Optical activity diastereomeric helicities

Optically Active Polysilanes with Diastereomeric Helicities Containing Opposite Screw Senses and Different Screw Pitches... 

This knowledge and understanding may be helpful to characterize local conformations of other optically active polysilanes in solution. For example, poly(methyl-(-)-(3-pinanylsilane) [(+)-7 Mw = 10,200] prepared by Shinohara and co-workers.281 showed a bisignate CD band at 280 and 303 nm, associated with a broad UV absorption at 300 nm in chloroform at 15°C. Since the spectroscopic features are quite similar to those of i,28d-28e it is possible that the main chain in 7 may contain diastereomeric helical motifs with opposite screw senses and different screw pitches. 

Acylnitroso compounds 197 (R = Me, Ph or Bn) react in situ with 1-methoxycarbonyl-1,2-dihydropyridine to yield solely the bridged adducts 198 quantitatively. On the other hand, 1 1 mixtures of the regioisomers 199 and 200 were formed from the nitroso-formates 187 (R = Me or Bn) (equation 110)103. The chiral acylnitroso compounds 201 and 202, which are of opposite helicity, add to cyclohexadiene to give optically active dihydrooxazines in greater than 98% diastereomeric excess (equations 111 and 112)104. Similarly, periodate oxidation of the optically active hydroxamic acid 203 in the presence of cyclopentadiene, cyclohexa-1,3-diene and cyclohepta-1,3-diene affords chiral products 204 (n = 1, 2 and 3, respectively) in 70-88% yields and 87-98% de (equation 113)105. 

Our template synthesis of knots implies that the target molecules are obtained as cationic dicopper(I) complexes. Therefore we considered the possibility of interconverting both enantiomers into a pair of diastereomeric salts [137, 138] by combining them with an optically active anion. Binaphthyl phosphate (BNP") [139] drew our attention because its chirality arises from the binaphthyl core, which is twisted. This helical structure is of the same type as that of die copper double helix, precursor of the knot. Besides, both compounds are aromatic and, thus, we could expect some potentially helpful stacking interactions [87],... 

One may have a question of whether an optically active helical polymer obtained from an enantiopure monomer adopts a purely P- (or M-) screw sense helical main chain in solution at a given temperature, or is composed of an ensemble of pseudo-diastereomeric mixed helical motifs containing P- and M-screw senses. Fluorescence (FL) studies combined with circular dichroism (CD), UV, and NMR spectra of the main chain constitute a powerful probe in identifying the main chain chirality (screw sense, uniformity, and rigidity) and optical purity of helical polymers, since the photoexcited energy above... 

Diisocyanobenzene derivatives yield helical polymers via a cyclopolymerization mechanism by the polymerization with Pd and Ni complexes. Optically active polymers were initially obtained by the method illustrated in Figure 8.139 143 Monomer 66 was reacted with an optically active Pd complex to form diastereomeric pentamers 67, which were separated into (+)- and (—)-forms by HPLC. The polymerization of 68 using the separated 69 led to a one-handed helical polymer.139 The polymerization of 68 using the initiators having chiral binaphthyl groups, 69—71, also produced optically active polymers.142 The helix-sense selectivity in the polymerization using 69... 

A remarkable recent example of the influence of electropolymerization temperature on the properties of PAn products is observed in the potentiostatic polymerization of aniline in the presence of chiral (+)-HCSA to give optically active PAn/(+)-HCSA films. The CD spectra of the polymers electrodeposited at < 25°C were inverted compared to the spectra of analogous emeraldine salts deposited at > 40°C, indicating an inversion of the preferred helical hand for PAn chains.38 The observations may be rationalized in terms of a temperature-induced interconversion between two initially (kinetically) formed diastereomeric PAn products. 

An optically aaive single-handed helical stereocomplex is obtained by the interaction of it-PMMA with the above-described optically active st-PMMA/Cgo complex or st-PMMA (Figure 6). When the optically active st-PMMA/Cgo complex free from PEA was mixed with it-PMMA the encapsulated Cso molecules were gradually replaced with the added it-PMMA to afford an optically active stereocomplex. From the comparison of experimental VCD of the products and theoretical VCD obtained by DFT calculations for triple-stranded helical stereocomplex stmetures with various combinations of stereostructures of double-helical 9i it-PMMAs and 18i single-handed st-PMMAs (diastereomeric helical assemblies composed of the same or opposite handed it- and st-PMMAs), it was concluded that it-PMMA interacts with the outer st-PMMA helix and folds into a double helix with the same handedness as that of the st-PMMA helix. An optically active stereocomplex can also be formed by the interaaion between it-PMMA and optically aaive st-PMMA with single-handed helicity, which was induced by (R)- or (S)-PEA and memorized after removal of the alcohol. 

Helix-sense-selective polymerization has been achieved for several other optically active isocyanide monomers. In the polymerization of l-105 with NiCl2, helix sense was controlled by solvent and temperature. The spectra of poly-105 obtained in nonpolar solvents such as CCI4 and toluene were almost mirror images of those of poly-105 obtained in polar THF, indicating that the polymers have opposite helical senses, that is, the polymers are diastereomeric helices. The poly-105s with opposite helical senses showed cholesteric LC phases with opposite twist senses. 

ECD, VCD, Raman optical activity (ROA), and ORD are at disposal in a spectral range between the IR and the vacuum UV. ACD and AORD, the CD and ORD of chiral anisotropic phases are only available in the UV/vis spectral range. The specific rotation, as a standard, is measmed with the sodium D-line. For special applications other lines, e.g., mercury lines, have been taken. Indirect methods for chiroptical analyses are the nuclear magnetic resonance (NMR) spectroscopy of diastereomeric compounds and the chiral induction of cholesteric phases (helical twisting power (HTP)) combined with ACD/ CD and the corresponding selective reflection. 

Like hypericin the E. exhibit photodynamic activity and inhibit protein kinase C. The E. exist in solution as tautomeric mixtures, some E. can be separated into diastereomeric pairs. On account of their helical chirality, some E. exhibit very high optical rotation. 

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