Helical arrays of pendant fullerenes on optically active poly(phenylacetylene)s

Novel, optically active, stereoregular poly(phenylacetylene)s bearing the bulky fullerene as the pendant were synthesized by copolymerization of an achiral phenylacetylene bearing a [60]fullerene unit with optically active phenylacetylene components in the presence of a rhodium catalyst. The C60-bound phenylacetylene was prepared by treatment of C60 with N-(4-ethynylbenzyl)glycine in a Prato reaction. The obtained copolymers exhibited induced circular dichroism (ICD) in solution both in the main-chain region and in the achiral fullerene chromophoric region, although their ICD intensities were highly dependent on the structures of the optically active phenylacetylenes and the solution temperature. These results indicate that the optically active copolymers form one-handed helical structures and that the pendant achiral fullerene groups are arranged in helical arrays with a predominant screw sense along the polymer backbone. The structures and morphology of the copolymers on solid substrates were also investigated by atomic force microscopy.     

Temperature dependence of helical structures of poly(phenylacetylene) derivatives bearing an optically active substituent

The temperature dependence of the helical conformations for the homopolymers of phenylacetylene derivatives bearing an optically active substituent, such as the (R)-((1-phenylethyl)carbamoyl)oxy and (R)-((1-(1-naphthyl)ethyl)carbamoyl)oxy groups at the phenyl group, and their copolymers with achiral phenylacetylenes were investigated in solution using circular dichroism (CD) and absorption spectroscopies. The magnitude of the induced CD (ICD) of the optically active homopolymers increased with decreasing temperature and was accompanied by a blueshift in their absorption maxima. On the other hand, the copolymers with achiral phenylacetylenes exhibited interesting ICD changes with temperature, depending on the bulkiness of the achiral comonomers. The copolymers with a less bulky phenylacetylene had a very intense ICD at low temperatures, the ICD pattern was almost opposite to those of the chiral homopolymers, while the copolymers with the most bulky phenylacetylene bearing a tert-butyldiphenylsiloxy group at the para position showed an ICD change similar to that of the optically active homopolymers. However, the copolymers with the phenylacetylene bearing a tert-butyldimethylsiloxy group with intermediate bulkiness at the para position showed no ICD change with temperature. These results indicate that the prevailing helix-sense of the chiral-achiral random copolymers of the phenylacetylenes is determined by a delicate interaction between the chiral and achiral side chains. The thermodynamic stability parameters for the helical conformations of the homopolymers and copolymers of the phenylacetylenes were estimated from the temperature dependence of the ICDs.     

Optically active helical structure and magnetic interaction of poly(phenylacetylene)-based polyradicals

We synthesized optically active polyradicals possessing an excess of one-handed helical backbone by helix-sense-selective polymerization (HSSP) of achiral monomers which was promoted by rhodium complex catalyst in the presence of (R)- or (S)-1-phenylethylamine. The monomer, which had two hydroxy groups and a hydrogalvinoxyl unit, gave the corresponding polymer with an optically active helical conformation stabilized by intramolecular hydrogen bonding. The chemical oxidation of the polymer yielded the corresponding optically active helical polyradical with high spin concentration. The static magnetic susceptibility of the chiral polyradical was measured using a SQUID magnetometer. We have found that the stronger antiferromagnetic interaction was observed for the polyradical synthesized by HSSP in comparison with the polyradical via polymerization in the presence of racemic phenylethylamine.     

Synthesis of helical poly(phenylacetylene)s bearing cinchona alkaloid pendants and their application to asymmetric organocatalysis

