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Chemistry100%

Selected Abstracts

Chemo-Enzymatic Approach to Statin Side-Chain Building Blocks

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6-7 2003
R. Öhrlein
Abstract A versatile statin side-chain building block is obtained by an enzymatic desymmetrisation of the symmetrical glutaric acid diethyl ester. The monoacid is produced in almost quantitative yield in the desired high optical purity. The monoacid is easily converted to the corresponding acid chloride, which is a key compound to be elaborated to some statin side-chain derivatives. The optically active C-5 chain is subsequently elongated by two carbon atoms and syn -reduced to the final diol fragment. [source]

Quantifying the dynamics of sugar concentration in berries of Vitis vinifera cv. Shiraz: a novel approach based on allometric analysis

AUSTRALIAN JOURNAL OF GRAPE AND WINE RESEARCH, Issue 2 2007
VICTOR O. SADRAS
Abstract Concentrations of key compounds (e.g. sugar) in berries are the net result of relative changes in the amount of compound per berry and berry size. The complex nature of concentrations is widely recognised, but the widespread use of chronological scales for comparisons implies that ontogenetic drift or size-dependent effects are often overlooked. This paper presents an allometric analysis of sugar concentration in berries of cv. Shiraz as a way to formally account for ontogenetic drift. Our starting point is the double-sigmoid growth pattern of a grape berry where we distinguish Phase 1, from flowering to veraison; Phase 2, from veraison to peak berry fresh mass, and Phase 3, after peak fresh mass. Phase 3 explicitly accounts for the late season shrinkage typical of Shiraz berries. We advance an allometric model of sugar per berry with berry fresh mass, rather than time, as descriptor. The condition for an increase in sugar concentration in Phase 2 is that the relative rate of sugar accumulation per berry (RSB) exceeds the relative rate of berry net accumulation of fresh mass (RFM). This is equivalent to an allometric coefficient, calculated as the slope of the regression between amount of sugar per berry and berry mass in a log-log scale, being greater than 1. For Phase 3, the condition for increase of sugar concentration is that a large reduction in berry mass offsets any putative change of sugar per berry, yielding an allometric coefficient < 1. Such an allometric model was tested against measured data from sixteen contrasting crops resulting from the combination of eight water regimes and two seasons. Berry mass peaked between 96 and 105 days after anthesis, and these dates were used to separate Phases 2 and 3. In Phase 2, the relative rate of increase in sugar per berry varied from 0.01 to 0.02 d -1 in comparison to the relative rate of increase in berry fresh mass that varied from 0.0038 to 0.0066 d -1. Sugar per berry thus increased 2.4,3.3 times faster than berry mass, with allometric coefficients between 1.98 and 2.91 accounting for 78% of the variation in the relative rate of change of sugar concentration. In Phase 3, the relative rate of change in sugar per berry was not different from zero (P 0.05) in most cases, whereas the rate of change in berry size ranged from ,0.0013 to ,0.0035 d -1 and was significant (P < 0.05) in 14 out 16 cases. The small changes in sugar per berry and the net loss of berry material yielded allometric coefficients between 0.17 and 1.11, which accounted for 72% of the variation in the relative rate of change in sugar concentration. We conclude that a model, which pivots around peak berry mass, with allometric coefficients above 1 in Phase 2 and below 1 in Phase 3, is suitable to quantitatively account for ontogenetic drift in the dynamics of sugar concentration in berries of Shiraz. This allometric approach demonstrated that sugar per berry during the stage of berry shrinkage is a plastic trait under significant environmental influence. For the same genotype, environmental conditions could determine either, a putative backflow of water accounting for net loss of berry fresh mass (RFM < 0) that could also carry some sugar from berries back to the parent vine (RSB < 0) or a small gain of sugar (RSB 0) closely coupled with a net loss of berry fresh mass (P= 0.003). [source]

,-Substituted Terthiophene [2]Rotaxanes

CHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2009
Taichi Ikeda Dr.
Abstract Towards polythiophene polyrotaxanes: The ,-substituted terthiophene [2]rotaxanes have been synthesized (see figure). Basic optical and electrochemical properties of the synthesized [2]rotaxanes are also reported. Two kinds of ,-substituted terthiophene [2]rotaxanes were synthesized using the host-guest pairs of the electron-deficient cyclophane cyclobis(paraquat- p -phenylene) (CBPQT4+) and the electron-rich terthiophenes with diethyleneglycol chains at the ,-position. One is made from the ,-position non-substituted terthiophene (3,T-,-Rx) and the other is made from the ,-dibromo-substituted terthiophene (3,TBr-,-Rx). The binding constants of the ,-substituted terthiophene threads were confirmed to be smaller than that of the ,-substituted terthiophene analogue. By UV/Vis absorption measurements, we confirmed the charge-transfer (CT) band in the visible region with an extinction coefficient of ,102 (M,1,cm,1). Strong, but not quantitative, quenching of the terthiophene fluorescence was confirmed for the [2]rotaxanes. Although the ,-substituted terthiophene thread was electrochemically polymerizable, the [2]rotaxane 3,T-,-Rx was not polymerizable. This result indicates that the interlocked CBPQT4+ macrocycle effectively suppresses the electrochemical polymerization of the terthiophene unit because electrostatic repulsive and steric effects of CBPQT4+ hinder the dimerization of the terthiophene radical cations. In the electrochemical measurement, we confirmed the shift of the first reduction peak towards less negative potential compared to free CBPQT4+ and the splitting of the second reduction peak. These electrochemical behaviors are similar to those observed for the highly-constrained [2]rotaxanes. The ,-substituted terthiophene [2]rotaxanes reported herein are important key compounds to prepare polythiophene polyrotaxanes. [source]

Organocatalytic Asymmetric Mannich Reactions with N -Boc and N -Cbz Protected ,-Amido Sulfones (Boc: tert -Butoxycarbonyl, Cbz: Benzyloxycarbonyl)

CHEMISTRY - A EUROPEAN JOURNAL, Issue 29 2007
Olindo Marianacci
Abstract Different malonates and ,-ketoesters can react with N - tert -butoxycarbonyl- (N -Boc) and N -benzyloxycarbonyl- (N -Cbz) protected ,-amido sulfones in an organocatalytic asymmetric Mannich-type reaction. The reaction makes use of a simple and easily obtained phase-transfer catalyst and proceeds under very mild and user-friendly conditions. The optimised protocol avoids the preparation and the isolation of the relatively unstable N -Boc and N- Cbz imines that are generated in situ from the bench-stable ,-amido sulfones. The corresponding Mannich bases are generally obtained in good yields and enantioselectivities, and can be readily transformed into key compounds, such as optically active ,3 -amino acids in one easy step. Enantioenriched N -Boc and N -Cbz protected ,-amino acids that are suitable for peptide synthesis are also available from the Mannich adducts through simple manipulations. Control experiments showed the dual role of the enolate,catalyst ion pair in this reaction, as well as the crucial role of the presence of water to achieve high enantioselectivities. [source]