Synthesis and polymerization of racemic and optically active substituted β-propiolactones. VI. α-Phenyl β-propiolactone

The synthesis and optical resolution of α-phenyl β-amino-ethylpropionate led to the preparation of optically active α-phenyl β-propiolactones (PhPL) of different optical purities. The enantiomeric excess of PhPL was determined using 200 MHz ¹H-NMR spectroscopy, after complexation with tris[3-(trifluoromethyl hydroxymethylene)-d-camphorato]europium III. It was then polymerized, in bulk and in solution, using a potassium acetate/crown ether complex as initiator. The optically active poly(PhPL)s thus obtained are insoluble in most organic solvents, whereas atactic poly(PhPL)s are soluble in CCl₄, CHCl₃, and dichloroethane. Several differences are observed between the physical properties of optically active and atactic poly(PhPL)s. However, atactic poly(PhPL)s are semi-crystalline polymers, similar to poly(α-disubstituted β-propiolactone)s, but in contrast with poly(α-methyl β-propiolactone). Melting (T_f) and glass transition temperatures, as well as enthalpy of fusion (ΔH), vary with the optical purity of the polymers. For example, atactic poly(PhPL) exhibits a T_f = 94°C and ΔH = 9 J/g as compared to T_f = 119°C and ΔH = 37 J/g for a poly(PhPL) having an enatiomeric excess of 50%.     

Preparation,characterization, and porperties of optically active poly(α-methyl-α-ethyl-β-propiolactones)

S(?) and R(+) enantiomers of α-methyl-α-ethyl-β-propiolactone (MEEPL) were prepared in an eight-step synthesis with respective optical purities of 99 and 97% determined by ¹H-NMR (250 MHz) spectroscopy. Polymers (PMEPL) of different enatiomeric compositions were prepared with an anionic-type initiator. Substantial differences in physical properties were observed between the racemic and optically pure polymers; for example, the melting point of the latter is 42°C higher than that of the former. Chiroptical properties of PMEPLs are reported. The ¹³C-NMR (100.62 MHz) spectra of the polymers indicated that the distribution of configurational units in the macromolecular chain is random.     

Lipophilic Functionalized Cobyrinic-Acid Derivatives. Part 1. Cobester α-monoacids (α,α,α,β,β,β-hexamethyl α-hydrogen Coα, Coβ-dicyanocobyrinates)

The four α,α,α, β,β,β,-hexamethyl α-hydrogen Coα, Coβ-dicyanocobyrinates 2b, d–f , with a free b-, d-, e-, and f-propionic-acid function, respectively, were prepared by partial hydrolysis of heptamethyl Coα, Coβ-dicyanocobyrinate (cobester; 1 ) in aqueous sulfuric acid. The cobester monoacids 2b, d–f were obtained as a ca. 1:1:1:1 mixture which was separated. The monoacids were purified by chromatography and isolated in crystalline form. The position of the free propionic-acid function was determined by an extensive analysis of 2b, d–f using 2D-NMR techniques; an analysis of the C,H-coupling network topology resulted in an alternative assignment strategy for cobyrinic-acid derivatives, based on pattern recognition. Additional information on the structure of the most polar of the four hexamethyl cobyrinates, of the b-isomer 2b , was also obtained in the solid state from a single-crystal X-ray analysis. Earlier structural assignments based on 1D-NMR spectra of the corresponding regioisomeric monoamides 3b, d–f (obtained from crystalline samples of the monoacids 2b, d–f ) were confirmed by the present investigations.     

Study of segmental orientation in poly (vinyl chloride)/poly (α-methyl-α-n-propyl-β-propiolactone) blends by fourier-transform infrared spectroscopy

Fourier-transform infrared (FTIR) spectroscopy has been used to investigate the segmental orientation of poly(vinyl chloride) (PVC)/poly (α-methyl-α-n-propyl-β-propiolactone) (PMPPL) blends in uniaxially stretched samples over a wide range of compositions and draw ratios. The results indicate that for pure PVC, syndiotactic segments reach a higher degree of orientation than isotactic segments and gauche conformations. Similarly, for pure PMPPL, crystalline segments orient more than amorphous segments at any given elongation. Thus, for both polymers, the higher orientation is obtained for the more rigid segments or those located in a more rigid (crystalline) phase. The addition of the macromolecular plasticizer PMPPL has no effect on the orientation of PVC syndiotactic segments, but it lowers the orientation of PVC gauche conformations, suggesting that the polyester is located in the amorphous regions of PVC. Finally, the PMPPL orientation function initially decreases with the addition of PVC and thereafter remains constant. The results are discussed in terms of interpenetrating networks and relaxation processes.     

