Structure-based evaluation of the resonance interactions and effectiveness of the charge transfer in nitroamines

Structural data for five nitroamines of general formula Me₂N–G–NO₂ show effectiveness of the ground-state charge transfer to be most and least efficient in N,N-dimethylnitramine and in 4-N,N-dimethylamino-β-nitrostyrene, respectively. Electron-donor power of the amino nitrogen atom in the latter compound is less than that in 4-nitro-β-N,N-dimethylaminostyrene (these two compounds are isomers). Natural population analysis shows that the charge transfer from the amino to the nitro oxygen atoms is most effective in N,N-dimethylnitramine, Me₂N–NO₂. The nitro oxygen atoms are not the only acceptors of the negative charge lost by the amino nitrogen atom. The nitro group in two substituted nitrobenzenes studied was found to be independent on substituent (nitro group attached to the benzene ring withdraws a constant electron density regardless the substitution).     

Small-angle neutron-scattering study of bis(quaternaryammonium bromide) surfactant micelles in water. Effect of the long spacer chain on micellar structure

The microstructure of the normal micelles formed by dimeric surfactants with long spacers, [Br⁻(CH₃)₂N⁺(C _m H_{2 m +1})-(CH₂) _S  -(C _m H_{2 m +1})N⁺(CH₃)₂Br⁻, m = 10 and s = 8, 10 and 12], has been investigated by small-angle neutron scattering and compared with previously reported results for micelles of the same dimeric surfactants with shorter spacers (m = 10 and s = 2, 3, 4 and 6). It was found that for dimeric surfactants with long spacers (s = 8 and 10), both micellar growth and variation in shape occur to only a small extent, if at all, compared with dimeric surfactants with short spacers. However, for the dimeric surfactant with the longest spacer, s = 12, the extent of micellar growth and shape variation is also large. These results are due to the differences in conformation of dimeric surfactants with short spacers (s = 2–6) compared with that of the surfactants with long spacers (s = 8–12). Received: 15 June 1998 Accepted: 22 July 1998     

Syntheses and crystal structures of group 6 metal complexes containing allyl and cyanotrihydroborate groups

Compounds M(η³-C₃H₅)(CO)₂(NCCH₃)₂(NCBH₃) and [N(CH₃)₄]₂[M(η³-C₃H₅)(CO)₂(NCBH₃)₃] (M = Mo, W) were prepared and structurally characterized. In the solid state, the allyl group orients its open face to the two carbonyl groups producing an endo form in the above compounds. In solution, an exo form coexists with an endo form in compound Mo(η³-C₃H₅)(CO)₂(NCCH₃)₂(NCBH₃). The cyanotrihydroborate ligand bonds to the metal through a nitrogen atom. Both of the IR and the 11B NMR spectroscopic data suggest the negative charge of the cyanotrihydroborate ligand on the complex is almost localized on the BH₃ and this negative charge only has small effect on the metal-nitrogen interaction.     

Quantum chemical calculations of N@Cn endofullerenes (n ≤ 60)

The geometric parameters and energy characteristics of small endofullerenes N@C_n (n = 20, 24, 30, 32, 40, 50) and N@C₆₀ in the quartet ground state were calculated by the B3LYP/6-31G* method. The N atom is located at the center of the carbon cage in all molecules except N@C₃₀, where it is bound to the cage wall. Encapsulation of nitrogen atom has little effect on the fullerene cage geometry for n = 40, 50, and 60. No significant charge transfer from the N endo-atom to the cage was revealed for all the N@C_n endofullerenes studied. The calculated spin density on the nitrogen endo-atom increases as the size (n) of the carbon cage increases. The relative stabilities of C_n fullerenes and corresponding endofullerenes N@C_n are discussed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 15–20, January, 2006.     

Supramolecular interactions and structural transformations in the metal-organic sorbent-acetone nanoreactor system

By the ¹H NMR and Raman spectroscopy data it is shown that in the porous inclusion compound of Zn₂(C₈H₄O₄)₂[(N₂(CH₂)₆))]·n(CH₃)₂CO (n ≈ 0–4.7) acetone molecules exist in two structural forms: ketonic (CH₃)₂CO, for which the ¹H NMR chemical shift value is δ^ket = 0.8 ppm, and enolic CH₃C(O)=CH₂, for which δ^en(OH) = 11 ppm, δ^en(CH₂) = 8.9 ppm, and δ^en(CH₃) = 1.6 ppm are found, the average value over three proton sites being <δ^en> = 5.6 ppm. A sharp difference in chemical shift values for the keto and enol forms of acetone in the inclusion compounds can be assigned to the effect of structural chemical conditions in two types of adsorption centers.     

