Solution-phase synthesis precipitations

Fragment coupling method was first applied to glycopeptide 11 (Scheme 3). The C-terminal peptide 12 was prepared by traditional solution-phase synthesis, while the activated -terminal glycopeptide 13 was prepared by solution-phase synthesis with solid-phase workup as described. The coupling between 12 and 13 was achieved in NMP and the product was readily isolated by precipitation. Target glycopeptide 11 was obtained in 89% overall yield (28). 

Like conventional solid supports, soluble polymers may have a potential to function as scavengers in solution-phase synthesis. The approach has been applied to the synthesis of p-amino alcohols that are structurally related to propranolol. The crude products (prepared from the corresponding epoxides and amines) and an added PEG-bound borohy-dride reagent formed a complex which was precipitated and separated from unreacted starting materials and unwanted by-products. Cleavage with HC1 in MeOH/DCM and precipitation of the polymer gave a solution of pure (>92%) p-amino alcohols [75]. 

Soluble resins combine the advantages of solution phase synthesis with the simple work-up of solid phase synthesis. The soluble resin and the reagents form a homogeneous system during the reaction. After each coupling step the polymer is precipitated and carefully washed to remove excess reagents. Incomplete precipitation results in a loss of resin after each precipitation step. A further drawback is the limited temperature range, since most soluble resins precipitate below -45 °C. 

DCC is a waxy solid that is often difficult to remove from a bottle. Its vapors are extremely hazardous to inhalation and to the eyes. It should always be handled in a fume hood. The isourea by-product of a DCC-initiated reaction, dicyclohexyl urea (DCU) (Figure 3.5), is also water-insoluble and must be removed by organic solvent washing. For synthesis of peptides or affinity supports on insoluble matrices this is not a problem, because washing of the support material can be done without disturbing the conjugate coupled to the support. For solution phase chemistry, however, reaction products must be removed by solvent washings, precipitations, or recrystallizations. 

DIC is a moderately expensive liquid that is employed in solid-phase synthesis to avoid the obstacles presented by the use of DCC. Both the reagent and the corresponding urea are soluble in organic solvents, and hence there is no bulky precipitate to contend with. The urea cannot be removed from an organic solution by aqueous extraction however, it is soluble enough in water that final traces can be removed from a precipitated peptide by washing the latter with a water-ether mixture. Clean up of spills of DIC can cause temporary blindness if utmost care is not exercised. 

For synthesis of peptides or affinity supports on insoluble matrices, this is not a problem because washing of the support material can be done without disturbing the conjugate coupled to the support. For solution phase chemistry, however, reaction products must be removed by solvent washings, precipitations, or recrystallizations. 

Solution-phase synthetic methods, as they were described for synthetic organic libraries, can also be applied to materials science and are devoid of the diffusion problems encountered in thin-film deposition. The reagent solutions are mixed and incubated following an appropriate procedure, and the final products are usually isolated by precipitation or crystallization. Automated liquid dispensing units with extreme precision and high rehabiUty can be used in synthetic protocols. No major differences are presented in respect to solution-phase organic library synthesis (see Section 8.2.4). Several examples are briefly illustrated below to provide a quick overview of the currently reported synthetic methods in solution for materials libraries. 

For their rich potential in various applications described in the previous section, the synthesis and assembly of various ZnO micro and nanostructures have been extensively explored using both gas-phase and solution-based approaches. The most commonly used gas-phase growth approaches for synthesizing ZnO structures at the nanometer and micrometer scale include physical vapor deposition (40, 41), pulsed laser deposition (42), chemical vapor deposition (43), metal-organic chemical vapor deposition (44), vapor-liquid-solid epitaxial mechanisms (24, 28, 29, 45), and epitaxial electrodeposition (46). In solution-based synthesis approaches, growth methods such as hydrothermal decomposition processes (47, 48) and homogeneous precipitation of ZnO in aqueous solutions (49-51) were pursued. 

After the completion of the solid-phase synthesis, the peptide resin was treated with HF that contained 10% anisole for 30 minutes at 0°C. After rapid removal of HF under a stream of nitrogen, free, depretected peptide was precipitated by addition of ether, and it was washed and extracted into 2M acetic acid. This solution was applied to a column (2.5 x 95 cm) of Sephadex G-25, which was eluted with 2M acetic acid. Material that emerged just after the void volume as a major peak (254 nm) contained the major component confirmed by TEC. This material was injected onto a column (2.5 x 45 cm) of ODS silica LRP-1 (Whatman) (13-24 p,m) and was eluted with a linear gradient of 15 and 35% 1-propanol in 0.1 M ammonium acetate (pH 4) at a flow rate of 5mL/min and a pressure about 60psi. Tractions were examined by TEC and analytical HPEC, and the partially resolved peak emerging at 375 mL that contained the major component was found identical to a standard sample of secretin (Fig. 7). The peptide was lyophilized to give about 120 mg final product. 

Solution phase supports have the problem of product recovery. An alternative approach is to couple the substrate to a water-soluble polymer that can be removed from solution by precipitation of the polymer or size exclusion chromatography. Water-soluble supports have been used in the enzymatic synthesis of pseudo-GM3 (118). 

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