Cycloadditions of N-substituted C-(diethoxyphosphoryl)nitrones to N-allylated quinazoline-2,4-diones functionalized at N3 with substituted benzoyl or benzyl groups proceeded with moderate to good diastereoselectivities (d.e. 28-68%). The synthesized isoxazolidine phosphonates were assessed for the antiviral activity against a broad range of DNA and RNA viruses. Compounds trans-13c, cis-13c/trans-13c (86:14), cis-15b/trans-15b (87:13) and trans-15d/cis-15d (95:5) exhibited the highest activity toward both TK and TK VZV strains (mean EC values in the range of 3.0-8.7 μM). The EC's for isoxazolidines trans-12a, cis-12a, cis-13a, trans-13d, cis-15a/trans-15a (50:50) ranged between 6.9 and 8.5 μM for VZV TK strain and between 10.7 and 13.2 μM for VZV TK strain. The isoxazolidine phosphonates cis-15/trans-15 having benzyl substituents both at N3 of the quinazoline-2,4-dione skeleton and at N2 of the isoxazolidine ring displayed some anti-cytomegalovirus potency but at the same time showed significant cytostatic activity for human embryonic lung fibroblasts (used to carry out the antiviral assays) as well as for other cell lines (i.e. CEM, L1210, HeLa and HMEC-1).

Moderate diastereoselectivities (d.e. 2-62%) of isoxazolidine homonucleotides were observed for cycloadditions between N-methyl-C-(diethoxyphosphoryl)nitrone and N-allyl nucleobases, with trans-isoxazolidines predominating. The stereochemistry of the substituted isoxazolidines was established based on 2D NOE experiments performed for uracil-containing cycloadducts. The cis- and trans-isoxazolidine phosphonates obtained herein were evaluated in vitro for activity against a broad range of DNA and RNA viruses. None of the compounds were endowed with antiviral activity at subtoxic concentrations, but some of them were found to inhibit the proliferation of L1210 cells with IC50 values in the range of 33-100 µM.

Dipolar cycloaddition of the N-substituted C-(diethoxyphosphonyl)nitrones with N3-allyl-N1-benzylquinazoline-2,4-diones produced mixtures of diastereoisomeric 3-(diethoxyphosphonyl)isoxazolidines with a N1-benzylquinazoline-2,4-dione unit at C5. The obtained compounds were assessed for antiviral and antibacterial activities. Several compounds showed moderate inhibitory activities against VZV with EC50 values in the range of 12.63–58.48 µM. A mixture of isoxazolidines cis-20c/trans-20c (6:94) was found to be the most active against B. cereus PCM 1948, showing an MIC value 0.625 mg/mL, and also was not mutagenic up to this concentration.