroxy-Δ⁵ structure and an oxo group at C-17 or C-20. The nature of this fragment was investigated by the use of perdeuteriotrimethylsilyl ether derivatives and of [4-¹⁴C], [3-¹⁸O], [4,4-²H₂] and [2,2,4,4-²H] labelled derivatives of 3β-hydroxy-5-androsten-17-one and 3β-hydroxy-5-pregnen-20-one. Evidence is presented to show that the neutral fragment of mass 56 is composed of carbon atoms 1, 2 and 3, the oxygen at C-3 and four hydrogen atoms. During the fragmentation process, the trimethylsilyl group and one of the hydrogens at C-2 are transferred to the fragment that carries the charge.