A compound of the formula: wherein: R 1 is hydroxy, monophosphate, diphosphate, triphosphate, α-thiotriphosphate or polyphosphate; R 2 is hydrogen or hydroxy; R 3 is alkyl (C≤8) or substituted alkyl (C≤8); R 4 is hydrogen, hydroxy, halo, amino, nitro, cyano, azido or mercapto; alkyl (C≤6), acyl (c≤6), alkoxy (C≤6), acyloxy (C≤6), alkylamino (C≤6), dialkyl-amino (C≤6), amido (C≤6), or a substituted version of any of these groups; R 5 and R 6 are each independently: hydrogen, hydroxy, halo, amino, nitro, cyano, azido or mercapto; alkyl (C≤6), alkenyl (C≤6), alkynyl (C≤6), aryl (C≤6), aralkyl (C≤8), heteroaryl (C≤6), acyl (C≤6), alkoxy (C≤6), acyloxy (C≤6), alkylamino (C≤6), dialkyl-amino (C≤6), amido (C≤6), or a substituted version of any of these groups; a group of formula: or wherein X is -O-, -S-, or -NH-; or alkanediyl (C≤12), alkenediyl (C≤12), alkynediyl (C≤12), or a substituted version of any of these groups; Y is -O-, -NH-, alkanediyl (C≤12) or substituted alkane-diyl (C≤12); n is an integer from 0-6; and m is an integer from 0-6; or a -linker-reporter; or a salt, tautomer, or optical isomer thereof. R 1 is hydroxy, monophosphate, diphosphate, triphosphate, α-thiotriphosphate or polyphosphate; R 2 is hydrogen or hydroxy; R 3 is alkyl (C≤8) or substituted alkyl (C≤8); R 4 is hydrogen, hydroxy, halo, amino, nitro, cyano, azido or mercapto; alkyl (C≤6), acyl (c≤6), alkoxy (C≤6), acyloxy (C≤6), alkylamino (C≤6), dialkyl-amino (C≤6), amido (C≤6), or a substituted version of any of these groups; hydrogen, hydroxy, halo, amino, nitro, cyano, azido or mercapto; alkyl (C≤6), acyl (c≤6), alkoxy (C≤6), acyloxy (C≤6), alkylamino (C≤6), dialkyl-amino (C≤6), amido (C≤6), or a substituted version of any of these groups; R 5 and R 6 are each independently: hydrogen, hydroxy, halo, amino, nitro, cyano, azido or mercapto; alkyl (C≤6), alkenyl (C≤6), alkynyl (C≤6), aryl (C≤6), aralkyl (C≤8), heteroaryl (C≤6), acyl (C≤6), alkoxy (C≤6), acyloxy (C≤6), alkylamino (C≤6), dialkyl-amino (C≤6), amido (C≤6), or a substituted version of any of these groups; a group of formula: or wherein X is -O-, -S-, or -NH-; or alkanediyl (C≤12), alkenediyl (C≤12), alkynediyl (C≤12), or a substituted version of any of these groups; Y is -O-, -NH-, alkanediyl (C≤12) or substituted alkane-diyl (C≤12); n is an integer from 0-6; and m is an integer from 0-6; or a -linker-reporter; hydrogen, hydroxy, halo, amino, nitro, cyano, azido or mercapto; alkyl (C≤6), alkenyl (C≤6), alkynyl (C≤6), aryl (C≤6), aralkyl (C≤8), heteroaryl (C≤6), acyl (C≤6), alkoxy (C≤6), acyloxy (C≤6), alkylamino (C≤6), dialkyl-amino (C≤6), amido (C≤6), or a substituted version of any of these groups; a group of formula: or wherein X is -O-, -S-, or -NH-; or alkanediyl (C≤12), alkenediyl (C≤12), alkynediyl (C≤12), or a substituted version of any of these groups; Y is -O-, -NH-, alkanediyl (C≤12) or substituted alkane-diyl (C≤12); n is an integer from 0-6; and m is an integer from 0-6; or X is -O-, -S-, or -NH-; or alkanediyl (C≤12), alkenediyl (C≤12), alkynediyl (C≤12), or a substituted version of any of these groups; -O-, -S-, or -NH-; or alkanediyl (C≤12), alkenediyl (C≤12), alkynediyl (C≤12), or a substituted version of any of these groups; Y is -O-, -NH-, alkanediyl (C≤12) or substituted alkane-diyl (C≤12); n is an integer from 0-6; and m is an integer from 0-6; or a -linker-reporter;