- Stereoselective [4+2]-Cycloaddition with Chiral Alkenylboranes
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A method for the stereoselective [4+2]-cycloaddition of alkenylboranes and dienes is presented. This transformation was accomplished through the introduction of a new strategy that involves the use of chiral N-protonated alkenyl oxazaborolidines as dieneophiles. The reaction leads to the formation of products that can be readily derivatized to more complex structural motifs through stereospecific transformations of the C?B bond such as oxidation and homologation. Detailed computation evaluation of the reaction has uncovered a surprising role of the counterion on stereoselectivity.
- Brown, M. Kevin,Houk, K. N.,Ni, Dongshun,Witherspoon, Brittany P.,Zhang, Hong,Zhou, Chen
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supporting information
p. 11432 - 11439
(2020/05/18)
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- Alkylhalovinylboranes: a new class of Diels-Alder dienophiles
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The Diels-Alder reactions of alkylhalovinylboranes have been investigated theoretically and experimentally. Alkylhalovinylboranes presented higher reactivity than the corresponding dialkylvinylboranes. Although endo/exo selectivities were high for the reactions with cyclopentadiene, facial selectivities for the chiral analogues were low. Our results demonstrate that the replacement of an alkyl group on the boron atom by a halogen increases the dienophilicity considerably.
- Pisano, Pablo L.,Pellegrinet, Silvina C.
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p. 33864 - 33871
(2018/10/20)
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- α-Hydroxyacids accelerate the Diels-Alder reaction of dibutyl vinylboronate with cyclopentadiene: Experimental results and mechanistic insights
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We have found that α-hydroxyacids accelerate the Diels-Alder reaction of dibutyl vinylboronate with cyclopentadiene. When stoichiometric quantities are used, excellent yields are obtained, while catalytic activities are moderate. DFT calculations suggested that the activation of the dienophile occurs by ligand exchange with both functionalities of the α-hydroxyacid.
- Grimblat,Sarotti,Pisano,Pellegrinet
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p. 1966 - 1969
(2016/03/22)
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- Diels-Alder reactions of pinacol alkenylboronates: An experimental and theoretical study
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We have studied the Diels-Alder reactions of pinacol alkenylboronates with cyclopentadiene under two different sets of conditions: thermal heating at 170 °C in a pressure tube and with catalytic TFA (5 mol%) at 80 °C. Yields varied significantly from system to system and also for the uncatalyzed and catalyzed methodologies. Moderate to excellent exo-stereoselectivities were obtained in all cases. The theoretical study of the thermal reactions sheds some light on the intriguing substituent effects observed experimentally. A variety of substituted 5-norbornen-2-ols were easily generated by subsequent in situ oxidation of the cycloadducts with alkaline hydrogen peroxide. This journal is the Partner Organisations 2014.
- Vallejos, Margarita M.,Grimblat, Nicolas,Pellegrinet, Silvina C.
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p. 36385 - 36400
(2014/12/09)
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- A facile microwave-assisted Diels-Alder reaction of vinylboronates
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The Diels-Alder reaction of vinylboronates can be easily performed using microwave irradiation giving excellent yields of the cycloadducts. Pinacol vinylboronate was the reagent of choice due to its stability towards hydrolysis, operational simplicity and yields of Diels-Alder products. To the best of our knowledge, this is the first example of microwave-assisted Diels-Alder reaction of boron-substituted dienophiles. Subsequent in situ oxidation of the cycloadducts with alkaline hydrogen peroxide afforded the alcohols efficiently.
- Sarotti, Ariel M.,Pisano, Pablo L.,Pellegrinet, Silvina C.
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supporting information; experimental part
p. 5069 - 5073
(2010/12/25)
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- Stereoselectivity in the rhodium-catalysed reductions of nonconjugated dienes
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The stereochemical course of rhodium-catalysed addition of hydrogen and catecholborane to bicyclo[2.2.1]heptadiene, and of hydrogen to a range of cyclic dienes has been analysed. For hydroboration, the overall catalytic reaction possesses exo-selectivity, but the initial step is endo-selective. For hydrogenation (deuteration), the first step may occur with either exo- or endo- selectivity, depending on the structure of the diene. This enables a distinction to be made between pathways involving prior dissociation of the diene, and direct addition to the complexed diene without full dissociation. The relative ease of hydrogenation of the first and second doublebonds varies markedly with reactant structure, and also depends on the choice of catalyst ligands. For dicyclopentadiene, hydrogenation of the cyclopentene double bond is accompanied by rapid alkene isomerisation, as revealed by deuterium addition. The asymmetric hydrogenation of acyclic skipped mesodienes is reported, demonstrating control of relative rates of the two sequential steps, with ees of up to 53% after the first reduction.
