Screw Sense Research Articles

Helix-Sense-Selective Polymerization of Achiral Phenylacetylenes with Two N-Alkylamide Groups to Generate the One-Handed Helical Polymers Stabilized by Intramolecular Hydrogen Bonds.

In this study, we investigated the helix-sense-selective polymerizations of newly synthesized achiral phenylacetylenes having two N-alkylamide groups, such as 3,5-bis(dodecylamide)phenylacetylene (DAPA), 3,5-bis(octylamide)phenylacetylene (OAPA), and 3,5-bis(butylamide)phenylacetylene (BAPA), by using a chiral rhodium catalyst system ([Rh(nbd)Cl]2-(R)-(+)-1-phenylethylamine [(R)-PEA]). Poly(BAPA) was insoluble in any solvents. On the other hand, poly(DAPA) and poly(OAPA) were soluble in toluene, THF, and CHCl3, and the obtained polymers showed intense circular dichroism signals at the absorption region of the main chain in the UV-vis region. This result suggested those polymers were present in helical conformations with an excess of one-handed screw sense. The chiral helix structures of the formed polymers were stable in toluene at room temperature for a long time because of intramolecular hydrogen bonds. This result is the second example about polyacetylenes with one-handed helical conformation stabilized by intramolecular hydrogen bonds.

Chiral microspheres constructed by helical substituted polyacetylene: A new class of organocatalyst toward asymmetric catalysis

This article reports the first microspheres consisting of helical substituted polyacetylene and showing interesting asymmetric catalysis ability. For preparing such chiral microspheres, two acetylene-based monomers, M1L with l-proline pendant group and MA, underwent precipitation copolymerization in the presence of rhodium catalyst in a solvent mixture of butanone/n-heptane. Regular microspheres (approx. 1μm in diameter) were successfully prepared under suitable conditions. According to CD and UV spectroscopy measurements, the microspheres exhibit considerable optical activity, resulting from the substituted polyacetylene chains adopting helical structures of preferred one-handed screw sense. The microspheres were used as a chiral organocatalyst to heterogeneously catalyze the asymmetric Aldol reaction of p-nitrobenzaldehyde and cyclohexanone in aqueous media. Remarkable catalytic efficiency was obtained (yield 76% and ee 80%). After catalysis, the microspheres can be easily recovered. The methodology created in this study provides opportunities for developing a new class of chiral polymer organocatalyst.

Chiral Microspheres Consisting Purely of Optically Active Helical Substituted Polyacetylene: The First Preparation via Precipitation Polymerization and Application in Enantioselective Crystallization

This article reports on a novel type of microspheres (∼720 nm in diameter) prepared via precipitation polymerization and constructed by optically active helical substituted polyacetylene (PSA). The microspheres were obtained in high yield (>80%), with regular morphology and narrow size distribution. PSA forming the microspheres was found to adopt helices with predominant one-handed screw sense, according to circular dichroism and UV–vis absorption spectroscopies and specific optical rotation measurements. The helical conformations of PSA endowed the microspheres thereof with considerable optical activity. The chiral microspheres feature in combining in one entity the advantages of both chiral polymers and the micrometer-sized particles in scale and spherical morphology and thus are expected to find some significant applications. This is well exemplified by successful induction of enantioselective crystallization with the chiral microspheres. Such chiral microspheres efficiently induced enantioselective cr...

Induced circular dichroism in chiral N-methyl amides possessing an achiral binaphthyl chromophore and its application to absolute configuration determination of aliphatic chiral amines

Induced circular dichroism (ICD) was observed in binaphthyl N-methyl amides of chiral primary amines. The CD spectra showed split curves centered at 225nm. A good correlation was found between the sign of the exciton chirality of the binaphthyl derivative and the absolute configuration of the original amine. Furthermore, the screw sense of the two naphthyl groups in the most stable conformer obtained from molecular mechanics (MM) calculation was in agreement with those expected from the exciton chirality, indicating that the method based on MM calculations can be used to determine unknown absolute configurations of aliphatic chiral amines. The practical use of the present method was demonstrated by the determination of the absolute configurations of the natural products, d- and l-cycloserines.

