2,7-Di-tert-butyl-9-fluorenylmethyl Chloro-formate1

[287381-46-8]  · C23H27ClO2  · (MW 370.92)

(base-labile protecting group for primary and secondary amines with the resulting carbamates having improved solubility in organic solvents over those of the Fmoc protecting group)

Alternate Name: 2,7-di-tert-butyl-9-fluorenylmethoxycarbonyl chloride; carbonochloridic acid, [2,7-bis(1,1-dimethylethyl)-9H-fluoren-9-yl]methyl ester; Fmoc*-Cl.

Physical Data: mp 63-65 °C.

Solubility: soluble in organics (CH2Cl2, CHCl3, pentane, etc.); reacts with alcohols, amines, and water.

Form Supplied in: 2,7-di-tert-butyl-9-fluorenylmethanol chloroformate and its immediate synthetic precursor (2,7-di-tert-butyl-9-fluorenylmethanol) are available from Aldrich, with the immediate synthetic precursor priced considerably more economically.

Analysis of Reagent Purity: 1H NMR, IR, elemental analysis.

Preparative Methods: the title reagent can be prepared in three steps from fluorene1 by Friedel-Crafts alkylation with (CH3)3 CCl and FeCl3 to give 2,7-di-tert-butyl-9-fluorene, hydroxymethylation with n-BuLi and paraformaldehyde to give 2,7-di-tert-butyl-9-fluorenylmethanol, and conversion to the chloroformate with phosgene.

Purification: low-temperature (-78 °C) recrystallization from pentane.

Handling, Storage, and Precautions: treat with precautions typical of nonvolatile acid chlorides and chloroformates—avoid skin and eye contact; avoid breathing fumes, which may contain HCl, by working in a fume hood; open bottles carefully, as pressure may develop; protect from moisture; reactions with nucleophiles, such as amines, may be exothermic and should be performed with adequate dilution and cooling.

2,7-Di-tert-butyl-9-fluorenylmethyl chloroformate (Fmoc*-Cl) is useful as a base-labile protecting group for primary and secondary amines for applications in which the 9-fluorenylmethyl-oxycarbonyl (Fmoc) protecting group2 would generally be used but is precluded by the low solubility of the Fmoc carbamates.3 It may be coupled with amines in the presence of a suitable base, such as aqueous sodium carbonate, and the resulting carbamate may be subjected to various synthetic transformations (1).1 At an appropriate point, the Fmoc* protecting group may be cleaved with a suitable base, such as piperidine, to liberate the amine group (2).1 As such, 2,7-di-tert-butyl-9-fluorenylmethyl chloroformate may be viewed as an alternative to 9-fluorenylmethyl chloroformate. An added benefit of Fmoc* is that the piperidine adduct that forms upon deprotection is highly lipophilic, allowing the removal of this adduct by extraction of a DMF solution of the deprotected amine with several portions of hexanes.

The Fmoc-protecting group has emerged as one of the two most important a-amino protecting groups in peptide synthesis. Although Fmoc-protected versions of the 20 proteinogenic amino acids with good solubilities in organic solvents are available, the growing importance of combinatorial and peptidomimetic chemistry has created a demand for many other protected amino acids. In some cases, the Fmoc-protected versions of these amino acids or their synthetic precursors have poor solubility in organic solvents. The Fmoc*-protecting group provides what may be the only practical method of preparing and incorporating these amino acids in Fmoc-based peptide synthesis. Such is the situation with the unnatural amino acid Hao (5-HO2CCONH-2-MeO-C6H3-CONHNH2).4 Hao is readily prepared as its Fmoc*-protected derivative (Fmoc*-Hao) by the condensation of Fmoc*-hydrazine with 5-methoxy-2-nitrobenzoyl chloride, reduction of the resulting Fmoc*-protected nitrobenzoic hydrazide to the corresponding aminobenzoic hydrazide, followed by condensation with ethyl oxalyl chloride and saponification of the ethyl ester group (3). Fmoc*-Hao is readily incorporated into standard solid-phase peptide synthesis, as illustrated by the preparation of i-PrCO-Phe-Hao-Val-NHBu (4).

Improvements in solubility of Fmoc*-protected amines of up to one-to-two orders of magnitude over their Fmoc-protected counterparts have been reported.1 Thus, the Fmoc-protected nitrobenzoic hydrazide 5-NO2-2-MeO-C6H3-CONHNH-Fmoc (1) exhibits extremely limited solubility in CHCl3 (1 mg mL-1) and limited solubility in THF (8 mg mL-1), while the corresponding Fmoc*-protected compound (5-NO2-2-MeO-C6H3-CONHNH- Fmoc*) (2) is freely soluble in both of these solvents (> 100 mg mL-1). The limited solubility of the Fmoc-protected compound precludes its use in the synthesis of Fmoc-Hao by a route analogous to that shown in 3, and the use of the Fmoc*-protected compound is essential.

An alternative to the Fmoc*-protecting group, which bears trimethylsilyl groups in place of the two tert-butyl groups, has been reported to have even better solubility properties than Fmoc*.5 The removal of the 2,7-bis(trimethylsilyl)fluorenylmethyl protecting group with amine bases proceeds in a rate almost comparable to that of the removal of Fmoc, while the removal of Fmoc* has been reported to proceed at either a nearly comparable rate to that of Fmoc1 or somewhat more slowly.5 A major drawback of the 2,7-bis(trimethylsilyl)fluorenylmethyl protecting group is that 2,7-bis(trimethylsilyl)fluorenylmethyl chloroformate is not commercially available and its preparation from fluorene requires seven steps (as opposed to three for Fmoc*-Cl).5,6,7

Related Reagents.

9-Fluorenylmethyl chloroformate (Fmoc-Cl); 2,7-bis(trimethylsilyl)-9-fluorenylmethyl chloroformate.5

1. Stigers, K. D.; Koutroulis, M. R.; Chung, D. M.; Nowick, J. S., J. Org. Chem. 2000, 65, 3858.
2. Carpino, L. A.; Han, G. Y., J. Org. Chem. 1972, 37, 3404.
3. Kocienski, P. J., Protecting Groups. Thieme, New York, 1994, p 202.
4. Nowick, J. S.; Chung, D. M.; Maitra, K.; Maitra, S.; Stigers, K. D.; Sun, Y., J. Am. Chem. Soc. 2000, 122, 7654.
5. Carpino, L. A.; Wu, A.-C., J. Org. Chem. 2000, 65, 9238.
6. Eaborn, C.; Shaw, R. A., J. Chem. Soc. 1955, 1420.
7. Smith, W. K.; Hardin, J. N.; Rabideau, P. W., J. Org. Chem. 1990, 55, 5301.

James S. Nowick

University of California, Irvine, California, USA

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