Propynyl groups in duplex DNA: stability of base pairs incorporating 7‐substituted 8‐aza‐7‐deazapurines or 5‐substituted pyrimidines

摘要: ABSTRACTOligonucleotides incorporating the 7-propynylderivatives of 8-aza-7-deaza-2¢-deoxyguanosine (3b)and 8-aza-7-deaza-2¢-deoxyadenosine (4b) weresynthesized and their duplex stability was com-pared with those containing 5-propynyl deriva-tives 2¢-deoxycytidine (1) 2¢-deoxyuridine (2).For this purpose phosphoramidites 8-aza-7-deazapurine (pyrazolo[3,4-d]pyrimidine) nucleo-sides were prepared employed in solid-phasesynthesis. All propynyl nucleosides exert a positiveeffect on DNA because theincreased polarizability nucleobase thehydrophobic character group. Thepropynyl residues introduced into 7-position ofthe 8-aza-7-deazapurines are generally more stabil-izing than at 5-position pyrimidinebases. The stabilization propynylderivative 4b higher for bromo nucleo-side 4c. extraordinary duplexescontaining 7-propynyl derivative 8-aza-7-deazapurin-2,6-diamine (5b) is attributed to theformation third hydrogen bond, which appar-ently not present base pair purin-2,6-diamine 2¢-deoxyribonucleoside dT.INTRODUCTIONThe thermal oligonucleotide duplexes depends ontheir length composition (1). Efforts wereundertaken increase by chemicalmodification constituents. sugar moiety (2)or (1,3–7) constituents as well asthe backbone (8) structurally modified.These changes have broadened application oligonucle-otides field antisense technology (9–11) or inhybridization techniques used diagnostic purposes (12).All approaches focused either strengthening hydrogenbonds between bases and/or stackinginteractions among them. Our search stable pairsled us pyrazolo[3,4-d]pyrimidine (8-aza-7-deazapurine)2¢-deoxyribonucleosides (13–16), can be considered asideal shape mimics parent purine nucleosides. Their 7-halogenated alkynylated derivatives induce (1,17,18).Among various groups oligonucleotidesfor stabilization, group found particularattention (19–21). This 5-positionof pyrimidine nucleosides, e.g. (1)(19–23) (2) (24–26) 7-positionof pyrrolo[2,3-d]pyrimidine (27). Also, 8-propy-nylated 2¢-deoxyadenosine 2¢-deoxyguanosine derivativeswere studied (26,28). However, 8-substituted destabilize due steric hindrance. As 7-substituents freedomwithin DNA-duplex, now introducedinto oligonucleotides pyrazo-lo[3,4-d]pyrimidines (8-aza-7-deazapurines) (1,13,18). Thisstudy reports synthesis the8-aza-7-deaza-2¢-deoxyguanosine (3b) corresponding2¢-deoxyadenosine (4b). work also paysattention incorpor-ating (18) compares thestabilities ‘dA–dT’ versus ‘dG–dC’ pairs replacingeither one two propynylated analogs shown inScheme 1.MATERIALS AND METHODSGeneralFor general remarks see Seela Becher Thin-layerchromatography (TLC) performed TLC aluminiumsheets coated silica gel 60 F