The third generation of cyclo Sal-pronucleotides, 5-diacetoxymethyl-cyclo Sal-d4T- monophosphates (5-di-AM-cyclo Sal-d4TMPs), is reported as a new class of “lock-in”- modified cyclo Sal-pronucleotides. These compounds bear an esterase-cleavable geminal dicarboxylate (acylal) attached to the aromatic ring of the saligenyl unit. The conversion into a strong acceptor group (aldehyde) leads to a strong decrease in hydrolytic stability. As a ...