DiTOX acylation

Page et al. (see [298] and references therein) have shown that generally excellent stereocontrol in organic reactions can be obtained by using DITOX (1,3-dithiane-l-oxide) derivatives as chiral auxiliaries. The one-pot stereo-controlled cycloalkanone synthesis given here outlines some aspects of the chemistry worked out for efficient acylation-alkylations steps. Of note are the use of N-acyl imidazoles under mixed base (sodium hexamethyldisilazide/n-butyllithium) conditions to yield the lithium enolates of 2-acyl-l,3-dithiane-l-oxides) and the sequential alkylation-cyclization of the latter (steps (iv) and (v)). 

Unless otherwise stated, racemic 2-acyl-2-alkyl-DiTOX derivatives were used as substrates for the applications described in this review. The preparation of enan-tiomerically pure 2-acyl-2-alkyl-DiTOX systems are described in a subsequent section. 

Asymmetric sulfoxidation of2-acyl-1,3-dithianes Preparation of optically pure DiTOX substrates. The preliminary investigations of 1,3-dithiane derivatives as asymmetric building blocks and chiral auxiliaries described in this review... 

Enantiomerically pure DiTOX systems (95) and (96) were used to prepare a-hydroxy acids [132], Page had previously described the stereoselective reduction of 2-acyl-2-alkyl-l,3-dithiane 1-oxides with DIBAL [113], and normally observed [132] a reversal of selectivity upon addition of zinc chloride. In this case (Scheme 4.73), THF solutions of the substrates were treated at -78°C with either DIBAL or DIBAL/ZnCl2 reducing systems. As expected, the DIBAL and DIBAL/ZnClz reducing systems gave products of opposite stereoselectivity in most cases only one product diastereoisomer was observed. 

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