Abstract
The biologically important l-hexoses, which are less widespread than d-hexoses, cannot be obtained from natural sources or can only be extracted very costly.
Due to the complexity of their synthesis, their commercially available derivatives
(which are sold mostly in free form) are also very expensive, which is further exacerbated
by the current rapid rise in prices. In the present work, starting from the cheapest
d-hexose, d-glucose, using inexpensive and readily available chemicals, a reaction pathway was
developed in which the three most expensive l-hexoses (l-idose, l-altrose, and l-talose) were successfully prepared in orthogonally protected thioglycoside form,
ready for glycosylation. The l-ido and l-talo derivatives were synthesized by C-5 epimerization of the corresponding 5,6-unsaturated
thioglycosides. From the l-ido derivatives, the orthogonally protected thioglycosides of l-altrose were then prepared by C-4 epimerization. Different approaches to the preparation
of the key intermediates, 5,6-unsaturated thioglycoside derivatives, were systematically
investigated in the presence of various protecting groups (ether and ester) and using
commercially available reagents.
Key words
l-hexoses - thioglycosides - epimerization - elimination - hydroboration