- Nguyen, Bao,Brown, John M.
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supporting information; scheme or table
p. 1333 - 1343
(2009/12/27)
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- Design of very transparent fluoropolymer resists for semiconductor manufacture at 157 nm
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Photolithography at 157 nm requires development of new photoresists that are highly transparent at this wavelength. Transparent fluoropolymer platforms have been identified which also possess other materials properties required for chemically amplified imaging and aqueous development. Polymers of tetrafluoroethylene (TFE), a fluoroalcohol-substituted norbornene and an acid-labile acrylate ester show the best combination of properties. A solution, semibatch, free-radical polymerization process was developed allowing synthesis of the terpolymers on a multikilogram scale. Further property enhancements may arise from replacing the norbornene with functionalized tricyclononenes. Formulated resists have been imaged in a 157 nm microstepper.
- Feiring, Andrew E.,Crawford,Farnham,Feldman,French,Leffew,Petrov,Schadt III,Wheland,Zumsteg
-
-
- Ring-opening metathesis - Cross-metathesis reactions (ROM-CM) of substituted norbornadienes and norbornenes
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Ring-opening metathesis - cross-metathesis reactions (ROM-CM) of substituted norbornadienes and norbornenes were investigated. The reactions with symmetrical 2,3-disubstituted norbornadienes were found to be highly chemoselective, with the ROM reactions occurring only on the less substituted or less sterically hindered double bonds regardless of the electronic nature of the substituents, giving highly substituted cyclopentenes in moderate to good yields. This study provides an efficient method for the stereoselective synthesis of highly substituted cyclopentenoids. Long-range electronic effect of a remote substituent on unsymmetrical norbornenes in the ROM-CM reactions was also investigated. Low levels of regioselectivities were observed (50:50 to 69:31) with various remote substituents on the norbornenes.
- Mayo, Peter,Tam, William
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p. 9513 - 9525
(2007/10/03)
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- Remote substituent effects on the oxymercuration of 2-substituted norbornenes: An experimental and theoretical study
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The effect of a remote substituent on regioselectivity in the oxymercuration of 2-substituted norbornenes has been investigated experimentally and theoretically using density functional theory (DFT). Regioselectivities of 1:1 to 14:1 were observed with various 2-substituted norbornenes. Exo-2-substituted norbornenes always gave greater regioselectivities compared to the corresponding endo-2-substituted norbornenes. The effects of solvents on the regioselectivity have also been examined, and ethereal solvents were found to be the best choice giving the optimal yield and regioselectivity. The relative rate of oxymercuration was estimated by competition experiments. The least reactive substrate (X = OAc) gave the highest regioselectivity. According to DFT predictions, the increased difference between the reaction barriers that results in the greater regioselectivity is correlated directly with the larger polarity of the C=C double bond, which is attacked by the mercury and oxygen. A number of stable exo and endo conformers were predicted. All exo conformers show the same polarity of the double bond, while some endo conformers have a reversal of this polarity. All the conformers except those with the OAc substituent are very close in energy and thus should react. The existence of a mixture of endo conformers with the C=C double bond of opposite polarity clearly explains a decrease in regioselectivity for the endo species. The origin of the greatest regioselectivity for the OAc-2-norbornenes lies in the fact that the conformer with the largest polarity is notably lower in energy than others due to an internal C-H-O hydrogen bond.
- Mayo,Orlova,Goddard,Tam
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p. 5182 - 5191
(2007/10/03)
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- Convenient Enzymatic Resolution of Alcohols Using Highly Reactive, Nonharmful Acyl Donors, 1-Ethoxyvinyl Esters
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1-Ethoxyvinyl esters 3, a new type of acyl donors for enzymatic resolution of racemic alcohols, were disclosed to be superior to the contemporary major reagents, vinyl esters 1 and isopropenyl esters 2. Three features of 3 are noticeable: (1) 3 generates ethyl acetate as a single coproduct, which does not affect any enzymes, while acetaldehyde liberated from 1 deactivates some kinds of lipases. (2) The reactivity of 3 was not less than that of 1 and much higher than that of 2, and the optical purity of the products was as high as that of 1 and 2. Especially, it was generally observed that 3 showed higher reactivity than 1 for reactions using Candida rugosa lipases, one of the most commonly employed lipases, having liberal applicability to substrates but sensitive to acetaldehyde. Twelve examples of the kinetic resolution of racemic secondary alcohols (5 and 10) and one desymmetrization of meso-alcohol 7 were presented employing the acetate 3a or the octanoate 3b and four types of lipases. (3) A one-pot procedure for the preparation of 3 from the corresponding carboxylic acid and the subsequent enzymatic resolution of alcohols, which has not been reported using 1 or 2, was elucidated. The chemical and optical yields of the products by this procedure were similar to those obtained using isolated 3.
