The meta photocycloaddition of anisole to oxa- and dioxa-cyclopentenes and to ethyl vinyl ether

The meta photocycloaddition of anisole to 2,3- and 2,5-dihydrofuran, 1,3-dioxole and ethyl vinyl ether is described. The NMR data of the adducts are presented. The reaction yields mixtures of endo and exo adducts. The endo/exo ratio is strongly influenced by the presence of one or two oxygen atoms next to the double bond of the alkene. A mechanism is presented in which a zwitterionic intermediate is formed from the excited anisole and the alkene.     

Synthesis of endo-(3-azabicyclo[3.1.0]hex-6-yl)-methanol and derivatives as new geometric/charge mimics of glycosyltransfer transition states

The syntheses of azabicyclo[3.1.0]hexane transition state (TS) analogs are described. Cyclopropanation of N-Boc-3-pyrroline with ethyl diazoacetate afforded a 1.6:1 exo/endo ratio of the resulting ester. Reduction of the endo-isomer with LiAlH₄ yielded the alcohol which was phosphorylated, or iodinated to provide access to aglycon containing TS analogs.     

The behavior of two thiosugar thioglycosides towards oxidation

Oxidation of 4-cyanophenyl 2,5-anhydro-1,6-dithio-- -glucoseptanoside with magnesium monoperoxyphthalate (MMPP) resulted in a mixture containing two endo- and two exo-monosulfoxides as well as two endo-exo bis-sulfoxides differing in the chirality of the sulfoxide groups. Besides the aforementioned six products a further endo–exo bis-sulfoxide isomer as well as an exo-sulfone was obtained via oxidation with NaIO₄. Oxidation of 4-nitrophenyl 1,5-dithio-β- -arabinopyranoside with MMPP yielded only two endo-sulfoxides, while oxidation with NaIO₄ in turn led to cleavage of the carbohydrate ring. The formed dialdehyde was stabilized by cyclisation to a hemiacetal, which on further treatment with MMPP afforded an exo-monosulfoxide. The position of oxidation as well as the chirality of the respective sulfoxide groups was established by NMR spectroscopy and X-ray crystallography.     

Proton magnetic resonance and stereochemistry of some 5,6-disubstituted bicyclo[2.2.2]oct-2-enes and their oxidation products

The two adducts obtained from the Diels-Alder condensation between 1,3-cyclohexadiene and trans-p-Chloro-β-nitrostyrene have been characterized from the PMR spectra of the respective cyclohexane dicarboxylic acids obtained from chromic acid oxidation of the adducts. The Diels-Alder condensation yielded a larger quantity of the isomer with the nitro group endo to the double bond. Hydrogenation of the double bond in the two adducts yielded one product, the PMR spectrum of which is consistent with the expected product. The PMR spectra of the adducts, their oxidation products and the hydrogenation product are reported and discussed.     

Highly stereoselective synthesis of exo and endo indolotropanes

Highly stereoselective routes to exo and endo indolotropanes have been developed. This provides a facile route to these bicyclic analogues of the pharmaceutically active indolopiperidine motif.     

Indium trichloride: a useful catalyst for ionic Diels–Alder reactions

Indium trichloride (20 mol%) in nitromethane permits ionic Diels–Alder reaction of a variety of 2,3-olefinic acetals to form the corresponding cycloadducts in good yields with good endo selectivities.     

Asymmetric 1,3-dipolar cycloaddition of chiral sulphinylethenes with 1-methyl-3-oxido-pyridinium and some nitrones

1,3-Dipolar cycloaddition of (R)-(+)-p-tolyl vinyl sulphoxide 1 with 1-methyl-3-oxidopyridinium 6 proceeded in a diastereoselective manner to afford the exo and endo cycloadducts 11a,b and 12a in 36%, 7% and 29% yield, respectively. The absolute configuration of 11a was determined by its transformation to (1S)-(−)-2-tropanol (−)-15. Attempts to the cycloaddition of the sulphinylethenes 17–19 with the pyridinium 6 were nsuccessful under several conditions. The reaction of the sulphoxide 20 with pyrroline 1-oxide 21 gave an inseparable mixture of products. The cycloaddition of 20 with 3,4,5,6-tetrahydropyridine 1-oxide22 afforded a mixture of four adducts in ca. 90% yield. High level of diastereoselectivity was achieved for the endo cycloaddition affording the adduct 23 in 33% isolated yield. The absolute configuration of 23 was confirmed by a single-crystal X-ray diffraction study. The stereochemical course of the reaction was discussed based on the absolute configuration of the products.     

