Fluorenylmethoxycarbonyl protection

In the first step, Fmoc (9-fluorenylmethoxycarbonyl)-protected dithiocarbazate resin 43 was prepared by a three-component reaction of Merrifield resin 1 and carbon disulfide with Fmoc-protected hydrazine in the presence of sodium hydride in DMF at room temperature. Deprotection of the Fmoc group of resin 43 with 5% piperidine produced the corresponding free dithiocarbazate resin 42." In this step, the use of 5% piperidine was essential because a higher concentration caused loss of the desired substrate from resin 43. In addition, we have developed a more convenient synthetic route to the resin 42, compared to the previous report using hydrazine monohydrate and carbon disulfide with potassium hydroxide in ethanol solvent. Under these reaction conditions, we obtained the polymer-bound dithiocarbazate 42 without the Fmoc protection step of the hydrazine. 

This is a very important and well tested method for the quantitative determination of loading of Fmoc protected compounds particularly that of Fmoc (fluorenylmethoxycarbonyl) amino acids on solid support. Fmoc groups can... 

For a review of the use of Fmoc protection in peptide synthesis, see E. Atherton and R. C. Sheppard, The Fluorenylmethoxycarbonyl Amino Protecting Group, in The... 

In SPPS, there are two main protecting groups commonly used for Ai -protection [3] ferf-butoxycarbonyl (Boc) [7] and 9-fluorenylmethoxycarbonyl (Fmoc) [8] (Fig. 2). 

To overcome these difficulties in the selective deprotection and chain extension, several carboxyl-protecting groups, namely, allyl (16,32), benzyl (43,44), tert-butyl (42), 2-bromoethyl (45), 2-chloroethyl (45), heptyl (46), 4-nitrophenyl (47,48), and pentafluorophenyl (49) for L-serine/L-threonine have been introduced or applied. Similarly, amino-protecting groups for L-serine/L-threonine that have proved useful for the synthesis of glycopeptides are tm-butyloxycarbonyl (50), 9-fluorenylmethoxycarbonyl (43,44,48), 2-(2-pyridyl)ethoxycarbonyl (51), 2-(4-pyridyl)ethoxycarbonyl (44,52), and 2-triphenylphosphonioethoxycarbonyl (53). Some applications of these groups have been discussed in earlier reviews (7-11). 

C-D Chang, M Waki, M Ahmad, J Meienhofer, EO Lundell, JD Haug. Preparation and properties of N -9-fluorenylmethoxycarbonyl amino acids bearing tert. -butyl side chain protection. Int J Pept Prot Res 15, 59, 1980. 

L Lapatsanis, G Milias, K Froussios, M Kolovos. Synthesis of A-2,2,2,-(trichloro-ethoxy carbonyl)-L-amino acids and A-(fluorenylmethoxycarbonyl)-L-amino acids involving succinimidoxy anion as a leaving group in amino acid protection. Synthesis 671, 1983. 

A Paquet. Introduction of 9-fluorenylmethoxycarbonyl, trichloroethoxycarbonyl, and benzyloxycarbonyl amino protecting groups into O-unprotected hydroxyamino acids using succinimidyl carbonates. Can J Chem 60, 976, 1982. 

E Atherton, RC Sheppard. The fluorenylmethoxycarbonyl amino protecting group, in The Peptides Analysis, Synthesis, Biology, Vol. 9, pp 1-38, Academic Press, New York, 1987. 

For a,a-dialkylamino acids enantiomerization is not a problem. The preparation of 4,4-dimethyl-2-[(9-fluorenylmethyl)oxy]-5(4F/)-oxazolone, an intermediate used in the synthesis of ( )-mirabazole C has been described. Recently, two new 2-aIkoxy-5(4F/)-oxazolones derived from Toac (2,2,6,6-tetramethyl-4-amino-l-oxy-piperidine-4-carboxylic acid) that incorporate Z or 9-fluorenylmethoxycarbonyl (Fmoc) protection at C-2 have been described. The Toac analogues were synthesized as part of a study of the crystal structure and ab initio calculations for these interesting systems. 

Scheme 15 Synthesis of Tyrosine 0-Sulfate Peptides with the 9-Fluorenylmethoxycarbonyl Group for Temporary N"-Protection 421...

In the synthesis of analogues of calicheamicin 71 and esperamicin Ajb, Moutel and Prandi employed the glycosyla-tion of a nitrone with a trichloroacetimidate as a key step - /3-N-O glycosidic bond formation. Preparation of the nitrone begins with the alkylation of the known alcohol 69 <1992CC1494> with 1,4-dibromobutane in the presence of sodium hydride. Subsequent aminoalkylation, amine protection with 9-fluorenylmethoxycarbonyl (Fmoc), and reduction with NaBHsCN were followed by nitrone 70 formation with 4-methoxybenzaldehyde (Scheme 8) <2001J(P1)305>. 

Scheme 14 shows a typical example in a series of reactions in which a supported amino acid reacted with fluorenylmethoxycarbonyl isothiocyanate to give a supported (on Rink s amide)35 thiourea.36 Removal of the protection followed by 5-alkylation gave supported isothioureas. Reaction of these with amines, then cleavage from the resin, afforded substituted guanidines. For 10 examples the purities were between 40 and 92%. An aryl group separates the resin from the guanidine, just as in the sequences shown in Schemes 11 and 12. 

The hydrazone link is formed by the reaction of two linkers, J (5,5-dimethoxypentanoic acid, 23) and [l-(9-fluorenylmethoxycarbonyl)-2-(isopropylidene)hydrazino]acetic add (25). J (23) is obtained by hydrolyzing the corresponding methyl ester, which is commercially available or can be prepared by following the method of Stevens and Lee1 43 (Scheme 11). Compound 25 is prepared by protecting commercially available ethyl hydrazinoacetate hydrochloride (24) (Scheme 12)J1211... 

Scheme 11 The Four 9-Fluorenylmethoxycarbonyl/Benzhydryloxycarbonyl Protected Peptide Nucleic Acid Monomers...

The 9-fluorenylmethoxycarbonyl group, developed by Carpino and co-workers in 1972 [257], has become one of the most widely used protective groups for aliphatic or aromatic amines in solid-phase synthesis. For solid-phase peptide synthesis in particular, this protective group plays an important role [258] (Section 16.1). The Fmoc group is not well suited for liquid-phase synthesis because non-volatile side products are formed during deprotection. 

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