Synthesis and polymerization of racemic and optically active substituted ß-propiolactones. V. α-methyl ß-propiolactone

R(+) and S(?) enantiomers of α-methyl β-propiolactone (MPL) have been synthesized from the corresponding α-methyl β-hydroxymethylpropionates and racemic MPL from methyl methacrylate. The optical purity and absolute configuration of these lactones were determined using ¹H-NMR spectroscopy after complexation with a chiral compound: 2,2,2-trifluoro-1-(9-anthryl)-ethanol. Optical purities of 100% were obtained for both the S(?) ([α₀] = ?10.4°, c = 1.3 g/dL in CHCl₃) and the R(+) ([α₀] = +10.5°, c = 1.0 g/dL in CHCl₃) enantiomers. The corresponding racemic and optically active polylactones [poly(MPL)] were prepared by anionic polymerization, in bulk and in solution, as well as poly(MPL)s of intermediate optical purities. The polymers thus obtained are optically active ([α₀] = 16.2° in CHCl₃ for the optically pure polymer, S configuration) and exhibit significant differences. For example, the racemic poly(MPL) is soluble in several organic solvents such as tetrahydrofuran, benzene, CCl₄, CH₂Cl₂, hexafluoroisopropanol, and CHCl₃, whereas the optically active poly(MPL)s are soluble in CHCl₃ and hexafluoroisopropanol only. Moreover, racemic poly(MPL) is amorphous whereas optically active poly(MPL)s are semicrystalline for optical purities larger than 51%. Melting temperatures and enthalpies of fusion of the semicrystalline polylactones vary with optical purity whereas glass transition temperatures remain invariant for all polymers, at about ?28°C. The poly(MPL) of highest optical purity exhibits a melting temperature of 95°C and an enthalpy of fusion of 61 J/g.     

Synthesis and polymerization of racemic and optically active β-substituted β-propiolactones. III. β-monosubstituted monomers and polymers

Optically active β-(1,1-dichloroethyl)-β-propiolactone (CH₃CCl₂-PL), β-(1,1-dichloropropyl)-β-propiolactone (C₂H₅CCl₂-PL), and β-(1,1-dichlorobutyl)-β-propiolactone (C₃H₇CCl₂-PL) were synthesized with enantiomeric excesses of 100, 100, and 84%, respectively. Polymerization was conducted in bulk and toluene solution, under vacuum, using mainly ZnEt₂/H₂O as initiator. Osmometry analyses indicate molecular weights in the range 10,000–25,000. The polymers thus prepared are semi-crystalline and show large optical rotation values.¹³ C-NMR was used to show that they have a high degree of isotacticity, indicating that little or no racemization occurs in the course of polymerization. Glass transition, melting and decomposition temperatures are given as a function of the size of the substituent, and their variations are discussed.     

Stereoregularity of poly(alkyl α-chloroacrylates) and mechanism of isotactic polymerization

The catalytic activity of the complexes prepared by the reaction of Grignard reagents with ketones, esters, and an epoxide as polymerization catalysts of methyl and ethyl α-chloroacrylates was investigated. The modifiers which gave isotactic polymers were α,β-unsaturated ketones such as benzalacetophenone, benzalacetone, dibenzalacetone, mesityl oxide, and methyl vinyl ketone, and α,β-unsaturated esters such as ethyl cinnamate, ethyl crotonate, and methyl acrylate. Catalysts with butyl ethyl ketone, propiophenone, and propylene oxide as modifiers produced atactic polymers but no isotactic polymers. It was revealed that the complex catalysts having a structure ? C?C? O? MgX (X is halogen) gave isotactic polymers. The mechanism of isotactic polymerization was discussed. In addition, for radical polymerization of ethyl α-chloroacrylate, enthalpy and entropy differences between isotactic and syndiotactic additions were calculated to give ΔH_i^* ? ΔH_s^* = 910 cal/mole and ΔS_i^* ? ΔS_s^* = 0.82 eu.     

