Fmoc-p-alanine

A reduction in reaction time by virtue of the improved transport properties in eledroosmotic-driven microreadors was demonstrated for the P-dipeptide synthesis using pentafluorophenyl O-activation. Fmoc-P-alanine was preactivated by introducing the pentafluorophenyl fundion as ester group [9]. Dmab-P-alanine and the pentafluorophenyl ester of Fmoc-P-alanine readed, and the synthesis of the corres-... 

In the next step, the preparation oflonger chain peptides was done in electroosmotic microreactors. For this purpose, deprotection and peptide bond forming reactions had to be developed resulting in a yield of 30% [6,9]. In this way, the formation of tripeptides was achieved. Dmab-P-alanine and Fmoc-P-alanine were reacted in the first step to a dipeptide [6]. After cleavage of the Fmoc function, Fmoc-[3-alanine was added to such a dipeptide that resulted in tripeptide formation in 30% yield. 

Attachment and deavage To resin 173 were added DIC (3 equiv.), Fmoc-P-alanine (3 equiv.), DIEA (3 equiv.), DMAP (catalyst), and HOBt (catalyst) in DMF. The yield for the esterification was 32%, as checked by the release of the Fmoc group from an aliquot. 

The spot-synthesis technique is simple and easily accessible for practical use. There are some limitations to the scope of chemistries that can be used with cellulose as a solid support. To eliminate some shortcomings of cellulose, Gao and Esnouf [58,59 proposed using a different membrane, Immobilon AV-1, for the spot synthesis. Before synthesis of peptide arrays can be carried out, this polyvinylidene fluoride-based material is derivatized with ethylene diamine, followed by coupling of Fmoc-p-alanine as a spacer. In most other aspects, the peptide array synthesis is very similar to the spot synthesis proposed by Frank. 

Many reports confirm notable reductions in reaction times when carrying out reactions under micro flow conditions. Concerning p-dipeptide synthesis, for example, a comparison between batch and micro-reactor processing was made for the reaction of Dmab-P-alanine and Fmoc-i-P-homophenylalanine [158]. While the micro reactor gave a 100% yield in 20 min, only about 5% was reached with the batch method. Even after 400 h, only 70% conversion was achieved. 

P 12] The micro channels were primed with anhydrous N,N-dimethylformamide (DMF) to remove air and moisture before carrying out the reaction [88]. A 50 pi volume of a solution of Fmoc-y9-alanine (0.1 M) in anhydrous DMF was placed in one reservoir of a micro chip, driven by electroosmotic flow. A 50 pi volume of a solution of EDCl (0.1 M) in anhydrous DMF and 50 pi of a solution of Dmab-y9-alanine (0.1 M) in anhydrous DMF were inserted in the other two reservoirs. Anhydrous DMF was placed in the fourth reservoir, for collection of the product. Room temperature was applied for reaction for a 20 min period. The voltage was set in the range 500-700 V. 

P 13] The micro channels were primed with anhydrous DMF to remove air and moisture before carrying out the reaction [5, 88]. A 50 pi volume of a solution of the pentafluorophenyl ester of Fmoc-y9-alanine (0.1 M) in anhydrous DMF was placed in one reservoir of a micro chip, driven by electroosmotic flow (Figure 4.42). 

Peptide cleavage from the allyl-linker resin is achieved with morpholine in large excess in the presence of tetrakis(triphenylphosphine)palladium (10mol% based on the allylic substitution of the resin). (3-HYCRAM resin is a modification of HYCRAM containing a P-alanine residue as a spacer between the crotonoyl unit and the resin. It has been used for the solid-phase synthesis of glycopeptides by the Boc and Fmoc strategies.b 1 Difficulties may be... 

Microreaction technology has also been applied to peptide synthesis. Haswell and coworkers demonstrated that, using a borosihcate glass microreactor, the desired dipeptide was obtained by the reaction of N-Fmoc-(3-alanine with P-alanine Dmab ester in the presence of DCC (1,3-dicydohexykarbodiimide) (Scheme 5.30) [41,42]. It also was demonstrated that the Fmoc group could be removed by DBU... 

OS 16] ]R 5] ]P 13] Using continuous flow in an electroosmotic-driven micro reactor gave a quantitative yield of the dipeptide in only 20 min (600 V for Dmab-/ -alanine 700 V for the Fmoc ester) [5, 88]. Batch synthesis under the same conditions gave only a 40-50% yield [5] (46% in [5]), needing 24 h. 

In 2004, Podlech and coworkers have reported some further advances in the photochemical treatment of Fmoc-protected diazoketones A and B, (Scheme 76), derived from leucine and alanine, respectively, with /V-(benzylidene)glycine and leucine methyl ester to produce a mixture of the corresponding diastereomeric trans-substituted p-lactams [175],... 

C-Terminal cysteine residues linked via ester bonds to the solid support suffer extensive epimerization in the repetitive piperidine-mediated Fmoc cleavage steps of SPPS.P Moreover, p-elimination occurs to some extent from the S-protected cysteine residues with formation of dehydroalanine that reacts with the excess piperidine to form -(piper-idino)alanine as an adduct.These cysteine-related side reactions are thoroughly discussed in Section 2.6.6. 

In addition to the problems of racemization of S-protected cysteine residues during both their anchoring to hydroxymethyl-type resins and formation of peptide bonds (see Section 2.6.6.2), the synthesis of C-terminal Cys(Acm)-containing peptides using Fmoc/tBu chemistry has been observed to produce peptides with C-terminal P-(l-piperidyl)alanine (see Section 2.6.6.2).t l This type of side reaction can be reduced by nninimal exposure to piperidine, but it can not be suppressed completely. 

Burke. T.R., Jr., Smyth, M.S., Otaka, A., and Roller, P.P., Synthesis of 4-phosphono(difluoromethyl)-D.L-plieiiy I alanine and A-Boc and 7V-Fmoc derivatives suitably protected for sohd-phase synthesis of nonhydrolysable phosphotyrosyl peptide analogues, Tetrahedron Lett., 34, 4125, 1993. 

Syntheses of rac-la [8-10], (2 >la [11-13], (S)-la [13], (R)-P-(dimethylphenylsilyl)alanine [12], and (7 )-p-(methyldiphenylsilyl)alanine [12] have already been reported. In addition, the ethyl ester and the A -Fmoc and W-Boc derivatives of (K)-la, as well as renin inhibitory peptides bearing (7 )-la, have been described in the literature [14]. In contrast, germanium-containing a-amino acids have not yet been reported. 

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