Über die Depolymerisierung von Poly-(R)-3-hydroxy-buttersäureester (PHB)

On the Depolymerization of Poly-(R)-3-hydroxy-butanoate (PHB) From cell-free PHB or from dried cells of Alcaligenes eutrophus H 16, which had been grown in an aqueous fructose solution, enantiomerically pure methyl, ethyl, butyl or β-methoxyethyl (R)-3-hydroxy-butanoates are obtained in yields ranging from 75–90% (0.1 to 70 g scale). The depolymerization is achieved by heating the PHB-containing materials to temperatures of 80–160° in the corresponding alcohol with or without the cosolvent dichloroethane in the presence of either sulfuric acid or tetraethoxytitanium catalyst. Since (S)-3-hydroxy-butanoates are also readily obtained (by yeast reduction of aceto-acetates), starting materials derived from 3-hydroxy-butyric acid now belong to the especially useful group of synthetic building blocks which are available in both enantiomeric forms.     

Isozeaxanthine: Chiralität und enantioselektive Synthese von (4R,4′R)-Isozeaxanthin ((−)-(4R, 4′R)-β, β-Carotin-4,4′-diol)

Isozeaxanthin: Chirality and Enantioselective Synthesis of (4R,4′R)-Isozeaxanthin ((?)-(4R,4′R)-β, β-Carotin-4,4′-diol) The absolute configuration of optically active isozeaxanthin was established by synthesis using (?)-(R)-4-hydroxy-β-ionon ( 2 ) [18] as starting material.     

Zur Darstellung und Reaktivität von Tetraalkylcyclobutadienplatin(II)-Komplexen [PtCl2(C4R4)L]

Preparation and Reactivity of Platinumcyclobutadiene Complexes [PtCl₂(C₄R₄)L] H[PtCl₃(C₄H₈)], prepared by reduction of H₂[PtCl₆] with n-butanol reacts with 2-pentyne to give equal amounts of the regioisomers [PtCl₂(C₄Et₂Me₂)] ( 3 a, 3 b ). An equimolar mixture of 2-butyne/3-hexyne reacts under the same conditions to give [PtCl₂(C₄Me₄)] ( 1 ), [PtCl₂(C₄Et₄)] ( 2 ) and [PtCl₂(C₄Et₂Me₂)] ( 3 a ) in a molar ratio 1:1.3:6.6. 1 and 2 react with ligands L (L = py a , p-tol b , PPh₃ c , AsPh₃ d , SbPh₃ e ) to give complexes of the type [PtCl₂(C₄R₄)L]. The complexes were characterized by microanalysis as well as by i.r., ¹H- and ¹³C-n.m.r. spectroscopy.     

Cyclische Oligomere von (R)-3-Hydroxybuttersäure: Herstellung und strukturelle Aspekte

