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Synlett 2004(2): 0301-0305  
DOI: 10.1055/s-2003-44999
LETTER
© Georg Thieme Verlag Stuttgart · New York

Lanthanide Triflates/N-Iodosuccinimide for Chemoselective Coupling of n-Pentenyl Donors in Oligosaccharide Assembly

K. N. Jayaprakash, Bert Fraser-Reid*
Natural Products and Glycotechnology Research Institute, Inc., (NPG) 4118 Swarthmore Road, Durham, North Carolina 27706, USA
e-Mail: Dglucose@aol.com;
Further Information

Publication History

Received 8 August 2003
Publication Date:
16 December 2003 (online)

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Abstract

Armed n-pentenyl glycosides (NPGs) react (a) readily with scandium and indium (III) triflates, (b) moderately with samarium and lanthanum counterparts, and (c) not at all with ytterbium counterpart. However the last salt reacts readily with n-pentenyl orthoesters (NPOE). These chemoselectivities of n-pentenyl donors towards lanthanide salts therefore permit hydroxyl-bearing armed NPGs to function as acceptors towards NPOE donors. This concept is demonstrated by synthesis of a tetrasaccharide.

Key words

lanthanide triflates - N-Iodosuccinimide - n-pentenyl glycosides

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13

Recent calculations have shown that the charges can sometimes be distributed to the pyran oxygen thereby giving tri, as well as di, oxolenium ions. [14]

17

A typical procedure for the glycosidation is as follows: Acceptor (1 equiv) and and n-pentenyl donor (3 equiv) were dissolved together in a small amount of toluene and azeotroped to dryness. The residue was dried overnight under vacuum, and then dissolved in anhyd CH2Cl2 (5 mL) at 0 °C under Ar atmosphere. NIS (4 equiv) was added to the solution and after stirring for few minutes, the Lewis acid (0.3-0.5 equiv) was added and slowly warmed to r.t. The reaction was monitored by TLC, and when complete, quenched with NaHCO3 and sodium thiosulfate solutions. The organic layer was extracted with CH2Cl2, washed with water, and dried over Na2SO4. The solvents were removed, and the residue was purified by column chromatography.

18

When 1.3 equiv of donor were used, the yields with 15a, 15b, and 18a fell to 50%, 75% and 74%, respectively.