Recent advances in controlled/living radical polymerization in emulsion and dispersion

Effective ways to conduct controlled/living radical polymerization (CRP) in emulsion systems are necessary for commercial latex production without significant modification of current industrial facilities. Conducting CRP in emulsion media is more complicated and more challenging than its application in homogeneous bulk. These challenges come from the intrinsic kinetics of emulsion polymerization. They include mass transport, slow chain growth mechanism, and exit of short radicals from polymeric particles. This review describes the recent developments of CRP in heterogeneous dispersion, including miniemulsion, microemulsion, dispersion, and especially emulsion. Various approaches for conducting emulsion CRP are detailed, including controlled seeded emulsion polymerization, nanoprecipitation, use of short oligomers as macroinitiators for in situ block copolymerization, and RAFT‐mediated self‐assembly. In addition many remaining challenges of the current methods barring wide spread industrial application of emulsion CRP are also suggested. © Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6983–7001, 2008     

Preparation, characterization and catalytic activity of Bi−Mo-based catalysts for the oxidative dehydrogenation ofn-butene to 1,3-butadiene

Multicomponent bismuth molybdates were prepared by a co-precipitation method for use in the oxidative dehydrogenation ofn-butene to 1,3-butadiene. The effect of divalent and trivalent metals on the catalytic performance of multicomponent bismuth molybdate catalysts was investigated. It was found that the metal ratio of Fe/Bi/Mo=3∶1∶12 was favorable for the reaction. The successful formation of Ni _X Co_8?X Fe₃Bi₁Mo₁₂O₅₀ (X=0?8) catalysts was well confirmed by XRD and ICP-AES measurements. The multicomponent bismuth molybdate catalysts showed a better catalytic performance than the pure γ-Bi₂MoO₆ catalyst. Among the Ni _X Co_8?X Fe₃Bi₁Mo₁₂O₅₀ (X=0?8) catalysts, Ni₃Co₅Fe₃Bi₁Mo₁₂O₅₀ showed the highest yield of 1,3-butadiene.     

Rapid determination of floral aroma compounds of lilac blossom by fast gas chromatography combined with surface acoustic wave sensor

A novel analytical method using fast gas chromatography combined with surface acoustic wave sensor (GC/SAW) has been developed for the detection of volatile aroma compounds emanated from lilac blossom (Syringa species: Syringa vulgaris variginata and Syringa dilatata). GC/SAW could detect and quantify various fragrance emitted from lilac blossom, enabling to provide fragrance pattern analysis results. The fragrance pattern analysis could easily characterize the delicate differences in aromas caused by the substantial difference of chemical composition according to different color and shape of petals. Moreover, the method validation of GC/SAW was performed for the purpose of volatile floral actual aroma analysis, achieving a high reproducibility and excellent sensitivity. From the validation results, GC/SAW could serve as an alternative analytical technique for the analysis of volatile floral actual aroma of lilac. In addition, headspace solid-phase microextraction (HS-SPME) GC-MS was employed to further confirm the identification of fragrances emitted from lilac blossom and compared to GC/SAW.     

Fluoride-catalyzed hydroalkoxylation of hexafluoropropene with 2,2,2-trifluoroethanol

Trifluoroethoxylation of hexafluoropropene with 2,2,2-trifluoroethanol (TFE) were conducted using an alkali metal fluoride catalyst to produce CF₃CHFCF₂OCH₂CF₃. KF exhibited the highest yield and selectivity of CF₃CHFCF₂OCH₂CF₃, whereas LiF and NaF were inactive for the trifluoroethoxylation reaction. The same reaction also proceeded well in the presence of RbF or CsF, but yielded large amounts of olefinic and high molecular weight side products, implying that the size of alkali metal cation or the degree of MF dissociation plays an important role in determining the activity and the product composition. FT-IR and NMR experiments revealed that CsF interacts with TFE more strongly than KF through a hydrogen bonding. The experimental and spectroscopic results suggest that the degree of MF dissociation should be in the medium range for the selective production of CF₃CHFCF₂OCH₂CF₃ in high yield and selectivity.     