Cyclic Oligomers of (R)-3-Hydroxybutanoic Acid: Preparation and Structural Aspects The oligolides containing three to ten (R)-3-hydroxybutanoate (3-HB) units (12-through 40-membered rings 1–8 ) are prepared from the hydroxy acid itself, its methyl ester, its lactone (‘monolide’), or its polymer (poly(3-HB), mol. wt. ca. 10⁶ Dalton) under three sets of conditions: (i) treatment of 3-HB ( 10 ) with 2,6-dichlorobenzoyl chloride/pyridine and macrolactonization under high dilution in toluene with 4-(dimethylamino)pyridine (Fig. 3); (ii) heating a solution (benzene, xylene) of the β-lactone 12 or of the methyl ester 13 from 3-HB with the tetraoxadistanna compound 11 as trans-esterification catalyst (Fig. 4); (iii) heating a mixture of poly(3-HB) and toluene-sulfonic acid in toluene/1,2-dichloroethane for prolonged periods of time at ca. 100° (Fig. 6). In all three cases, mixtures of oligolides are formed with the triolide 1 being the prevailing component (up to 50% yield) at higher temperatures and with longer reaction times (thermodynamic control, Figs. 3–6). Starting from rac-β-lactone rac- 12 , a separable 3:1 to 3:2 mixture of the l,u- and the l,l-triolide diasteroisomers rac- 14 and rac- 1 , respectively, is obtained. An alternative method for the synthesis of the octolide 6 is also described: starting from the appropriate esters 15 and 17 and the benzyl ether 16 of 3-HB, linear dimer, tetramer, and octamer derivatives 18–23 are prepared, and the octamer 23 with free OH and CO₂H group is cyclized (→ 6 ) under typical macrolactonization conditions (see Scheme). This ‘exponential fragment coupling protocol’ can be used to make higher linear oligomers as well. The oligolides 1–8 are isolated in pure form by vacuum distillation, chromatography, and crystallization, an important analytical tool for determining the composition of mixtures being ¹³C-NMR spectroscopy (each oligolide has a unique and characteristic chemical shift of the carbonyl C-atom, with the triolide 1 at lowest, the decolide 8 at highest field). The previously published X-ray crystal structures of triolide 1 , pentolide 3 , and hexolide 4 (two forms), as well as those of the l,u-triolide rac- 14 , of tetrolide ent- 2 , of heptolide 5 , and of two modifications of octolide 6 described herein for the first time are compared with each other (Figs. 7–10 and 12–15, Tables 2 and 5–7) and with recently modelled structures (Tables 3 and 4, Fig. 11). The preferred dihedral angles τ₁ to τ₄ found along the backbone of the nine oligolide structures (the hexamer and the larger ones all have folded rings!) are mapped and statistically evaluated (Fig. 16, Tables 5–7). Due to the occurrence of two conformational minima of the dihedral angle O? CO? CH₂? CH (τ₃ = + 151 or ?43°), it is possible to locate two types of building blocks for helices in the structures at hand: a right-handed 3₁ and a left-handed 2₁ helix; both have a ca. 6 Å pitch, but very different shapes and dispositions of the carbonyl groups (Fig. 17). The 2₁ helix thus constructed from the oligolide single-crystal data is essentially superimposable with the helix derived for the crystalline domains of poly(3-HB) from stretched-fiber X-ray diffraction studies. The absence of the unfavorable (E)-type arrangements around the OC? OR bond (‘cis-ester’) from all the structures of (3-HB) oligomers known so far suggests that the model proposed for a poly(3-HB)-containing ion channel (Fig. 2) must be modified.     

Citrus-Carotinoide. 2. Mitteilung. Synthese von (3R)-β-Citraurin, (3R)-β-Citraurol und (3R)-β-Citraurinin; Aufklärung der Konfiguration von Citrus-Carotinoiden

Citrus-Carotenoids. Synthesis of (3R)-β-Citraurin, (3R)-β-Citraurol and (3R)-β-Citraurinene; Determination of the Configuration of Citrus-Carotenoids (3R)-β-Citraurin, (3R)-β-citraurol and (3R)-β-citraurinene were prepared starting from (3R)-3-acetoxy-β-ionone. Comparison of the chiroptical data of β-citraurin from orange peels with those of the synthetic compound confirmed the 3R-configuration of the natural pigment.     

Spektralphotometrische Bestimmung von Eisen(III) und Chrom(VI) mit Helasolviolett 4R

Ohne Zusammenfassung

---

Spectrophotometric determination of iron(III) and chromium(VI) by means of helasol violet 4R

     

Zur polarographischen Reduktion von Molybdän(VI) und Molybdän(V) in Gegenwart von Citronensäure

According to the ratio of citric acid to molybdenum different citratomolybdenum(VI) complexes exist at pH <2. Only one citratomolybdenum(VI) complex exists in solutions with a great excess of citric acid (20:1). In such solutions the polarographic reduction of Mo^VI proceeds in two waves: First Mo^VI → M^V, then Mo^V → Mo^IV, which disproportionates into Mo^V and Mo^III however. Furthermore, it is possible that the Mo^IV reacts with Mo^VI forming Mo^V, so that also a catalytic character can be attributed to the second wave besides a kinetic one.     

Stereospezifische Synthese von (+)-(3R, 4R)-4-Methyl-3-heptanol,das Enantiomere eines Pheromons des kleinen Ulmensplintkäfers (Scolytus multistriatus)

Stereospecific Synthesis of (+)-(3R, 4R)-4-Methyl-3-heptanol, the Enantiomer of a Pheromone of the Smaller European Elm Bark Beetle (Scolytus multistriatus) Reduction of 2 with actively fermenting baker's yeast gave (?) -3. Stereospecific alkylation [3] of (?) -3 with propyl iodide furnished ethyl (+)-(2R, 3R)-2-propyl-3-hydroxypentanoate ((+) -4 , 58%) which was converted to the tetrahydropyranyl ether (?) -5 , then the alcohol 6 , the p-toluenesulfonate 7 and the thiophenyl ether 8 to give the title compound (+) -1. The latter consisted of 97% of the threo- and 3% of the erythro-isomer. The above synthesis also correlates the absolute configuration of (?)-(R) -3 with that of (+)-(R)-citronellic acid (see [2]).     

