7-Deaza-2′-Deoxy-O⁶-Methylguanosine: Selective N²-Formylation Via a Formamidine, Phosphoramidite Synthesis and Properties of Oligonucleotides

Abstract

Hexa- and dodecanucleotides containing 7-deaza-2′-deoxy-O⁶-methylguanosine (m⁶c⁷G_d, 2) have been prepared by solid-phase synthesis employing methyl- as well as cyanoethyl phosphoramidites of 2. As the N²-isobuturyl group of protected 2 was difficult to remove after oligonucleotide synthesis the more labile formyl group was introduced. Regioselective N²-formylation was carried out without sugar protection by in-situ hydrolysis of a formamidine intermediate. Replacement of dG within the oligomer d(CGCGAATTCGCG) next to dA increased hairpin formation, whereas duplexes were formed if one of the outer dG residues was replaced by m⁶c⁷G_d. Tertiary structure changes of the oligomers were detected by comparing the melting curves at 260 and 280 nm or duplex specific cleavage by the endodeoxyribonuclease Eco RI.