Chiral crystallization of aromatic helical foldamers via complementarities in shape and end functionalities

Biomolecules such as α-helices are exclusively right-handed without contamination by their left-handed counterparts. Within the abiotic world, chiral or external perturbations have to be applied to the abiotic helical foldamers or polymers to control and bias their helical screw sense. Otherwise, it is not possible to separate racemic helical foldamers composed of achiral building blocks. By designing two complementary “sticky” groups, incorporating them into the ends of helical pentamers and taking advantage of the energetically more favored full overlap, which involves helical backbones, we succeeded in demonstrating, for the first time, a spontaneous resolution of racemic helices into their enantiopure single-handed helical forms via chiral crystallization without the use of chiral auxiliary or external stimuli.

Optically Active Helical Substituted Polyacetylenes Showing Reversible Helix Inversion in Emulsion and Solution State

This Communication reports two substituted polyacetylenes that can adopt helical structures of preferential screw sense in both emulsion (nanoparticle) and solution; however, the handedness of the macromolecular helices is just opposite in the two states. More interestingly, the helical screw sense of the polymers demonstrated a reversible transition between the two states. The unprecedented findings are of significant importance for acquiring new insights into helical polymers and for developing novel advanced chiral materials.

Hydrogen‐Bond‐Assisted Helical Folding of Propeller‐Shaped Molecules: Effects of Extended π‐Conjugation on Chiral Selection, Conformational Stability, and Exciton Coupling

AbstractCooperative interaction between multiple chiral centers dictates the absolute handedness of structural folding. We have designed and prepared a series of chiral C3‐symmetric tris(N‐salicylidenamine) derivatives that adopt three‐blade propeller‐like conformations. Synthetic access to an expanded family of such constructs was aided by enzymatic resolution and C–C cross‐coupling reactions of aryl‐substituted chiral propargylic alcohol derivatives. These key structural components were integrated into molecular propellers of predetermined helical screw sense. Through comparative studies on a homologous set of molecules, we found that installation of phenylene‐ethynylene‐derived π‐conjugation profoundly affected the stabilities of the helically folded structures, as evidenced by UV/Vis and circular dichroism (CD) studies. Increasing the number of hydrogen bonds through additional substitution also enhanced the populations of the folded conformations in solution. In addition to introducing steric bias to control structural folding, linearly π‐conjugated groups function as spatially well‐defined chromophores that give rise to characteristic exciton‐coupled circular dichroism. Absolute configurations of chiral centers could thus be further confirmed by comparing the torsional relationships between pairs of chromophores on adjacent subunits, which are fully consistent with the computationally predicted structural models.

Determination of the Helical Screw Sense and Side‐Group Chirality of a Synthetic Chiral Polymer from Raman Optical Activity

Splitting it up: excellent agreement between the experimental and the quantum-chemically simulated Raman optical activity (ROA) spectrum of (+)-poly(trityl methacrylate) shows that the polymer backbone adopts a left-handed helical conformation while the trityl side groups display a left-handed propeller conformation. Thus ROA can be used to determine the complete structure of synthetic chiral polymers in solution.

Synthesis of polymers bearing proline moieties in the side chains and their application as catalysts for asymmetric induction

AbstractNovel polyphenylacetylene and polystyrene derivatives carrying L‐proline moieties at the side chains were synthesized by the rhodium‐catalyzed and radical polymerizations of the corresponding monomers. The polyphenylacetylene derivatives showed Cotton effects at the absorption region of the main chain, indicating that the polymers adopt helical conformations with predominantly one‐handed screw sense. The polymers catalyzed the asymmetric aldol reactions of acetone with aromatic aldehydes, and cyclohexanone with p‐nitrobenzaldehyde. The enantioselectivities largely depended on the reaction conditions. In the asymmetric aldol reaction of acetone with aromatic aldehydes, the R‐enantiomeric products were predominantly obtained except the cases with the polymer catalyst in CHCl3. The ee of the products became higher as the reaction temperature was decreased. The polymeric catalysts were recoverable from the reaction mixture by filtration, and the recovered ones catalyzed the asymmetric aldol reaction of acetone with p‐nitrobenzaldehyde without decreasing the product yield and ee. The ee was improved using the copolymers of L‐proline‐based and nonchiral monomers as catalysts. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Isovaline in naturally occurring peptides: A nondestructive methodology for configurational assignment