Neue Synthese von (−)-(R)-Cembren A,Synthese von (+)-(R)-Cembrenen und (+)-(S)-Cembren

Novel Synthesis of (?)-(R)-Cembrene A, Synthesis of (+)-(R)-Cembrenene and (+)-(S)-Cembrene A novel synthesis of (?)-(R)-cembrene A ((?)- 3 ) was developed using the Sharpless epoxidation for the introduction of the chiral center. Furthermore, the synthesis of (+)-(R)-cembrenene ((+)- 4 ) showed that this cembranoid must have the (R)-configuration and not, as previously reported, the (S)-configuration. Selective hydrogenation of (+)- 4 afforded (+)-(S)-cenibrene ((+)- 5 ).     

Synthese von (−)-(R)-Nephthenol und (−)-(R)-Cembren A

Synthesis of (?)-(R)-Nephthenol and (?)-(R)-Cembren A Starting for L -serine,(?)-(R)-nephthenol((?)- 2 ) and (?)-(R)-cembren A((?)- 3 ) were synthesized.     

Periphere Anbindung von Quecksilber(II)-iodid an dreikernige Molybdän-Schwefel-Dithiophosphinatocluster: [Mo3S4(R2PS2)4HgI2] (R = Et,Pr)

Peripheral Bonding of Mercury(II) Iodide to Trinuclear Molybdenum-Sulfur-Dithiophosphinato Clusters: [Mo₃S₄(R₂PS₂)₄HgI₂] (R = Et, Pr) Reaction of Mo₃S₄(R₂PS₂)₄ 1 (a : R = Et, b : R = Pr) with HgI₂ in THF yields the diamagnetic title complexes [Mo₃S₄(R₂PS₂)₄HgI₂] 3 . The crystal structure of [ 3a (H₂O)] · 2 CH₂Cl₂ shows the complexes to consist of a triangular array of Mo atoms which are bridged by μ₂? S atoms and capped by a μ₃? S atom. Each of the Mo atoms is chelated by a dithiophosphinato ligand Et₂PS₂^? and in addition two Mo atoms are bridged by a Et₂PS₂^? ligand while the H₂O molecule is bonded weakly to the third Mo atom. Thus, all Mo atoms reveal a distorted octahedral coordination sphere. HgI₂ is ?peripherally”? bonded to the cluster via two S atoms, one of which belongs to a chelating ligand and the other one to the bridging ligand. Space group P1 , lattice constants a = 12.157(2), b = 15.284(3), c = 16.049(3) Å, α = 115.56(1), β = 107.35(1), and γ = 94.62(1)°; Z = 2, d_calc = 2.23 mg/mm³; 4 236 observed reflections, R = 0.068. In organic solvents complexes 3 are strong electrolytes. VT-³¹P NMR data suggest a stepwise dissociation of 3 with formation of [Mo₃S₄(R₂PS₂)₃] ⁺[(R₂PS₂)HgI₂]^? and elimination of the bridging ligand from the cluster.     

Synthese von optisch aktiven,natürlichen Carotinoiden und strukturell verwandten Naturprodukten. VII. Synthese von (3R)-3-Hydroxyretinol, (3R)-3-Hydroxyretinal und (3R)-3-Hydroxyretinsäure

Synthesis of optically active natural carotenoids and structurally related compounds. VII. Synthesis of (3R)-3-hydroxyretinol, (3R)-3-hydroxyretinal and (3R)-3-hydroxyretinoic acid The synthesis of (3R)-3-hydroxyretinol, ( 7 ), (3R)-3-hydroxyretinal ( 9 ) and (3R)-3-hydroxyretinoic acid ( 5 ) according to the building principle C₁₅ + C₅ = C₂₀ is reported utilizing the optically active C₁₅-phosphonium salt 2 and the C₅-aldehyde ester 3 .