Homologation Of Esters Research Articles

Reagent-Controlled Asymmetric Homologation of Boronic Esters by Enantioenriched Main-Group Chiral Carbenoids

[reaction: see text] Putative enantioenriched carbenoid species, (R)-1-chloro-2-phenylethylmagnesium chloride (9) and (S)-1-chloro-2-phenylethyllithium (26), generated in situ by sulfoxide ligand exchange from (-)-(R(S),R)-1-chloro-2-phenylethyl p-tolyl sulfoxide (8), effected the stereocontrolled homologation of boronic esters. sec-Alcohols derived from the product boronates by oxidation with basic hydrogen peroxide exhibited % ee closely approaching that of sulfoxide 8 in examples employing Li-carbenoid 26.

Physiology and Phylogeny of Green Sulfur Bacteria Forming a Monospecific Phototrophic Assemblage at a Depth of 100 Meters in the Black Sea

The biomass, phylogenetic composition, and photoautotrophic metabolism of green sulfur bacteria in the Black Sea was assessed in situ and in laboratory enrichments. In the center of the western basin, bacteriochlorophyll e (BChl e) was detected between depths of 90 and 120 m and reached maxima of 54 and 68 ng liter(-1). High-pressure liquid chromatography analysis revealed a dominance of farnesyl esters and the presence of four unusual geranyl ester homologs of BChl e. Only traces of BChl e (8 ng liter(-1)) were found at the northwestern slope of the Black Sea basin, where the chemocline was positioned at a significantly greater depth of 140 m. Stable carbon isotope fractionation values of farnesol indicated an autotrophic growth mode of the green sulfur bacteria. For the first time, light intensities in the Black Sea chemocline were determined employing an integrating quantum meter, which yielded maximum values between 0.0022 and 0.00075 micromol quanta m(-2) s(-1) at the top of the green sulfur bacterial layer around solar noon in December. These values represent by far the lowest values reported for any habitat of photosynthetic organisms. Only one 16S rRNA gene sequence type was detected in the chemocline using PCR primers specific for green sulfur bacteria. This previously unknown phylotype groups with the marine cluster of the Chlorobiaceae and was successfully enriched in a mineral medium containing sulfide, dithionite, and freshly prepared yeast extract. Under precisely controlled laboratory conditions, the enriched green sulfur bacterium proved to be capable of exploiting light intensities as low as 0.015 micromol quanta m(-2) s(-1) for photosynthetic 14CO2 fixation. Calculated in situ doubling times of the green sulfur bacterium range between 3.1 and 26 years depending on the season, and anoxygenic photosynthesis contributes only 0.002 to 0.01% to total sulfide oxidation in the chemocline. The stable population of green sulfur bacteria in the Black Sea chemocline thus represents the most extremely low-light-adapted and slowest-growing type of phototroph known to date.

Enantioselective Total Synthesis of (‐)‐Microcarpalide.

Abstract The enantioselective total synthesis of the actin-targeting metabolite (−)-microcarpalide is described. Key steps include ring-closing metathesis (RCM) for the final construction of the 10-membered lactone framework and stereoselective homologation of boronic esters for the insertion of all stereocentres with the desired absolute configuration. In particular, the acidic fragment was prepared in seven steps from a suitable chiral bromomethane boronate by means of two sequential stereoselective homologations to install the two stereocentres with the correct final R stereochemistry, employing (−)-pinanediol as the chiral director. Subsequent elaboration to the required C 7 backbone entailed nucleophilic displacement with a vinyl Grignard reagent, oxidative cleavage of the boronic scaffold and protection–deprotection manipulations. Interestingly, when the tribenzyloxy diene ester resulting from DCC-mediated coupling of the two key synthons was subjected to RCM in the presence of Grubbs' catalyst, the reaction proceeded stereoselectively to yield the desired trans oxecin-2-one, albeit with poor conversion.

