One of the most unusual purine riboside in nature is isoguanosine. Most of isoguanosine derivatives have been reported to be found in sponge. The new marine natural product doridosine(1-methylisoguanosine) has several biological activities in mammals. Some doridosine analogues, such as 1-ethyl-3-methyl-$N^6$-imino-2-oxo-9-$\beta$-D-ribofuranosylpurine($\underline{2}$), 2-mercaptoadenosine($\underline{3}$), 1-methyl-2-thioadenosine($\underline{4}$), and 1-$\beta$-D-ribofuranosyl-5-alkyl-4-amino-6-oxopyrazolo[3,4-d]pyrimidines($\underline{8a}$,$\underline{8b}$), and acyclic nucleosides of these compounds($\underline{5}$,$\underline{6}$,$\underline{9}$,$\underline{10}$) were prepared and their physical and biological properties were examined. In amino-imino and enol-keto tautomerism, compound $\underline{2}$ is a fixed model compound of 6-imino-2-keto form. Compound $\underline{2}$ was synthesized by several steps started from 5-aminoimidazolo-4-carbamide riboside. Compounds $\underline{3}$ and $\underline{4}$ were prepared from 6-amino-2-mercaptopurine hemisulfate, and compounds $\underline{8a}$, $\underline{8b}$ were synthesized from 5-amino-4-cyanopyrazole via several steps. In the field of biological activity, Compounds $\underline{2}$, $\underline{3}$, and $\underline{4}$ showed coronary vasodilating effects ($\underline{2}$: 36.5\%, $\underline{3}$: 57.5\%, $\underline{4}$: 23.4\%) at 10$\mu$g/heart, and compounds $\underline{3}$ and $\underline{4}$ showed powerful hypotensive effect ($\underline{3}$: 40 mmHg, $\underline{4}$: 50 mmHg fall in 1 mg/kg) which was accompanied by slightly increase in heart rate and respiratory rate. Compound $\underline{4}$ did not show any toxicity on liver within dose of 10 mg/kg. These preliminary results demonstrated that the sulfur analogues of doridosine, $\underline{3}$ and $\underline{4}$ are promising product of a rational drug design.