Anhydrous mercaptolysis of agar: an efficient preparation of 3,6-anhydro-l-galactose diethyl dithioacetal

Mercaptolysis of agar in a 2:1 mixture of 0.5 M ethanethiolic HCl and 0.5 M methanolic HCl (v/v) at 60 °C for 12 h liberated component sugars almost quantitatively as their dithioacetals without any accompanying methyl glycosides. Simple extraction procedures afforded 3,6-anhydro-l-galactose diethyl dithioacetal in 86% yield.