A series of N4-alkyl-1,6,7,11b-tetrahydro-2H-pyrimido[4,3-a]isoquinolinamine hydroiodides with triazasterol-related structures was designed and synthesized to mimic, as stable analogues, native high energy intermediates (HEI) of ergosterol biosynthesis. The title compounds can be regarded as 8,13,15-triaza-13,17-secosteroids with aromatic ring A bearing the positive charge in the guanidinium moiety. Hence, these compounds present structural similarities with corresponding carbocationic intermediates occurring during the enzyme catalyzed transformation of squalene into ergosterol. The N4-alkylaminopyrimidoisoquinolinium salts were prepared by reaction of respective S-methylthiotetrahydropyrimidoisoquinoline hydroiodides with octylamine, and appropriately methyl-branched alkyl- and alkenylamines. In order to prepare (3R)-6-isopropyl-3-methyl-6-hepten-1-amine several synthetic routes were investigated. The structures of all reported compounds were proved and completely assigned on the basis of homo- and heteronuclear correlated 1D and 2D NMR spectroscopy. The in vitro antifungal susceptibility tests of the title compounds with a standard panel of eight pathogenic fungi revealed especially against the used dermatophytes and yeasts with MICs in the range of 1-32 microg/ml moderate to good antimycotic effects. Depending on the nature of the N4-alkyl substituents structure-activity relationships were found with a maximum of antifungal efficacy of the N4-3,7-dimethyloctylaminopyrimidoisoquinolinium iodide.