Chemical bond inside endohedral complexes H@C59B and H@C59P

The geometries, electronic structures, and spin densities on the atoms of paramagnetic heterofullerenes C₅₉X (X = B, P) and their endohedral derivatives H@C₅₉X were obtained from B3LYP/6-31G* density functional calculations. The encapsulated hydrogen atom can form a C-H bond inside the fullerene sphere. The energies of the C-H(endo) bonds are 40–50 kcal mol⁻¹ lower than those of the corresponding exo-bonds. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1239–1243, July, 2007.     

Ion–molecule reactions in the gas phase. XVIII.—nucleophilic substitution of diastereomeric norborneols,norbornyl acetates and benzoates under ammonia chemical ionization

The comparative behaviour of the endo- and exo-norborneols and diastereomeric derivatives (acetates and benzoates) towards the NH₃/NH₄⁺ system was investigated. It appears that the proton affinity (PA) of the substrate relative to Pa(NH₃) strongly influences competition between the protonation and nucleophilic substitution processes yielding the MH⁺ and [M + NH₄ ? H₂O]⁺ ions, respectively. Tandem mass spectrometry was used to compare collision-activated dissociation spectra of [M + NH₄ ? H₂O]⁺ with those of analogous endo- and exo-norbornylamines protonated in the source. This demonstrates that an S_Nimechanism occurs specifically for the isomeric norborneols; in contrast, for acetates and benzoates, stereospecific S_Ni and S_N2 pathways take place for exo and endo derivatives, respectively. This particular behaviour is explained by considering the steric effect induced by the endo-H at C(6). In addition, the competitive decompositions of [M + NH₄ – H₂O]⁺ into NH₄⁺ and [C₇H₁₁]⁺ daughter ions are consistent with the formation of a proton-bound complex intermediate. The observed stereochemical effects for these dauther ions are rationalized by means of arguments based on the estimated heats of formation of the transition states, which is lower for the exo-norbonyl protonated amine, consistent with anchimeric assistance, rather than a stepwise pathway which is proposed for the endoisomer.     

Nanomolar determination of hydrazine by TiO2 nanoparticles modified carbon paste electrode

In the present paper, the use of a novel carbon paste electrode modified by N,N′(2,3-dihydroxybenzylidene)-1,4-phenylene diamine (DHBPD) and TiO₂ nanoparticles prepared by a simple and rapid method for the determination of hydrazine (HZ) was described. In the first part of the work, cyclic voltammetry was used to investigate the redox properties of this modified electrode at various solution pH values and at various scan rates. A linear segment was found with a slope value of about 48 mV/pH in the pH range 2.0–12.0. The apparent charge transfer rate constant (k _s) and transfer coefficient (α) for electron transfer between DHBPD and TiO₂ nanoparticles-modified carbon paste electrode were calculated. In the second part of the work, the mediated oxidation of HZ at the modified electrode was described. It has been found that under optimum condition (pH 8.0) in cyclic voltammetry, a high decrease in overpotential occurs for oxidation of HZ at the modified electrode. The values of electron transfer coefficients (α) and diffusion coefficient (D) were calculated for HZ, using electrochemical approaches. Differential pulse voltammetry exhibited a linear dynamic range from 1.0 × 10⁻⁸ to 4.0 × 10⁻⁶ M and a detection limit (3σ) of 9.15 nM for HZ. Finally, this method was used for the determination of HZ in water samples, using standard addition method.     

The 1H NMR spectra of some norbornene derivatives,a LIS study

The ¹H NMR spectra of 2-exo-hydroxymethyl-3-endo-methylnorbornene and the corresponding 2-endo-3-exo isomer have been completely assigned with the aid of lanthanide reagents. This enabled a full analysis of the unusual spectrum of 2-exo-hydroxy-methyl-3-endo-methylnorbornene to be undertaken, confirming the proposed structure. The ΔEu values for 2-exo-hydroxymethyl-3-endo-methylnorbornene and the 2-endo-3-exo-isomer have been used to test the effect of rotational averaging on the calculated pseudo contact shifts. Good agreement is obtained for a dynamic model in which the Eu atom exchanges between two sites on the oxygen atom of the OH bond, and in which the rotational equilibrium about the CH? CH₂OH bond is explicitly considered.     

