2-HALOGENO-1,3-DITHIANE 1,3-DIOXIDE - A DIASTEREOSELECTIVE CARBONYL ANION EQUIVALENT IN REACTIONS WITH ALDEHYDES

The metal anions of 2-halogeno-1,3-dithiane trans-1,3-dioxide react di astereoselectively with aldehydes, The scope and limitations of this r eaction have been studied through variations in the metal counter-ion, halogen, aldehyde and reaction temperature, with a view to achieving umpolung asymmetric Darzens type reactions, Chlorohydrins were obtaine d with high diastereoselectivity by reaction of the lithium anion of 2 -chloro-1,3-dithiane dioxide 4 with aromatic aldehydes under equilibra ting conditions, Aliphatic aldehydes gave poor selectivities, Attempte d stereospecific conversion of the chlorohydrins into epoxides resulte d only in reversion to 4 and aldehyde, Use of sodium or potassium base s gave lower yields of chlorohydrins; epoxides were not isolated, It w as possible to obtain aromatic bromohydrins with high selectivity usin g the magnesium anion of 2-bromo-1,3-dithiane 1,3-dioxide 5 as the nuc leophile, but once again ring closure to form epoxides failed to compe te with the facile retro-aldol process, Use of sodium bases with 5 res ulted in capricious reactions from which a single isomer of epoxide co uld be isolated, There is evidence of decomposition of the target epox ides under the reaction conditions to a-substituted carbonyl compounds and 1,2-dithiolane 1-oxide.