Asymmetric Diels–Alder reactions in supercritical carbon dioxide catalyzed by rare earth complexes

The rare earth(III) salt catalysed asymmetric Diels–Alder reaction of cyclopentadiene with a chiral dienophile in supercritical carbon dioxide (scCO₂) proceeded rapidly to give the adduct with a higher diastereoselectivity than that in dichloromethane; Yb(ClO₄)₃ gave the endo adduct with value up to 77% de at 40°C, 8 MPa. The chiral rare earth diketonate catalyzed hetero Diels–Alder reaction of the Danishefsky's diene with benzaldehyde gave a higher yield and an enantioselectivity in scCO₂ than that in dichloromethane. Scandium/pybox 8a complex catalysed asymmetric Diels–Alder reaction of 3-crotonoyl-2-oxazolidinone with cyclopentadiene in the presence of MS4A proceeded smoothly in scCO₂ to give the endo adduct 10 in a good yield with up to 88% ee.     

Novel glass-forming liquid crystals V. Nematic and chiral-nematic systems with an elevated glass transition temperature

The formerly implemented molecular design concept behind glass-forming liquid crystals (gLCs) was generalized by increasing the volume of the non-mesogenic central core, with an attendant increase in the number of nematic pendants, using 5-hydroxyisophthalic acid as the bridging unit. New nematic gLCs were synthesized and characterized, showing an elevation in T_g by 30 to 40°C with no definite trend in T_c over the benzene, cis, cis-cyclohexane, and exo, endo-bicyclo[2.2.2]oct-7-ene base cores. The exo, exo-configured gLC showed a higher T_g and a higher T _c than the exo, endo-counterpart. Morphological characterization with X-ray diffractometry revealed the non-crystalline nature of pristine samples and the morphological stability of thermally processed gLC films against recrystallization for six months. Nematic gLC films were prepared for characterization by FTIR linear dichroism, resulting in an orientational order parameter in the range 0.52 to 0.63. A chiral-nematic gLC derived from exo, exo-bicyclo[2.2.2.]oct-7-ene also showed an elevation in T_g by 10 to 20°C over the cyclohexane-based systems reported previously. With (S)-(-)-1-phenylethylamine as the chiral moiety, the left-handed, chiral-nematic gLC film yielded a selective reflection band centred around 375 nm. Tunability of selective reflection from the UV to visible region was demonstrated by mixing the chiral-nematic and nematic gLCs at varying ratios.     

Ab initio and DFT studies on the mechanism of the 1,3-dipolar cycloaddition reaction between nitrone and sulfonylethene chloride

The molecular mechanism for the 1,3-dipolar cycloaddition of nitrone with sulfonylethene chlorides has been studied using ab initio and DFT methods at the HF, MP2 and B3LYP levels together with the 6-31G* basis set. Relative rates, stereo and regioselectivity, have been analysed and discussed. For this cycloaddition four reactive channels associated with the formation of two pairs of diastereoisomeric regioisomers have been characterized. Analysis of the geometries of the corresponding transition structures shows that the cycloaddition takes place along a concerted but asynchronous mechanism. Activation energies as asynchronicity are dependent on the computation level. Thus, while HF calculations gave large barriers, MP2 calculations tend to underestimate them. DFT calculations gave reasonable values. These 1,3-dipolar cycloadditions present an endo stereoselectivity while the meta regioselectivity depends on the computational level. Thus, while HF and DFT calculations predict meta path, in agreement with the experimental results, MP2 calculation predict ortho regioselectivity. The frontier molecular orbitals analysis shows that the reaction is controlled by the (HOMO_dipole–LUMO_{dipolarophile}) interaction in agreement with the charge transfer analysis carried out at the transition structures. Inclusion of diffuse functions at the B3LYP/6-31+G* level increases the energy barriers about 4 kcal/mol, giving a similar endo/meta selectivity. Solvent effects have been taken into account, by means of self-consistent reaction field.     

