Propylene oxide enantiomers

Polymerization in which a tactic polymer is formed. However, polymerization in which stereoisomerism present in the monomer is merely retained in the polymer is not to be regarded as stereospecific. For example, the polymerization of a chiral monomer, e.g., R)-propylene oxide ((i )-methyloxirane), with retention of configuration is not considered to be a stereospecific reaction however, selective polymerization, with retention, of one of the enantiomers present in a mixture of R)- and (S)-propylene oxide molecules is so classified. 

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 first example of kinetic resolution catalyzed by an organometallic compound was the partially enantiomer-selective polymerization of racemic propylene oxide induced with a diethylzinc optically active alcohol system (50). 

In the presence of chiral polymerization catalysts, enantiomeric monomers are consumed at different rates (Scheme 75). Enantiomer-selective polymerization of racemic propylene oxide catalyzed by a diethylzinc-(-f)-bomeol system is a classical example of such kinetic resolution H 176). The polymeric product has an [a]D of +7.4°. The mechanism... 

The first enantiomer-selective polymerization was performed with propylene oxide (172) as a monomer [245], The polymerization was carried out with a ZnEt2/(+)-bor-neol or ZnEt2/(-)-menthol initiator system. The obtained polymer was optically active and the unreacted monomer was rich in (S)-isomer. Various examples are known concerning the polymerization and copolymerization of 172 [246-251 ]. A Schiff base complex 173 has been shown to be an effective catalyst In the polymerization at 60°C, the enantiopurity of the remaining monomer was 9% ee at 50% monomer conversion [250],... 

As shown in Scheme 9, various organic compounds can act as a chiral initiator of asymmetric auto catalysis. 2-Methylpyrimidine-5-carbaldehyde 9 was subjected to the addition of z-Pr2Zn in the presence of chiral butan-2-ol, methyl mandelate and a carboxylic acid [74], When the chiral alcohol, (S)-butan-2-ol with ca. 0.1% ee was used as a chiral initiator of asymmetric autocatalysis, (S)-pyrimidyl alkanol 10 with 73% ee was obtained. In contrast, (,R)-butan-2-ol with 0.1% ee induced the production of (A)-10 with 76% ee. In the same manner, methyl mandelate (ca. 0.05% ee) and a chiral carboxylic acid (ca. 0.1% ee) can act as a chiral initiator of asymmetric autocatalysis, therefore the S- and IC enantiomers of methyl mandelate and carboxylic acid induce the formation of (R)- and (S)-alkanol 10, respectively. Chiral propylene oxide (2% ee) and styrene oxide (2% ee) also induce the imbalance of ee in initially forming the zinc alkoxide of the pyrimidyl alkanol in the addition reaction of z-Pr2Zn to pyrimidine-5-carbaldehyde 11 [75]. Further consecutive reactions enable the amplification of ee to produce the highly enantiomerically enriched alkanol 12 (up to 96% ee) with the corresponding... 

The R sites accept (/ )-propylene oxide in preference to the (5)-enantiomer, resulting in the formation of RRR isotactic sequences of polymer chains. On the other hand, the S sites accept (.Sj-propylcnc oxide in preference to the (R)-enantiomer, resulting in the formation of SSS isotactic sequences of polymer chains. 

For this reason, the two enantiomers of propylene oxide are commonly used as chiral pool starting materials. These epoxides react with the appropriate Giignard reagent to give either enantiomer of the sulcatol. 

Schmidt and co-workers reported the first stereoselective synthesis of both enantiomers of a nonactic acid derivative, as well as nonactin itself.As depicted in Scheme 4.30, 2-lithiofuran was reacted with (S)-propylene oxide to... 

Both enantiomers of a member of the new chiral pool, propylene oxide 44, can be made from lactic acid 33. The idea is to reduce the acid and cyclise in two different ways. One is simple enough. Ethyl lactate 41 is mesylated, to turn the secondary alcohol into a leaving group 42, and then the ester is reduced. Cyclisation uses the primary alcohol of 43 as the nucleophile in an internal SN2 reaction so that inversion gives (R) -propylene oxide15 44. 

Derived from them and from other chiral pool members is a small collection of epoxides are members of the new chiral pool both enantiomers of propylene oxide 44, of glycidol 231 235, and of epichlorhydrin 236 and 237, and the bis-epoxide 117. 

The amine 49, made from one enantiomer of propylene oxide by simple reactions, is the source of the chirality. A cross-metathesis with 47, using the Grubbs catalyst 50 gave a surprisingly excellent yield (89%) of the complete carbon skeleton 49 of monomorine. 

Figure 5.27 summarizes our synthesis of (/ )-(+)-143, the opposite enantiomer of the naturally occurring (-)-nocardione B. Commercially available 5-methoxy-l-tetralone (A) was treated with lithium hexam-ethyldisilazide (LiHMDS), followed by (S )-propylene oxide (B) in the presence of scandium triflate in dry toluene to give hydroxy ketone C. The hydroxy group of C was protected as 2,2,2-trichloroethoxycarbonate to give D, and its alicyclic ring was oxidized with selenium dioxide to give E. 

An interesting problem arises in polymerization of propylene oxide. This cyclic monomer possesses an assymetric carbon atom and therefore it exists in two enantiomor-phic forms. The question arises whether, under the same conditions, propagation of a pure enantiomer proceeds with the same rate as the propagation of the racemic mixture. Such a study was reported by Price53,448>. Polymerization initiated by potassium r-butox-ide in dimethyl sulphoxide or in hexamethyl phosphoric-triamide was investigated at several temperatures and the following results were reported ... 

Following this exploratory work, the Schmidt group (13) reported the first homochiral synthesis of methyl (-)- and (+)-nonactate using (S)-propylene oxide as the only enantiomerically pure starting material (Scheme 4). This gave the mixture of diastereoisomeric methyl nonactates 38, 39,40 and 41, all with the ( -configuration at C-8, and chromatographic separation provided 25% of the natural (-) ester 38. The mixture of the other three diastereoisomers (39+40+41) was converted by Walden inversion into a mixture of the same three compounds with C-8 inverted, from which (+)-methyl (25,35,6R,8/J)-nonactate 44 could be isolated in approximately the same amount as its enantiomer 38. 

A stereospecific synthesis of both enantiomers of 8-hydroxy-3-methyl-3,4-dihydroisocoumarin, mellein, from propylene oxide has been described <97TA2153>. 

Only recently, the diastereoselective synthesis and preparative separation of the enantiomers of galaxolide (Givaudan) have been described. [169] The titanium tetrachloride-catalysed Friedel-Crafts alkylation of 1,1,2,3,3-pentamethylin-dane with (S)-propylene oxide produces two epimeric alcohols (whereby no racemisation is observed) with paraformaldehyde and catalytic amounts of sulfuric acid, these are converted into the desired isochroman diastereomers. The separation of the epimers is accomplished by means of the corresponding chro-... 

Cationic polymerizations of oxiranes are much less isospecific and regiospecific than are anionic polymerizations. In anionic and coordinated anionic polymerizations, only chiral epoxides, like propylene oxide, yield stereoregular polymers. Both pure enantiomers yield isotactic polymers when the reaction proceeds in a regiospecific manner with the bond cleavage taking place at the primary carbon. 

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