Four novel dynamic helical poly(phenylacetylene)s bearing cinchona alkaloids as pendant groups were synthesized starting from the commercially available cinchona alkaloids, cinchonidine, cinchonine, quinine, and quinidine, by the polymerization of the corresponding phenylacetylene monomers with a rhodium catalyst. These polymers exhibited an induced circular dichroism (ICD) in the UV–visible region of the polymer backbones in solution, resulting from the preferred‐handed helical conformation induced by the optically active cinchona alkaloid pendants. In response to the solvent used, their Cotton effect patterns and intensities were significantly changed accompanied by the changes in their absorption spectra probably due to the changes in their helical conformations, such as the inversion of the helical sense or helical pitch of the polymers. When these helical polymers were used as polymeric organocatalysts for the asymmetric conjugated addition and Henry reactions, the optically active products with a modest enantiomeric excess were obtained whose enantioselectivities were comparable to those obtained with the corresponding cinchona alkaloid‐bound monomers as the catalysts. However, we observed a unique enhancement of the enantioselectivity and a reversal of the stereoselectivity for some helical polymers, suggesting the important role of the helical chirality during the asymmetric organocatalysis. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Switching of macromolecular helicity of optically active poly(phenylacetylene)s bearing cyclodextrin pendants induced by various external stimuli

A series of novel phenylacetylenes bearing optically active cyclodextrin (CyD) residues such as alpha-, beta-, and gamma-CyD and permethylated beta-CyD residues as the pendant groups was synthesized and polymerized with a rhodium catalyst to give highly cis-transoidal poly(phenylacetylene)s, poly-1alpha, poly-2beta, poly-3gamma, and poly-2beta-Me, respectively. The polymers exhibited an induced circular dichroism (CD) in the UV-visible region of the polymer backbones, resulting from the prevailing one-handed helical conformations. The Cotton effect signs were inverted in response to external chiral and achiral stimuli, such as temperature, solvent, and interactions with chiral or achiral guest molecules. The inversion of the Cotton effect signs was accompanied by a color change due to a conformational change, such as inversion of the helicity of the polymer backbones with a different twist angle of the conjugated double bonds, that was readily visible with the naked eye and could be quantified by absorption and CD spectroscopies. The dynamic helical conformations of poly-2beta showing opposite Cotton effect signs in different solvents could be further fixed by intramolecular cross-linking between the hydroxy groups of the neighboring beta-CyD units in each solvent. The cross-link between the pendant CyD units suppressed the inversion of the helicity; therefore, the cross-linked poly-2betas showed no Cotton effect inversion, although the polymer backbones were still flexible enough to alter their helical pitch with the same handedness, resulting in a color change depending on the degree of intramolecular cross-linking.     

Layer-by-layer assembly of charged poly(phenylacetylene)s with induced macromolecular helicity

The macromolecular helicity of charged poly(phenylacetylene)s induced by small chiral guests in water can be retained by the alternative deposition of achiral polyelectrolytes with opposite charges, resulting in optically active multilayer thin films with a macromolecular helicity memory.     

Optically active poly(hydroxyurethane)s derived from cyclic carbonate and L-lysine derivatives

The polyaddition of L -lysine hydrochloride and bifunctional five-membered cyclic carbonate 1 was carried out in DMAc in the presence of 1 equiv of DBU or 2 equiv of sodium hydride for 24 h to afford optically active polyurethane 2 bearing hydroxy and carboxyl group. The polyaddition of L -lysinol and 1 was also carried out in NMP to afford higher molecular weight of optically active polyurethane 3 bearing hydroxy group in quantitative yield. These polyurethanes reacted with cupric acetate, sodium tetrahydroborate, and titanium tetraisopropoxide to afford the corresponding crosslinked gels immediately. © 1996 John Wiley & Sons, Inc.     

Helical polymer brushes with a preferred-handed helix-sense triggered by a terminal optically active group in the pendant

Helical polymer brushes with a preferred-handed helix-sense composed of a poly(phenylacetylene) backbone and poly(phenyl isocyanate) pendants are synthesized. The helix-sense of the backbone is effectively controlled by the helical chirality of the pendants, which is triggered by an optically active group introduced at the pendant terminal.     

Asymmetric multicomponent copper catalyzed synthesis of chiral propargylamines

A three-component stereoselective reaction between an aldehyde, an amine and phenylacetylene to afford optically active propargyl amines in good yields was developed. The reaction is catalysed by copper complexes of enantiomerically pure bis-imines. The best results were obtained with imines readily prepared in very high yields from the commercially available binaphtyl diamine.

A very simple experimental procedure at room temperature allowed to obtain optically active propargyl amines in very good yields and enantioselectivity up to 75%. The extremely simple methodology and the mild reaction conditions, as well as the possibility of a modular approach for developing new and more efficient bis-imine-based chiral ligands make the present methodology very attractive.     