- Kita, Yasuyuki,Takebe, Yasushi,Murata, Kenji,Naka, Tadaatsu,Akai, Shuji
-
-
- Quadricyclane hydration kinetics in natural waters
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Hydration rates of quadricyclane were measured over a pH range of 3 to 8 in phosphate buffer, carbonate buffer, and nitric acid solutions. Norbornadiene was a minor impurity in the quadricyclane, and its hydration rate was also measured. Hydration products were exo-5-norbornen-2-ol (bicyclo[2.2.1]hept-5-ene-2-ol) and nortricyclyl alcohol (tricyclo[2.2.1.03,5]heptane-2-ol). Diol formation was not detected. Quadricyclane hydration in solutions containing chloride ions formed the alcohols and their chlorinated analogs. A mechanism consistent with observed kinetics and reaction products involving two carbocation intermediates is proposed. Comparison of reaction rates was used to determine the role of buffer catalysis and chlorination reactions in natural waters. Phosphate buffer catalysis was shown to be important, and rate constants for all phosphate species were calculated. Comparison hydration rates in nitric acid and carbonate buffer indicated that catalysis by carbonate species for this reaction is unimportant in surface waters. Catalytic effects of other buffers in some natural waters appear possible. Proper buffer selection is important if laboratory-measured hydration rates of quadricyclane are to be applied to natural water systems.
- Stauffer, Thomas B.,Antworth, Christopher P.,Burr, Eila M.,MacIntyre, William G.
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p. 2237 - 2242
(2007/10/03)
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- Effect of a remote substituent on regioselectivity in oxymercuration of unsymmetrically substituted norbornenes
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The effect of a remote substituent on the regioselectivity in the oxymercuration of unsymmetrical substituted norbornenes has been investigated. Moderate to high levels of regioselectivity were observed with both exo- and endo-substituents at C-2 of norbornenes.
- Mayo, Peter,Poirier, Marc,Rainey, Jan,Tam, William
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p. 7727 - 7730
(2007/10/03)
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- Experiments towards the formation of 1,6-dehydroquadricyclane and density functional calculations on this and related molecules
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1,6-Dibromoquadricyclane (6) was obtained from norbornadiene (11) by hydroboration, oxidation of the diol 12 to the diketone 14 and its conversion into 2,6-dibromonorbornadiene (20) using tribromodioxaphosphole 16b followed by treatment of the mixture 17/18 with potassium tert-butoxide in DMSO and photocyclization of 20. Reaction of 6 with tBuLi (2 equiv.) led to the formation of 1-bromo-6-lithioquadricyclane 7, the NMR spectra of which were observed up to 0°C. 7 did not lose LiBr to give 4, but could be trapped with H2O and chlorotrimethylsilane to give 21e (53%) and 21f (64%). Reaction of 6 with fBuLi (> 4 equiv.) gave rise to 1,6-dilithioquadricyclane (21c), whose NMR spectra could also be recorded. 21c was converted into the corresponding 1,6-disubstituted quadricyclanes with D2O (87%), chlorotrimethylsilane (92%), dimethyl sulfate (55%), methyl chloroformate (45%), iodine monochloride (62%), and p-toluenesulfonyl chloride (48%). - Density functional calculations using the B3LYP/6-31G* level of theory showed that 1,6-dehydroquadricyclane (4) is a local energy minimum in its singlet electronic state. 4 contains a unique structure with 4 condensed cyclopropane units. The parent hydrocarbons 27 and 28, hitherto unknown, are also local energy minima in their singlet electronic states.
