Threo structures

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

On the basis of the assignments given in Table II, one may conclude that the threo structure is slightly predominant in the hydrogenated 1,4-polydimethylbutadiene sample. This result taken in conjunction with the microstructure of the starting material (74% trans-1,4, 23% cis-1,4, and 3% 1,2) indicates a nonstereospecific addition of hydrogen to the unsaturated 1,4 units of polydimethylbutadi ene. 

G. aabbitt In the same spectrum, you showed pairyof methylene signals and attributed the doubling to erythro and threo structures of the H-Cl units. Three units were pictured an ethylene unit in the center with H-Cl containing units on either end. It is possible that both ends can be erythro, or both ends can be threo, or be mixed. Yet, only doublets are obtained. It seems to me there should be more multiplicity in the methylene signals. 

Problem 5.35 Designate the following compounds as erythro or threo structures. 

In the case of poly(l,2-cycloalkylene)s containing symmetrical rings such as poly(l,2-cyclobutylene)s, the erythro and threo structures are also referred to as meso (M) and racemo (R) structures for denoting the stereochemistry of the rings the relative stereochemistry between the rings is denoted by meso (m) and racemo (r) configurations corresponding to diisotactic and disyndiotactic structures respectively [21,22]. 

The CPK space-filling molecular model of the threo structure represents a helical loop in which all oxygen atoms point toward the center (Fig. 1) whereas that of the erythro structure tends to form an extended rigid chain in which oxygen atoms tend to alternate along the chain because of steric crowding of the methine hydrogens (Fig. 2). As to the threo polymer, it is reasonable to assume by... 

One of the major aspects of these additions is the stereoselection which is achieved to create ( )- or (Z)-vinylmetals from alkynes, or to create erythro or threo structures from alkenes. The latest developments of this approach provide a route to non-racemic, chiral, metallated structures from common organometallics (RLi, RMgX), either by transmetallation with asymmetrically ligated transition metals (as Zr complexes) or simply... 

The effects of changes in structure, temperature, and reaction medium on the kinetics of alkaline hydrolysis of aryl esters of phosphinothioic acids (182 R = R = Me or Ph R = Me, R = Ph X = H, Br, Me, or NO2) have been investigated. The effect of the nature of the leaving group on the kinetics of alkaline hydrolysis of aryl diphenylphosphinates has also been reported on. Indirect evidence has been advanced for the participation of the azaphospholi-dines (183) in the solvolysis of A/-[amino(methyl)phosphinyl]-L-phenylalanine derivatives, and it includes the transfer of the P-amino group to the phenyl-alanyl carbonyl. The products from the hydrolysis or ammonolysis of fosfomycin have the threo structure and have been shown to possess no antibiotic activity. ... 

Several possibilities exist for ditacticity in macromolecules formed by polymerizing 1,2-disubstituted ethylenes of the type RHC=CHR, structures (XVIII)-(XX). It can be seen that each unit contains two different asymmetric carbon atoms in the chain. The original definitions of tacticity have been extended to include these modifications and Newman s (1956) definitions of erythro and threo structures. Structures (XVIII)-(XX) illustrate ifereo-diisotactic, erythro-diisotactic, and disyndiotactic poisoners, respectively. 

By a stereochemically controlled synthesis the C40 cyclopropane carotenoid precursor prephytoene alcohol [(57), as racemate] has been obtained (47). The total synthesis of azafrin [(6), as racemate] (175,176), of lutein [(14), as racemate] and 3 -epilutein [(58), as racemate] (176) and of 3,6(3, 6 )-cis (59) and 3,6(3, 6 )-trans ,e-carotene-3,3 -diol (60), both as racemates (176), have been announced in symposium reviews. Experimental details have not yet been published. The NMR data of racemic (6) were consistent with the threo structure of natural azafrin (6) that has been documented in other ways, while the H NMR properties of the models (59) and (60) were consistent with the previously established... 

Hexadiene was found to polymerize in DCA and apoCA canals on heating over 100°C for 10 to 20 days after Y-ray irradiation. Particullary trans, trans-2,4-hexadiene polymerized in an inclusion state via radical mechanism for the first time. The polymers from the monomer prefer erythro structure to threo structure in a DCA canal, while they do slightly threo to erythro in an apoCA canal. It is considered that the polymerization proceeded preferentially in the canals through trans opening to yield erythro diisotactic structure in a DCA canal [ll]. 

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