Regiocontrolled, Palladium-Catalyzed Bisfunctionalization of Allenyl Esters. Multicomponent Coupling Approaches to Highly Substituted α,β-Unsaturated δ-Lactones

A palladium-catalyzed regioselective bisfunctionalization of allenyl esters with boronic acids (nucleophiles) and aldehydes (electrophiles) was demonstrated. The three-component coupling afforded alpha,beta-unsaturated delta-lactones under mild conditions and with excellent chemo-, regio-, and diastereoselectivity. Aromatic, heteroaromatic and vinylic boronic acids (R1B(OH)2) reacted with ethyl 2,3-butadienoate and benzaldehyde to afford the corresponding 4-R(1),6-Ph-disubstituted alpha,beta-unsaturated delta-lactones in 62-78% yields. Lactones derived from aromatic, heteroaromatic, and vinylic aldehydes were isolated in 51-58% yields, while aliphatic aldehydes were less reactive. The regiochemistry of bisfunctionalization of allenyl ester homologues remained controlled by the ester substituent, and the reactions afforded cis-4,5,6-trisubstituted alpha,beta-unsaturated delta-lactones and esters of (Z)-syn-3,4,5-trisubstituted-5-hydroxy-2-pentenoic acids in combined 47-65% yields. The superior performance of a pi-allylpalladium(II) dimer catalyst featuring an auxiliary allyl ligand derived from beta-pinene, among diverse palladium(II) catalysts, was demonstrated. A catalytic cycle involving an unsymmetrical bis-pi-allylpalladium complex as the key intermediate was proposed, and the communication highlights the synthetic potential of such intermediates. However, the efficiency of asymmetry transfer remained low (<20%).

Enantioselective total synthesis of (−)-microcarpalide

The enantioselective total synthesis of the actin-targeting metabolite (−)-microcarpalide is described. Key steps include ring-closing metathesis (RCM) for the final construction of the 10-membered lactone framework and stereoselective homologation of boronic esters for the insertion of all stereocentres with the desired absolute configuration. In particular, the acidic fragment was prepared in seven steps from a suitable chiral bromomethane boronate by means of two sequential stereoselective homologations to install the two stereocentres with the correct final R stereochemistry, employing (−)-pinanediol as the chiral director. Subsequent elaboration to the required C 7 backbone entailed nucleophilic displacement with a vinyl Grignard reagent, oxidative cleavage of the boronic scaffold and protection–deprotection manipulations. Interestingly, when the tribenzyloxy diene ester resulting from DCC-mediated coupling of the two key synthons was subjected to RCM in the presence of Grubbs' catalyst, the reaction proceeded stereoselectively to yield the desired trans oxecin-2-one, albeit with poor conversion.

Kowalski Ester Homologation. Application to the Synthesis of β‐Amino Esters.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Clathrates of dianilinegossypol with lower acetate ester homologues: Structure, isomorphism, and morphotropic transitions

The structure of dianilinegossypol clathrates is shown to be sensitive to size of the ester molecule included. The change in the guest component from methyl acetate to amyl acetate is accompanied by two morphotropic transitions in the acetic ester clathrate series and one transition into a nonsolvate dianilinegossypol form.

Kowalski ester homologation. application to the synthesis of beta-amino esters.

The Kowalski ester homologation protocol has been applied to a representative range of alpha-amino esters to provide beta-amino esters with excellent levels of enantio- and diastereocontrol. A key feature of this chemistry is the nature of the N-protecting group that is employed.

Concise formal synthesis of the bryostatin southern hemisphere (C17-C27).

An efficient synthesis of Hale and co-workers' C17-C27 bryostatin southern hemisphere intermediate has been accomplished in six steps and 33% overall yield from (R)-2-(benzyloxy)propanal. The synthesis features a one-pot DIBALH/HWE ester homologation as well as a novel acetonide rearrangement/glycal formation cascade.

