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; <EOS>