3-Hydroxyglutarate and β,γ-Epoxy Esters as Substrates for Pig Liver Esterase (PLE) and α-Chymotrypsin

The pH dependence of the α-chymotrypsin-catalyzed hydrolysis of dimethyl 3-hydroxyglutarate ( 3 ) has been studied. The e.e. was determined by HPLC analysis of diastereoisomeric camphanoic-acid derivatives. Kinetic resolution of the β,α-epoxy esters 10 and 24 by pig liver esterase has been shown to provide an alternative access to chiral β-hydroxy esters and acids of high optical purity. By this latter method, the unnatural enantiomer of γ-amino-β-hydroxybutyric acid (GABOB) has been synthesized. Finally, dimethyl meso-3,4-epoxyadipate ( 19 ) was hydrolyzed by pig liver esterase with almost 100% selectivity.     

Cationic polymerization of α,β-disubstituted olefins. VI. Steric structure of poly(methyl propenyl ether) obtained by cationic catalysts

The steric structure of poly(methyl propenyl ether) obtained by cationic polymerization was studied by NMR spectra. From the analysis of β-methyl and α-methoxyal spectra, it was found that the tacticities of the α-carbon were different from those of the β-carbon in all polymers obtained. In the crystalline polymers obtained from the trans isomer by homogeneous catalysts, BF₃·O(C₂H₅)₂ or Al(C₂H₅)Cl₂, and from the cis isomer by a heterogeneous catalyst, Al₂(SO₄)₃–H₂SO₄ complex, the structure of polymers was threo-di-isotactic. Though the configurations of all α-carbons were isotactic, a small amount of syndiotactic structure was observed in the β-carbon. On the other hand, in the amorphous polymer obtained from cis isomer by the homogeneous catalyst, the configuration of the α-carbon was isotactic, but that of the β-carbon was atactic. These facts suggest that the type of opening of a monomeric double bond is complicated, or that carbon–carbon double bond in an incoming monomer rotates in the transition state. From these experimental results, a probability treatment was proposed from the diad tacticity of α,β-disubstituted polymers. It shows that the tacticity is decided by a polymerization mechanism different from that proposed by Bovey.     

Synthesis and polymerization of racemic and optically active β-substituted β-propiolactones. II: β-monosubstituted and β-disubstituted monomers and polymers with different optical purities

β-(trichloromethyl)-β-propiolactone (CCl₃-PL), β-(trifluoromethyl,methyl)-β-propiolactone (CF₃, Me-PL) and β-(trifluoromethyl,ethyl)-β-propiolactone (CF₃,Et-PL) have been obtained by the reaction of ketene with chloral, 1,1,1-trifluoroacetone and 1,1,1-trifluorobutanone, respectively. Chiral catalysis lead to optically active monomers. The enantiomeric excess of the lactones has been measured by ¹H-NMR spectroscopy, in the presence of 2,2,2-trifluoro-1-(9-anthryl)ethanol or an europium chiral shift reagent. Polymerizations have been carried out in bulk or in toluene, at 60°C or 80°C, using mainly organometallic initiators. The Polymers become insoluble and crystalline at enantiomeric excesses over 80% for CCl₃-PL and 70% for CF₃,Me-PL. Melting temperatures were recorded from 238 to 268°C for poly(CCl₃-PL) and from 78 to 100°C for poly(CF₃,Me-PL), depending upon the molecular weight and the enantiomeric excess. The ¹³C-NMR specroscopy of poly(CCL₃-PL) indicates that the polymerization of the corresponding lactone leads to polymers of increasing degrees of isotacticity with the enantiomeric excess of the monomer.     

Synthesis of β-pyrazinyl-L-alanine (Paa) and of peptide derivatives

The title compound was prepared via a malonic ester synthesis starting with α-chloromethylpyrazine [2], and ending, after an asymmetric enzymatic hydrolysis of the racemic N-acetyl-β-2-pyrazinylalanine to the L -form of the new amino acid. The optical purity was ascertained by ¹H-NMR analysis at 360 MHz of the diastereoisomeric dipeptides L -pyrazinylalanine-L -leucine and D -pyrazinylalanine-L -leucine. Hydrophobic, steric and electronic parameters for its side chain were also estimated, which can be useful for the quantitative study of structure-activity relationships of biologically active peptide derivatives. The new amino acid could be introduced in the place of phenylalanine in the enkephalin-like pentapeptide [D -alanyl², leucine⁵]enkephalin, thus showing good stability towards the classical methods of peptide synthesis.     

