SYNTHESIS AND STEREOELECTRONIC PROPERTIES OF SUGAR-SHAPED POLYHYDROXYLATED HEXAHYDROPYRIMIDINE-2-THIONES

The reactivity and conformations in water solution of polyhydroxylated hexahydropyrimidine-2-thiones structurally related to biologically ac tive nonulosonic acids and azaoctitols, obtained by intramolecular cyc lization of sugar-derived gamma-oxothioureas, have been investigated. The total preference for the axial orientation of the pseudoanomeric C -O bond and the high tendency to undergo acid-promoted intramolecular glycosylation are interpreted in terms of stabilizing hyperconjugative interactions in both the aminoketalic derivatives and the reactive az acarbenium cations. Moreover, repulsion between lone pairs and vicinal C-H bonds leads to a preference for structures bearing axial alpha-ca rbon substituents. The present results provide strong experimental evi dence for the predominance of molecular orbital over electrostatic int eractions in the stereoelectronic properties Of these systems, (C) 199 8 Elsevier Science Ltd. All rights reserved.