Useful enantioselective bicyclization reactions using an N-protonated chiral oxazaborolidine as catalyst

[reaction: see text] Nine examples are reported of enantioselective [4 + 2] cycloaddition reactions of achiral, acyclic substrates to form chiral bicyclo[4.3.0]nonane or bicyclo[4.4.0]decane derivatives.     

Applications of molecular mechanics calculations in carbohydrate chemistry. I. Conformational and constitutional equilibria of tetraoxabicyclo[4.4.0]- and -[5.3.0] decanes,bicyclic diacetals of alditols

The energies of various conformations have been calculated by molecular mechanics for cis and trans isomers of 2,4,7,9-tetraoxabicyclo[4.4.0] decane and 3,5,8,10-tetraoxabicyclo[5.3.0]decane and their methyl derivatives. These molecules are models for reaction products from formaldehyde and the tetrols, pentitols, and hexitols. The conformational equilibria were analyzed for the cis-bicyclo [4.4.0] and cis-bicyclo[5.3.0] systems and compared with available experimental data. The thermodynamic stability of bicyclo[4.4.0] products was found to be higher than that of bicyclo[5.3.0] derivatives in the gas phase in every case studied. Discrepancies with experimental data that exist in a few cases can be ascribed to solvent effects.     

Effects of ring strain on gas-phase rate constants. 2. OH radical reactions with cycloalkenes

Relative rate constants for the gas-phase reactions of OH radicals with a series of cycloalkenes have been determined at 298 ± 2 K using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with isoprene of 9.60 × 10^?11 cm³ molecule^?1 s^?1, the rate constants obtained were (X 10¹¹ cm³ molecule^?1 s^?1): cyclopentene 6.39 ± 0.23, cyclohexene 6.43 ± 0.17, cycloheptene 7.08 ± 0.22, 1,3-cyclohexadiene 15.6 ± 0.5, 1,4 cyclohexadiene 9.48 ± 0.39, bicyclo[2.2.1]-2-heptene 4.68 ± 0.39, bicyclo[2.2.1] 2,5 heptadiene 11.4 ± 1.0, and bicyclo[2.2.2] 2 octene 3.88 ± 0.19. These data show that the rate constants for the nonconjugated cycloalkenes studied depend on the number of double bonds and the degree of substitution per double bond, and indicate that there are no obvious effects of ring strain energy on these OH radical addition rate constants. A predictive technique for the estimation of OH radical rate constants for alkenes and cycloalkenes is presented and discussed.     

Synthesis of 1‐azabicyclic systems by double cyclization

5‐Cyano‐1‐azabicyclo[3.3.0]octane ( 1 ) was prepared in one step from 1,7‐dichloro‐4‐heptanone ( 4 ) under mild conditions. The application of this method for the preparation of 5‐cyano‐4,6‐dimethyl‐1‐azabicy‐clo[3.3.0]octane ( 11 ) gave two diastereomers in equilibrium. The NMR measurements of 11 and its reduced compound 15 showed that the major isomer is the cis‐exo form, and the minor isomer is the trans form. Molecular orbital calculations indicated that the cis‐exo form is more stable than the trans form, in agreement with the experimental results. Furthermore, 6‐cyano‐1‐azabicyclo[4.3.0]nonane ( 17 ) and 1‐azabicy‐clo[4.4.0]decane ( 19 ), both including a six‐membered ring, were prepared from appropriate haloketones by using this double cyclization method.     

Kinetics of the reactions of O3 and OH radicals with furan and thiophene at 298 ± 2 K

Rate constants for the reactions of O₃ and OH radicals with furan and thiophene have been determined at 298 ± 2 K. The rate constants obtained for the O₃ reactions were (2.42 ± 0.28) × 10^?18 cm³/molec·s for furan and <6 ×10^?20 cm³/molec·s for thiophene. The rate constants for the OH radical reactions, relative to a rate constant for the reaction of OH radicals with n-hexane of (5.70 ± 0.09) × 10^?12 cm³/molec·s, were determined to be (4.01 ± 0.30) × 10^?11 cm³/molec·s for furan and (9.58 ± 0.38) × 10^?12 cm³/molec·s for thiophene. There are to date no reported rate constant data for the reactions of OH radicals with furan and thiophene or for the reaction of O₃ with furan. The data are compared and discussed with respect to those for other alkenes, dialkenes, and heteroatom containing organics.     

