A study on the conformation-anomeric effect-stereoselectivity relationship in anomeric radical reactions,using conformationally restricted glucose derivatives as substrates

We previously theorized that, since the stereoselectivity of anomeric radical reactions is significantly influenced by the kinetic anomeric effect, which can be controlled by restricting the conformation of the radical intermediate, the proper conformational restriction of the pyranose ring of the substrates would therefore make highly alpha- and beta-stereoselective anomeric radical reactions possible. This theory was based on our previous results of the anomeric radical reactions with d-xylose derivatives as the substrates. We herein report the anomeric radical deuteration reactions with the conformationally restricted 1-phenylseleno-d-glucose derivatives, 2g and 3g, restricted in a (4)C(1)-conformation by an O-cyclic diketal moiety, and 4g, 5g, 6g, 7g, and 8g, restricted in a (1)C(4)-conformation by bulky O-silyl protecting groups. The radical deuterations with Bu(3)SnD, using the (4)C(1)-restricted substrates 2g and 3g, afforded the corresponding alpha-products (alpha/beta = 98:2) highly stereoselectively, whereas the (1)C(4)-restricted substrate 6g, having a trigonal (sp(2)) carbon substituent, i.e., -CHO, at the 5-position, selectively gave the beta-products (alpha/beta = 0:100). Thus, the stereoselectivity was significantly increased by the conformational restriction and was completely inverted by changing the substrate conformation from the (4)C(1)-form to the (1)C(4)-form. On the other hand, the deuterations with the (1)C(4)-restricted substrates 4g and 5g showed that the 1,5-steric effect due to the tetrahedral carbon substituent (-CH(2)OTIPS or -CH(2)OH) at the 5-axial position dominantly prevented the hydride transfer from the beta-face competing with the kinetic anomeric effect. This study suggests that, depending on the restricted conformation of the substrates to the (4)C(1)- or the (1)C(4)-form, the alpha- or beta-products would be obtained highly stereoselectively via anomeric radical reactions of hexopyranoses.     

Triethylborane-mediated hydrogallation and hydroindation: novel access to organogalliums and organoindiums

Hydrogallation of carbon[bond]carbon multiple bonds proceeds in the presence of triethylborane as a radical initiator. Several functionalities do not interfere with this reaction. Resulting alkenyl- and alkylgallium species can be trapped by several electrophiles. Highly regioselective radical addition of an indium hydride reagent to alkynes is also achieved. Various functionalities are tolerant under the reaction conditions. The reaction proceeds with complete anti stereoselectivity. Alkenylindiums obtained via hydroindation can be employed for the following cross-coupling reaction with aryl halides in one pot.     

2-Silyl group effect on the reactivity of cyclopentane-1,3-diyls. Intramolecular ring-closure versus silyl migration

Generation of singlet and triplet 2-silylcyclopentane-1,3-diyls and their reactivity have been investigated in the thermal and photochemical denitrogenation of 2,3-diaza-7-silylbicyclo[2.2.1]hept-2-ene. 5-Silylcyclopentene (silyl migration product) is quantitatively obtained, while 5-silylbicyclo[2.1.0]pentane (intramolecular ring-closure product) is not detected in the denitrogenation reactions. Deuterium labeling studies clarify that 5-silylcyclopentene is formed by a suprafacial [1,2] silyl migration in singlet 2-silylcyclopentane-1,3-diyl. UDFT calculations closely reproduce the observed reactivity of the singlet diradical: The enthalpic barriers of the intramolecular ring-closure are calculated to be DeltaH++exo468 = 5.8 kcal/mol and DeltaH++endo468 = 6.7 kcal/mol, which are much higher than the energy barrier for the [1,2] silyl migration, DeltaH++468 = 2.7 kcal/mol. The notable effect of the silyl group on raising the energy barrier of the intramolecular cyclization is rationalized by an electronic configuration of the lowest singlet state of 2-silylcyclopentane-1,3-diyls.     

Radiation-induced addition reaction of carbon tetrachloride onto 1,2-polybutadiene

The γ-ray-induced addition reaction of carbon tetrachloride onto syndiotactic 1,2-polybutadiene film and liquid 1,2-polybutadiene was carried out at room temperature. In the film gelation was pronounced and the rate of addition increased as the crystallinity of the polymer decreased. In the liquid gelation, which makes the polymer insoluble in carbon tetrachloride, did not take place, although a definite crosslinking reaction was noticed. In this case the appearance of the product changed from a viscous liquid to a white powder as the reaction proceeded. Its structure was compared with that of chlorinated 1,2-polybutadiene. The addition of carbon tetrachloride to the vinyl group in liquid 1,2-polybutadiene caused an anti-Markownikoff-type reaction and was accompanied by an unexpectedly large vinyl depletion in the polymer. The total decrease in the vinyl group was found to be much larger than that brought about by the addition of carbon tetrachloride. This discrepancy was attributed to a cyclization and crosslinking reaction ascribed to the vinyl group bound by the main chain. Cyclization and crosslinking were less noticeable in the chlorination than in the carbon tetrachloride.     