Photoaddition reactions of 1,2-diketones with silyl ketene acetals. formation of beta-hydroxy-gamma-ketoesters

Photochemical reactions taking place between 1,2-diketones and silyl ketene acetals and their excited state reaction mechanisms have been explored. Irradiation of benzene, acetone, or acetonitrile solutions containing 1,2-diketones and silyl ketene acetals is observed to promote formation of 1,4-dioxenes, resulting from [4 + 2]-cycloaddition, oxetanes, arising by Paterno-Buchi processes, and beta-hydroxy-gamma-ketoesters, generated by SET-promoted Claisen-type condensation. These competitive pathways leading from the excited states of the 1,2-diketones to these products are influenced by solvent polarity and the nature of the silyl ketene acetal and 1,2-diketone. The Claisen-type condensation process, following an SET desilylation pathway and predominating when the photoreactions are carried out in the polar solvent acetonitrile, represents an efficient method to prepare a variety of diversely substituted beta-hydoxy-gamma-ketoesters.     

Single electron transfer-promoted photocyclization reactions of linked acceptor-polydonor systems: effects of chain length and type on the efficiencies of macrocyclic ring-forming photoreactions of tethered alpha-silyl ether phthalimide substrates

Results of an investigation, aimed at gaining information about the factors governing the efficiencies of single electron transfer (SET)-promoted photocyclization reactions of linked acceptor-polydonor systems, are described. One set of substrates used in this effort includes alpha-trimethylsilyl ether terminated, polymethylene- and polyethylenoxy-tethered phthalimides and 2,3-naphthalimides. Photocyclization reactions of the polyethylenoxy-linked phthalimides and naphthalimides were observed to take place in higher chemical yields and with larger quantum efficiencies than those of analogs containing polymethylene tethers of near equal length. These findings show that the rates of formation of 1,omega-zwitterionic biradicals that serve as key intermediates in the photocyclization processes are enhanced in substances that contain oxygen donor sites in the chain. The findings suggest that these donor sites facilitate both initial SET to acceptor excited states and ensuing intrachain SET, resulting in migration of the cation radical center to the terminal alpha-trimethylsilyl ether position. In addition, an inverse relationship was observed between the quantum yields of photocyclization reactions of the tethered phthalimides and naphthalimides and the length of the polyethylenoxy chain. Finally, the roles played by chain type and length in governing photoreaction efficiencies were investigated by using intramolecular competition in photoreactions of polyethylenoxy and polymethylene bis-tethered phthalimides. Mechanistic interpretations and synthetic consequences of the observations made in this study are discussed.     

Biosynthesis and structures of cyclomarins and cyclomarazines, prenylated cyclic peptides of marine actinobacterial origin

Two new diketopiperazine dipeptides, cyclomarazines A and B, were isolated and characterized along with the new cyclic heptapeptide cyclomarin D from the marine bacterium Salinispora arenicola CNS-205. These structurally related cyclic peptides each contain modified amino acid residues, including derivatives of N-(1,1-dimethylallyl)-tryptophan and delta-hydroxyleucine, which are common in the di- and heptapeptide series. Stable isotope incorporation studies in Streptomyces sp. CNB-982, which was first reported to produce the cyclomarin anti-inflammatory agents, illuminated the biosynthetic building blocks associated with the major metabolite cyclomarin A, signifying that this marine microbial peptide is nonribosomally derived largely from nonproteinogenic amino acid residues. DNA sequence analysis of the 5.8 Mb S. arenicola circular genome and PCR-targeted gene inactivation experiments identified the 47 kb cyclomarin/cyclomarazine biosynthetic gene cluster (cym) harboring 23 open reading frames. The cym locus is dominated by the 23 358 bp cymA, which encodes a 7-module nonribosomal peptide synthetase (NRPS) responsible for assembly of the full-length cyclomarin heptapeptides as well as the truncated cyclomarazine dipeptides. The unprecedented biosynthetic feature of the megasynthetase CymA to synthesize differently sized peptides in vivo may be triggered by the level of beta oxidation of the priming tryptophan residue, which is oxidized in the cyclomarin series and unoxidized in the cyclomarazines. Biosynthesis of the N-(1,1-dimethyl-2,3-epoxypropyl)-beta-hydroxytryptophan residue of cyclomarin A was further illuminated through gene inactivation experiments, which suggest that the tryptophan residue is reverse prenylated by CymD prior to release of the cyclic peptide from the CymA megasynthetase, whereas the cytochrome P450 CymV installs the epoxide group on the isoprene of cyclomarin C post-NRPS assembly. Last, the novel amino acid residue 2-amino-3,5-dimethylhex-4-enoic acid in the cyclomarin series was shown by bioinformatics and stable isotope experiments to derive from a new pathway involving condensation of isobutyraldehyde and pyruvate followed by S-adenosylmethionine methylation. Assembly of this unsaturated, branched amino acid is unexpectedly related to the degradation of the environmental pollutant 3-(3-hydroxyphenyl)propionic acid.     

