Acids progressive identification

Fig. 5.5 Progressive identification of acids and anhydrides in roasted coffee volatiles...

Although the identification of tetrahedrally coordinated, tetra- and tripodal Ti4+ ions on the surface of titanosilicates, as the likely active sites in reactions that require Lewis acidity, seems convincing, the structure and role of the sites active in catalytic oxidation, presumably oxo-titanium species, formed by the interaction of H202 (or H2 + 02) with these surface Ti ions, are not clear. In recent years, this problem has been investigated by FTIR (133), Raman (39,40), XANES (46-48), electronic (54-57), and EPR (51-54) spectroscopies. This is one of the areas in which major progress has been made since the reviews of Notari (33) and Vayssilov (34). Zecchina et al. (153) recently summarized some of the salient features of this progress. 

IR spectroscopy is not a very sensitive analytical tool and is, therefore, not well suited to the detection of small amounts of material. If, however, intermediates have intense and well-resolved IR absorptions, the progress of their chemical transformation can be followed by IR spectroscopy [83,88,91-93], Near-infrared spectroscopy, in combination with an acousto-optic tunable filter, can be sufficiently sensitive to enable the on-bead identification of polystyrene-bound di- and tripeptides, even if the peptides have very similar structures (e.g., Leu-Ala-Gly-PS and Val-Ala-Gly-PS) or differ only in their amino acid sequence (e.g., Leu-Val-Gly-PS and Val-Leu-Gly-PS) [94]. Special resins displaying an IR and Raman barcode have been developed, which may facilitate the deconvolution of combinatorial compound libraries prepared by the mix-and-split method [48]. 

The 4,7-oxaeunicellane skeleton of the eleutherobins is also found in the eleuthosides (2, 3), the sarcodictyins (4), and the valdivones (5) (Figure 1) [4-6]. While the arabinosyl residue is not required for an antitumor effect, the methylurocanic acid ester side chain bound to C8 is part of the pharmacophore. Recent progress in the total synthesis of eleutherobin is discussed together with the identification of a common pharmacophore for tubulin-binding natural products, and a combinatorial way to determine the structure/activity relationship and drug optimization. 

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