Monolayer properties of mixtures of poly(n-stearyl methacrylate), poly(n-lauryl methacrylate), and their corresponding copolymers

The monolayer properties of poly(n-stearyl methacrylate), poly(n-lauryl methacrylate), and their mixtures at various ratios of the two polymers have been studied from the measurements of their surface pressure–area isotherms at air–water interface. The monolayer properties of their mixtures have been compared with those of their corresponding copolymers. The results show that the isotherms of the mixed monolayers have two break points at higher pressures than that of poly(n-lauryl methacrylate). This suggests that the mixtures may form more stable films that consist of separate phases of the two homopolymers, although each phase may contain a small amount of the other. The isotherms of the copolymer monolayers indicate a phase transition from liquid condensed to solid film between 50 segment mole % and 70% poly(n-stearyl methacrylate). The monclayer of these copolymers has properties that differ from those of the corresponding mixtures of two pure homopolymers and is more compatible than the mixtures of pure homopolymers.     

Some structural features induced by the space-time metrical fluctuation in the theory of gravitational fields

Under the assumption that the so-called space-time fluctuationy(x) in a classical sense, attached to each point of the gravitational field at some microscopic stage, is summarized as the metrical fluctuation in the formg _λκ (x)=g_λκ (x)·exp2σ(y(x)), some new physical aspects induced by the conformal scalarσ(x) (≡σ(y(x))) are found: By introducing the torsionT _κ ^λμ (x) from a general standpoint, the resulting micro-gravitational field is made to have a conformally non-Riemannian structure, where a special form ofT _κ ^λμ (i.e.,T _κ ^λμ =δ _κ ^λ σ_μ?δ _κ ^μ σ_λ(σ_μ=?σ/?x ^μ)) shows some peculiar features. An averaging process with respect toy is taken into account, by which the spatial structure of the corresponding macro-field is shown, in general, to have a somewhat “non”-Riemannian structure due to the contributions of the torsionT _κ ^λμ .     

m-Diethynylbenzene macrocycles: syntheses and self-association behavior in solution

m-Diethynylbenzene macrocycles (DBMs), buta-1,3-diyne-bridged [4(n)]metacyclophanes, have been synthesized and their self-association behaviors in solution were investigated. Cyclic tetramers, hexamers, and octamers of DBMs having exo-annular octyl, hexadecyl, and 3,6,9-trioxadecyl ester groups were prepared by intermolecular oxidative coupling of dimer units or intramolecular cyclization of the corresponding open-chain oligomers. The aggregation properties were investigated by two methods, the (1)H NMR spectra and the vapor pressure osmometry (VPO). Although some discrepancies were observed between the association constants obtained from the two methods, the qualitative view was consistent with each other. The analysis of self-aggregation by VPO revealed unique aggregation behavior of DBMs in acetone and toluene, which was not elucidated by the NMR method. Namely, the association constants for infinite association are several times larger than the dimerization constant, suggesting that the aggregation is enhanced by the formation of dimers (a nucleation mechanism). In polar solvents, DBMs aggregate more strongly than in chloroform due to the solvophobic interactions between the macrocyclic framework and the solvents. Moreover, DBMs self-associate in aromatic solvents such as toluene and o-xylene more readily than in chloroform. In particular, the hexameric DBM having a large macrocyclic cavity exhibits extremely large association constants in aromatic solvents. By comparing the aggregation properties of DBMs with the corresponding acyclic oligomers, the effect of the macrocyclic structure on the aggregation propensity was clarified. Finally, it turned out that DBMs tend to aggregate more readily than the corresponding phenylacetylene macrocycles, acetylene-bridged [2(n)]metacyclophanes, owing to the withdrawal of the electron density from the aromatic rings by the butadiyne linkages which facilitates pi-pi stacking interactions.     

Unambiguous Determination of the Absolute Configurations of Acetylene Alcohols by Combination of the Sonogashira Reaction and the CD Exciton Chirality Method – Exciton Coupling between Phenylacetylene and Benzoate Chromophores

The CD exciton chirality method was applied to various phenylacetylene alcohols to determine their absolute configurations; the long axis polarized –^* transition (_max=252nm) of the 4-methoxyphenylacetylene chromophore couples with the transition (_max=257nm) of the 4-methoxybenzoate group to generate intense exciton split CD Cotton effects, from the signs of which the absolute configurations of phenylacetylene alcohols were unambiguously determined. As an extension of the results, a new methodology for determining the absolute configurations of acetylene alcohols having the HCCCH(OH)-moiety by combination of the Sonogashira reaction and the CD exciton chirality method has been developed and applied. Since the –^* transition of acetylene triple bond is located below 180nm, it is difficult to observe ideal bisignate CD Cotton effects due to the exciton coupling between acetylene and benzoate chromophores. To observe the ideal exciton split Cotton effects necessary for the unambiguous determination of absolute configuration, the terminal acetylene group was converted, by the Sonogashira reaction, to the 4-methoxyphenylacetylene moiety, which exhibits an intense –^* absorption band polarized along the long axis of the chromophore at 252nm. As a partner of exciton coupling, 4-methoxybenzoate showing a –^* band at 257nm was introduced into the alcohol moiety, and the benzoates formed showed intense bisignate CD Cotton effects, from the signs of which the absolute configurations of original acetylene alcohols could be determined in an unambiguous manner.