Reactions of p-Nitrophenyloxirane with Amines Containing Fragments with Bicyclic Skeleton

Reactions of p-nitrophenyloxirane with amines containing fragments with bicyclic skeleton of norbornene, norbornane, epoxynorbornane (stereoisomeric exo- and endo-5-aminomethylbicyclo[2.2.1]hept-2-enes, N-benzyl-endo-5-aminomethylbicyclo[2.2.1]hept-2-ene, endo-5-(2-aminoethyl)bicyclo[2.2.1]hept-2-ene, stereoisomeric exo- and endo-2-aminomethylbicyclo[2.2.1]heptanes, 2-(1-aminoethyl)bicyclo[2.2.1]heptane, exo-5-aminomethyl-exo-2,3-epoxybicyclo[2.2.1]heptane) were investigated. The aminolysis of p-nitrophenyloxirane occurred regioselectively according to Krasusky rule as was proved by ¹H and ¹³C NMR data. As shown by ¹H and ¹³C NMR spectroscopy the oxyalkylation product obtained from N-benzyl-endo-5-aminomethylbicyclo[2.2.1]hept-2-ene was composed of two diastereomers originating from the presence of a chiral nitrogen atom in the rear part of the rigid bicyclic skeleton. New products of amino groups transformation in the molecules of hydroxyamines were obtained by reaction with p-methylbenzoyl chloride and p-nitrophenylsulfonyl chloride. Regioselectivity of the attack of electrophilic reagents on the nitrogen in the hydroxyamines was confirmed by IR and ¹H NMR spectra of the products. The data on pharmacological activity tests of N-2-hydroxyethyl(p-nitrophenyl)-5-aminomethylbicyclo[2.2.1]hept-2-ene are reported.     

Volume and compressibility effects in the formation of metal-EDTA complexes

We used precise measurements of ultrasonic velocity and density to study the complexation of ethylendiaminetetraacetic acid (EDTA) with Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ at 25‡C and pH 12. From these measurements we obtained the changes in the molar concentration increment of the ultrasonic velocity δA, the apparent molar adiabatic compressibility δK_sΦ, and the apparent molar volume δV_Φ of complex formation. The hydration contributions δ(AV_h) to the volume effect of binding range from 39.6 to 46.6 cm³-mol^-1 while the hydration contribution to the adiabatic compressibility change in the binding, δ(δK_h), ranges from 103.9X 10^-4 to 131.1 X 10^-4 cm³-mol^-1-bar^-1. These data are interpreted in terms of dehydration of interacting molecules,i.e., transfer of water molecules from the hydration shells of cations and EDTA into the bulk water. The ratio δ(δV_h)/ δ(δV_h) is in the range 0.35 to 0.38 bar, indicating a dominant contribution from the dehydration of charged atomic groups in the volume and the compressibility effects of complex formation.     

Exo- and endo-Isomers of Plumbylenes Supported by 1,3-Diethers of Calix[4]arene

Calix[4]arene-plumbylene complexes Pb[Calix^t-Bu(O)₂(OR)₂], where R=benzyl, 9-fluorenyl, Si^tBuMe₂, and SiⁱPr₃, have been synthesized by the reaction of Lappert's plumbylene Pb[N(SiMe₃)₂]₂ with the corresponding 1,3-diethers of calix[4]arene. The products have been fully characterized by ¹H, ¹³C{¹H}, and ²⁰⁷Pb NMR spectroscopy, elemental analysis, and X-ray diffraction analysis. The products adopt two different forms depending on the R groups: the alkyl derivatives were obtained as exo-isomers in which the lead atom resides outside the cavity of the calix[4]arene, while the silyl derivatives were isolated as endo-isomers where the lead atom is situated inside the cavity. X-ray diffraction studies revealed that the lead(II) atoms in the exo- and endo-isomers are coordinated by the ethereal oxygen atoms (OR) and the aromatic rings (ArOR), respectively, which are maintained in solution as evidenced by ²⁰⁷Pb NMR studies.     