Asymmetric α-Acetoxylation of Carboxylic Esters. Preliminary Communication

Using the readily accessible chiral auxiliaries 1 – 3 the sulfonamide-shielded O-silylated esters 5 underwent π-face-selective α-acetoxylation on successive treatment with Pb(OAc)₄ and NEt₃ HF to give after recrystallization α-acetoxy ester 6 in 55–67% yields and in 95–100% d.e. Starting from conjugated enoates addition of RCu and subsequent acetoxylation 10 → 11 → 12 yielded α,β-bifunctionalized esters 12 with >95% configurational control at both C_α and C_β. Nondestructive removal of the auxiliary ( 6 → 7 , 6 → 8 and 12 → 13 ) gave either α-hydroxycarboxylic acids or terminal α-glycols in high enantiomeric purity. The prepared glycols 8c and 13a are key intermediates for previously reported syntheses of the natural products 16 and 17 , respectively.     

Molecular deformation mechanisms of isotactic polypropylene in α‐ and β‐crystal forms by FTIR spectroscopy

The orientation behavior of isotactic polypropylene (iPP) in α‐ and β‐crystal form was investigated by rheo‐optical Fourier transformed infrared (FTIR) spectroscopy. This method enabled quantification of the degree of orientation as a feature of structural changes during uniaxial elongation in not only the crystalline phase but also the amorphous one. Molecular orientation mechanisms can be successfully derived from experimental results. Generally, three mechanisms were detected for iPP: (1) interlamellar separation in the amorphous phase, (2) interlamellar slip and lamellar twisting at small elongations, and (3) intralamellar slip at high elongations. The third mechanism was favored by α‐PP, whereas β‐PP favored the second mechanism, which, in fact, was responsible for the different mechanical properties of both materials at the macroscopic level. On the other hand, crystallization conditions may have significantly affected the amorphous orientation. Nevertheless, for both iPP types the chains in the amorphous phase always oriented less than did those in the crystalline phase. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4478–4488, 2004     

Stereoregularity of polystyrene-β,β-d2

The 100-MHz methine proton spectra of polystyrene-β,β-d₂ obtained by radical and cationic initiators consisted of four peaks at 2.35, 2.25, 2.17, and 2.03 ppm, the proportion of which changed with polymerization conditions such as catalyst, solvent, and temperature. The spectrum was interpreted in terms of pentad sequences assuming Bernoullian statistics and the stereoregularity was determined. Polystyrene-β,β-d₂ prepared by radical initiators had a syndiotactic-rich configuration, independent of polymerization temperature. Polymers obtained by cationic initiators had lower racemic dyads. Cationic polymerization in toluene at 0°C gave a polymer of an almost random configuration. It was revealed that nondeuterated polystyrene of a random configuration can be distinguished from syndiotactic-rich polystyrene as well as the isotactic polymer by 100 MHz NMR spectroscopy.     

Polymerization of α,α-disubstituted-β-propiolactones and lactams. 10. Crystalline structure,thermal and mechanical studies of statistical and block copolymers of pivalolactone and α-methyl-α-propyl-β-propiolactone

AB and ABA block copolyesters based on racemic poly(α-methyl-α-n propyl-α-propiolactone) (PMPPL) as a “soft” or elastomeric segment and polypivalolactone (PPL) as a “hard” or crystallizable segment have been synthesized and compared with random copolymers of the same composition. X-ray studies show the coexistence of polymorphic crystal forms for a given polymer in a given sample. Thermal and dynamic mechanical properties give clear evidence of heterophase structure corresponding to segregation of PPL and PMPPL. The crystalline phase clearly provides thermally reversible crosslinking in the ABA block copolymers. On stretching, the planar zigzag form of PMPPL is observed. Because of the domain structure, moduli of ABA samples are higher than those of PMPPL and their tensile strengths are similar to those of comparable styrene-butadiene block copolymers. The polymer synthesis was achieved by sequential monomer addition with tetrahexyl ammonium benzoate as initiator. For the ABA polymers the diammonium salt of sebacic acid provided a di-functional initiator. The agreement between calculated and observed molecular weights testify to the “living” character of this polymerization reaction.     