Electron transfer from highly strained polycyclic molecules

The details of electron transfer from strained, saturated hydrocarbons in electrochemical oxidations on platinum have been studied. Among the systems investigated were tetracyclo[3.2.0.0^2,7.0^4,6]heptane, tricyclo [4.1.0.0^2,7]heptane, pentacyclo[4.3.0.0^2,4.0^5,7]nonane, pentacyclo[4.4.0.0^2,5.0^3.8.0^4,7] decane, pentacyclo[4.2.0.0^2,5.0^3,8.0^4,7]octane, pentacyclo[4.3.0.0^2,5.0^3,8.0^4,7]nonane, bicyclo[2.1.0]pentane, and a variety of bicyclo[1.1.0]butane derivatives. The oxidation of 1,2,3-trimethylbicyclo [1.1.0]butane is discussed in detail.     

Kinetics of the gas-phase reactions of OH radicals with a series of α,β-unsaturated carbonyls at 299 ± 2 K

Relative rate constants for the reaction of OH radicals with a series of α,β-unsaturated carbonyls have been determined at 299 ± 2 K, using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with propene of 2.52 × 10^?11 cm³/molec·s, the rate constants obtained are (× 10¹¹ cm³/molec·s: acrolein, 1.83 ± 0.13; crotonaldehyde, 3.50 ± 0.40; methacrolein, 2.85 ± 0.23; and methylvinylketone, 1.88 ± 0.14). These data, which are necessary input to chemical computer models of the NO_x–air photooxidations of conjugated dialkenes, are discussed and compared with literature values.     

Samarium(II) iodide-induced cascade reaction for tricyclic γ-lactone synthesis from acyclic keto diesters

Cascade reaction involving reductive cyclization, Dieckmann condensation, and lactonization of E- and Z-dimethyl 2-methyl-8-oxoundec-2-enedioates and Z-dimethyl 2-methyl-7-oxodec-2-enedioate with samarium(II) iodide was found to stereospecifically produce cis and trans bicyclo[4.4.0]decane (decalin) ring systems and trans bicyclo[4.3.0]nonan (perhydroindane) ring system each consisting of γ-lactone, respectively.     

Lewis acid-mediated generation of bicyclo[5.3.0]decanes and bicyclo[4.3.0]nonanes

[reactions: see text] Fragmentation of the cyclobutane-containing adducts generated from intramolecular cycloadditions of cyclobutadiene with olefins provides rapid entry into bicyclo[5.3.0]decane and bicyclo[4.3.0]nonane ring systems. Whereas earlier studies featured thermal methods to achieve the desired rearrangements, a mild, Lewis acid-mediated fragmentation has been identified for substrates with appropriate functionality adjacent to the strained ring system. The substrate scope and stereochemical outcome of the acid-mediated fragmentation are complementary to the thermal ring expansions, particularly in the case of the bicyclo[5.3.0]decanes.     

The Novel Adamantane Isomer Tricyclo[4.4.0.03,9]decane (2-Homotwistbrendane)

Two synthetic approaches to the novel C₁₀H₁₆ hydrocarbon tricyclo[4.4.0.0^3,9]decane ( 1 ; 2-homotwistbrendane), one of the 19 members of the adamantaneland, and its Lewis-acid-catalyzed rearrangement are described. One route starts from tricyclo[4.3.0.0^3,8]nonan-2-one ( 2 ; 2-twistbrendanone). The missing tenth C-atom is introduced by ring enlargement (Tiffeneau-Demjanov method). Starting from methyl 8,9,10-trinorborn-5-ene-2-endo-carboxylate ( 8 ), ring enlargement by one C-atom, regio- and stereoselective introduction of a C₁ unit to a 2-endo,6-endo-disubstituted bicyclo[3.2.1]octane, and ring closure by acyloin condensation are the key steps in the second approach.     

Effects of ring strain on gas-phase rate constants. I. Ozone reactions with cycloalkenes

Rate constants for the gas-phase reactions of O₃ with a series of cycloalkenes and with cis-2-butene have been determined at 297 ± 1 K. The rate constants obtained were (in units of 10^?16 cm³/molecule·s): cis-2-butene, 1.38 ± 0.16; cyclopentene, 2.75 ± 0.33; cyclohexene, 1.04 ± 0.14; cycloheptene, 3.19 ± 0.36; 1,3-cyclohexadiene, 19.7 ± 2.8; 1,4-cyclohexadiene, 0.639 ± 0.074; bicyclo[2.2.1]-2-heptene, 21.4 ± 3.5; bicyclo[2.2.1]-2,5-heptadiene, 46.8 ± 12.9; and bicyclo[2.2.2]-2-octene, 0.728 ± 0.090. These data for cis-2-butene, cyclopentene, and cyclohexene are compared with previous literature data, and the effects of ring strain on the rate constants are discussed.     

