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(+)-( S ) and (−)-( R )-5-methyl-Wieland-Miescher ketone (+)- 1  and (−)- 1 , are important synthons in the diastereo and enantioselective syntheses of biological and/or pharmacological interesting compounds. A key step in these syntheses is the chemoselective C(1)O acetalization to (+)- 5  and (−)- 5 , respectively. Various procedures for this transformation have been described in the literature. Among them, the classical procedure based on the use of 1,2-ethanediol and TsOH in refluxing benzene in the presence of a Dean-Stark apparatus. Within our work on bioactive natural products, it occurred to us to observe the partial racemization of (+)- 5  in the course of the acetalization of (+)- 1  by means of the latter methodology. Aiming to investigate this drawback, which, to our best knowledge, has no precedents in the literature, we acetalized with 1,2-ethanediol and TsOH in refluxing benzene and in the presence of a Dean–Stark apparatus under various experimental conditions, enantiomerically pure (+)- 1 . It was found that the extent of racemization depends on the TsOH/(+)- 1  and 1,2-ethanediol/(+)- 1  ratios. Mechanism hypotheses for this partial and unexpected racemization are provided.

international journal of molecular sciences

Unexpected Racemization in the Course of the Acetalization of (+)-(S)-5-Methyl-Wieland–Miescher Ketone with 1,2-Ethanediol and TsOH under Classical Experimental Conditions