- Glueck-Walther, Susanne,Jarosch, Oliver,Szeimies, Guenter
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p. 493 - 500
(2007/10/03)
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- Enantiomers of cis- and trans-3-(4-propyl-cyclopent-2-enyl) propyl acetate. A study on the bioactive conformation and chiral recognition of a moth sex pheromone component
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The enantiomers of cis- and trans-3-(4-propyl-cyclopent-2-enyl) propyl acetate, which are conformationally constrained analogues of (Z)-5-decenyl acetate (1), a sex pheromone component of the turnip moth, Agrotis segetum, have been synthesized and tested using the elcctrophysiological single-sensillum technique. The analogues mimic a cisoid and transoid conformation of 1, respectively. In addition, the enantiomers of each of the cis- and trans-isomers are conformationally constrained analogues of enantiomeric cisoid and transoid conformations of 1. Thus, the compounds prepared and tested are well suited to investigate the nature of the bioactive conformation of the natural pheromone component 1 and the chiral sense of its interaction with the receptor. Electrophysiological single-sensillum recordings show that the activity of the most active cis-isomer, which has a (1S,4R)-configuration, is more than two orders of magnitude higher than that of the most active trans-isomer. Furthermore, the (1S,4R)-isomer is at least 100 times more active than its enantiomer. These results strongly support a previously proposed cisoid bioactive conformation of 1. Furthermore, the (1S,4R)-configuration of most active stereoisomer identifies the chiral sense of the interaction between the natural pheromone component 1 and its receptor.
- Gustavsson, Anna-Lena,Larsson, Mattias C.,Hansson, Bill S.,Liljefors, Tommy
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p. 2173 - 2183
(2007/10/03)
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- Fate of a highly strained hydrocarbon in aqueous soil environment
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The most important factor affecting the reactivity of quadricyclane in the soils was pH, while organic matter content was not important. In soils of high pH, quadricyclane behaved as a light nonaqueous phase liquid except that it had a propensity to form microemulsions. In soils of low pH, the reactions producing the alcohols and their subsequent dissolution in the groundwater became significant.
- Hill,Szechi,Hofstee,Dane
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p. 651 - 655
(2007/10/03)
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- 4S,5S-[Bis(carbethoxy)]-2-ethenyl-1,3,2-dioxaborolane: A novel enantioselective dienophile
-
The novel chiral vinyl boronate (+)-1 was utilized as a dienophile in a series of [4+2] cycloadditions. The intermediate cyclized products obtained were oxidized directly (H2O2, NaOH). The resulting alcohols displayed enantiomeric excesses of 7-33%. A slight preference for the endo configuration was observed, in agreement with previous results.
- Bonk, Jason D.,Avery, Mitchell A.
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p. 1149 - 1152
(2007/10/03)
-
- Stereoselectivity of 2-methyl-2-norpinyl cations
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Optically active (1S)-1-methylbicyclo[2.2.1]heptan-2-one (25) was prepared by starting from (1R)-bicyclo[2.2.1]heptan-2-one (21). Photolysis of the tosylhydrazone 8 of 25 generates, in part, 1-methylbicyclo[2.2.1]heptane-endo-2-diazonium ions (10) from which 1-methylbicyclo[2.2.1]heptan-endo-2-ol (14) and 2-methylbicyclo[3.1.1]heptan-2-ol (16) arise. The ee of both products (42-45%) was found to agree within experimental error. These data point to the bridged 2-methyl-2-norpinyl cation (13) as the most stable structure. Racemization of 13 via the open 2-methyl-2-norpinyl cation (17) as the transition state competes with nucleophilic capture. VCH Verlagsgesellschaft mbH, 1996.
- Herrmann, Roland,Kirmse, Wolfgang
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p. 435 - 438
(2007/10/03)
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- Bacillus stearothermophilus alcohol dehydrogenase: A new catalyst to obtain enantiomerically pure bicyclic octen- and hepten-ols and -ones
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From the cells of Bacillus stearothermophilus a new enzyme has been isolated, which catalyses the stereospecific redox reactions of bicyclic octen- and hepten- ols and -ones.
- Giovannini, Pier Paolo,Hanau, Stefania,Rippa, Mario,Bortolini, Olga,Fogagnolo, Marco,Medici, Alessandro
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p. 1669 - 1676
(2007/10/03)
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- Kinetic Resolution via Oxidation of endo-Bicyclic Octen- and Heptenols with Bacillus stearothermophilus
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Key words: Kinetic resolution, Microbial oxidation, 6-endo-Bicyclohept-2-en-6-ol, 2-endo-Bicyclooct-7-en-2-ol, 2-endo-Norborn-5-en-2-ol Kinetic resolution of the racemic endo-bicyclic octenol and heptenols via oxidation with Bacillus stearothermopilus is described.The enantiomerically pure ketones and alcohols can be obtained varying the oxidation time.
