A New Protocol for Selective Cleavage of Acyl Protecting Groups in 2′-O-Modified 3′,5′-O-(Tetraisopropyldisiloxane-1,3-diyl)ribonucleosides

Abstract

The stability of tetraisopropyldisiloxane-1,3-diyl (TIPDS) protection in nucleosides in ammonia/amine solutions in methanol and ethanol was studied. In ammonia-methanol at ambient temperature significant partial cleavage of TIPDS was observed. When ethanol was used instead of methanol this undesired side reaction was completely inhibited. It was found that commercially available 8 M methylamine-ethanol solution is the reagent of choice for selective deacylation of N-or/and O-acyl protected nucleosides without notable cleavage of 3′,5′-TIPDS group. Several examples of the developed protocol for the preparation 2′-O-modified nucleosides with overall high yields are presented.The 1,1,3,3-tetraisopropyldisiloxane-1,3-diyl (TIPDS) group developed by Markiewicz 1 is widely used for the simultaneous protection of the 3′,5′-dihydroxy groups in ribonucleosides and subsequent manipulation of the 2′-OH group, such as deoxygenation, 2 oxidation, 3 alkylation, 4-6 glycosylation, 7-12 protection, 13-18 preparation of 2′-amino-2′-deoxynucleosides, 19,20 and many others. This group is one of the most popular and useful protecting group in nucleoside chemistry. Moreover simultaneous protection of 3′-and 5′-hydroxy groups can be carried out with both Nunprotected and N-acyl-protected ribonucleosides. It is assumed that the reaction of 1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane with the ribonucleoside starts with silylation of the primary 5′-hydroxy group followed by the formation of an eight-membered ring. As a result, the 2′-hydroxy group remains free for further chemical modification. The TIPDS group is usually removed with tetrabutylammonium fluoride trihydrate in tetrahydrofuran. 21Another valuable feature of this group is the conformational rigidity of the 3′,5′-O-TIPDS-ribonucleosides; they have a small J 1′,2′ < 1.5 Hz coupling which is widely used for configurational assignments in sugar moieties. 22 We have used 3′,5′-O-(tetraisopopyldisiloxane-1,3-diyl)ribonucleosides extensively for the preparation of 2′-O-pentafuranosylnucleosides and 2′-O-(alkoxymethyl)nucleosides. [23][24][25][26] Additional hydroxy groups within the residue were usually protected with acyl blocking groups. When using common deacylation procedures, such as ammonia in methanol or sodium methoxide in methanol, we have found that in several cases the TIPDS group is not stable. As a result, the yields of the corresponding deacylated derivatives were moderate.It is known that TIPDS protection has limited stability both in acidic and basic media. The reactions that take place under these conditions are shown in Scheme 1.