Polyesters containing bicyclo[2.2.2]octane and bicyclo[3.2.2]nonane rings

Polyesters containing bicyclo[2.2.2]octane and bicyclo[3.2.2]nonane rings are prepared from 1,4-bis(carboethoxy)bicyclo[2.2.2]octane, 1,4-bis(hydroxymethyl)bicyclo[2.2.2]-octane and the 1,5-disubstituted bicyclo[3.2.2]nonane analogs. These polyesters are compared to the related polymers containing 1,4-phenylene and trans-1,4-cyclohexylene rings in terms of their melting point, thermal stabilities and oxidative stabilities. The lower symmetry of the bicyclo[3.2.2]nonane ring produces lower-melting polymers than the other ring systems. The remaining three rings are approximately equivalent in their effect on the melting point of a polymer provided that no more than one bicyclo[2.2.2]octane ring is present per polymer repeat unit. Two such rings produce a highermelting polymer than any other combination. Both the thermal and oxidative stabilities of the polyesters is improved by the presence of the bicyclo rings. This is attributed to the rings providing an approximation of a ladder polymer.     

Kinetics of the gas-phase reactions of OH radicals with alkyl nitrates at 299 ± 2 K

Relative rate constants for the gas-phase reactions of OH radicals with a series of alkyl nitrates have been determined at 299 ± 2 K, using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with cyclohexane of 7.57 × 10^?12 cm³/molec·s, the rate constants obtained are (× 10¹² cm³/molec·s): 2-propyl nitrate, 0.18 ± 0.05; 1-butyl nitrate, 1.42 ± 0.11; 2-butyl nitrate, 0.69 ± 0.10; 2-pentyl nitrate, 1.87 ± 0.12; 3-pentyl nitrate, 1.13 ± 0.20; 2-hexyl nitrate, 3.19 ± 0.16; 3-hexyl nitrate, 2.72 ± 0.22; 3-heptyl nitrate, 3.72 ± 0.43; and 3-octyl nitrate, 3.91 ± 0.80. These rate constants, which are the first reported for the alkyl nitrates, are significantly lower than those for the parent alkanes, and a formula, based on the numbers of the various types of C? H bonds in the alkyl nitrates, is derived for rate constant estimation purposes.     

Photochemische Umwandlungen. XV [1] Photochemische Isomerisierung des exo-Tricyclo[4.2.1.02′5]nona-3,7-dien-Systems. Vorläufige Mitteilung

The acetone sensitized isomerization of two exo-tricyclo[4.2.1.0^2,5]nonadiene derivatives, of the corresponding tricyclo[4.3.0.0^2,5]nonadienes, and the photoisomerization of two bicyclo[4.3.0]nonatrienes by direct excitation are described.     

Effects of ring strain on gas-phase rate constants. 3. NO3 radical reactions with cycloalkenes

Rate constants for the gas-phase reactions of NO₃ radicals with a series of cycloalkenes have been determined at 298 ± 2 K, using a relative rate technique. Using an equilibrium constant for the NO₂ + NO₃ ? N₂O₅ reactions of 3.4 × 10^?11 cm³ molecule^?1, the following rate constants (in units of 10^?13 cm³ molecule^?1 s^?1) were obtained: cyclopentene, 4.52 ± 0.52; cycloheptene, 4.71 ± 0.56; bicyclo[2.2.1]-2-heptene, 2.41 ± 0.28; bicyclo[2.2.2]-2-octene, 1.41 ± 0.17; bicyclo[2.2.1]-2,5-heptadiene, 9.92 ± 1.13; and 1,3,5-cycloheptatriene, 12.6 ± 2.9. When combined with previous literature rate constants for cyclohexene and 1,4-cyclohexadiene, these data show that the rate constants for the nonconjugated cycloalkenes studied depend to a first approximation on the number of double bonds and the degree and configuration of substitution per double bond. No obvious effects of ring strain energy on these NO₃ radical addition rate constants were observed. Our previous a priori predictive techniques for the alkenes and cycloalkenes can now be extended to strained cycloalkenes.     