Response of flame ionization detectors to different homologous series

Hydrogen flame-ionization detectors (FIDs) are the most widely used type of detector in gas chromatography. The FID signals is proportional to the number of carbon atoms in a hydrocarbon molecule; the presence of heteroatoms usually reduces the signal. If the extent of the signal-reducing effect of heteroatoms were known, it would be possible to measure compounds which are not available as pure standards, or cannot be prepared, or their preparation is very expensive. The sensitivity of a detector to an organic molecule containing heteroatoms is referred to normal hydrocarbons by means of the effective carbon-atom number (ECN) value. By use of the values of increments inECN for heteroatoms and functional groups, theECN can be calculated for any organic molecule. For this, exact values of theECN increments are needed, and the effects of different factors on the increments must be known. In this study a wide range of homologues of normal paraffins, alcohols, amines, and esters was investigated, with emphasis on differences between the behaviour of lower and higher homologues. Studies were extended to theECN values of ketones, and aromatic and halogenated compounds. For all types of compound investigated the difference between the actual carbon number and the calculated effective carbon number (dECN) was compared with literature data, and an attempt was made to interpret the differences.

Mesomorphic Properties and Hydrogen Bonding Formation of Asymmetrical Triphenylene Discotic Liquid Crystals

Asymmetrical triphenylene derivatives containing ester or carboxylic acids as terminal substituents were synthesized and characterized by DSC, optical microscopy and X-ray diffraction. It was found that in the ester homologues, only one compound exhibits a monotropic nematic (ND) mesophase, while most of the carboxylic acid derivatives show a hexagonal columnar (Colh) mesophase. Temperature dependent infrared spectra of the carboxylic acid derivatives indicate the existence of strong hydrogen bonding interactions in the isotropic, mesomorphic and solid phases.

Total synthesis of (-)-cylindrocyclophanes A and F exploiting the reversible nature of the olefin cross metathesis reaction.

Efficient total syntheses of the C(2)-symmetric (-)-cylindrocyclophanes A and F (1a and 1f) have been achieved. The initial strategy featured the use of a common advanced intermediate to assemble in stepwise fashion the required macrocycle of 1f, exploiting in turn a Myers reductive coupling followed by ring-closing metathesis. In a second-generation strategy, a remarkable cross olefin metathesis dimerization cascade was discovered and exploited to assemble the requisite [7,7]-paracyclophane macrocycles of both 1a and 1f from dienyl monomers. The successful syntheses also featured the effective use of the Danheiser annulation to construct substrates for both the Myers reductive coupling and the metathesis dimerizations strategies. Finally, the Kowalski two-step chain homologation of esters to siloxyalkynes proved superior over the original one-step protocol.

Asymmetric homologation of boronic esters bearing azido and silyloxy substituents.

In the asymmetric homologation of boronic esters with a (dihalomethyl)lithium, substituents that can bind metal cations tend to interfere. Accordingly, we undertook the introduction of weakly basic oxygen and nitrogen substituents into boronic esters in order to maximize the efficiency of multistep syntheses utilizing this chemistry. Silyloxy boronic esters cannot be made efficiently by direct substitution, but a (hydroxymethyl)boronic ester has been silylated in the usual manner. Conversion of alpha-halo boronic esters to alpha-azido boronic esters has been carried out with sodium azide and a tetrabutylammonium salt as phase-transfer catalyst in a two-phase system with water and either nitromethane or ethyl acetate. These are safer solvents than the previously used dichloromethane, which can form an explosive byproduct with azide ion. Boronic esters containing silyloxy or alkoxy and azido substituents have been shown to react efficiently with (dihalomethyl)lithiums, resulting in efficient asymmetric insertion of the halomethyl group into the carbon-boron bond.

Switched enantiopreference of Humicola lipase for 2-phenoxyalkanoic acid ester homologs can be rationalized by different substrate binding modes

Humicola lanuginosa lipase was used for enantioselective hydrolyses of a series of homologous 2-phenoxyalkanoic acid ethyl esters. The enantioselectivity ( E-value) of the enzyme changed from an ( R)-enantiomer preference for the smallest substrate, 2-phenoxypropanoic acid ester, to an ( S)-enantiomer preference for the homologous esters with longer acyl moieties. The E-values span the range from E=13 ( R) to E=56 ( S). A molecular modeling study identified two different substrate-binding modes for each enantiomer. We found that the enantiomers favored different modes. This discovery provided a model that offered a rational explanation for the observed switch in enantioselectivity.

