Propylene oxide stereochemistry

The stereochemistry of ring-opening polymerizations has been studied for epoxides, episul-fides, lactones, cycloalkenes (Sec. 8-6a), and other cyclic monomers [Pasquon et al., 1989 Tsuruta and Kawakami, 1989]. Epoxides have been studied more than any other type of monomer. A chiral cyclic monomer such as propylene oxide is capable of yielding stereoregular polymers. Polymerization of either of the two pure enantiomers yields the isotactic polymer when the reaction proceeds in a regioselective manner with bond cleavage at bond 1. 

The same concept is presented by Tsuruta, Inoue, Ishimori and Yoshida (95) in their study of propylene oxide polymerization. They concluded that the stereochemistry of the monomeric units in the isotactic structure is regulated by the asymmetric center of the proceeding monomer unit. 

N-alkylated valine may be slmllarlly synthesized from an epoxide and valine. Figure 15A. Synthesis of DL-mlxtures of N-(2-hydroxyethyl)- and N-(2-hydroxypropyl)-vallne have been performed using o-bromo-lsovalerlc acid and the appropriate alky lamine, Figure 15B (2 ). The reactions of propylene oxide and N-acetyl-cystelne methyl ester, valine methyl ester and N-benzoyl-hlstldlne methyl ester respectively were recently thoroughly Investigated and the stereochemistry discussed (257). 

White has also completed two syntheses of racemic methyl nonactate (154). The first approach controlled the C-8 alcohol stereochemistry, and the second provided a rapid entry into the ring system. The first sequence, outlined in Scheme 4.28, began with the opening of propylene oxide by 2-lithiofuran. The Friedel-Crafts acylation that followed also resulted in protection of the alcohol as the acetate to give 175 in 81% overall yield. Hydrogenation of the furan ring over rhodium on charcoal gave a 96% yield of tetrahydrofuran diastereomers... 

Now one of the two OH groups must be replaced by NH2 with retention of configuration. This can be achieved by the ortho-ester method like that summarised in chapter 23 for propylene oxide. The intermediates 188 and 189 can be isolated but the process works better if 189 is hydrogenated to give the oxazoline 184 in 89% yield over the three steps. Note that the stereochemistry of 189, and hence 184, comes from a double inversion at the position a to the ester group. 

Thionylaniline reacted with ethylene oxide at low temperatures to afford the oxathiazolidine (127), which was converted into a piperazine (128) at 160°C. The same workers also studied the stereochemistry of the products formed in a similar reaction using propylene oxide. A similar... 

In poly(propylene oxide), both crystalline and amorphous polymer can occur. Amorphous polymer, or amorphous segments in the polymer chain, can arise either from atactic sequences along the chain (stereochemistry) or from head-to-head isomerism in an otherwise tactic sequence. Both Vandenberg (76) and Price (77) found up to 30 percent head-to-head structures in some poly(propylene oxide)s. Polymer from mixed abnormal and normal ring opening of propylene oxide is most likely to occur when there is considerable cationic character to a coordination polymerization initiator. 

The range of polymer properties that can be achieved depending on the stereochemistry of the poly(alkylene oxide) is in part exemplified by poly(propylene oxide), but can be extended to the entire spectrum of polymers of alkylene oxides (Table 8). 

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