New 1,3-oxathianes derived from Myrtenal: Synthesis and reactivity

2-Methyl- and 2-phenyl-substituted oxathianes derived from Myrtenal have been synthesized in satisfying yields. Lithiation of 2-methyl-substituted oxathiane could not be done, but lithiation of 2-phenyl-substituted and non-substituted oxathianes could be performed with s-BuLi. Quenching with D ₂O, TMSCI, and/or a carbonyl compound always provides the equatorial product in consistency with a prefered equatorial orientation of the lithium in the lithiated derivatives. A model is proposed to rationalize the diastereoselectivities observed at C5′ during reaction of aldehydes with lithiated non-substituted oxathiane. The model is based on the hypothesis that the lithium, being linked simultaneously to the carbon and the oxygen, is shifted toward the oxygen side, making the steric hindrance of this side more effective. Dimeric side products were observed during formation of these oxathianes (condensation of various aldehydes with the corresponding hydroxythiol), which had not been reported for other oxathianes (derived from pulegone and/or camphorsulfonic acid).