- Fantin, Giancarlo,Fogagnolo, Marco,Medici, Alessandro,Pedrini, Paola,Rosini, Goffredo
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p. 1635 - 1638
(2007/10/02)
-
- Tuning of vinylborane dienophilicity. Optimization of reactivity, regioselectivity, endo stereoselectivity, and reagent stability
-
Simple syntheses of some vinylboranes are reported, and their properties in Diels-Alder reactions are compared. Vinyl-3,6-dimethylborepane was the most stable simple vinylborane examined, and appears to be indefinitely stable at 25°C. Surprisingly, trivinylborane is the most reactive, and reacts about 18 times faster than the vinyldialkylboranes with cyclopentadiene. Vinyl-9-BBN is the most regioselective dienophile, in keeping with principally steric control of regioselectivity in Diels-Alder reactions of vinylboranes. All the dienophiles display high endo-stereoselectivity with piperylene, but vinyl-3,6-dimethylborepane displays significantly improved stereoselectivity with cyclopentadiene. In general, by choice of alkyl-substituents on boron, the reactivity, regioselectivity, endo-stereo selectivity, and stability of vinylboranes can be optimized.
- Singleton, Daniel A.,Martinez, Jose P.,Watson, Jose V.,Ndip, Grace M.
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p. 5831 - 5838
(2007/10/02)
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- 'Immunization' of Lipase against Acetaldehyde emerging in Acyl Transfer Reactions from Vinyl Acetate
-
Immobilization of Candida cylindracea lipase by covalent linkage involving the ε-amino residues of lysine leads to a five-fold increase of selectivity which is entirely preserved against deactivation caused by acetaldehyde, an unavoidable by-product in acyl transfer reactions with vinyl acetate.
- Berger, Brigitte,Faber, Kurt
-
p. 1198 - 1200
(2007/10/02)
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- Hydroboration. 82. Asymmetric Hydroboration of Representative Cis Disubstituted and Heterocyclic Olefins with Dicaranylboranes of High Enantiomeric Purity
-
Dicaranylboranes (2-dIcr2BH and 4-dIcr2BH) are prepared by the reaction of borane-methyl sulfide with (+)-2- and (+)-3-carene, respectively.The reagents readily hydroborate prochiral cis disubstituted olefins to yield the corresponding trialkylboranes, which, upon oxidation, give chiral alcohols of 77-93percent ee (2-dIcr2BH) and 36-75percent ee (4-dIcr2BH). 2-dIcr2BH and 4-dIcr2BH convert cis-alkenes into alcohols of the opposite absolute configurations.The trialkylborane hydroboration products, upon treatment with benzaldehyde, eliminate (+)-2- or (+)-3-carene, providing the corresponding benzyl boronates.Thus, the chiral auxiliary is recovered, and the alcohols can be obtained by oxidation of the benzyl boronates or alternatively by their hydrolysis and oxidation of the boronic acids.Trans disubstituted and trisubstituted olefins are hydroborated by both reagents with partial elimination of (+)-2- or (+)-3-carene and low asymmetric induction.On the other hand, the hydroboration-oxidation of five-membered ring heterocyclic olefins proceeds rapidly to give the corresponding chiral alcohols of 11-85percent ee.Convenient procedures for the preparation of high purity (+)-2- and (+)-3-carene are described.
- Brown, Herbert C.,Prasad, J.V.N. Vara,Zaidlewicz, Marek
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p. 2911 - 2916
(2007/10/02)
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- ENZYMATIC RESOLUTION OF NORBORNANE-TYPE ESTERS
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Chiral norbornane-type alcohols of high optical purity were prepared via enzymatic resolution of their racemic esters using lipases from Candida cylindracea and Pseudomonas sp.This method presents a short and efficient access to a number of chiral building blocks on a molar scale for the synthesis of optically active cyclopentane systems.
- Oberhauser, Th.,Bodenteich, M.,Faber, K.,Penn, G.,Griengl, H.
