Diastereoreactivity of a chiral oxathiane derived from 5-hydroxy-1-tetralone

The diastereoreactivity at the CH₂ bridge of a cis-oxathiane derived from 5-hydroxy-1-tetralone (unprotected and protected at the OH) was studied, and it appeared that all reactions at C-2 provided equatorial substitution. It was also found that: i) reductions of the equatorial benzoyl derivative with selectrides provided high diastereoselectivities at C-2' while LAH did not. Methyl Grignard addition was less selective and the selectivity of the propanoyl analogue was smaller; ii) condensation of aldehydes and/or ketones with the lithio derivative provided high equatorial diastereoselectivities at C-2 but low of nil diastereoselectivities at C-2′; iii) lithiation at C-2 was possible only with the oxathiane unsubstituted at C-2 (CH₂) and not possible with derivatives substituted at C-2 (CHR), perhaps due to the conformational rigidity of this oxathiane in the lithiation transition state and to the equatorial demand of all substituents at C-2, including lithium.