Meso structures

The molecular scaffolds also provide an unusual opportunity for the identification and recognition of meso structures. For example, successive acylation of L and D phenylalanine with the acridine diacid gives the structure 56 and we anticipate that... 

There is little variation in the shape of the isotherms for a series of meso structural isomers (C-15 3,3 -C-15 12,12 ) upon moving the carbonyl group up the hydrocarbon chain. There is an expansion of the film, as would be expected if the head groups were allowed to drift apart at large areas (> 240 A2) per molecule. A comparison of the IT//1 isotherms for the series of structural isomers shows that the meso-diastereomers become more expanded at low surface pressures as the carbonyl group is moved up the hydrocarbon chain. 

New reactor technologies are currently under development, and these include meso- and micro-structured reactors or the use of membranes. Among meso-structured reactors, monolithic catalysts play a pre-eminent role in environmental applications, initially in the cleaning of automotive exhaust gases. Beside this gas-solid application, other meso-structures such as membranes [57, 58], corrugated plate or other arranged catalysts and, of course, monoliths can be used as multiphase reactors [59, 60]. These reactors also offer a real potential for process intensification, which has already been demonstrated in commercial applications such as the production of hydrogen peroxide. 

A systematic investigation of the free amino acids of the Leguminosae led to the isolation of a novel ninhydrin-positive compound from the leaves of Derris elliptica Benth. (Papilionidae) (93). This substance was analyzed as C6H,3N04 (microanalysis and high resolution mass spectrometry) and was shown to be an amino alcohol. The absence of a carbonyl in the 1R, the loss of 31 mass units in the mass spectrum, and a positive periodate cleavage reaction were best embodied into a dihydroxydihydroxymethylpyrrolidine structure. The relative simplicity of the NMR spectra (three peaks in the 13C spectrum four spin-system in the H spectrum) pointed out a symmetrical structure. Inasmuch as the material was optically active ([a]D 56.4, c = 7, H20), meso structures were ruled out, and the 2R, 3R, 4R, 5R relative configuration was retained (93). This structure (53) was further confirmed by an X-ray determination (94). 

Note that the alcohol portion of hyoscyamine, namely tropine, also contains two chiral centres, but it is a symmetrical molecule and is optically inactive it can be considered as a meso structure (see Box 3.21). Thus, the optical activity of hyoscyamine stems entirely from the chiral centre in the acid portion, tropic acid. 

The polymer obtained from 9 by y-radiation was soluble in chloroform despite a high crystallinity. The alternating molecular stacking of 9 led to stereoregular polymer formation with a disyndiotactic structure. The racemo and meso structures of the resulting polymers were confirmed by NMR spectroscopy. A comparison of the NMR data of related polymers concludes that the chemical shifts for a series of the polymers are predominantly determined by the meso-racemo structure rather than the diisotactic-disyndiotactic one. 

Structures VII and VIII are identical because rotating either one 180° in the plane of the paper makes is superposable with the other one. VII possesses a symmetry plane and is achiral. Achiral stereoisomers which have chiral centers are called meso. The meso structure is a diastereomer of either of the enantiomers. The meso structure with two chiral sites always has the (RS) configuration. 

Stereoisomerism in compounds with two stereo centres diastereomers and meso structure In compounds whose stereoisomerism is due to tetrahedral stereocentres, the total number of stereoisomers will not exceed 2", where n is the number of tetrahedral stereocentres. For example, in 2,3,4-trihydroxybutanal, there are two chiral carbons. The chiral centres are at C-2 and C-3. Therefore, the maximum number of possible isomers will be 2 = 4. All four stereoisomers of 2,3,4-trihydroxybutanal (A-D) are optically active, and among them there are two enantiomeric pairs, A and B, and C and D, as shown in the structures below. 

Now, let us consider another similar molecule, tartaric acid, where there are two chiral carbons. In tartaric acid, four isomeric forms are theoretically expected (2 = 4). However, because one half of the tartaric acid molecule is a mirror image of the other half, we get a meso structure. This means this compound and its mirror image are superimposable, i.e. they are the same compound. Thus, instead of four, we obtain only three stereoisomers for tartaric acid. 

Structures 1 and 2 are enantiomers, and both are optically active. In structures 3 and 4, there is a plane of symmetry, i.e., there is a mirror image within a single molecule. Such a structure is called a meso structure. Structures 3 and 4 are superimposable, and essentially are the same compound. Hence, we have a meso-tartaric acid and it is achiral (since it has a plane of symmetry, and it is superimposable on its mirror image). Meso-tartaric acid is optically inactive. Therefore, for tartaric acid, we have (-f), (—) and meio-tartaric acid. 

