The synthesis of different cycloSal-phosphotriesters of the acyclic nucleoside analogues acyclovir (ACV), penciclovir (PCV) and T-penciclovir (T-PCV) as potential new lipophilic, membrane-soluble pronucleotides is described. The introduction of the cycloSal moiety was achieved by using reactive cyclic chlorophosphane reagents. In addition to the cycloSal-PCV monophosphate (MP) phosphotriesters, a second derivative bearing an acetyl group at the second primary alcohol function was prepared. In hydrolysis studies the cycloSal-ACVMPs showed the expected range of hydrolytic stability dependent on the substituent in the masking group (8-17 h). In contrast, the cycloSal-PCVMP derivatives exhibited a 11- to 15-fold increase in hydrolytic lability as compared to the corresponding cycloSal-ACVMP derivatives. We demonstrated that the free primary alcohol group is responsible for this rate acceleration because cycloSal-OAc-PCVMP, in which the hydroxyl group was blocked by acetylation, did not show the aforementioned acceleration. Unexpectedly, the hydrolysis product was not PCVMP but according to NMR and mass spectrometry it was cycloPCVMP (cPCVMP). The title compounds were evaluated in vitro for their ability to inhibit herpes simplex virus type 1 (HSV-1) and thymidine kinase-negative (TK-) HSV-1 replication in Vero cells. The cycloSal-ACVMP compounds exhibited high antiviral activity in HSV-1-infected cells. More importantly, one derivative retained all activity from the wild-type virus strain in HSV-1/TK(-)-infected Vero cells. The PCV derivatives were markedly less active. The reason for the failure of the cycloSal-PCVMPs seems to be due to the formation of cPCVMP instead of the desired PCVMP.