The thermooxidative degradation of poly(vinylchloride): 3. The stabilising action of dibutyltin maleate and trisnitro (1,3-dihydroxy-2-hydroxymethyl-2-nitropropane). Effect of outdoor exposure on PVC

The effects of the stabilising action of admixed mixtures of dibutyltin maleate and trisnitro and its ester homologues on polyvinyl chloride exposed to natural atmosphere were studied. The degradation and stabilisation reactions were monitored by i.r., colour formation, tensile strength, elongation at break, reduced viscosity as well as determination of time to embrittlement. Observed reductions in carbonyl group formation, lack of colour formation and generally improved mechanical properties of the stabilised PVC sample indicated effective reduction in degradation reactions. The stabilising agents containing 7.2×10 −3 mol% DBTM in admixture with 10×10 −4 mol% of stearyl ester of trisnitro was most effective.

Asymmetric Homologation of Cedranediol Boronic Esters and X-Ray Structure of Cedranediol 1-Chloro-1- Phenylmetylboronate

Homologation of cedranediol boronic esters 1, RBO,C15H,4l with (dichloromethyl)lithium resulted in the formation of (αR)-α-chloro boronic esters 3, RCHCIBO2C15H24, consistently yielding (R)/(S)ratios over 25:1. The absolute configuration of cedranediol (R)-1-chloro-1-phenylmethylboronate 3dR was determined by X-ray diffraction. The distortion of the five-membered 1,3,2-dioxaborolane ring of 3dR from planarity was found. The reason why the cedranediol boronic esters are prone to hydrolysis has been discussed.

Ligand exchange reaction of sulfoxides in organic synthesis: A versatile procedure for one-carbon homologation of methylesters to esters, thioesters, carboxylic acids and amides

Abstract A novel two-step procedure for one-carbon homologation of methylesters to esters, thioesters, carboxylic acids and amides is described. Methylesters are reacted with lithium carbanion of chloromethyl phenyl sulfoxide to give α-chloro α-sulfinyl ketones in 70 to 90% yields. Potassium enolate of the α-chloro α-sulfinyl ketone was treated with tert-butyllithium at −78 °C to give alkynolate via alkylidene carbenoid. This intermediate was treated with alcohols, thioles, 5% aqueous NaOH, and amine hydrochlorides to afford one-carbon homologated esters, thioesters, carboxylic acids and amides, respectively, in good to excellent yields.

Responses to ultraviolet‐B radiation (280–315 nm) of pea (Pisum sativum) lines differing in leaf surface wax

To test the hypothesis that leaf surface wax influences plant responses to UV‐B, 6 lines of cultivated pea (Pisum sativum L.), selected as having more or less wax, were grown at 0 or 6.5 kJ m‐2 day‐1 plant‐weighted UV‐B against a background of 850–950 μmol m‐2 s‐1 photosynthetically active radiation. In the 4 lines with least leaf surface wax the amount of wax on adaxial and abaxial leaf surfaces was increased following exposure to 6.5 kJ m‐2 day‐1 UV‐B, but UV‐B decreased surface wax in Scout, which had the greatest wax deposits. On the adaxial leaf surface, UV‐B radiation caused a shift in wax composition from alcohols to esters and hydrocarbons and the ratio of short to long chain length alkyl ester homologues was increased. There was no evidence of a shortening in carbon chain length of hydrocarbons, primary alcohols or fatty acids due to UV‐B and no significant correlation between wax amount and UV reflectance from leaves.UV‐B induced significant increases in UV‐absorbing compounds in the expanded leaves and buds of most lines. UV‐B reduced the growth of all lines. Foliage area (leaves plus stipules) declined by 5–30%, plant dry weight by 12–30%, and plant height by 24–38%. Reductions in growth occurred in the absence of any changes in chlorophyll fluorescence or photosynthetic rate. UV‐B also had no major effect on carbon allocation patterns. The effects of UV‐B on growth appeared to be due to changes in tissue extension and expansion. Indeed, many of the responses to UV‐B observed in this study of pea appear more consistent with indirect effects being expressed in developing tissues rather than through the direct action of UV‐B on mature tissues.There was no evidence that wax amount or biochemistry was associated with the sensitivity of the lines to UV‐B radiation. Furthermore, induction of pigments was not correlated with changes in growth. However, lines with the greatest constitutive amounts of pigments in unexpanded bud tissues were most tolerant of elevated UV‐B.

