9-fluorenylmethyloxycarbonyl protecting group

It has been found that the tris(tert-butyloxycarbonyl) protected hydantoin of 4-piperidone 2, selectively hydrolyses in alkali to yield the N-tert-butyloxycarbonylated piperidine amino acid 3. The hydrolysis, which is performed in a biphasic mixture of THF and 2.0M KOH at room temperature, cleanly partitions the deprotonated 4-amino-N -(tert-butyloxycarbonyl)piperidine-4-carboxylic acid into the aqueous phase of the reaction with minimal contamination of the hydrolysis product, di-tert-butyl iminodicarboxylate, which partitions into the THF layer. Upon neutralization of the aqueous phase with aqueous hydrochloric acid, the zwitterion of the amino acid is isolated. The Bolin procedure to introduce the 9-fluorenylmethyloxycarbonyl protecting group efficiently produces 4.8 This synthesis is a significant improvement over the previously described method9 where the final protection step was complicated by contamination of the hydrolysis side-product, di-tert-butyl iminodicarboxylate, which is very difficult to separate from 4, even by chromatographic means. 

The details of the solid-phase technique have been improved substantially over the years, but the fundamental idea remains the same. The most commonly used resins at present are either the Wang resin or the PAM (phenyl-acetamidomethyl) resin, and the most commonly used N-protecting group is the fluorenylmethyloxycarbonyl, or Fmoc group, rather than Boc. 

In the case of vancomycin [72], an original study was performed to obtain a well-defined stationary phase structure, since it was reasonably assumed that the antibiotic is randomly linked to the silica by one or both of its amino groups, one belonging to the disaccharide portion (primary), and the other one to the heptapeptide core (secondary). Thus, alternate fluorenylmethyloxycarbonyl (FMOC)-amino-protected derivatives were prepared and immobilized in a packed column, and then vancomycin was recovered by cleavage of the protecting groups. The two defined CSPs obtained, when compared with the CSP produced from native randomly linked vancomycin, showed lower retention and enantioselectivity, also if they still separated the same compounds. Thus, no advantages could be found to choose these phases as an alternative to the native vancomycin CSP. 

One of the most important limitations in the use of these diamino acids is the need of an additional orthogonal protection for the amino groups. Using the t-butoxycarbonyl/benzyl (Boc/Bzl) strategy of solid-phase peptide synthesis, this additional orthogonality can be easily provided by a base-labile protecting group such as the 9-fluorenylmethyloxycarbonyl... 

Standard amino acid protecting groups that were used routinely in BOC-peptide synthesis. For Lys the -nitrogen has the potential to cyclize. We sometimes protect this with the fluorenylmethyloxycarbonyl (FMOC) protecting group. 

Glycopeptides are more difficult to synthesize than the conventional peptides, because common protection-deprotection reactions used in the peptide synthesis can cause serious problems to the protective groups used for carbohydrates. Many of these problems have been solved by recent technical innovations [21]. The combination of 9-fluorenylmethyloxycarbonyl (Fmoc) group for N-protection and pentafluo-rophenyl (PFP) group as the activating group for the carboxylic acid allows GlcNAc-Asn... 

This is the reason why peptide chemists, to decrease the problems of purification prefer for long peptides to use protecting groups (tert-butyloxycarbonyl (t-Boc), benzyloxycarbonyl (Z), fluorenylmethyloxycarbonyl (FMOC).) and classical reagents such as T.B.T.U. (0-lH-benzotriazol-l-yl)-l,l,3,3-tetramethyl uronium tetrafluoroborate), B.O.P.(benzotriazol-l-yl-oxy-tris (dimethylamino) phosphonium hexafluorophosphate and so on in polar solvents such as N,N-dimethylformamide or N-methylpyrrolidone. But this solvents are not compatible with the acidic deprotection reagents such as trifluoroacetic acid and... 

Fmoc/Bu chemistry, Sheppard tactics, a widely applied alternative approach to the Boc-based SPPS Merrifield tactics) that makes use of the base lability of the 9-fluorenylmethyloxycarbonyl group in SPPS. The semipermanent side-chain-protecting groups are mostly of the tert.-butyl type, and can be cleaved under relatively mild reaction conditions with TFA. Linker moieties displaying comparable acid lability are mainly used. 

Several important peptide-protecting groups such as 9-fluorenylmethyloxycarbonyl, benzyl, 4-nitrobenzyl, 2,2,2-trichloroethyl, and acetonyl (eq 23) can be removed by TBAF under mild conditions. 

In order to design syntheses of complex peptides containing several functional side chains, it is necessary to have a variety of methods of protection and deblocking at disposal. Therefore, it was a major step forward when in addition to reduction and acidolysis deprotection with weak bases, under mild conditions, became a practical possibility. The 9-fluorenylmethyloxycarbonyl (Fmoc) group (Carpino and Han 1970) is removed from the amino group by proton abstraction with secondary amines. A carbamoic add is generated, that, in turn, loses carbon dioxide and affords the free amine ... 

Figure 4, 9-Fluorenylmethyloxycarbonyl - protected 5 -aminothymidine phosphoramidite for the synthesis of oligonucleotides containing an aliphatic amino group at the 5 terminus, R CH3 or -CH2CH2CN.

Fluorenylmethyloxycarbonyl (Fmoc)-protected amino acids with appropriate side-chain protecting groups... 

Fig. 3. A mild two-dimensional orthogonal protection scheme for solid-phase synthesis. Temporary -amino protection is provided by the 9-fluorenylmethyloxycarbonyl (Fmoc) group, removed by a base-catalyzed elimination mechanism, typically with piperidine. Permanent tert-bulyl-based side-chain-protecting groups and the p-alkoxybenzyl (PAB) ester handle linkage are both cleaved by treatment with TFA to 5deld the free peptide acid at the end of the synthesis. Details in text.

Amino groups are often protected as their terf-butyloxycarbonyl amide (Boc) or fluorenylmethyloxycarbonyl amide (Fmoc) derivatives. The Boc protecting group is introduced by reaction of the amino acid with di-fi rt-butyl dicarbonate in a nucleophilic acyl substitution reaction and is removed by brief treatment with a strong acid such as trifluoroacetic acid, CF3CO2H. The Fmoc protecting group is introduced by reaction with an acid chloride and is removed by treatment with base. 

Miscellaneous. Fluoride ion from anhydrous TBAF undergoes nucleophilic displacement of tosylates, halides, and aryl nitro compounds to give fluorinated products. When used with N-Bromosuccinimide, bromofluorination products are obtained. Several important peptide-protecting groups such as 9-fluorenylmethyloxycarbonyl, benzyl, 4-nitrobenzyl, 2,2,2-trichloroethyl, and acetonyl (eq 23) can be removed by TBAF under mild conditions. 

The definitive paper on the 9-fluorenylmethyloxycarbonyl group (FMOC) for protection of an amino group has been published. ... 

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