A new method has been established for the preparation of C 2-oxidized
5,5-spiroacetals, which are key intermediates for the synthesis of the
bis-spiroacetal moiety of the spirolides. A bridged
orthoester was used as a masked carboxylic acid in the preparation of these
bicyclic oxaspirolactones. The synthesis of chiral lactone (12), a building
block for the synthesis of the spirolides, is also reported. The two chiral
centres in lactone (12) were assembled by addition of a chiral crotyl borane
to an aldehyde. The structure of lactone (12) was determined by single-crystal
X-ray diffraction; orthorhombic space group
P212121
(No. 19), a 12.437(2), b
23.881(4), c 7.545(1) Å, V
2240.9(5) Å3, R(F) 0.0460,
and Rw(F)
0.0458.