Tan and coworkers [137, 138] have described several methods for the selective
formation of either the kinetic or thermodynamic spiroacetal resulting from intramolecular ring opening of carbohydrate-derived epoxides 259–260 (Scheme 65).
The epoxides themselves were obtained by treatment of the parent glycal with
DMDO. The selectivity of the threo glycal-derived epoxides 259 for either the
“retention” spiroacetal 261a (i.e., retention of stereochemistry at C1) or “inversion”
spiroacetal 261b could be controlled by the choice of reagent, with the thermodynamic preference for 261a able to be reversed through the use of hydrogen
bond-driven methanol catalysis. In the case of the erythro glycal-derived epoxides
260, the inversion spiroacetal 262a is favored under each set of conditions, though
the selectivity is diminished somewhat when the spirocyclization is performed in
the presence of methanol.
Scheme 63 Sharma et al.’s DIHMA approach towards dinemasone A [136]
Scheme 64 Ring opening of epoxides in the synthesis of spiroacetals
234
M.A. Brimble and L.A. Stubbing