Synthesis of 4-substituted 5-amino-2-(.beta.-D-ribofuranosyl)thiazoles and 4-substituted 5-amino-2-(.beta.-D-ribofuranosyl)selenazoles, and their respective conversion into 2-(.beta.-D-ribofuranosyl)thiazolo[5,4-d]pyrimidines and 2-(.beta.-D-ribofuranosyl)selenazolo[5,4-d]pyrimidines. A new synthesis of tiazofurin and selenazofurin