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p. 3931 - 3944
(2007/10/02)
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- Endo-Selectivity and Kinetic Control in Alkali Metal-NH3-NH4(1+) Reductions of Bicycloheptan-2-ones
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Reduction of five bicycloheptan-2-ones (6), (9), (12), (15), (18) and the parent bicyclohept-5-en-2-one (21) by alkali metals dissolving in NH3 and a cosolvent saturated with NH4Cl affords in every case predominantly (83 -> 99percent) the corresponding endo-alcohol (7), (10), (13), (16), (19), and (22), and since two of these endo-alcohols are the thermodynamically more stable and two the less stable isomers, these reductions are probably all kinetically controlled; a correlation is made between this endo-selectivity and exo-hydrogen-exchange in the ketones (6), (9), (12), (15), and (21).
- Rautenstrauch, Valentin
-
p. 1558 - 1560
(2007/10/02)
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- 221. Asymmetric Diels-Alder Reactions of Neopentyl-Ether-Shielded Acrylates and Allenic Esters: Syntheses of (-)-Norbornenone and (-)-β-Santalene
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Starting from (+)- or (-)-camphor, the antipodal alcohols 14 and 18, respectively, have been prepared; the corresponding acrylates 15 and 19 underwent TiCl2(i-PrO)2-mediated Diels-Alder additions to cyclopentadiene to give adducts 20a and 22a respectively, with 95percent endo- and 99.2percent ?-face selectivities.Adduct 22a was converted to enentiomerically pure norbornenone 26.Addition of 1,3-butadiene to acrylate 15 in the presence of TiCl4 afforded 3-cyclohexenyl carboxylate 29 with > 95.6percent stereodifferentiation.The TiCl2(i-PrO)2-promoted cycloaddition of cyclopentadiene to allenic ester 43 proceeding with 99percent face differentiation served as the key step for an efficient enantioselective synthesis of (-)-β-santalene ((-)-41) with concomitant recovery of the chiral control alcohol 14.
- Oppolzer, Wolfgang,Chapuis, Christian,Dupuis, Dominique,Guo, Maodao
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p. 2100 - 2114
(2007/10/02)
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- An Unusual Occurrence of Chemiluminescence resulting from Metal-Ammonia Reductive Dehalogenation Reactions
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Birch type reductive dehalogenation of some members of the series of norbornenes (2a - h), (4i) proceeds with chemiluminescence; an explanation for this phenomenon is advanced.
- Cotsaris, Evangelo,Paddon-Row, Michael N.
-
-
- REGIO- AND STEREO-SELECTIVITIES IN THE TITANIUM COMPLEX CATALYZED HYDROBORATION OF CARBON-CARBON DOUBLE BONDS IN VARIOUS UNSATURATED COMPOUNDS
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The titanium complex prepared from TiCl3 and NaBH4 in THF in the presence of 18-crown-6 promotes the catalytic hydroboration of carbon-carbon double bonds in alkenes, alkadienes and unsaturated ethers with NaBH4, giving sodium organoborates which can be converted into alcohols by oxidation with H2O2/CH3ONa.The reaction proceeds with high regio- and stereo-selectivities.
- Lee, Hyung Soo,Isagawa, Kakuzo,Toyoda, Hiromu,Otsuji, Yoshio
-
p. 673 - 676
(2007/10/02)
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- THE OBSERVATION OF REMARKABLE EFFECTS OF REMOTELY CONNECTED BUT SPATIALLY PROXIMATE HYDROXY-GROUPS ON THE RATES AND REGIOCHEMISTRY OF THE BRICH REDUCTION OF AROMATIC RINGS AND DOUBLE BONDS
-
Product and competitive-rate studies of Brich reduction (Li, ButOH, NH3) of a series of alicyclic compounds, (6)-(13), are described.The hydrocarbons (6a)-(8a) and the syn-methoxy derivative (6c) are slowly reduced to give the unconjugated dienes such as (16a).In contrast the syn-alcohols (6b)-(8b) were rapidly reduced to give the monoenes (17), (19), and (22), respectively.Reduction of the syn-alcohol (10b) was also extremely rapid but that of the norbornenols (11a) and (12a) only showed moderate rate enhancements.Each of the above alcohols displays intramolecular OH...? bonding.Brich reductions of (1), (6a), (10a), (11b), and (12b), none of which possess intramolecular OH...? bonds, appear to obey third-order kinetics whereas those of the OH...?-bonded alcohols (6b), (10b), (11a), and (12a) followed a combination of second- and third-order kinetics .The rate and product data for the reduction of the alcohols (6b)-(8b), (10b), (11a), and (12a) are explained in terms of the presence of intramolecular OH...? bonding in these substrates.The observed second-order kinetics are explained in terms of intramolecular protonation of the anion-radical intermediate and is supported by the data for reduction of (6c) and (9).Geometric features which affect the efficacy of intramolecular proton transfer are discussed.Full STO-3G geometry optimisations on the anion radicals of (11a and b) reveal the presence of strong OH...? bonding in the anion-radical of (11a) amounting to some 27 kJ mol-1.