Three-component chiral derivatizing protocols for NMR spectroscopic enantiodiscrimination of hydroxy acids and primary amines

The novel three-component chiral derivatization protocols have been derived for (1)H and (19)F NMR spectroscopic discrimination of a series of chiral hydroxy acids by their coordination and self-assembly with optically active α-methylbenzylamine and 2-formylphenylboronic acid. In addition, the optically pure (S)-mandelic acid in combination with 2-formylphenylboronic acid permits visualization of enantiomers of primary amines. These protocols have been demonstrated on enantiodiscrimination of chiral amines and hydroxy acids.     

Halogenative kinetic resolution of β-amido alcohols: chiral BINAP-mediated SN2 displacement of hydroxy groups by chlorides with inversion of stereochemistry

A series of optically active cyclic trans-β-amido alcohols were obtained by the non-enzymatic kinetic resolution of the corresponding racemic amido alcohols using commercially available (S)-BINAP and NCS by S_N2 halogenation of the hydroxy group. The product, cis-β-amido chloride, was also obtained in optically active form with an inversion of stereochemistry.     

Halogenative kinetic resolution of β-aryloxy cyclic alcohols: chiral BINAP-mediated SN2 displacement of hydroxy groups by chlorides with inversion of stereochemistry

A series of optically active cyclic trans-β-aryloxy alcohols have been obtained by non-enzymatic kinetic resolution of the corresponding racemic aryloxy cyclic alcohols using commercially available (S)-BINAP and NCS by S_N2 halogenation of a hydroxy group. The product, cis-β-aryloxy chlorides, was also obtained in optically active form with inversion of the stereochemistry.     

Hierarchical amplification of macromolecular helicity of dynamic helical poly(phenylacetylene)s composed of chiral and achiral phenylacetylenes in dilute solution, liquid crystal, and two-dimensional crystal

Optically active poly(phenylacetylene) copolymers consisting of optically active and achiral phenylacetylenes bearing L-alanine decyl esters (1L) and 2-aminoisobutylic acid decyl esters (Aib) as the pendant groups (poly(1L(m)-co-Aib(n))) with various compositions were synthesized by the copolymerization of the optically active 1L with achiral Aib using a rhodium catalyst, and their chiral amplification of the macromolecular helicity in a dilute solution, a lyotropic liquid crystalline (LC) state, and a two-dimensional (2D) crystal on the substrate was investigated by measuring the circular dichroism of the copolymers, mesoscopic cholesteric twist in the LC state (cholesteric helical pitch), and high-resolution atomic force microscopy (AFM) images of the self-assembled 2D helix-bundles of the copolymer chains. We found that the macromolecular helicity of poly(1L(m)-co-Aib(n))s could be hierarchically amplified in the order of the dilute solution, LC state, and 2D crystal. In sharp contrast, almost no chiral amplification of the macromolecular helicity was observed for the homopolymer mixtures of 1L and Aib in the LC state and 2D crystal on graphite.     

Facile synthesis of 6a-carbaprostaglandin i2

The optically active 6a-carbaprostaglandin I₂ (2), a stable mimic of natural prostacylin (1), was synthesized from the lactone 4 or the hydroxy acid 5, which were general synthetic intermediates for natural prostaglandins.     

Macromolecular helicity inversion of poly(phenylacetylene) derivatives induced by various external stimuli

Unique macromolecular helicity inversion of stereoregular, optically active poly(phenylacetylene) derivatives induced by external achiral and chiral stimuli is briefly reviewed. Stereoregular, cis-transoidal poly(phenylacetylene)s bearing an optically active substituent, such as (1R,2S)-norephedrine (poly- 1 ) and β-cyclodextrin residues (poly- 2 ), show an induced circular dichroism (ICD) in the UV-visible region of the polymer backbone in solution due to a predominantly one-handed helical conformation of the polymers. However, poly- 1 undergoes a helix-helix transition upon complexation with chiral acids having an R configuration, and the complexes exhibit a dramatic change in the ICD of poly- 1 . Poly- 2 also shows the inversion of macromolecular helicity responding to molecular and chiral recognition events that occurred at the remote cyclodextrin residues from the polymer backbone; the helicity inversion is accompanied by a visible color change. A similar helix-helix transition of poly((R)- or (S)-(4-((1-(1-naphthyl)ethyl)carbamoyl)phenyl)acetylene) is also briefly described.     