To improve the meso-structural order and stability of the mesoporous silica ropes, a postsynthesis ammonia hydrothermal treatment (at 100 °C) was invoked. As indicated by the XRD profile in Fig. 3A, 4-5, sharp features are readily observed in ammonia hydrothermal treated samples. Moreover, after the post-synthesis ammonia treatment, the sample also possesses a sharp capillary condensation at p/po 0.35(Fig. 3B) corresponding to a much narrower BJH pore size distribution of ca. 0.12 nm (at FWHM). In other words, the mesostructures are not only more uniform but also more stable when subjected to the post-synthesis treatment. The morphology of the silica ropes remained unchanged during the ammonia hydrothermal process. The mesostructures remain intact under hydrothermal at 100 °C in water even for extended reaction time (> 12 h). 

The Sharpless epoxidation is sensitive to preexisting chirality in selected substrate positions, so epoxidation in the absence or presence of molecular sieves allows easy kinetic resolution of open-chain, flexible allylic alcohols (Scheme 26) (52, 61). The relative rates, kf/ks, range from 16 to 700. The lower side-chain units of prostaglandins can be prepared in high ee and in reasonable yields (62). A doubly allylic alcohol with a meso structure can be converted to highly enantiomerically pure monoepoxy alcohol by using double asymmetric induction in the kinetic resolution (Scheme 26) (63). A mathematical model has been proposed to estimate the degree of the selectivity enhancement. 

Epoxides that have meso structures can be converted to optically active chiral compounds via enantioselective ring opening (Scheme 125). 

In the presence of a cinchona alkaloid, certain cyclic carboxylic anhydrides with meso structures are converted to the chiral diacid monoesters in up to 76% ee (Scheme 10) 31). Quinine or cinchonidine and quinidine or cinchonine show opposite asymmetric induction. 

Notice further that, consistent with the principle developed in Section 7.9, optically inactive starting materials (achiral alkenes and bromine) yield optically inactive products (a racemic mixture or a meso structure) in these reactions. 

A different experimental approach to the relative importance of one-center and two-center epimerizations in cyclopropane itself was based on the isomeric l-13C-l,2,3-d3-cyclopropanes165"169. Here each carbon has the same substituents, one hydrogen and one deuterium, and should be equally involved in stereomutation events secondary carbon-13 kinetic isotope effects or diastereotopically distinct secondary deuterium kinetic isotope effects may be safely presumed to be inconsequential. Unlike the isomeric 1,2,3-d3-cyclo-propanes (two isomers, only one phenomenological rate constant, for approach to syn, anti equilibrium), the l-13C-l,2,3-d3-cyclopropanes provide four isomers and two distinct observables since there are two chiral forms as well as two meso structures (Scheme 4). Both chiral isomers were synthesized, and the phenomenological rate constants at 407 °C were found to be k, = (4 l2 + 8, ) = (4.63 0.19)x 10 5s l and ka = (4kl2 + 4, ) = (3.10 0.07) x 10 5 s 1. The ratio of rate constants k, kl2 is thus 1.0 0.2 both one-center and two-center... 

Meso-Structures of Polymer Melts Confined in Curved 184... 

MESO-STRUCTURES OF POLYMER MELTS CONFINED IN CURVED SURFACES... 

In order to develop a theoretical method for describing the meso-structures of diblock copolymer confined in curved surfaces, MC simulation was first used to find the possible phase separation structures of diblock copolymer melt. 

To establish the molecular thermodynamic model for uniform systems based on concepts from statistical mechanics, an effective method by combining statistical mechanics and molecular simulation has been recommended (Hu and Liu, 2006). Here, the role of molecular simulation is not limited to be a standard to test the reliability of models. More directly, a few simulation results are used to determine the analytical form and the corresponding coefficients of the models. It retains the rigor of statistical mechanics, while mathematical difficulties are avoided by using simulation results. The method is characterized by two steps (1) based on a statistical-mechanical derivation, an analytical expression is obtained first. The expression may contain unknown functions or coefficients because of mathematical difficulty or sometimes because of the introduced simplifications. (2) The form of the unknown functions or unknown coefficients is then determined by simulation results. For the adsorption of polymers at interfaces, simulation was used to test the validity of the weighting function of the WDA in DFT. For the meso-structure of a diblock copolymer melt confined in curved surfaces, we found from MC simulation that some more complex structures exist. From the information provided by simulation, these complex structures were approximated as a combination of simple structures. Then, the Helmholtz energy of these complex structures can be calculated by summing those of the different simple structures. 

The macroscopic properties such as mechanical behavior of block copolymers or polymer blends depend directly on the relative concentrations of different constituents and their meso-structures. How to predict the exact macroscopic properties of polymer blends or block copolymers with meso-phase separation structures from pure component properties remains a big challenge. Some theoretical efforts have been explored. For example, Buxton et al. found that the deformations and fractures of polymer blends can be described by the... 

FIGURE 5.12 Racemic and meso structures of edge-sharing dinuclear complexes of [M2(OH)2(LL)4] type with absolute configurations with absolute configurations (above) and hydroxoaqua complexes (below). 

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