Diastereoselective Coupling of N‐(tert‐Butyl)sulfinyl Imines and Dimethyl Malonate. Synthesis of Enantiomerically Enriched β‐Amino Esters and β‐Lactams

A diastereoselective coupling of dimethyl malonate with N‐(tert‐butyl)sulfinyl imines under solvent‐free conditions was developed, using NaHCO₃ or NaI as base promoters. The resulting dimethyl 2‐(1‐aminoalkyl)malonates could be easily transformed successively to β‐amino esters and the corresponding β‐lactams with high optical purity.     

Two New Diterpens Phenols-7α-Methoxydeoxocryptojaponol and 7β-Hydroxydeoxocryptojaponol

Six crystalline compounds, α-cedrol, β-sitosterol, cryptojaponol sugiol and two new diterpene phenols were isolated from the hexane extractive of wood of Juniperus formosana Hayata. Two new phenolic derivatives of deoxocryptojaponoi were established as 7α-methoxydeoxocryptojaponol and 7β-hydroxydeoxocryptojaponol, respectively, by chemical and physical evidence.     

Two New Diterpens Phenols-7α-Methoxydeoxocryptojaponol and 7β-Hydroxydeoxocryptojaponol

Six crystalline compounds, α-cedrol, β-sitosterol, cryptojaponol sugiol and two new diterpene phenols were isolated from the hexane extractive of wood of Juniperus formosana Hayata. Two new phenolic derivatives of deoxocryptojaponol were established as 7α-methoxydeoxocryptojaponol and 7β-hydroxydeoxocryptojaponol, respectively, by chemical and physical evidence.     

Synthesis and characterization of poly-β-hydroxybutyrate. I. Synthesis of crystalline DL-poly-β-hydroxybutyrate from DL-β-butyrolactone

The polymerization of β-butyrolactone was investigated as a possible monomer for a proposed synthesis of the naturally occurring polyester, D -poly-β-hydroxybutyrate (D -PHB). The racemic DL -monomer was used in this initial study to determine the best conditions and catalyst system for use in a subsequent study of the polymerization of optically active β-butyrolactone. In so doing it was found that certain organometallic catalysts (Et₂Zn and Et₃Al) plus a cocatalyst of water produced highly crystalline samples of polyester from the racemic monomer. This paper describes the synthesis and characterization of the racemic polymer obtained using these catalyst systems, and compares the results obtained with certain other catalysts that were also investigated for this purpose. Examination of the DL -PHB by infrared, NMR, x-ray, and electron microscopy shows that it is possible to synthesize a crystalline racemic polymer that is virtually identical (excepting optical activity) to the naturally occurring polymer, D -PHB.     

Asymmetric selective polymerization of racemic methacrylates with the cyclohexylmagnesium bromide-(−)-sparteine system

Asymmetric selective (or stereoelective) polymerization of various racemic methacrylates with cyclohexylmagnesium bromide (c-HexMgBr)-(-)sparteine (1/1.2) catalyst was studied in toluene at ?78°C. The methacrylates of α-ethylbenzyl (EBMA), α-isopropylbenzyl (i-PBMA), α-tert-butylbenzyl (t-BBMA), sec-butyl (s-BuMA), 1-methylallyl (1-MAMA), 2,3-epoxypropyl (2,3-EPMA), 2-phenylpropyl (2-PPMA), and menthyl (MentMA) alcohols were used as racemic monomers. In the polymerization of EBMA and i-PBMA (S) enantiomers were consumed preferentially and the optical purity of initially polymerized i-PBMA was as high as 97%. Optically pure (R) monomers were recovered at about 60% conversion for i-PBMA and 80% for EBMA. In t-BBMA, however, the (R) monomer was consumed preferentially over the (S) isomer. In the polymerization of s-BuMA and 1-MAMA (S) monomers were consumed in excess and the optical purity of the polymers formed in the early stage was about 30%. In 2,3-EPMA and 2-PPMA, which have asymmetric centers at the β position from the ester oxygen, (R) antipodes were more reactive. MentMA did not polymerize at ?78°C. Enantiomer selectivity ratios r_S and r_R were determined in the polymerization of EBMA, i-PBMA, and 1-MAMA. All polymers except poly(t-BBMA) were highly isotactic, but the tacticity of poly(t-BBMA) could not be estimated. Circular dichroism spectra of optically active polymers of α-substituted benzyl esters were measured.