Rate constants for the gas-phase reactions of alkanes with Cl atoms at 296 ± 2 K

Rate constants for the gas-phase reactions of the Cl atom with a series of alkanes have been determined at 296 ± 2 K using a relative rate method. Using a rate constant for the Cl atom reaction with n-butane of 1.94 × 10^?10 cm³ molecule^?1 s^?1, the rate constants obtained (in units of 10^?11 cm³ molecule^?1 s^?1) were: 2-methylpentane, 25.0 ± 0.8; 3-methylpentane, 24.8 ± 0.6; cyclohexane, 30.8 ± 1.2; cyclohexane-d₁₂, 25.6 ± 0.8; 2,4-dimethylpentane, 25.6 ± 1.2; 2,2,3-trimethylbutane, 17.9 ± 0.7; methylcyclohexane, 34.7 ± 1.2; n-octane, 40.5 ± 1.2; 2,2,4-trimethylpentane, 23.1 ± 0.8; 2,2,3,3-tetramethylbutane, 15.6 ± 0.9; n-nonane, 42.9 ± 1.2; n-decane, 48.7 ± 1.8; and cis-bicyclo[4.4.0]decane, 43.1 ± 0.8, where the indicated errors are two least-squares standard deviations and do not include the uncertainties in the n-butane rate constant. These data have been combined with rate constants obtained previously for ten C₂? C₇ alkanes and this entire data set has been used to develop an estimation method allowing the room temperature rate constants for the reactions of the Cl atom with alkanes to be calculated. © 1995 John Wiley & Sons, Inc.     

Rate constants for the gas-phase reactions of O3 with a series of cycloalkenes and α,β-unsaturated ketones at 296 ± 2 K

The kinetics of the gas-phase reactions of O₃ with a series of alkenes and two α,β-unsaturated ketones have been investigated at atmospheric pressure (ca. 740 torr) of air and 296 ± 2 K, using a relative rate method in the presence of sufficient cyclohexane to scavenge OH radicals generated in these reactions. Combined with our previous relative rate measurements (Int. J. Chem. Kinet., 24, 803 (1992)), the rate constants obtained relative to k(O₃ + propene) = 1.00 were: 3-penten-2-one, 3.62 ± 0.16; 2-cyclohexen-1-one, <0.19; bicyclo[2.2.2]-2-octene, 7.44 ± 0.48; 1,3-cycloheptadiene, 16.1 ± 1.1; cycloheptene, 23.7 ± 1.6; 1,3-cyclohexadiene, 134 ± 13; bicyclo[2.2.1]-2-heptene, 170 ± 16; and bicyclo[2.2.1]-2,5-heptadiene, 390 ± 36. The resulting rate constants then lead to a self-consistent set of room temperature data for the reactions of O₃ with these alkenes and α,β-unsaturated ketones. These rate constants are compared with the literature data, and the effects of ring size discussed. © 1994 John Wiley & Sons, Inc.     

Tricycloclavulone and clavubicyclone, novel prostanoid-related marine oxylipins, isolated from the Okinawan soft coral Clavularia viridis

Two novel prostanoid-related marine oxylipins, tricycloclavulone (1) and clavubicyclone (2), were isolated from the Okinawan soft coral Clavularia viridis. The structures of 1, having a tricyclo[5.3.0.0(1,4)]decane ring system, and 2, having a bicyclo[3.2.1]octane ring system, were elucidated on the basis of spectroscopic analysis. Clavubicyclone showed a moderate growth inhibition activity against tumor cells in vitro.     

Intramolecular diels-alder reaction of iminothiol esters

Diels-Alder reaction of dienyl α-methacrylthioimidates has been investigated under thermal or Lewis acid or protonic acid catalysed conditions. The utility of the reaction is shown by desulfurative ring contraction of bicyclo[4.4.0] to bicyclo[4.3.0]system.     

Solvolysis of 2-Bicyclo

The solvolysis rate of 2-bicyclo[3.2.2]nonyl p-toluenesulfonate (6-OTs) was nearly equal to that of cycloheptyl p-toluenesulfonate in 2,2,2-trifluoroethanol (TFE). This indicates that the ethylene bridge in 6-OTs does not significantly enhance the rate and that 6-OTs ionizes without anchimeric assistance. The solvolysis of [1-(13)C]-2-bicyclo[3.2.2]nonyl p-toluenesulfonate in methanol or TFE gave 2-substituted bicyclo[3.2.2]nonane, exo-2-substituted bicyclo[3.3.1]nonane, 2-bicyclo[3.2.2]nonene (10), and 2-bicyclo[3.3.1]nonene (11), whose distributions of (13)C labels were determined by quantitative (13)C NMR analysis using a relaxation reagent. The (13)C labels were exclusively placed at only two positions, the ratios of them were not unity, and the labels in 10 were less extensively scrambled than those in other products. These results indicate that the 2-bicyclo[3.2.2]nonyl cation is classical and that 10 is formed at a former ionization stage than 2-substituted bicyclo[3.2.2]nonane. The (13)C redistributions for both exo-2-substituted bicyclo[3.3.1]nonane and 11, which are yielded via 1,3-hydride shift, were similar to that of 2-substituted bicyclo[3.2.2]nonane, suggesting that 1,3-hydride shift occurs mainly at the solvent-separated ion pair.