Mixed synthesis methods

OBOC combinatorial bead-libraries can be considered as chemical microarrays that are spatially separable but non-addressable. The identity of the chemical compound on the positive beads can be determined directly with an automatic sequencer if it is an N-terminally unprotected peptide, by mass spectroscopy, or through chemical encoding. i A synthetic scheme for the OBOC library is shown in Figure 2. Using the highly efficient split-mix synthesis method,literally hundreds of thousands to millions of compounds can be prepared within a week. The recent... 

Figure 2 Synthetic scheme of the split-mix synthesis method to generate an OBOC combinatorial library...

Figure 1 The "split-mix synthesis" method to generate a one-bead one-compound combinatorial library (a) and a number of permutations for random peptide libraries (b). P, , and T are building blocks (in this case amino acids).

In this method, peptides or small molecules are prepared by the split-mix synthesis method and cleaved from the resin to form an encoded solution-phase library such that each library compound is tethered to a PNA code via a hydrophilic linker (11). The library then is mixed with the target protein and later exposed to planar oligonucleotide microarrays of predetermined sequences. Alternatively, the encoded soluble library can be hybridized to the oligonucleotide microarrays before incubation with the target protein. 

The principal variations on the normal crown synthesis methods were applied in preparing mixed crowns such as those shown in Eq. (3.55) and in forming isomers of the dibinaphthyl-22-crown-6 systems. The latter has been discussed in Sect. 3.5 (see Eq. 3.21) . The binaphthyl unit was prepared to receive a non-naphthyl unit as shown in Eq. (3.57). Binaphthol was allowed to react with the tetrahydropyranyl ether or 2-chloroethoxyethanol. Cleavage of the THP protecting group followed by tosyla-tion of the free hydroxyl afforded a two-armed binaphthyl unit which could serve as an electrophile in the cyclization with catechol. Obviously, the reaction could be accomplished in the opposite direction, beginning with catechol". ... 

Taramasso et al. (5) had originally reported two methods for the hydrothermal synthesis of TS-1. The first method (mixed alkoxide method) involves the preparation of a solution of mixed alkoxides of titanium and silica (preferably ethoxides) followed by hydrolysis with alkali-free solution of tetrapropylammonium hydroxide (TPAOH), distillation of the alcohol and crystallization of the resulting gel at 448 K. In the second method (dissolved or hydrolyzed titanium method) a soluble tetrapropylammonium peroxo-titanate species was prepared initially and then colloidal SiC>2 (Ludox AS-40) was added. This entire operation had to be carried out at 278 K. The TS-1 samples obtained by these two synthesis routes differed, particularly because of the presence of impurities such as Al3+ usually present in colloidal silica (33). 

Similar procedures adopted for the synthesis of TS-1 (the mixed alkoxide method, dissolved titanium method, pre-hydrolysis method, wetness impregnation method, and promoter induced synthesis method) were also used for the synthesis of TS-2. Tetrabutylammonium hydroxide (TBAOH) instead of TPAOH was used as the template (6,7,305-308). 

Mixed alkoxide method. Synthesis using TPAOH and HF and wetness-impregnation method using TPABr and NH4F... 

The reagents and methods employed for coupling in solid-phase synthesis are the same as for synthesis in solution, but a few are excluded because they are unsuitable. The mixed-anhydride method (see Section 2.6) and l-ethoxycarbonyl-2-ethoxy-l,2-dihydroquinoline (see Section 2.15) are not used because there is no way to eliminate aminolysis at the wrong carbonyl of the anhydride. Acyl azides (see Section 2.13) are too laborious to make and too slow to react. The preparation of acyl chlorides (see Section 2.14) is too complicated for their routine use this may be rectified, however, by the availability of triphosgene (see Section 7.13). That leaves the following choices, bearing in mind that a two to three times molar excess of protected amino acid is always employed. 

E Izeboud, HC Beyerman. Synthesis of substance P via its sulfoxide by the repetitive excess mixed anhydride method, (iodide for reduction) Rec Trav Chim Pays-Bas 97, 1, 1978. 

HC Beyerman, EWB De Leer, J Floor. On the repetitive excess mixed anhydride method for the synthesis of peptides. Synthesis of the sequence 1-10 of human growth hormone. Rec Trav Chim Pays-Bas 92, 481, 1973. 

M Bodanszky, JC Tolle. Side reactions in peptide synthesis. V. A reexamination of the mixed anhydride method. Ini Pept Prot Res 10, 380, 1977. 

Fe(CO)s], [Fe2(CO)g], [Co2(CO)8] and [Os3(CO)i2]) have been reacted with dicyanobenzene to form intrazeolite [M(Pc)] complexes [140]. Another class of materials prepared by the intrazeolite template synthesis method has been mixed ligand metal carbonyls and metal carbonyl clusters, frequently by reductive car-bonylation of metal ions in zeolite cages [175]. However, because these are frequently decomposed in situ to form, for example, nanoparticles, they are outside the scope of this chapter, and will be considered here only when they are used as precursors for metal complexes. 

Nucleoside 5 -phosphorothioates have also been employed as activated nucleotide derivatives for synthesis of pyrophosphates.321 The interaction of tributylammonium 2, 3 -di-0-benzoyluridine 5 -phosphorothioate (73) with silver a-D-glucopyranosyl and a-D-galac-topyranosyl phosphates in pyridine solution, with subsequent de-benzoylation, gave the corresponding glycosyl esters in 60-70% yield. This procedure can probably be classified as a variant of the mixed-anhydride method, the driving force of the reaction being the formation of insoluble silver sulfide. 

The Z-protected derivative, again prepared by standard methods using benzyl chloroformate,t208 may serve in the case of racemic pipecolic acid for resolution into the pure enantiomers by fractional crystallization with L-tyrosine hydrazide/208 Acylation with N-protected pipecolic acid or of pipecolyl peptides is performed by standard procedures via the active ester methods, e.g. A-hydroxysuccinimide ester/121 by the mixed anhydride method, e.g. with isobutyl chloro-formate 95-114 or pivalic acid chloride/121 as well as by DCC/HOBt/118 In the synthesis on solid support, longer coupling times are required when compared to N-protected proline.1[235 ... 

Preparation of [NHCHj] pseudodipeptides from a A-acetyl gem-diamine 88 and an a-aldehydo ester 89 was unsuccessful.185 The desired pseudodipeptide was unstable and eliminated the acetamide moiety. To overcome this instability a pseudotripeptide was prepared that incorporates the reduced retro isostere from a retro-inverso pseudodipeptide 85 the synthesis of this compound is shown in Scheme 16. Coupling of Z-L-Phe-OH with 32 using the mixed anhydride method afforded the retro-inverse pseudodipeptide 33. Reaction of 33 after hydrogenation with an a-aldehydo ester provides the reduced-retro pseudotripeptide 35 as a pair of diastereomers. 

Synthesis of Z-Gly-DL-Alap from Alap by the Mixed Anhydride Method Typical Procedure 171... 

Scheme 11 Synthesis of Peptides with C-Terminal 1-Aminoalkylphosphonates by the Mixed Anhydride Method 21-25 ...

Synthesis of Peptides 17 with C-Terminal Phosphonate by the Mixed Anhydride Method General Procedure 1251... 

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