chapter 2 - Bentham Science

Oligonucleotide-Carbohydrate Conjugates Synthesis and Biological Applications of Glycoconjugates 147
Figure 2: Monovalent, multivalent 5’-glycoconjugates and phosphoramidites for their preparation.
Adsorption experiments highlighted slight difference of behavior of glycoconjugates in each cell type. For instance,
the binding of tetravalent conjugate 10 was found to be less efficient with Hela cells than that with U87. The best
cellular uptake was found for divalent 9 and maltose 7 conjugates. Conjugates linked through longer spacer (7 and
9) showed better incorporation than these linked through shorter spacers (6 and 8). Surprisingly, tetravalent glucose
conjugate 10 showed a bad incorporation presumably due to cell uptake hindering.
Synthesis of heteroglycoconjugates [32], i.e. oligonucleotides bearing three different glycosyl groups (Fig. 3, 14) has
also been achieved using appropriate orthogonally protected branching phosphoramidite 15 and carbohydratephosphoramidites
(16-18). A derivative of bis(hydroxymethyl)-N,N’-bis(3-hydroxypropyl)malondiamide 15 was
introduced during solid-phase synthesis. It contained a DMT protected hydroxyl and a phosphoramidite group
allowing oligonucleotide elongation, and a levulinyl protected hydroxyl and a t-butyldiphenylsilyl protected
hydroxyl for connecting glycosides residues. Conjugate syntheses were performed as follows: building block 15 was
incorporated at the 5’ end of the oligonucleotide, DMT group was removed under acidic conditions and nucleotides
were incorporated. At the end of the elongation, a carbohydrate-phosphoramidite was incorporated at the 5’ end.
Levulinyl group was removed by hydrazinolysis allowing the incorporation of another carbohydrate
phosphoramidite and finally TBDPS was removed prior to incorporation of the last glycosyl-phosphoramidite.
This method allows the synthesis of heteroglycoclusters such as 14 in a reasonable yield. Melting temperature
measurements showed that the presence of the bulky glycocluster at the 5’ terminus destabilises the duplex.
In Sequence-Conjugation
In the last example, glycosides were incorporated at the 5’ terminus, but building block 15 should allow in-sequence
incorporation. This can also be performed using modified nucleoside phosphoramidite. This strategy has been used