ADDITION OF ORGANOMETALLIC REAGENTS TO ACYLOXATHIANES - DIASTEREOSELECTIVITY AND MECHANISTIC CONSIDERATION

The addition of methyl- and phenylmagnesium bromide and phenyllithium to 2-(methoxyacetyl)-, 2-[(tri-isopropylsiloxy)acetyl]-, and 2-[(methylthio)acetyl]hexahydro-4,4,7-trimethyl-4H-1,3-benxozathiin and the corresponding 2'-methyl, 2-(triisopropylsiloxy)propionyl, and 2-(methylthio)butyryl homologs has been Depending on the 2'-substituent, the reagent and (in case of the higher homologs) the configuration at C(2'), these reactions may or may not be highly diastereoselective and may or may not yield the product of Cram's chelate rule involving the oxygen moiety of the oxathiane ring. Explanations for the different stereochemical outcome of the various reactions are suggested.