6 chromonytmethyl m nitrophenyl 3 5 dimethoxybenzyl 1 methyl 1 3 5 dimethoxyphenyl ethyl o nitrobenzyl u03b1 methylnitropiperonyl 3 4 dimethoxy 6 nitrobenzyl n benzenesulfenyl n o nitrobenzenesulfenyl n 2 4 dinitrobenzenesulfenyl n pentachlorobenzenesulfenyl n 2 nitro 4 methoxybenzenesulfenyl n triphenylmethylsulfenyl n 1 2 2 2 trifluoro 1 1 diphenyl ethylsulfenyl n 3 nitro 2 pyridinesulfenyl n p toluenesulfonyl n benzenesulfonyl n 2 3 6 trimethyl 4 methoxybenzenesulfonyl n 2 4 6 trimethoxybenzene sulfonyl n 2 6 dimethyl 4 methoxybenzenesulfonyl n pentamethyibenzenesulfonyl n 2 3 5 6 tetramethyl 4 methoxybenzenesulfonyl and the like b u2014 c o or 20 where r 20 is selected from alkyl substituted alkyl aryl and more specifically r 20 methyl ethyl 9 fluorenylmethyl 9 2 sulfo fluorenylmethyl 9 2 7 dibromo fluorenylmethyl 17 tetrabenzo a c g i fluorenylmethyl 2 chloro 3 indenylmethyl benz f inden 3 ylmethyl 2 7 di t butyl 9 10 10 dioxo 10 10 10 10 tetrahydrothloxanthyl methyl 1 1 dioxobenzo b thiophene 2 ylmethyl 2 2 2 trichloroethyl 2 trimethylsilylethyl 2 phenylethyl 1 1 adamantyl 1 methylethyl 2 chloroethyl 1 1 dimethyl 2 haloethyl 1 1 dimethyl 2 2 dibromoethyl 1 1 dimethyl 2 2 2 trichloroethyl 1 methyl 1 4 biphenylyl ethyl 1 3 5di tert butylphenyl 1 methylethyl 2 2 u2032 pyridyl ethyl 2 4 u2032 pyridyl ethyl 2 2 bis 4 u2032 nitrophenyl ethyl n 2 pivaloylamino 1 1 dimethylethyl 2 2 nitrophenyl dithio 1 phenylethyl tert butyl 1 adamantyl 2 adamantyl vinyl allyl.