Threo-trans-erythro configuration

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... 

Propylene oxide-has two asymmetric carbon atoms, and exists as cis- and trans-isomers. If the polymerization of cis- or trans-propylene oxide- -dj proceeds exclusively by /3-opening and by either retention or inversion of the attacked /3-carbon atom, the polymer composed of the monomeric unit having exclusively an erythro or a threo configuration is expected to be formed. 

As outlined above, MeBmt contains three contiguous asymmetric carbon atoms and an (E)-trans double bond. The MeNH and OH groups are arranged in a threo configuration, whereas the OH and CH3 groups are oriented in the erythro form. 

Stereoregular polymers that can be afforded by 2,4-hexadiene and other symmetric terminally disubstituted butadienes (of the CHR CH CH CHR type) exhibit still more complex stereoisomerism, since each monomeric unit in these polymers possesses three sites of isomerism. The formation of these polymers involves 1,2- and 1,4-polymerisation. The 1,2-polymers derived from the CHR=CH—CH=CHR monomers exhibit the same type of stereoisomerism as polymers with a 3,4 structure obtained from monomers of the CH2 CH CH=CHR type. However, owing to the presence of the R substituent at the double bond in the side group of the polymer derived from a monomer of the CHR=CH—CH=CHR type, two types of eryt/zro-diisotactic, t/zraz-diisotactic and disyndiotactic polymer are foreseeable, each type with either cis or trans configuration of the double bond, as in the 1,2-polymer derived from a monomer of the CH2 CH CH CHR type. Thus, six stereo-isomeric forms of 1,2-polymer are possible for the CHR CH CH CHR monomer. The 1,4 monomeric units in the polymers formed by the polymerisation of CHR CH CH CHR monomers contain one double bond (in either cis or trans configuration) and two tertiary carbon atoms and therefore can exist as two sets of enantiomers, erythro and threo ... 

H NMR correlation studies suggest that bis-THF portion of uvaricin possesses threo/trans/threo/trans/erythro relative configuration among C(15)-C(24). Establishes general method for determining relative configuration. 

Iodide attack trans to the bridging phenyl at either carbon yields the threo isomer, but will yield the RS) or SR) isomer depending on which carbon is attacked. The basic assumption used in the interpretation of the erythro and threo isomer interconversions is that the configuration of the carbon a to the R group remains constant. Hence, if inversion occurs at both centers, it would not be detected. Thus, an observation interpreted as retention with the Ph(CHD)2-system should be viewed with suspicion nevertheless, racemization or inversion would be interpreted correctly. Studies with optically-active (CHD)Ph corroborate the observed inversion with bromination found with -(CHD)2f-Bu hence, care must be taken with interpretations using the (CHD)2Ph system, and it would appear that the butyl system provides a more reliable indication of processes at a primary carbon. 

If, during ring opening polymerization a tertiary carbon is involved in the reaction, a configurational inversion may occur in which case a change in the stereochemistry of the monomer unit with respect to the monomer will be observed. Thus, the cf5-2,3-epoxybutane gives rise to a polymer with threo monomer units, 39, whereas the trans isomer is transformed into erythro or meso units, 40 (Scheme 8) (81, 82). 

Cyanogen bromide reacts with threo- 2-piperidyl carbinol (278) to form the trans imine (279). The latter is also formed from the erythro 1-carbamate (280) using thionyl chloride for the cyclodehydration reaction. The latter reaction therefore proceeds by inversion of the configuration at the carbinol carbon atom (73AP284). 

To account for the production of bromo chlorides and bromo nitrates, it is proposed that the reaction of anions (Br, Cl , NO ) in step 5 is very fast. Independent evidence for the existence ot a metastable, cyclic bro-monium ion comes from the observation that the bromine atoms are usually in a threo (trans) orientation in the final product and that in the reverse reaction of acetolysis of dibromides, there is retention of configuration. This would be consistent with the Br-assisted SnI ionization of the dibromide. The assisting Br atom would attack the neighboring C atom from the back side to the leaving Br ion in a typical Walden inversion. Thus from irans-butene-2 we obtain erythro-2,3-dihromo butane ... 

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