Protection as chiral acetal

5-Exo radical cyclizations of bromoacetals that contain allylic chiral centers are often referred to as Ueno-Stork reactions [50]. It was recently discovered that stereochemistry in these cyclizations could also be controlled by the acetal center, as shown in Eq. (13.39) [51]. A chair-like transition state (125) is envisaged where the alkene occupies a pseudoequatorial position leading to predominantly cis product 126. Yields of approximately 70% and cis trans ratios of up to 98 2 are obtained. 

Sugars have also been used as chiral auxiliaries in acetal formation for diaste-reoselective radical cyclizations [52]. In Eq. (13.40) a chiral acetal is utilized to control the stereochemistry of a 5-exo-dig cyclization resulting in the formation of quaternary carbon-based stereocenters. Product 129 is formed as a single diaste-reomer in 35% yield. An allylic strain model is proposed to account for the stereochemical outcome of this reaction. 

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