Diazotization of α-amino acids in 48:52 (w/w) hydrogen fluoride/pyridine along with excess of potassium halide results in the corresponding α-halocarboxylic acids in good to excellent yields (Table 1 and 2). 相似文献
A convenient route with high stereo control to γ‐acetoxy dienoates is provided by the reaction of methyl propiolate with aldehydes in the presence of ZnEt2 and N‐methylimidazole at room temperature, followed by the catalytic conversion of the resulting γ‐hydroxy‐α,β‐acetylenic esters with p‐N,N‐dimethylaminopyridine (DMAP) in acetic anhydride (see scheme).
A one‐step transformation of γ‐ and δ‐(spiro)lactones into γ,δ‐ and δ,ε‐unsaturated aldehydes with an excess of formic acid in the vapor phase over a supported manganese catalyst is described for the first time. The scope and limitations of this new reaction are shown with different lactones as substrate, and a mechanistic rationale is proposed. 相似文献
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. 相似文献
The Ser, Cys, and His side chains play decisive roles in the syntheses, structures, and functions of proteins and enzymes. For our structural and biomedical investigations of β‐peptides consisting of amino acids with proteinogenic side chains, we needed to have reliable preparative access to the title compounds. The two β3‐homoamino acid derivatives were obtained by Arndt–Eistert methodology from Boc‐His(Ts)‐OH and Fmoc‐Cys(PMB)‐OH (Schemes 2–4), with the side‐chain functional groups' reactivities requiring special precautions. The β2‐homoamino acids were prepared with the help of the chiral oxazolidinone auxiliary DIOZ by diastereoselective aldol additions of suitable Ti‐enolates to formaldehyde (generated in situ from trioxane) and subsequent functional‐group manipulations. These include OH→OtBu etherification (for β2hSer; Schemes 5 and 6), OH→STrt replacement (for β2hCys; Scheme 7), and CH2OH→CH2N3→CH2NH2 transformations (for β2hHis; Schemes 9–11). Including protection/deprotection/re‐protection reactions, it takes up to ten steps to obtain the enantiomerically pure target compounds from commercial precursors. Unsuccessful approaches, pitfalls, and optimization procedures are also discussed. The final products and the intermediate compounds are fully characterized by retention times (tR), melting points, optical rotations, HPLC on chiral columns, IR, 1H‐ and 13C‐NMR spectroscopy, mass spectrometry, elemental analyses, and (in some cases) by X‐ray crystal‐structure analysis. 相似文献
A series of novel γ,γ‐difluorinated Goniothalamin analogues 4a – 4i and 6a – 6i were synthesized. The key steps included the construction of C‐5 stereocenter adjacent to gem‐difluoromethylene group by way of lipase AK catalyzed kinetic resolution, the introduction of aryl group via Stille coupling, and lactonization by 1,5‐oxidative cyclization. These γ,γ‐difluorinated Goniothalamin analogues 4a – 4i and their enantiomers 6a – 6i , together with several corresponding γ‐monofluorinated Goniothalamin analogues were biologically evaluated against four different cancer cell lines. Compound 7h showed a nearly equivalent potency as the parent (R)‐Goniothalamin in the micromolar range. The different fluorine effects between fluoromethylene and gem‐difluoromethylene on antitumor activity were discussed through the analysis of bioassay data. 相似文献
A preparatively useful one‐step transformation of γ,γ‐disubstituted α‐formyl‐γ‐lactones into trisubstituted γ,δ‐unsaturated aldehydes is described, by means of catalytic amounts of either AcOH or AcOEt in the vapor phase over a glass support. A mechanistic rationale is proposed. 相似文献
A structural comparison of three different crystalline forms of poly(β‐propiolactone) (PPL) was carried out by wide‐angle X‐ray diffraction, Fourier‐transform infrared spectroscopy, and differential scanning calorimetry. The α‐form in a hot‐drawn and annealed film represents a 21 helix conformation. The β‐form in a cold‐drawn and annealed film represents a planar zigzag conformation. The γ‐form in an oriented sedimented mat of solution‐grown chain‐folded lamellar crystals also implies a planar zigzag conformation. The solution‐cast film depicts similar outlines with the γ‐form in lamellar crystals in all the experimental measurements, suggesting that the molecular chain in the solution‐cast film has a planar zigzag conformation. While elongation at break decreased, tensile strength and Young's modulus increased with an increase in the crystallinity, independent of the crystalline forms. The influence of the enzymatic degradation of these crystal structures has been investigated by using an extracellular PHB depolymerase purified from Ralstonia pickettii T1. The rate of degradation was in the order of β‐form > α‐form > solution‐cast (γ‐form) film, and the different surface morphologies after partial enzymatic degradation were observed in scanning electron micrographs. It is suggested that the crystal structure is one of the important factors for determining the rate of degradation together with crystallinity.
