Fluorous amine protection

De Visser PC, van Helden M, Filippov DV, van der Marel GA, Drijfhout JW, van Boom JH, Noort D, Overkleeft HS (2003) A novel, Base-Labile Fluorous Amine Protecting Group Synthesis and Use as a Tag in the Purification of Synthetic Peptides. Tetrahedron Lett 44 9013-9016... 

Furthermore, multicomponent reactions can also be performed under fluorous-phase conditions, as shown for the Ugi four-component reaction [96], To improve the efficiency of a recently reported Ugi/de-Boc/cyclization strategy, Zhang and Tempest introduced a fluorous Boc group for amine protection and carried out the Ugi multicomponent condensation under microwave irradiation (Scheme 7.84). The desired fluorous condensation products were easily separated by fluorous solid-phase extraction (F-SPE) and deprotected by treatment with trifluoroacetic acid/tet-rahydrofuran under microwave irradiation. The resulting quinoxalinones were purified by a second F-SPE to furnish the products in excellent purity. This methodology was also applied in a benzimidazole synthesis, employing benzoic acid as a substrate. 

As fluorous Cbz-type hnkers are known to be partially deprotected in acidic medium, a base-labile amine-protecting group (19, Scheme 8.4) was developed as an orthogonal alternative. The peptide products were purified by both fluorous HPLC and FSPE [41]. The deprotection of the fluorous tag was carried out using 2% aqueous ammonia, a reagent that would raise concerns in the presence of common carbohydrate-protecting groups such as acetates. 

A perfluoro-tagged tert-butyloxycarbonyl group ( Boc) has been used for the protection of primary amino functions in the synthesis of a small amide library [29]. The Boc-protected amino acids were coupled wifh primary or secondary amines. The products were purified by preparative fluorous HPLC. After deprotection by add treatment, the products were isolated by conventional extractive work-up. 

Ladlow and coworkers have reported the use of fluorous-tagged aldehydes as a protecting group in the synthesis of a library of sulfonamides. The F-aldehyde was prepared via a simple alkylation of 4-hydroxy-2-meoxybanzaldehyde with a perfluo-roalkyl halide. The authors have protected various primary amines with the F-aldehyde followed by reduction, sulfonylation and Suzuki coupling and acid-mediated deprotection. Filtration via a fluorous SPE (solid phase extraction) was... 

Scheme 8.2 Fluorous-protecting groups for amine and phosphate used in oligosaccharide synthesis.

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