A proton NMR study on aggregation of cationic surfactants in water: effects of the structure of the headgroup

 ¹H NMR chemical shifts of solutions of the following cationic surfactants in D₂O were determined as a function of their concentrations: cetyltrimethylammonium chloride, CTACl, a 1 : 1 molar mixture of CTACl and toluene, cetylpyridinium chloride, CPyCl, cetyldimethylphenylam-monium chloride, CDPhACl, cetyldimethylbenzylammonium chloride, CDBzACl, cetyldimethyl-2-phenylethylammonium chloride, CDPhEtACl, and cetyldimethyl-3-phenylpropylammonium chloride, CDPhPrACl. Plots of observed chemical shifts versus [surfactant] are sigmoidal, and were fitted to a model based on the mass-action law. Satisfactory fitting was obtained for the discrete protons of all surfactants. From these fits, we calculated the equilibrium constant for micelle formation, K, the critical micelle concentration, CMC and the chemical shifts of the monomer, δ_mon and the micelle δ_mic. ¹H NMR-based CMC values are in excellent agreement with those which we determined by surface tension measurements of surfactant solutions in H₂O, allowing for the difference in structure between D₂O and H₂O. Values of K increase as a function of increasing the size of the hydrophilic group, but the free energy of transfer per CH₂ group of the phenylalkyl moiety from bulk water to the micellar interface is approximately constant, 1.9±0.1 kJ mol^-1. Values of (δ_mic–δ_mon) for the surfactant groups at the interface, e.g., CH₃–(CH₂)₁₅–N+(CH₃)₂ and within the micellar core, e.g., CH₃–(CH₂)₁₅–N⁺ were used to probe the (average) conformation of the phenyl group in the interfacial region. The picture that emerges is that the aromatic ring is perpendicular to the interface in CDPhACl and is more or less parallel to it in CDBzACl, CDPhEtACl, and CDPhPrACl. Received: 23 February 1996 Accepted: 29 August 1996     

2′‐Deoxy‐5‐propynyluridine: a nucleoside with two conformations in the asymmetric unit

The title compound, 1‐(2‐deoxy‐β‐d ‐erythro‐pentofuranosyl)‐5‐(prop‐1‐ynyl)pyrimidin‐2,4(1H,3H)‐dione, C₁₂H₁₄N₂O₅, shows two conformations in the crystalline state: conformer 1 adopts a C2′‐endo (close to ²E; S‐type) sugar pucker and an anti nucleobase orientation [χ = −134.04 (19)°], while conformer 2 shows an S sugar pucker (twisted C2′‐endo–C3′‐exo), which is accompanied by a different anti base orientation [χ = −162.79 (17)°]. Both molecules show a +sc (gauche, gauche) conformation at the exocyclic C4′—C5′ bond and a coplanar orientation of the propynyl group with respect to the pyrimidine ring. The extended structure is a three‐dimensional hydrogen‐bond network involving intermolecular N—H...O and O—H...O hydrogen bonds. Only O atoms function as H‐atom acceptor sites.     

Extensive hydrogen and halogen bonding,and absence of intramolecular hydrogen bonding between alcohol and nitro groups in a series of endo‐nitronorbornanol compounds

