Synthesis of an oxaspirolactone intermediate for the synthesis of spirolides

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 P2(1)2(1)2(1) (No. 19), a 12.437( 2), b 23.881( 4), c 7.545(1) Angstrom, V 2240.9(5) Angstrom (3), R(F)0.0460, and R-w(F) 0.0458.