Thioacetalization of aldehydes and ketones catalyzed by hexabromoacetone
Protection of p-anisaldehyde with 1,3-propanedithiol under UV irradiation without a catalyst resulted in 87% yield of 1,3-dithiane in 20 min. Addition of hexabromoacetone further reduced the reaction time and UV irradiation also accelerated the formation of dithianes, yielding 95% of 1,3-dithiane in only 1 min. Aromatic aldehydes or ketones with electron-donating substituents at the ortho or para position increased the yield of the corresponding dithianes, while electron-withdrawing group decelerated the reaction. Aldehydes were easier to protect than ketones, and aliphatic ketones were also more readily protected than aromatic ketones. The protection was highly selective towards an aldehyde compared to a ketone.