For each backbone, the pK a values are shown in Figure . The acidity strength of the reactants participating in reactions PT25, PT58, PT14, PT47, PT03, and PT36 decreases systematically, with the exception of backbones 2 and 8 where PT58 data overlaps with PT14 and PT25, respectively. The pK a of compound 13 involved in reaction PT36 is widely distributed. With all data taken into account, pK a ranges from -22 to 30.