Chemistry of bis-spiroacetals. Synthesis of the 1,6,8-trioxadispiro[4.1.5.3]pentadecane ring system

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Abstract
The syntheses of the unsaturated keto epoxide (4) and saturated keto epoxide (18) are described. In both cases, the key step was the nucleophilic addition of the acetylene (10) to δ-valerolactone. Cyclisation of the keto epoxide (18) in the presence of a catalytic amount of camphorsulphonic acid in dichloromethane led to isolation of a single saturated bis-spiroacetal (21). This selectivity can be explained by the preference of spiroacetals to adopt the conformation in which the ring oxygens are axial to the adjacent ring thus gaining stability from the anomeric effect. Analogous protic or Lewis acid-catalysed intramolecular cyclisation of the unsaturated keto epoxide (4) was not successful under a variety of reaction conditions. The unsaturated bis-spiroacetal (24), however, was prepared by an alternative strategy involving an acid-catalysed cyclisation of (22) to form the unsaturated spiroacetal (23) followed by subsequent conversion into a bis-spiroacetal utilising an oxy-radical generated by photolysis. Formation of the unsaturated bis-spiroacetal (24) from (23) was achieved via a Barton-type reaction involving irradiation of a solution of the spiroacetal (23), iodobenzene diacetate, and iodine in cyclohexane at room temperature.