The nonproteinogenic, C(α)-tetrasubstituted, helicogenic, chiral α-amino acid isovaline (Iva) is remarkably spread in the biosphere. Together with its achiral, lower homolog α-aminoisobutyric acid (Aib), it represents a characteristic marker of a class of naturally occurring peptide antibiotics, for which the acronym "peptaibiotics" became established. In these peptides, Iva occurs as the (S)-(= L) or the (R)-(= D) enantiomer, but peptide sequences containing both Iva enantiomers are also common. Here, we applied our recently developed (1)H-NMR method, which enables the nondestructive assignment of the configuration of each Iva residue in a peptide of known helical screw sense, to natural and synthetic peptaibiotics. Our method proved to be generally applicable and provided evidence that, in the peptaibiotic bergofungin A, the Iva(12) configuration is (R) and not (S) as reported previously. Moreover, we extended our NMR method by including a (13)C-NMR parameter. A statistical analysis of the preferred main- and side-chain conformations of the Iva residues in peptides, performed based on their published X-ray diffraction structures, allowed us to provide a sound rationale to the NMR criteria exploited to establish the configuration of this amino acid.

Structural Examination of Dissolved and Solid Helical Chiral Poly(trityl methacrylate) by VCD Spectroscopy

The structures of the helical chiral polymer poly(trityl methacrylate), PTrMA, in solid state and in chloroform solution are compared by using vibrational circular dichroism spectroscopy. The differences between the spectra are discussed in detail based on band assignments and density functional theory calculations. Therefore, VA and VCD spectra of oligomers of PTrMA were calculated. It is shown that in solution as well as in solid state the helical structure is maintained. Furthermore, it was possible to determine the helical screw sense of the dextrorotary enantiomer (+)-PTrMA to be left-handed.

Direct Access to Polyisocyanide Screw Sense Using Vibrational Circular Dichroism

We show that the screw sense of polyisocyanide helices can be determined in a simple manner from the vibrational circular dichroism (VCD) of their CN-stretching mode. The relation between VCD and molecular structure is obtained using the coupled-oscillator approximation. It is shown that since the C═N groups point approximately radially outward from the helical axis, the CN-stretch region of the VCD spectrum of a polyisocyanide helix consists of a single couplet, the sign of which is directly related to the screw sense of the helix. We use this method to determine the screw sense of poly(R)-2-isocyanooctane and poly(S)-2-isocyanooctane from their VCD spectrum.

Synthesis and Secondary Structure of Poly(1-methylpropargyl-N-alkylcarbamate)s

A series of novel 1-methylpropargyl-N-alkylcarbamates, HC≡C-(R)-CH(CH3)OCONHCH2CO2CH2CH2CH3 [(R)-1], HC≡C-(S)-CH(CH3)OCONHCH2CO2CH2CH2CH3 [(S)-1], HC≡C-(S)-CH(CH3)OCONH-(R)-CH(CH3)CH2CH2CH2CH2CH3 [(S),(R)′-2], HC≡C-(S)-CH(CH3)OCONH-(R)-CH(CH3)CO2CH2CH2CH3 [(S),(R)′-3], HC≡CCH2OCONH-(R)-CH(CH3)CO2CH2CH2CH3 [(R)′-4], HC≡CCH2OCONH-(S)-CH(CH3)CO2CH2CH2CH3 [(S)′-4], and HC≡CCH2OCONHCH2CO2CH2CH2CH3 (5) were synthesized and polymerized using (nbd)Rh+[η6-C6H5B−(C6H5)3] (nbd = 2,5-norbornadiene) as a catalyst. Substituted cis-stereoregular polyacetylenes with moderate molecular weights were obtained in good yields. Poly[(R)-1], poly[(S)-1], poly[(S),(R)′-2], and poly[(S),(R)′-3] exhibited large optical rotations (|[α]D| = 344°−508°), clear CD signals (Δe = 4.8−7.0 M−1 cm−1) in CHCl3, and high viscosity indices (0.82−0.87), demonstrating the formation of helical structures with predominantly one-handed screw sense, which were stable up to 55 °C in the solvent. The solution state IR measurement confirmed the presenc...

