Stereoselective Synthesis of Difructose Dianhydrides by Use of the Xylylene Group as Stereodirecting Element in Spiroketalisation Reactions

Abstract

AbstractThe use of the o‐xylylene protecting group as an element of remote stereochemical control in bis(spiroketalisation) reactions of D‐fructose has been investigated. The presence of the cyclic diether functionality favours the trans‐diequatorial disposition of oxygen substituents, a conformational arrangement that is encountered in the 3‐O/4‐O segment in β‐D‐fructopyranoside and β‐D‐fructofuranoside moieties in di‐D‐fructose dianhydrides (DFAs). Moreover, the presence of the o‐xylylene group in cis‐oriented vic‐diol segments slows down the interconversion rate between the two chair conformers in fructopyranose rings, which has a strong influence on the stereochemical outcome of the dimerisation reaction. Those features have been exploited to develop stereoselective syntheses of dipyranose and difuranose DFAs with the aid of triflic acid activation of 1,2‐O‐isopropylidene‐D‐fructose precursors. For difuranose DFAs, the combination of this strategy with the concept of rigid spacer‐mediated intramolecular spiroketalisation, with use of xylylene‐tethered D‐fructose precursors, further allows double stereocontrol. The conclusions of this study open new perspectives in the stereoselective synthesis of complex spiroketal derivatives in general and in the preparation of pure DFA standards with application in food chemistry in particular.