Total syntheses of tacamine-type indole alkaloids of Tabernaemontana eglandulosa

Total syntheses are described for seven tacamine-type indole alkaloids ( 1–7) found in Tabernaemontana eglandulosa. (±)-Tacamine ( 1), (±)-16-epitacamine ( 2), and (±)-apotacamine ( 3) were prepared from pentacyclic intermediates 18. Apotacamine ( 3) was also obtained from aldehyde 12 via epimerization of 20-epiapotacamine ( 19) by the Polonovski-Potier reaction. Homologation of ester 13 led to 20-epitacamonine ( 23), which was similarly converted to (±)-tacamonine ( 4). Reduction of 4 gave (±)-descarbomethoxytacamines 5 and 6. Aldehyde 11 was reacted with trimethylsilyl cyanide (TMSCN) to yield the two (±)-17-hydroxytacamonines ( 7), of which the isomer with an axial hydroxy group (17β-OH) was found to be identical with the natural compound. Finally, an attempt to prepare (±)-19 S-hydroxytacamine ( 8) is described. As an intermediated for this, a new synthesis of the pyridine alkaloid methyl 5-(1′-hydroxyethyl)nicotinate ( 34) was achieved.

Self quenching of chlorosome chlorophylls in water and hexanol-saturated water

The optical properties of a methyl ester homolog of bacteriochlorophylld (BChld M ) and bacteriochlorophyllc (BChlc) in H2O, hexanol-saturated H2O and methanol were studied by absorption, fluorescence emission, and circular dichroism (CD). In H2O, BChld M spontaneously forms an aggregate similar to that formed in hexane, with absorption maximum at 730 nm and fluorescence emission at 748 nm. For the pigment sample in hexanol-saturated H2O, while the absorption peaks at 661 nm, only slightly red-shifted compared to the monomer, the fluorescence emission is highly quenched. When diluted 2-3 fold with H2O, the absorption returns to around 720 nm, characteristic of an aggregate. The CD spectrum of the H2O aggregate exhibits a derivative-shaped feature with positive and negative peaks, while the amplitude is lower than that of chlorosomes. The Fourier transform infrared spectra of BChld M aggregates in H2O and hexane were measured. A 1644 cm(-1) band, indicative of a bonded 13(1)-keto group, is detected for both samples. A marker band for 5-coordinated Mg was observed at 1611 cm(-1) for the two samples as well. To study the kinetic behavior of the samples, both single-photon counting (SPC) fluorescence and transient absorption difference spectroscopic measurements were performed. For BChld M in hexanol-saturated H2O, a fast decay component with a lifetime of 10 to 14 ps was detected using the two different techniques. The fast decay could be explained by the concentration quenching phenomenon due to a high local pigment concentration. For the pigment sample in H2O, SPC gave a 16 ps component, whereas global analysis of transient absorption data generated two fast components: 3.5 and 26 ps. The difference may arise from the different excitation intensities. With a much higher excitation in the latter measurements, other quenching processes, e.g. annihilation, might be introduced, giving the 3.5 ps component. Finally, atomic force microscopy was used to examine the ultrastructure of BChld M in H2O and hexanol-saturated H2O. Pigment clusters with diameters ranging from 15 to 45 nm were observed in both samples.