Configurational Assignment of D‐ and L‐Isovalines in Intact, Natural, and Synthetic Peptides by 2D‐NMR Spectroscopy

We investigated, by means of 2D-NMR, the naturally occurring and chemically synthesized 16-mer integramides A and B, which belong to a group of bioactive, fungal peptides (peptaibiotics), that are characterized by an abundance of Aib as well as D- and L-Iva residues. The chemical shifts of the C(alpha)-alkyl groups in the Iva enantiomers depend on the alpha-C-atom configuration and on the helical screw sense of the peptides, the latter determined by CD. In the full-length, right-handed helical integramides, as well as in the partial sequences exploited for their total chemical syntheses, the gamma-Me H-atoms of the Et side chain of the D-Iva residues located near the C-terminus are significantly more shielded (delta<0.90 ppm) than those of the L-Iva residues (delta>0.95 ppm). The opposite behavior is observed for the left-handed, synthetic, intermediate Z-Aib-L-Hyp-L-Iva(14)-D-Iva(15)-O(t)Bu. Here, the gamma-Me H-atoms of L-Iva(14) are more shielded (0.838 ppm) than those of D-Iva(15) (0.905 ppm). The chemical-shift difference between the diastereotopic beta-CH(2) H-atoms of the Iva side chains in the right-handed helical peptides is much larger for D-Iva than for L-Iva. For D-Iva(14/15), the values range from 0.38 to 0.63 ppm, whereas, for D-Iva(1), the value is in the range of 0.26-0.31 ppm. In each case, the difference is always larger for the d-Iva than for the l-Iva residues (which is always <or=0.19 ppm). Again, an opposite behavior is seen for the left-handed tetrapeptide. Overall, our method enables the nondestructive assignment of the configuration of each Iva residue in peptides of known helical screw sense.

Unusual Chiral Transfer in Radical Copolymerization of Vinyl Monomers Bearing Side‐on p‐Terphenyl Pendants

Radical copolymerization of a chiral monomer, (+)-2,5-bis[4'-((S)-2-methylbutoxy)phenyl]styrene and an achiral monomer, 2,5-bis(4'-hexyloxyphenyl)styrene, is carried out in anisole at 90 degrees C with benzoyl peroxide as the initiator. The resultant optically active helical copolymers show an unusual linear relationship between optical activity and composition, in sharp contrast with Green's "sergeants-and-soldiers" and "majority" rules. Analysis of the polarimetry, circular dichroism, and chain extension results suggest that the weak steric interaction between the propagation helical radical and the incoming monomer drove the copolymer chain to grow in a specific direction. Moreover, the helical macroinitiator is successfully used to induce the helix-sense-selective atom transfer radical polymerization of an achiral analogous monomer to yield optically-active block copolymers, which indicate a convenient way to prepare helical polymers with an excess screw sense from an achiral monomer by means of radical polymerization.

N- versus C-Terminal Control over the Screw-Sense Preference of the Configurationally Achiral, Conformationally Helical Peptide Motif Aib8GlyAib8

Conformational control over the screw sense of a helical 17-mer of achiral amino acids, Aib(8)GlyAib(8), from a single chiral residue located at the N-terminus is better than that from a single amino acid located at the C-terminus. X-ray crystallography indicates that Aib(8)GlyAib(8) forms the longest 3(10) helical structure observed crystallographically to date.