Asymmetric Syntheses XXVI: Catalytic Enantioselective Syntheses of β-Hydroxy Esters via Double Chiral Induction in Asymmetric Reformatsky Reactions

Catalytic asymmetric Reformatsky reactions of benzaldehyde with optically active menthyl bromoacetates in the presence of Zn-Cu couple were performed using 0.25 equiv. of (1R,2S) or (1S,2R)-dimethyl-2-amino-1,2-diphenyl ethanol as chiral ligand to obtain β - hydroxy esters with enantioselectivities up to 60.2%. The obvious double chiral induction effect was observed while chiral ligands matched with optically active substrates.     

Enantioselective Syntheses of Substituted γ‐Butyrolactones

The previously described chiral 2‐acyloxathianes 5 (Scheme I) are used in two different enantioselective syntheses of γ‐butyrolactones. In one synthesis, Grignard addition, cleavage and reduction to carbinols RR'C(OH)CH₂OH is followed by tosylation, malonate homologation, lactonization, and removal of the carbomethoxy group to give optically active γ‐lactones. A modification of this synthesis (Scheme I) leads to optically active α‐methylene‐γ‐lactones. In the second synthesis, reaction of a bromomagnesium enolate with ketones 5 leads to β‐hydroxyesters, which, by appropriate sequences of reduction and cleavage (Scheme II) are converted to optically active α‐ or β‐hydroxy‐γ‐lactones.     

Synthesis and bifunctional asymmetric organocatalysis of helical poly(phenylacetylene)s bearing cinchona alkaloid pendants via a sulfonamide linkage

Four novel helical poly(phenylacetylene)s with amino‐functionalized cinchona alkaloid pendant groups connecting to the phenyl rings through a sulfonamide linkage were synthesized by the polymerization of the corresponding phenylacetylene monomers using Rh⁺(2,5‐norbornadiene)[(η⁶‐C₆H₅)B^?(C₆H₅)₃] (Rh(nbd)BPh₄) as the catalyst. The optically active sulfonamide‐linked polymers adopted a helical conformation with an excess of one‐handedness as supported by the appearance of the induced Cotton effects in the main‐chain chromophore regions, and efficiently catalyzed the enantioselective methanolytic desymmetrization of a cyclic anhydride and aza‐Michael addition of aniline to chalcone, thereby producing the corresponding optically active products up to 86% enantiomeric excess. However, their enantioselectivities from the methanolytic desymmetrization were slightly lower than those catalyzed by the corresponding cinchona alkaloid‐bound monomers. On the other hand, during the asymmetric aza‐Michael addition, a unique enhancement of the enantioselectivity was observed for several sulfonamide‐linked helical polymers, and thus affording a remarkably higher enantioselectivity compared to those of the corresponding monomers and nonhelical polymers bearing the identical cinchona alkaloid residues. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2869–2879     

Stereospecific novel glycosylation of hydroxy β-lactams via iodine-catalyzed reaction: a new method for optical resolution

Glycosylation of racemic and optically active α-hydroxy β-lactams by reaction with a few glycal derivatives in the presence of catalytic amounts of iodine has provided stereospecific formation of α-glycosides. This method has been extended for the preparation of optically active hydroxy β-lactams in excellent yields. The stereochemistry and nature of the glycals as well as the stereochemistry of the β-lactam ring has a profound influence for effective glycosylation.     

Simple Stereospecific Syntheses of (R)-2-Fluorohexanoic Acid Ethyl Ester

Simple stereospecific syntheses of 2-fluorohexanoic acid 5 were accomplished by conversion of optically pure L-(+)-norleucine to the optically pure hydroxy acid 3 via the classic diazotization reaction followed by substitution of the hydroxy functionality by the fluoro group. This was accomplished stereospecifically using DAST reagent or more practically by nucleophilic displacement of the corresponding mesyloxy group with fluoride ion.