Dithioacetalization of carbonyl compounds

One of the effective reagents for highly chemoselective dithioacetalization of carbonyl compounds is ceric ammonium nitrate (CAN) in chloroform. When a mixture of benzaldehyde and acetophenone was allowed to react with 1,2-ethanedithiol and a catalytic amount of CAN, the 1,3-dithiolane derived from the aldehyde was obtained in 84% yield while the ketone was recovered unchanged. It is noteworthy that aromatic ketones, 7-lactones, and acylic ketones did not react at all under these conditions and even at elevated temperatures for longer reaction times <1995T7823>. 

The ionic liquid based on the l- -butyl-3-methylimidazolium cation was also used as an efficient catalytic medium for the chemoselective dithioacetalization of carbonyl compounds. This reaction proceeded with both activated and weakly activated aromatic aldehydes in almost quantitative yields in short reaction times. In addition, an acid-sensitive substrate such as furfural also gave the corresponding 1,3-dithiolane without a formation of any side products (Table 8). Moreover, aromatic ketones did not produce dithioketals under the same reaction conditions (room temperature, 10-20 min) or even after a prolonged reaction time <2004ASC579>. 

Carbonyl protection/deprotection. Both dithioacetalization of carbonyl compounds and hydrolysis of acetals are effectively accomplished in the presence of WClg. 

Dithioacetalization of carbonyl compounds with thiols general procedure... 

Another example of polymer-supported A1 based Lewis acid is cross-linked polystyrene-supported aluminum triflate (79). Cross-linked polystyrene-supported AICI3 (72) was easily converted into (79) by treatment with triflic acid. This catalyst was applied to dithioacetalization of carbonyl compounds and transdithioacetal-ization of acetals (Scheme 19.19) [44]. From benzaldehyde the corresponding dithioacetal (84) was obtained in the presence of the polymeric catalyst (79) in 98% yield in 30 minutes. The same product was also obtained from the dimethyl acetal (85) in 94% yield. Chemoselectivity of the polymeric catalyst was also demonstrated in Scheme 19.19. Aldehydes reacted faster in the presence of ketone (34) to give the dithioacetals with (79). Aliphatic ketone (89) exclusively reacted with dithiol (81) in the presence of aromatic ketone (34). These chemoselectivities were higher than those obtained from the reactions using nonsupported Al(OTf)3. 

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