The influence of the substituent at the C2 position on the hydrogen‐bonding patterns is compared for a series of five related compounds, namely (±)‐3‐exo,6‐exo‐dibromo‐5‐endo‐hydroxy‐3‐endo‐nitrobicyclo[2.2.1]heptane‐2‐exo‐carbonitrile, C₈H₈Br₂N₂O₃, (II), (±)‐3‐exo,6‐exo‐dibromo‐6‐endo‐nitro‐5‐exo‐phenylbicyclo[2.2.1]heptan‐2‐endo‐ol, C₁₃H₁₃Br₂NO₃, (III), (±)‐methyl 3‐exo,6‐exo‐dibromo‐5‐endo‐hydroxy‐3‐endo‐nitrobicyclo[2.2.1]heptane‐2‐exo‐carboxylate, C₉H₁₁Br₂NO₅, (IV), (±)‐methyl 3‐exo,6‐exo‐dibromo‐7‐diphenylmethylidene‐5‐endo‐hydroxy‐3‐endo‐nitrobicyclo[2.2.1]heptane‐2‐exo‐carboxylate, C₂₂H₁₉Br₂NO₅, (V), and (±)‐methyl 3‐exo,6‐exo‐dibromo‐5‐endo‐hydroxy‐3‐endo‐nitro‐7‐oxabicyclo[2.2.1]heptane‐2‐exo‐carboxylate, C₈H₉Br₂NO₆, (VI). The hydrogen‐bonding motif in all five compounds is a chain, formed by O—H...O hydrogen bonds in (III), (IV), (V) and (VI), and by O—H...N hydrogen bonds in (II). All compounds except (III) contain a number of Br...Br and Br...O halogen bonds that connect the chains to each other to form two‐dimensional sheets or three‐dimensional networks. None of the compounds features intramolecular hydrogen bonding between the alcohol and nitro functional groups, as was found in the related compound (±)‐methyl 3‐exo,6‐exo‐dichloro‐5‐endo‐hydroxy‐3‐endo‐nitrobicyclo[2.2.1]heptane‐2‐exo‐carboxylate, (I) [Boeyens, Denner & Michael (1984b). J. Chem. Soc. Perkin Trans. 2, pp. 767–770]. The crystal structure of (V) exhibits whole‐molecule disorder.     

Inductive and Polarizability Effects of an Exocyclic Diene-Iron Tricarbonyl Group. The acetolyses of exo- and endo-irontricarbonyl complexes of 5,6-dimethylidene-2-exo-norbornyl and 2,3-dimethylidene-7-anti-norbornyl parabromobenzenesulfonates

The exo- and endo-irontricarbonyl complexes of 5,6-dimethylidene-2-exo-norbornyl alcohols 10x, 10n , p-bromobenzenesulfonates 11x, 11n , acetate 12x and of the 2,3-dimethylidene-7-anti-norbornyl alcohols 17x, 17n , p-bromobenzenesulfonates 19x, 19n and acetates 20x, 20n have been prepared. The S_N1 buffered acetolyses of 11x, 19x and 19n gave 12x, 20x and 20n , respectively (retention of configuration). The first-order rate constants of the acetolyses have been evaluated and compared with those of the acetolyses of the uncomplexed 5,6-dimethylidene-2-exo-norbornyl ( 14 ) and 2,3-dimethylidene-7-anti-norbornyl p-bromobenzenesulfonates ( 18 ). A rate retardation effect of ca. 1.5 · 10⁵ was measured for 11x → 12x (65°) compared with the acetolysis of 14 . The retardation effect is larger (> 5 · 10⁷) with 11n . Contrastingly, the acetolysis 19x → 20x was slightly accelerated with respect to that of the uncomplexed p-bromobenzenesulfonate 18 . An unsignificant rate-retardation effect was measured for the acetolysis 19n → 20n . The results are interpreted in terms of competitive inductive destabilization and charge-induced dipole stabilizing interaction by the exocyclic diene-iron tricarbonyl fragment. PMO. arguments give a rationale for the difference in polarizability between the diene-Fe(CO)₃ group in 19 and that in the endo-7-norbornadienyl-iron tricarbonyl system.     

Fragmentation of isomeric intrastrand crosslink lesions of DNA in an ion-trap mass spectrometer

The collision-induced dissociation pathways of isomeric cytosine-guanine and cytosine-adenine intrastrand crosslink-containing dinucleoside monophosphates were investigated with the stable isotope-labeled compounds to gain insights into the effects of chemical structure on the fragmentation pathways of these DNA modifications. A Dimroth-like rearrangement, which was reported for protonated 2′-deoxycytidine and involved the switching of the exocyclic N4 with the ring N3 nitrogen atom, was also observed for the cytosine component in the protonated ions of C[5–8]G, C[5–2]A, and C[5–8]A, but not C[5-N ²]G or C[5-N ⁶]A. In these two sets of crosslinks, the C5 of cytosine is covalently bonded with its neighboring purine base via a carbon atom on the aromatic ring and an exocyclic nitrogen atom, respectively. On the contrary, the rearrangement could occur for the deprotonated ions of C[5-N ²]G, C[5-N ⁶]A, and unmodified cytosine, but not C[5–8]G, C[5–2]A, or C[5–8]A. In addition, ammonia could be lost more readily from C[5-N ²]G and C[5-N ⁶]A than from C[5–8]G, C[5–2]A, and C[5–8]A. The results from the present study afforded important guidance for the application of mass spectrometry for the structure elucidation of other intrastrand/interstrand crosslink lesions.     

