Abstract
The use of iodobenzene diacetate and iodine under photolytic conditions provides and efficient method for the oxidative cyclization of spiroacetals bearing an hydroxyalkyl side chain to bis-spiroacetals. An overview is provided of the use of this reaction for the synthesis of several bis-spiroacetal containing natural products such as the polyether antibiotics salinomycin and CP44,161 and the shellfish toxins, the spirolides.
Highlights
Tricyclic bis-spiroacetal ring systems are present as sub-units in a diverse range of natural products and have attracted attention from synthetic chemists due to their stereochemical complexity and variety in their biological activity
Many of the synthetic methods adopted for the construction of spiroacetals rely on the control of stereochemistry at the anomeric centre being dictated by the relative thermodynamic stability of the different isomers formed using an acid-catalysed spiroacetalisation step
Aldehyde 26 provided the substituents required for the substituted tetrahydrofuran ring of the 6,5,5-bis-spiroacetal ring system and these were introduced in a stereocontrolled manner by crotyl metallation of a protected 3-hydroxypropanal that afforded the desired homoallylic alcohol 25 in 82% yield and >97% d.e
Summary
Tricyclic bis-spiroacetal ring systems are present as sub-units in a diverse range of natural products and have attracted attention from synthetic chemists due to their stereochemical complexity and variety in their biological activity. Molecules 2004, 9 isomer produced in this case is that which has the maximum number of anomeric effects with minimum steric interactions When these two factors work in opposite directions lower stereoselectivity is observed. We describe the subtle interplay of steric and electronic effects in the construction of several bis-spiroacetal ring systems in the context of our own work using a kinetically controlled oxidative radical cyclization rather than a thermodynamically controlled acid-catalysed cyclization to construct the representative bis-spiroacetals. This radical cyclization approach provides a convenient method to access the thermodynamically less stable isomers
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