Reaction of thiamine free base (1) with diethyl pyrocarbonate (DEPC) in pyridine resulted in the formation of a complex mixture containing 1-ethoxycarbonyl-2-hydroxy-3-(2-methyl-3-ethoxycarbonylthiotetrahydrofuran-2-yl)-7-methyl-1,2,3,4-tetrahydropyrimido [4,5-d] pyrimidine (IV) (26% yield), N-(2-methyl-4-ethoxycarbonylaminopyrimidine-5-yl) methyl-N-(2-methyl-3-ethoxycarbonylthiotetrahydrofuran-2-yl) formamide (V) (10.5% yield) and N-(2-methyl-4-ethoxycarbonylaminopyrimidine-5-yl) methyl-N-(2-methyl-3-mercaptotetrahydrofuran-2-yl) formamide (VI) (37.5% yield) were the major products. Reaction of 1 with ethyl chloroformate (ECF) in pyridine produced VI as the major product (42% yield), while reaction in Me_2CO-NaHCO_3 system it produced V as the major product (37% yield) and IV as the minor product (2.1% yield).