Reactions of mer(exo)‐ and mer(endo)‐[Co(dien)(dapo)X]2+ Isomers (X=OH,Cl, ONO2, N3)

Properties indirectly determined, or alluded to, in previous publications on the titled isomers have been measured, and the results generally support the earlier conclusions. Thus, the common five‐coordinate intermediate generated in the OH^?‐catalyzed hydrolysis of exo‐ and endo‐[Co(dien)(dapo)X]²⁺ (X=Cl, ONO₂) has the same properties as that generated in the rapid spontaneous loss of OH^? from exo‐ and endo‐[Co(dien)(dapo)OH]²⁺ (40±2% endo‐OH, 60±2% exo‐OH) and an unusually large capacity for capturing (R=[CoN₃]/[CoOH][]=1.3; exo‐[CoN₃]/endo‐[CoN₃]=2.1±0.1). Solvent exchange for spontaneous loss of OH^? from exo‐[Co(dien)(dapo)OH]²⁺ has been measured at 0.04 s^?1 (k₁, 0.50M NaClO₄, 25°) from which similar loss from the endo‐OH isomer may be calculated as 0.24 s^?1 (k₂). The OH^?‐catalyzed reactions of exo‐ and endo‐[Co(dien)(dapo)N₃]²⁺ result in both hydrolysis of coordinated via an OH^?‐limiting process =153 M ^?1 s^?1; =295 M ^?1 s^?1; K_H=1.3±0.1 M ^?1; 0.50M NaClO₄, 25.0°) and direct epimerization between the two reactants =33 M ^?1 s^?1; =110 M ^?1 s^?1; 1.0M NaClO₄, 25.0°). Comparisons are made with other rapidly reacting Co^III‐acido systems.     

Face Selectivity in the Diels-Alder Additions and Chelotropic Addition of Sulfur Dioxide to 2-(D)Methylidene-3-methylidenebicyclo[2.2.1]heptane

The stereoselectivity of the cycloadditions of 2-(D)methylidene-3-methylidenebicyclo[2.2.1]heptane ( 4 ) to various dienophiles has been determined. The exo- vs. endo-face selectivity depends on the type of dienophiles, and for olefinic ones, on the mode of attack (Alder- vs. anti-Alder endo rule). It is > 9:1 with N-phenyltriazolinedione (NPTAD) and ethylenetetracarbonitrile (TCNE), < 1:9 with dimethyl acetylenedicarboxylate (DMAD), 30 ± 5:70 ± 5 with DMAD in the presence of AlCl₃, 15 ± 5:85 ± 5 with dehydrobenzene and 40 ± 5:60 ± 5 with ¹O₂ generated photochemically (Table 1). With para-benzoquinone and maleic anhydride, the exo- vs. endo- face selectivity is < 1:9 and 20 ± 5:80 ± 5, respectively, for their anti-Alder mode of attack; it is 50 ± 5:50 ± 5 and 55 ± 5:45 ± 5, respectively, for their Alder mode of reaction. Under conditions of kinetic control, the chelotropic addition of SO₂ to 4 is endo-face selective.     

Synthese von Triafulven-Vorstufen für Retro-Diels-Alder-Reaktionen

Synthesis of Triafulvene Precursors for Retro-Diels-Alder Reactions Triafulvene precursors exo? 15 and endo? 15 have been prepared by addition of dibromocarbene to benzobarrelene 12 followed by a lithium-halogen exchange, methylation, and elimination of HBr ( 12→13→14→15 ), (Scheme 2). Gas-phase pyrolysis of exo/endo-mixtures of 15 above 400° gave minor amounts of naphthalene ( 16 ), traces of a hydrocarbon C₄H₄ identified by MS (presumably triafulvene 1 ) and predominantly (36%) the isomerization product 17 (Scheme 3). In a second synthetic approach the well-known cycloheptatriene-norcaradiene equilibrium of type 26?27 has been utilised to prepare various endo-trans-3-(X-methyl) tricyclo[3.2.2.0^2,4]nona-6,8-dienes 31 (Scheme 5). However, numerous elimination experiments 31→9 failed so far. The structure of two rearrangement products 33 and 34 (Scheme 6) has been elucidated.