Chemoselectivity and Unusual Internal Acetal Formation in the Synthesis of a Glycosidation Precursor

Chemoselective deprotection of a tert-butyldimethyl (TBS) silyl ether group in the presence of an acetal moiety within an advanced iridoid precursor using scandium trifluoromethanesulfonate at 25 °C unexpectedly leads to internal acetal formation in high yield (70%). The same reaction at 0 °C resulted in spontaneous lactonisation of the diol intermediate, which was further elaborated to an iridoid glycosidation precursor.