The synthesis of a bridgehead sulfonium salt analogue (7) of the indolizidine alkaloid castanospermine
has been achieved by a multistep procedure starting from 5-thio-d-glucopyranose pentaacetate. The compound
was intended to test the theory that glycosidase inhibitory activity of the indolizidine alkaloids might be due
to electrostatic stabilization of a positively charged species in the enzyme active site and that a sulfonium salt
carrying a permanent positive charge might be advantageous. The structure of the bicylic sulfonium salt (7)
[3(R),4(S),5(R),6(S)-3,4,5-trihydroxy-cis-1-thioniabicyclo[4.3.0]nonane perchlorate] was confirmed by X-ray
crystallography. Analysis of the 1H NMR spectrum of compound 7 indicated that a similar conformation was
adopted in solution. This conformational preference, with hydroxyl groups in the more sterically hindered
axial orientations, has been attributed to the dominance of stabilizing electrostatic interactions between the
oxygen atoms and the sulfonium center.
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