Lithiation-methylation of thiasilinane 1-oxides

Lithiation-methylation of 3,3-dimethyl-1λ⁴,3-thiasilinane 1-oxide and 4,4-dimethyl-1λ⁴,4-thiasilinane 1-oxide under the action of butyllithium or lithium diisopropylamide and methyl iodide was studied. In both cases, monomethylation proceeds selectively α to the sulfoxide group to form 2,3,3-trimethyl-1λ⁴,3-thiasilinane 1-oxide and 2,4,4-trimethyl-1λ⁴,4-thiasilinane 1-oxide, respectively. Subsequently, 2,3,3-trimethyl-1λ⁴,3-thiasilinane 1-oxide undergoes monomethylation into the same α position to give 2,2,3,3-tetramethyl-1λ⁴,3-thiasilinane 1-oxide, while 4,4-dimethyl-1λ⁴,4-thiasilinane 1-oxide is dimethylated into the neighboring α’ position to form two stereoisomers of 2,4,4,6-tetramethyl-1λ⁴,4-thiasilinane 1-oxide with axial-equatorial or equatorial-equatorial methyl groups in the 2 and 6 positions.