The molecular structure of bicyclo[1.1.0]tetraarsane from ab initio calculations using analytical gradients

The molecular structure of three different isomers of bicyclo[1.1.0]tetraarsane has been obtained by means of ab initio techniques including the use of non-empirical pseudo-potentials and analytical first derivatives of the total energy versus the nuclear coordinates. The results show unequivocally the existence of three different stable structures. The very small energy difference between the exo—exo and endo—exo is further evidence that these are in fact the two isomers found experimentally.     

Regioselective electrophilic additions to 2-alkoxy- and 2-alkoxymethyl-7-oxabicyclo [2.2.1] hept-5-ene derivatives

The influence of the stereochemistry at C-2 on the selectivity of the reaction between PhSeBr and 2-alkoxymethyl substituted oxanorbornenes has been studied. All endo isomers displayed complete regio- and stereocontrol with incorporation of the electrophile on C-5.     

Simulations of gas transport in membranes based on polynorbornenes functionalized with substituted imide side groups

This paper studies the diffusive and sorption steps of several gases across membranes cast from poly(N-phenyl-exo,endo-norbornene-5,6-dicarboximide) chloroform solutions. Chains packing effects on gas transport was investigated by conducting a parallel study on the permeation characteristics of membranes cast from hydrogenated poly(N-phenyl-exo,endo-norbornene-5,6-dicarboximide) chloroform solutions. The permeability coefficients of several gases in the two membranes were measured finding that hydrogenation of the norbornene moieties decreases gas permeability. The transition states approach was used to determine the trajectories of the gases in the two types of membranes from which the diffusion coefficients were obtained. Monte Carlo techniques based on the Widom method were used to simulate gas sorption process as a function of pressure. The values of the solubility coefficients thus obtained undergo a relatively sharp drop at low pressures approaching to a constant value as pressure increases. With the exception of carbon dioxide, pretty good agreement between the experimental and simulated values of the permeability coefficient is found for the gases studied.     

Pauson–Khand cycloaddition reactions of chiral ynamides. Observation of an unusual endo-addition with norbornadiene

Pauson–Khand cycloadditions using chiral ynamides are achieved in modest to good yields with excellent regioselectivity and modest stereoselectivity. An unusual endo addition is found when using norbornadiene and substituted ynamides, leading to cycloadducts that were not observed in previous studies using ynamides or ynamines.     

cis,cis-Spiro[4.4]nonane-1,6-diol: A new chiral auxiliary for the asymmetric Diels-Alder reaction

The use of a mono-pivalate mono-acrylate bis-ester of (+)-1S,5S,6S-spiro[4.4]nonane-1,6-diol in an asymmetric Diels-Alder reaction with cyclopentadiene (2 equiv. BCl₃, −85°C, CH₂Cl₂) provided the expected endo bicyclo adduct in >97% de. Iodolactonization of the bicyclo adduct provided the (+)-lactone (5) with a 1S,4S,6S,8R,9S configuration (97% ee). The de's obtained from using various types and amounts of Lewis acids, and both chiral and racemic bis-esters in the Diels-Alder reaction with cyclopentadiene are also reported.     

New cation-exchange resins with high reversed-phase character for solid-phase extraction of phenols

High capacity carboxylic acid functionalized resins, prepared by ring-opening metathesis polymerization, were used for solid-phase extraction (SPE) of phenols. Two resins, based on cross-linked poly-(endo,endo-norborn-2-ene-5,6-dicarboxylic acid) exhibited a capacity of 3.5 and 5.1 mequiv. COOH/g, respectively. Particle-loaded PTFE membranes were prepared from a 3.1 mequiv. COOH/g resin. The extraction behavior of the new materials versus two different EPA priority pollutant phenol standards was investigated. For most compounds, a quantitative recovery was observed. The extraction efficiency of the new resins was compared to those of other, commercially available materials such as silica C₁₈ or carboxypropylsilica. The general advantages of the new materials, the mechanism of extraction, the influence of polarity and acidity of the compounds investigated as well as differences between membranes and columns containing the new particles are discussed. Finally, the extraction efficiency of the new resins for phenols from spiked soil samples using both standard SPE as well as batch-techniques was investigated.     

