Butyllithium, reaction with cyclic acetals

Some of the problems associated with the synthesis of a-dicarbonyl starting materials have been alleviated by the use of propane-1,3-dithiol, which reacts with aldehydes to give cyclic thioacetals. With butyllithium the resulting stable dithiane anions (134) can be transformed into a-diketones or a-hydroxy ketones (Scheme 73). A further approach to such compounds is found in the reaction of a-ketonitrate esters with sodium acetate (Scheme 73), while aryl a-diketones are also available from a-ketoanils (prepared from the cyanide-ion-catalyzed transformation of aromatic aldimines) (70AHC(12)103). 

C=0. It is therefore susceptible to nucleophilic attack at carbon. Methods have been developed by which this polarization is effectively reversed so that the carbon atom itself becomes the nucleophilic centre. Such an inversion is known as umpolung. An example is provided by the 1,3-dithiane system. An aldehyde may be converted into a cyclic dithioacetal by reaction with propane-1,3-dithiol in the presence of an acid. The two adjacent electronegative sulphur atoms make the C—H bond of this acetal rather acidic. Treatment with butyllithium therefore affords a lithio derivative in which the carbon atom is susceptible to electrophilic attack. The 1,3-dithiane system is reconverted into a carbonyl group by acid hydrolysis in the presence of mercury(II) ions, which complex with the dithiol. The RCO group in the original aldehyde is thus equivalent to R—C=0 (Fig. 3.10). 

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