Prenylated flavonoids with PTP1B inhibitory activity from the root bark of Erythrina mildbraedii

Phytochemical study on an EtOAc-soluble extract of the root bark of Erythrina mildbraedii resulted in the isolation of six prenylated flavonoids 1-6. Based on physicochemical and spectroscopic analyses, their structures were determined to be new natural products licoflavanone-4'-O-methyl ether (1), 2',7-dihydroxy-4'-methoxy-5'-(3-methylbut-2-enyl)isoflavone (2), and (3R)-2',7-dihydroxy-3'-(3-methylbut-2-enyl)-2',2'-dimethylpyrano[5',6' :4',5']isoflavan (3), along with three known compounds erythrinin B (4), abyssinin II (5), and parvisoflavone B (6). All the isolates, except for compound 4, inhibited PTP1B activity in vitro with IC(50) values ranging from 5.3 to 42.6 microM. This result further suggests that the prenyl group on the B ring of flavonoids plays an important role in suppressing the enzyme PTP1B.     

Alterations of proliferative and differentiation potentials of human embryonic stem cells during long-term culture

Human embryonic stem cells (hESCs) are considered to be able to stably maintain their characteristics in vitro for prolonged periods, but we had previously encountered changes in proliferative ability and differentiation potential during extended culture of hESCs. Therefore, we investigated the proliferative ability and differentiation potential of hESCs during long-term culture. The hESCs, SNUhES3, were used to analyze population-doubling time, proliferation rate and differentiation potential. We classified hESCs into three groups according to culture period. Ten colonies of hESCs for each group were daily measured colony area and population-doubling time was assessed by the changes of colony area. Proliferation rate of hESCs was measured by 5-bromo-2'-deoxyuridine (BrdU) assay and telomerase activity. To evaluate differentiation potentials for hESCs, expression levels of undifferentiated and/or differentiated hESCs markers were examined by FACS, RT-PCR and immunostaining. Population-doubling time of early passage hESCs was longer than those of middle or late passage. Proliferative ability of hESCs was accelerated depending on culture periods. Cellular morphologies and the expression level of each three germ layer markers were obviously different from each passage of reattached embryoid bodies (EBs) after spontaneous differentiation. Differentiated cells of late passage expressed higher levels of undifferentiated markers such as Oct4 and SSEA4 than those of early and middle passage. But differentiated cells of early and middle passage expressed higher level of differentiated state markers, Nestin (ectoderm), Brachyury (mesoderm), HNF3beta (endoderm). From these results, it can be inferred that hESCs show higher proliferative abilities and reduced differentiation potentials as the passage number increased. Therefore, we conclude that early passage hESCs could be more suitable than middle and late passage hESCs in differentiation studies.     

Asymmetric synthesis of bicyclic beta-lactones via the intramolecular, nucleophile-catalyzed aldol lactonization: improved efficiency and expanded scope

[reaction: see text] The intramolecular, nucleophile-catalyzed, aldol lactonization (NCAL) process merges catalytic, asymmetric carbocycle synthesis with beta-lactone synthesis. The application of modified Mukaiyama reagents to this process led to greatly improved conversion and efficiency (70-82% yield) and shorter reaction times with no diminution of enantioselectivity (91-98% ee). The process was extended to several new aldehyde-acid substrates leading to new bicyclic-beta-lactones. This methodology uniquely provides beta-lactone-fused cyclopentanes and cyclohexanes readied for further transformations.