Controlling the helical screw sense of peptides with C‐terminal L‐valine

One chiral L-valine (L-Val) was inserted into the C-terminal position of achiral peptide segments constructed from alpha-aminoisobutyric acid (Aib) and alpha,beta-dehydrophenylalanine (Delta(Z)Phe) residues. The IR, (1)H NMR and CD spectra indicated that the dominant conformations of the pentapeptide Boc-Aib-DeltaPhe-(Aib)(2)-L-Val-NH-Bn (3) and the hexapeptide Boc-Aib-DeltaPhe-(Aib)(3)-L-Val-NH-Bn (4) in solution were both right-handed (P) 3(10)-helical structures. X-ray crystallographic analyses of 3 and 4 revealed that only a right-handed (P) 3(10)-helical structure was present in their crystalline states. The conformation of 4 was also studied by molecular-mechanics calculations.

Host-guest complexation-triggered chiroptical change of poly(phenylacetylene)s bearing binaphthocrown ether moieties on the main chain

Diacetylene monomers with respective lengths of the oxyethylene chains were cyclopolymerized with a rhodium catalyst to produce novel poly(phenylacetylene)s bearing a different cavity size of the chiral crown ether in the repeating units (2a–c). In the circular dichroism spectra of the resulting polymers, characteristic Cotton effects were observed in the range from 350 to 500 nm corresponding to the absorption of the conjugated polymer backbone, indicating that the polymers possessed a helical structure with an excess single screw sense induced by the covalently bonded binaphthyl units. The host–guest complexation of 2a–c with achiral guests produced a chiroptical change based on the fluctuation in the main chain conformation. The behavior of the complexation-induced chiroptical change was essentially dictated by the cavity size of the binaphthocrown ether units. Additionally, a chirality-responsive helicity change was observed in the case of the complexation of 2a–c with chiral guests, which also depended on the crown ether size. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1197–1206, 2010

An electric charge has no screw sense – a comment on the twistfree formulation of electrodynamics by da Rocha and Rodrigues

AbstractDa Rocha and Rodigues (RR) claim (i) that in classical electrodynamics in vector calculus the distinction between polar and axial vectors and in exterior calculus between twisted and untwisted forms is inappropriate and superfluous, and (ii) that they can derive the Lorentz force equation from Maxwell's equations. As to (i), we point out that the distinction of polar/axial and twisted/untwisted derives from the property of the electric charge of being a pure scalar, that is, not carrying any screw sense. Therefore, the mentioned distinctions are necessary ingredients in any fundamental theory of electrodynamics. If one restricted the allowed coordinate transformations to those with positive Jacobian determinants (or prescribed an equivalent constraint), then the RR scheme could be accommodated; however, such a restriction is illegal since electrodynamics is, in fact, also covariant under transformations with negative Jacobians. As to (ii), the “derivation” of the Lorentz force from Maxwell's equations, we point out that RR forgot to give the symbol F (the field strength) in Maxwell's equations an operational meaning in the first place. Thus, their proof is empty. Summing up: the approach of RR does not bring in any new insight into the structure of electrodynamics.

An electric charge has no screw sense - a comment on the twistfree formulation of electrodynamics by da Rocha and Rodrigues

Da Rocha and Rodigues (RR) claim (i) that in classical electrodynamics in vector calculus the distinction between polar and axial vectors and in exterior calculus between twisted and untwisted forms is inappropriate and superfluous, and (ii) that they can derive the Lorentz force equation from Maxwell's equations. As to (i), we point out that the distinction of polar/axial and twisted/untwisted derives from the property of the electric charge of being a pure scalar, that is, not carrying any screw sense. Therefore, the mentioned distinctions are necessary ingredients in any fundamental theory of electrodynamics. If one restricted the allowed coordinate transformations to those with positive Jacobian determinants (or prescribed an equivalent constraint), then the RR scheme could be accommodated; however, such a restriction is illegal since electrodynamics is, in fact, also covariant under transformations with negative Jacobians. As to (ii), the "derivation" of the Lorentz force from Maxwell's equations, we point out that RR forgot to give the symbol F (the field strength) in Maxwell's equations an operational meaning in the first place. Thus, their proof is empty. Summing up: the approach of RR does not bring in any new insight into the structure of electrodynamics.