Enzymatic degradation profiles of poly(β‐propiolactone) films. 相似文献
Carbon-13 spectra of a series of 26 unsaturated ketones (ortho- and para-cyclo-hexadienones and corresponding open-chain analogues) have been measured by Fourier-transform. Pulse spectroscopy. A complete analysis has been achieved by means of double resonance experiments using noise-modulated and coherent off-resonance proton irradiation and with the aid of non-decoupled spectra. Chemical shifts are interpreted in terms of charge distribution in the dienone system and of methyl substituent effects. Carbon chemical shifts were also obtained for O-protonated ortho- and para-cyclohexadienones. One-bond and long-range carbon-proton and carbon-fluorine spin coupling constants are reported for several compounds. 相似文献
Five β‐peptide thioesters ( 1 – 5 , containing 3, 4, 10 residues) were prepared by manual solid‐phase synthesis and purified by reverse‐phase preparative HPLC. A β‐undecapeptide ( 6 ) and an α‐undecapeptide ( 7 ) with N‐terminal β3‐HCys and Cys residues were prepared by manual and machine synthesis, respectively. Coupling of the thioesters with the cysteine derivatives in the presence of PhSH (Scheme and Fig. 1) in aqueous solution occurred smoothly and quantitatively. Pentadeca‐ and heneicosapeptides ( 8 – 10 ) were isolated, after preparative RP‐HPLC purification, in yields of up to 60%. Thus, the so‐called native chemical ligation works well with β‐peptides, producing larger β3‐ and α/β3‐mixed peptides. Compounds 1 – 10 were characterized by high‐resolution mass spectrometry (HR‐MS) and by CD spectroscopy, including temperature and concentration dependence. β‐Peptide 9 with 21 residues shows an intense negative Cotton effect near 210 nm but no zero‐crossing above 190 nm, (Figs. 2–4), which is characteristic of β‐peptidic 314‐helical structures. Comparison of the CD spectra of the mixed α/β‐pentadecapeptide ( 10 ) and a helical α‐peptide (Fig. 5) indicate the presence of an α‐peptidic 3.613 helix. 相似文献
Synthesis of Diastereo- and Enantioselectively Deuterated β,ε-, β,β-, β,γ- and γ,γ-Carotenes We describe the synthesis of (1′R, 6′S)-[16′, 16′, 16′-2H3]-β, εcarotene, (1R, 1′R)-[16, 16, 16, 16′, 16′, 16′-2H6]-β, β-carotene, (1′R, 6′S)-[16′, 16′, 16′-2H3]-γ, γ-carotene and (1R, 1′R, 6S, 6′S)-[16, 16, 16, 16′, 16′, 16′-2H6]-γ, γ-carotene by a multistep degradation of (4R, 5S, 10S)-[18, 18, 18-2H3]-didehydroabietane to optically active deuterated β-, ε- and γ-C11-endgroups and subsequent building up according to schemes \documentclass{article}\pagestyle{empty}\begin{document}${\rm C}_{11} \to {\rm C}_{14}^{C_{\mathop {26}\limits_ \to }} \to {\rm C}_{40} $\end{document} and C11 → C14; C14+C12+C14→C40. NMR.- and chiroptical data allow the identification of the geminal methyl groups in all these compounds. The optical activity of all-(E)-[2H6]-β,β-carotene, which is solely due to the isotopically different substituent not directly attached to the chiral centres, is demonstrated by a significant CD.-effect at low temperature. Therefore, if an enzymatic cyclization of [17, 17, 17, 17′, 17′, 17′-2H6]lycopine can be achieved, the steric course of the cyclization step would be derivable from NMR.- and CD.-spectra with very small samples of the isolated cyclic carotenes. A general scheme for the possible course of the cyclization steps is presented. 相似文献
