Erythro structures

The ROMP of the following compounds has been reported 130109,391, 131392, 132592,595 133394 i34395,396 135396,397 13 341,395,396,398-404 137397 i38405,406 139395,407 14() l,5 141-143337 jn a cases the reactive double bond is that in the C4 ring. The repeat units in the polymer have an erythro structure corresponding to the cis relationship of the bonds which attach the cyclobutene ring to the rest of the ring system in the monomer. 

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... 

The addition is stereospecific for example the adducts of the symmetrical cis olefins 199 and 201 have the three structure (equation 94) and symmetrical trans olefins (200, 202, 203) give adducts with the erythro structure (equation 95). Both forms undergo trans elimination... 

A more complicated picture emerges when the polymerization of 1,2-disubstituted ethylenes (CHR=CHR ) is considered because now each carbon atom in the chain becomes a chiral center. The resulting ditactic structures are illustrated in Figure 6.1(d,ed). Two isotactic structures are obtained, the erythro, in which all the carbon atoms have the same configuration, and the threo, in which the configuration alternates. Only one disyndiotactic structure is possible. The differences arise from the stereochemistry of the starting material if the monomer is cis-substimted the threo form is obtained, whereas a trans monomer leads to the erythro structure. 

The terms erythro and threo are shown in quotation marks for those structures in which tire labels would apply if the two groups that added to the olefiiuc carbon atoms were identical, even though they are not the same. In the structures below, the erythro structure would become a meso structure if the phenyl and methyl were made equivalent. In the "erythro" structure to its right, however, the bromine and acetoxy groups would also have to be made identical in order for the compoimd to become a meso structure. 

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]. 

Fig. 7. Structure of one member of the amphomycin family where Dab = D-erythro-Oc, /5-diaminobutyric acid Dab = L-threo-a, /5-diaminobutyric acid ...

Since the main chain in this representation is in an entirely staggered conformation, whereas in the Fischer projection formulas the conformation represented is completefy eclipsed, an anti relationsh between two adjacent substituents in an extended structure corresponds to being on the same side in a Fischer projection formula (erythro) whereas a syn relationship corresponds to being on opposite sides in die Fischer projection (three). 

Chemical Name 4-(hydroxy-2-piperidinylmethyl)-1,2-benZBnediol Common Name Erythro-3,4-dihydroxyphenyl-2-piperidinylcarbinol Structural Formula nn... 

It was noted (23) that the NMR spectrum of compound 32 was identical with that published (34) for the third product (assigned structure 29) isolated from the reaction of tetra-O-acetyl-2-hydroxy-D-glucal with acetic anhydride and zinc chloride. The identity of the compounds was fully established and a revised structure proposed for this third product. In the presence of zinc chloride, therefore, epimerization can occur at an allylic site and the quasi-equatorial C-4 acetoxy group in the erythro isomers 27 and 28 can assume the favored quasi-axial orientation (24). 

Bardet, M. Robert, D. Lundquist, K. von Unge, S. Distribution of erythro and threo forms of different types of P-O-4 structures in aspen lignin by carbon-13 NMR using the 2D inadequate experiment. Magn. Reson. Chem. 1998, 36, 597-600. 

Akiyama, T. Goto, H. Nawawi, D. S. Syafii, W. Matsumoto, Y. Meshitsuka, G. Erythro/threo ratio of P-O-4-structures as an important structural characteristic of lignin. Part 4 variation in the erythro/threo ratio in softwood and hardwood lignins and its relation to syringyl/guaiacyl ratio. Holzforschung 2005, 59, 276-281. 

NMR-spectroscopic studies and a single crystal X-ray structure determination of the reduced Co / form of 100 revealed the presence of a bridging 2,3-dibromo-3-phenyl-propionato ligand (threo dl pair). The complex bearing the erythro form of 2,3-dibromo-3-phenyl-propionate is only produced in minor yields <3%. Therefore, the bromination of the encapsulated alkene is a highly diastereoselective syu-addition. This is rather... 

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... 

The conformational properties of furanose rings have received considerable attention in recent years because of the impact these properties may have in mediating biological processes . Most notable in this respect are the p-D-ribofuranose 1 and 2-deoxy-p-D-erythro-pentose 2 (Scheme 1) components of ribo- (RNA) and deoxyribonucleic (DNA) acids. It is well known that the furanose ring adopts specific shapes depending on its local structural environment in a biopolymer. For example, in tRNA, the... 

---