each B independently is a base selected from A, G, T, C or U; each Q independently is H or OR??? wherein R??? is alkyl, substituted alkyl, alkenyl or a protecting group; each R independently is an aryl, methyl, substituted alkyl or alkenyl group; W is H, a protecting group or an H-phosphonate group of formula in which M+ is a monovalent cation; each X independently represent O or S; each Y represe