- Cotsaris, Evangelo,Paddon-Row, Michael N.
-
p. 1487 - 1500
(2007/10/02)
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- Hydroboration. 61. Diisopinocampheylborane of High Optical Purity. Improved Preparation and Asymmetric Hydroboration of Representative Cis-Disubstituted Alkenes
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The convenient preparation of diisopinocampheylborane of high enentiomeric purity (99.1percent) utilizing the commercially available relatively stable borane-methyl sulfide and α-pinene of 92percent enantiomeric purity is described.Methyl sulfide liberated in the hydroboration step interferes with the equilibration needed to improve the optical purity of the reagent.However, this difficulty is readily overcome by removal of methyl sulfide under vacuum following hydroboration of the α-pinene.The raegent is then equilibrated in THF with 15percent excess α-pinene at 0 deg C for3 days.During this equilibration period, the major isomer becomes incorporated selectively into the reagent.This high optical purity diisopinocampheylborane has been utilized for asymmetric hydroboration of representative cis-disubstituted alkenes such as cis-2-butene, cis-3-hexene, cis-2-pentene, norbornene, norbornadiene, cis-4,4-dimethyl-2-pentene, and cis-propenylbenzene.Oxidation of the intermediate organoboranes provides the coprresponding alcohols in enantiomeric purities of 60-98percent.
- Brown, Herbert C.,Desai, Manoj C.,Jadhav, Prabhakar K.
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p. 5065 - 5069
(2007/10/02)
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- The twist angle approach to the stereoselectivity in reduction of ketones
-
Semiempirical MO calculations show that the ?* orbital of the carbonyl carbon of asymmetric ketones deviates from orthogonality.These small deviations, measured by twist angles, are used to predict the configurations of the diastereoisomeric alcohols formed on reduction of the ketones with complex metal hydrides.The method is particularly suitable for predicting the changes in stereoselectivity when the carbonyl compound is substituted.This is illustrated by a number of examples of reduction of substituted rigid polycyclic ketones.It is proposed that a nucleophile approaches a carbonyl group preferentially from the side of obtuse angle with respect to the O-C bond.The twists incorporate the electronic nature of the substituent and help to understand the steric effect.Tha application of the twist angle approach to other reactions is briefly mentioned.
- Giddings, Michael R.,Hudec, John
-
p. 459 - 467
(2007/10/02)
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- Deamination Reactions. 35. Decomposition of 5-Norbornene-2-diazonium Ions. Derivatives of Bicyclohept-2-ene
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Photolysis of 5-norbornen-2-one tosylhydrazone (4) in aqueous sodium hydroxide afforded 4percent of 3-norpinen-2-ol (3) in addition to 5-norbornen-2-ol (5, 10percent) and nortricyclanol (6, 86percent). 3 arises from 5-norbornene-2endo-diazonium ions (2) as shown by the nitrous acid deamination of the epimeric amines 1 and 7. 3-Norpinen-2-one (12) was isolated by oxidation of the deamination products followed by preparative g.l.c. 3 and 12 were synthesized independently from norpinan-2-one (9) via sulfenylation and sulfoxide pyrolysis.The norbornene -> norpinene rearrangement is slightly enhanced by 1-OCH3.The deamination of 1-methoxy-5-norbornen-2endo-amine (14) produced 10percent of 12, as compared to 5.6percent of 3 from 1.
- Kirmse, Wolfgang,Knoepfel, Norbert,Loosen, Karin,Siegfried, Rainer,Wroblowsky, Heinz-Juergen
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p. 1187 - 1191
(2007/10/02)
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- Die Synthese von 6exo-substituierten p-Toluolsulfonsaeure-estern
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The preparation of hitherto unknown p-toluenesulfonates of 6exo-substituted 2endo-norbornanols is described.
- Fischer, Walter,Grob, Cyril A.,Sprecher, Georg von,Waldner, Adrian
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p. 816 - 823
(2007/10/02)
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