Chloroethoxy(trimethyl)silane: A hard-base trap which preserves tms ether groups and improves the wittig methylenation of gibberellins

A reagent, 2-chloroethoxy(trimethyl)silane (CETS), has been devised which ensures the preservation of TMS ether groups during Wittig methylenation in protic media and which makes this formerly capricious reaction with 16-keto gibberellins a reliable and straightforward procedure.     

The structures of three new Galbulimima alkaloids

The structures of three new alkaloids isolated from the bark of the rain forest tree Galbulimima belgraveana, have been determined by a combination of NMR spectroscopy and X-ray crystallography. One of the alkaloids, himgrine (5), was shown to be an oxygenated derivative of himbacine (1), while the second, GB16, (8) proved to be identical with a degradation product from himgaline (4). The remaining alkaloid, GB17 (12) possesses an entirely new and unexpected skeleton.     

Synthesis and confirmation of structure for the gibberellin GA131 (18-hydroxy-GA4)

A general method for the hydroxylation of the 18-methyl group in gibberellins has been developed, as demonstrated by the successful synthesis of 18-hydroxy GA(4) (GA(131)) by means of a tandem process involving the conjugate addition of alkoxides to the alpha-methylene lactone moiety of a ring A-seco-gibberellin followed by an intramolecular aldol reaction.     

Transesterification of Waste Cooking Oil by an Organic Solvent-Tolerant Alkaline Lipase from Streptomyces sp. CS273

The aim of this present study was to produce a microbial enzyme that can potentially be utilized for the enzymatic transesterification of waste cooking oil. To that end, an extracellular lipase was isolated and purified from the culture broth of Streptomyces sp. CS273. The molecular mass of purified lipase was estimated to be 36.55 kDa by SDS PAGE. The optimum lipolytic activity was obtained at alkaline pH 8.0 to 8.5 and temperature 40 °C, while the enzyme was stable in the pH range 7.0?～?9.0 and at temperature ≤40 °C. The lipase showed highest hydrolytic activity towards p-nitrophenyl myristate (C14). The lipase activity was enhanced by several salts and detergents including NaCl, MnSo₄, and deoxy cholic acid, while phenylmethylsulfonyl fluoride at concentration 10 mM inhibited the activity. The lipase showed tolerance towards different organic solvents including ethanol and methanol which are commonly used in transesterification reactions to displace alcohol from triglycerides (ester) contained in renewable resources to yield fatty acid alkyl esters known as biodiesel. Applicability of the lipase in transesterification of waste cooking oil was confirmed by gas chromatography mass spectrometry analysis.     

A highly convergent and flexible strategy for the synthesis of a-ring aromatic steroids

Using Hendrickson's criteria for systematic synthesis design, a strategy featuring a linear six carbon synthon (7) for bis-annulation has been devised for economic, flexible and highly convergent syntheses of A-ring aromatic steroids.     

Dipolar cross-relaxation modulates signal amplitudes in the (1)H NMR spectrum of hyperpolarized [(13)C]formate

The asymmetry in the doublet of a spin coupled to hyperpolarized (13)C has been used previously to measure the initial polarization of (13)C. We tested the hypothesis that a single observation of the (1)H NMR spectrum of hyperpolarized (13)C formate monitors (13)C polarization. Depending on the microwave frequency during the polarization process, in-phase or out-of-phase doublets were observed in the (1)H NMR spectrum. Even in this simple two-spin system, (13)C polarization was not reflected in the relative area of the J(CH) doublet components due to strong heteronuclear cross-relaxation. The Solomon equations were used to model the proton signal as a function of time after polarization and to estimate (13)C polarization from the (1)H NMR spectra.     

Studies toward the total synthesis of nominine

The construction of the hetisane group of alkaloids, of which the extensively bridged nominine 17 is the simplest member, poses the ultimate challenge for those interested in the synthesis of the C20 diterpene alkaloids. We describe the synthesis of an advanced intermediate toward this goal. The key steps include reductive acylation, reductive deoxygenation, Birch reduction, and an intramolecular Lewis acid-catalyzed 1,6-addition of a carbamate to a dienone.     

Improving a Natural Enzyme Activity through Incorporation of Unnatural Amino Acids

The bacterial phosphotriesterases catalyze hydrolysis of the pesticide paraoxon with very fast turnover rates and are thought to be near to their evolutionary limit for this activity. To test whether the naturally evolved turnover rate could be improved through the incorporation of unnatural amino acids and to probe the role of peripheral active site residues in nonchemical steps of the catalytic cycle (substrate binding and product release), we replaced the naturally occurring tyrosine amino acid at position 309 with unnatural L-(7-hydroxycoumarin-4-yl)ethylglycine (Hco) and L-(7-methylcoumarin-4-yl)ethylglycine amino acids, as well as leucine, phenylalanine, and tryptophan. Kinetic analysis suggests that the 7-hydroxyl group of Hco, particularly in its deprotonated state, contributes to an increase in the rate-limiting product release step of substrate turnover as a result of its electrostatic repulsion of the negatively charged 4-nitrophenolate product of paraoxon hydrolysis. The 8-11-fold improvement of this already highly efficient catalyst through a single rationally designed mutation using an unnatural amino acid stands in contrast to the difficulty in improving this native activity through screening hundreds of thousands of mutants with natural amino acids. These results demonstrate that designer amino acids provide easy access to new and valuable sequence and functional space for the engineering and evolution of existing enzyme functions.