Stereoselective Diels–Alder reactions of 3-phosphonopropenoyl derivatives of 1,3-oxazolidin-2-ones

Dienophiles of the general structure (EtO)₂P(O)CHCHCOX have been prepared, where X represents an oxazolidinone chiral auxiliary. Use of the (S)-4-isopropyl-5,5-diphenyl-1,3-oxazolidin-2-one auxiliary gave Diels–Alder adducts with several cyclic and acyclic dienes. The crystal structures of the main cyclohexa-1,3-diene and 2,3-dimethylbutadiene adducts formed during reactions in the presence of dialkylaluminium halides are consistent with a reaction, which is stereoselectively endo with respect to the carbonyl group and occurs on the less hindered face of the dienophile when aluminium is chelated between the two carbonyl groups.     

Crystal structure of (1rs, 14sr, 17rs, 18rs)-2,20-dioxa-17-methyltricyclo [16,3,0,0] heneicosa-15-ene-3,19,21-trione; a macrocyclic lac

An X-ray study of the major product of the intramolecular Diels-Alder reaction of the diene-anhydride (2) has confirmed it as being the desired cytochalasan analogue (3). In particular the cyclisation has been regioselective, and has taken place via the endo transition state to generate the adduct with the correct relative configurations at the four chiral centres. Details of the conformational geometry of (3) are discussed.     

Synthesis and Diels-Alder reactions of homochiral 2-sulfinylmaleates with cyclopentadiene

Enantiomerically pure 2-p-tolylsulfinylmaleates 1, 2 and 3 have been readily prepared by Knoevenagel reaction between (S)-menthyl p-toluenesulfinate and glyoxylic acid. Their asymmetric Diels-Alder reactions with cyclopentadiene have been studied under a wide range of uncatalyzed and catalyzed conditions and the stereochemical results have been explained by assuming a steric control approach, in term of S-cis or S-trans favoured conformations. Uncatalyzed Diels-Alder reactions of 1 and some Lewis acid catalyzed Diels-Alder reactions of 2 show high facial and endo selectivities. The facial selectivity of dienophile 2 highly depends on the Lewis acid, whereas reactivity of 1 and 3 is very sensitive to the solvent. These sulfinylmaleates 1, 2 and 3 act as synthetic equivalents of chiral acetylenedicarboxylates in Diels-Alder reactions after basic elimination of the sulfinylic moiety in the resulting adducts.     

Direkte und indirekte metallierung von endo-dicylopentadien. Sn- und C-NMR studie stannylierter folgeprodukte

endo-Dicyclopentadiene (1) can be metalated by use of simple procedures with good overall yields. The attack occurs at the various vinyl, rather than at the allyl, positions of 1 as was confirmed by trapping the carbanions with Me₃SnCl. When t-BuLi/TMEDA are used, the 8- and 9-stannyl derivatives (3 and 4) are formed, whereas an excess of n-BuLi/t-BuOK leads to doubly stannylated derivatives with Me₃Sn groups in position 4/8 (6), 4/9 (7), and 3/9 (8) in addition to 3 and 4. Furthermore the latter reaction yields 5,5-bis(trimethylstannyl)cyclopentadiene (5). With stoichiometric amounts of n-BuLi/t-BuOK the formation of 3 and 4 predominates over that of 5–8. 5 is obtained from 1 after deprotonation at the allyl position, followed by an extremely fast retro-Diels-Alder reaction and then by further deprotonation. This follows from two experiments: (1) exo- and endo-5-trimethylstannyl-endo-dicyclopentadiene (11 and 12) which are synthesized from 1 in three steps give cyclopentadienyllithium and 1 when treated with methyllithium at −78°C; (2) cyclopentadiene reacts with an excess of n-BuLi/t-BuOK and Me₃SnCl to give 5. When 12 is heated syn-10-trimethylstannyl-endo-dicyclopentadiene (13) is obtained. The eight stannyl derivatives of 1 are identified mainly from the following NMR parameters: δ(¹¹⁹Sn), δ(¹³C), δ(¹H), ⁿJ(^119/117Sn---¹³C), and ⁶J(¹¹⁹Sn---^119/117Sn). The ¹³C NMR satellite spectrum of 1 yields the isotope shifts ¹Δ¹³(i(¹³C(j)) and ¹J(¹³C---¹³C). The latter lead to the revision of earlier signal assignments.