9-Fluorenylmethoxycarbonyl synthesis

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... 

Albericio F, Kneib-Cordonier N, Biancalana S, Gera L, Masada RI, Hudson D, Barany G. Preparation and application of the 5-(4-(9-fluorenylmethoxycarbonyl)aminomethyl-3, 5-dimethoxyphenoxy)-valeric acid (PAL) handle for the solid-phase synthesis of C-terminal peptide amides under mild conditions. J Org Chem 1990 55 3730-3743. 

Urge, L., Kollat, E., Hollosi, M., Laczko, I., Wroblewski, K., Thurin, J., and Otvos Jr., L. (1991) Solid-phase synthesis of glycopeptides synthesis of Na-fluorenylmethoxycarbonyl L-asparagine Nb-glyco-sides. Tetrahedron Lett. 32, 3445-3448. 

C.G. Fields, G.B. Fields, Minimization of tryptophan alkylation following 9-fluorenylmethoxycarbonyl solid-phase peptide synthesis, Tetrahedron Letters 34(1993)6661-6664. 

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). 

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. 

E Atherton, M Caviezel, H Fox, D Harkiss, H Over, RC Sheppard. Peptide synthesis. Part 3. Comparative solid-phase synthesis of human P-endorphin on polyamide supports using t-butoxycarbonyl and fluorenylmethoxycarbonyl. 

E Atherton, JL Holder, MMeldal, RC Sheppard, RM Valerio. 3,4-Dihydro-4-oxo-l,2,3-benzotriazin-3-yl esters of fluorenylmethoxycarbonyl amino acids as self-indicating reagents for solid phase peptide synthesis. J Chem Soc Perkin Trans 1 2887, 1988. 

JD Wade, J Bedford, RC Sheppard, GW Tregear. DBU as an V -deprotecting reagent for the fluorenylmethoxycarbonyl group in continuous flow solid-phase peptide synthesis. Pept Res 4, 194, 1991. 

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. 

GB Fields, RL Noble. Solid phase synthesis utilizing 9-fluorenylmethoxycarbonyl amino acids. Int J Pept Prot Res 35, 161-214, 1990. 

R Sheppard. The fluorenylmethoxycarbonyl group in solid phase synthesis. J Pept Sci 9, 545-552, 2003. 

K Barlos, D Gatos. 9-Fluorenylmethoxycarbonyl/tbutyl-based convergent protein synthesis. Biopolymers (Pept Sci) 51, 266-278, 1999. 

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>. 

Reactions (T) through are necessary for the formation of each peptide bond. The 9-fluorenylmethoxycarbonyl (Fmoc) group (shaded blue) prevents unwanted reactions at the o-amino group of the residue (shaded red). Chemical synthesis proceeds from the carboxyl terminus to the amino terminus, the reverse of the direction of protein synthesis in vivo (Chapter 27). 

Unfortunately, A-(9-fluorenylmethoxycarbonyl)aziridine-2-carboxylic acid cannot be used in peptide synthesis, since N-deprotection of the respective peptides with secondary amines leads to oxazoline or dehydroamino acid side products. Similarly, N-(tert-butoxy-carbonyl)aziridine-2-carboxylic acid is inappropriate due to the instability of the aziridine moiety to TFA treatment. Attempts to convert A-tritylaziridine-2-carboxylic acid into homogenous and stable active esters as useful intermediates in peptide synthesis leads to positive results only in the case of the pentafluorophenyl ester. 47 Consequently, this active ester seems to be the method of choice for acylating peptides. The related Abhydroxysuc-cinimide and A-3-hydroxy-4-oxo-3,4-dihydro-l,2,3-benzotriazine ester could not be isolated in pure form and have therefore been used as crude products. 47 Access to 2-carbonylazir-idine peptides is also possible by carbodiimide-mediated coupling. Additionally, alkylamides of A-tritylaziridine-2-carboxylic acid are prepared by the azide method,1 5 yet this method fails in peptide coupling steps. 85 ... 

Scheme 12 Synthesis of Linker [l-(9-Fluorenylmethoxycarbonyl)-2-(isopropylidene)hydrazino]acetic Acidl1211...

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. 

Scheme 3. Synthesis of MGd-BSO conjugate. DEAD = diethyl azodicarboxylate Fmoc = 9-fluorenylmethoxycarbonyl.

In this chapter we will summarize chemical methods for the stereoselective attachment of carbohydrates to amino acids, with particular emphasis on the preparation of building blocks for use in solid-phase glycopeptide synthesis based on the 9-fluorenylmethoxycarbonyl (Fmoc) protective group strategy. 

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