endocyclic enol ether framework and was coupled to an aryl halide (bromide is
preferred). Electron-rich substrates gave lower yields of product due to side reactions in the spirocyclization step. A range of ring sizes could be accessed, though
the yields of 5,5-, 6,5-, and 7,6-spiroacetals were lower than the more common 5,6and 6,6-spiroacetals.
Papulacandin-like spiroacetal 116 could also be accessed using this method,
starting from boronate 114. Coupling with aryl chloride 115, followed by acidic
treatment gave the anomerically stabilized spiroacetal 116 in moderate yield.
2.4.2 Chiral Acid-Catalyzed Spirocyclization
The use of chiral phosphoric acids and their derivatives (e.g., 117–119) has recently
emerged [7] as a novel method for the stereoselective spirocyclization of simple
dihydropyrans. These Brønsted acids act as bifunctional catalysts, providing both
acidic and basic sites for complexation to the electrophile and nucleophile, respectively (Fig. 2). Two studies using such catalysts for the stereoselective synthesis of
spiroacetals have been reported.
Fig. 2 Chiral phosphoric acids
Synthesis of 5,6- and 6,6-Spirocyclic Compounds
211