Hybrid peptidic oligomers comprising natural and unnatural amino acid residues that can exhibit biomolecular folding and hydrogen‐bonding mimicry have attracted considerable interest in recent years. While a variety of hybrid peptidic helices have been reported in the literature, other secondary structural patterns such as γ‐turns and ribbons have not been well explored so far. The present work reports the design of novel periodic γ‐turns in the oligomers of 1:1 natural‐α/unnatural trans‐β‐norborenene (TNAA) amino acid residues. Through DFT, NMR, and MD studies, it is convincingly shown that, in the mixed conformational pool, the heterogeneous backbone of the hybrid peptides preferentially adopt periodic 8‐membered (pseudo γ‐turn)/7‐membered (inverse γ‐turn) hydrogen bonds in both polar and non‐polar solvent media. It is observed that the stereochemistry and local conformational preference of the β‐amino acid building blocks have a profound influence on accessing the specific secondary fold. These findings may be of significant relevance for the development of molecular scaffolds that facilitate desired positioning of functional side‐chains. 相似文献
The title compounds were prepared by aldol reaction of anisaldehyde and the respective N,N‐dibenzyl glycinates. Deprotection of the nitrogen atom with Pearlman’s catalyst delivered the unprotected β‐hydroxytyrosine esters, which were further N‐protected as N,N‐phthaloyl (Phth) and N‐fluorenylmethylcarbonyloxy (Fmoc) derivatives. The Friedel–Crafts reaction with various arenes was studied employing these alcohols as electrophiles. It turned out that the facial diastereoselectivitiy depends on the nitrogen protecting group and on the ester group. The unprotected substrates (NH2) gave preferentially syn‐products but the anti‐selectivity increased when going from NHFmoc over NPhth to NBn2. If the ester substituent was varied the syn‐preference increased in the order Me <Et <iPr. The reactions were shown to be fully stereoconvergent and proceeded under kinetic product control. A model is suggested to explain the facial diastereoselectivity based on a conformationally locked benzylic cation intermediate. The reactions are preparatively useful for the N‐unprotected isopropyl ester, which gave Friedel–Crafts alkylation products with good syn‐selectivity (anti/syn=21:79 to 7:93), and for the N,N‐dibenzyl‐protected methyl ester, which led preferentially to anti‐products (anti/syn=80:20 to >95:5). Upon acetylation of the latter compound to the respective acetate, Bi(OTf)3‐catalyzed alkylation reactions became possible, in which silyl enol ethers served as nucleophiles. The respective alkylation products were obtained in high yield and with excellent anti‐selectivitiy (anti/syn≥95:5). 相似文献
The 5,6:5′,6′-diepoxy-5,6:5′,6;-tetrahydro-β,β-carotene, isolated from tubers of a white-fleshed variety of sweet potato (Ipomoea batatas LAM .) has been assigned the (5R,6S,5′R,6′S)-chirality on the basis of its HPLC, UV/VIS, and CD data. 相似文献
A μ3‐η2:η2:η2‐silane complex, [(Cp*Ru)3(μ3‐η2:η2:η2‐H3SitBu)(μ‐H)3] ( 2 a ; Cp*=η5‐C5Me5), was synthesized from the reaction of [{Cp*Ru(μ‐H)}3(μ3‐H)2] ( 1 ) with tBuSiH3. Complex 2 a is the first example of a silane ligand adopting a μ3‐η2:η2:η2 coordination mode. This unprecedented coordination mode was established by NMR and IR spectroscopy as well as X‐ray diffraction analysis and supported by a density functional study. Variable‐temperature NMR analysis implied that 2 a equilibrates with a tautomeric μ3‐silyl complex ( 3 a ). Although 3 a was not isolated, the corresponding μ3‐silyl complex, [(Cp*Ru)3(μ3‐η2:η2‐H2SiPh)(H)(μ‐H)3] ( 3 b ), was obtained from the reaction of 1 with PhSiH3. Treatment of 2 a with PhSiH3 resulted in a silane exchange reaction, leading to the formation of 3 b accompanied by the elimination of tBuSiH3. This result indicates that the μ3‐silane complex can be regarded as an “arrested” intermediate for the oxidative addition/reductive elimination of a primary silane to a trinuclear site. 相似文献