2-Lithio-1,3-oxathiane

The lithiation of 1,3-oxathiane (296) takes place with s-BuLi at —78 °C to give 2-lithio-1,3-oxathiane (315), an analogue of 2-lithio-l,3-dithiane (161), but with lower stability487. This intermediate reacts with different electrophiles, such as alkyl halides, carbonyl compounds, benzonitrile, dimethyl disulfide, dimethyl diselenide, trimethylplumbyl acetate and trimethylsilyl, germyl and stannyl chlorides488,489. However, further deprotection of 2-substituted 1,3-oxathianes has not been reported yet. 

The lithio-derivative derived from cyclohexyl phenyl sulfone underwent 1,2-addition to cyclohexenylideneacetaldehyde or cinnamaldehyde to give the corresponding / -hydroxysulfones387. Reactions of 2,2-dimethyl-4-lithio-1,3-oxathiane 3,3-dioxide 308... 

From Carboxylic Acid Esters with 1-Lithiocyclopropyl Sulfoxides and from Cyclopropanecarbaldehydes with 2-Lithio-l,3-oxathianes... 

Like the 1,3-dithianes, 1,3-oxathianes can be metallated with alkyllithium reagents and they hydrolyse about 10,000 times faster with protic acids. However, their value is diminished by the limited stability of the lithio derivatives and their inherent lack of symmetry which introduces the complications of diaster-eoisomerism. Eliel and co-workers have exploited the diastereoisomerism of... 

Lithio-l,3-oxathiane - has proved to be much less reactive than phenylthiomethyllithium (Scheme 74, entry a).- Although the latter reacts well with alkyl or benzyl bromides and with alkyl iodides without distinction, 2-lithio-l,3-oxathiane works well only with alkyl iodides. ... 

Although this synthesis produces an essentially enantio-merically pure product, it hcis several drawbacks the difficulty of purifying the starting material, the need for sec-butyllithium to make the lithio derivative of and difficulties we encountered (22) at times in the methyl iodide cleavage. We decided therefore to look for yet another chiral oxathiane adjuvant - preferably one which would closely resemble the 4,4,6-trimethyloxathiane used in most of the preliminaury experiments shown in Table 1 - and for a better method of cleavage. 

The commonly encountered C-2 anions derived from 1,3-dithiane and 1,3-oxathiane and their derivatives can be generated by treatment with any of a wide range of bases, but typically n-butyllithium (for 1,3-dithianes) or yec-butyllithium (sometimes necessary for 1,3-oxathianes). There are many instances where the deprotonated heterocycles have been used in synthesis, usually as acyl anion equivalents <8977643, b-95MI 608-05>. Use of 1,3-dithiane derivatives is by far the most common, and the derived anions react with with a very wide variety of electrophiles <69AG(E)639, 8977643), whether or not the dithiane system is initially substituted at C-2. For example, 2-lithio-... 

The lithium in 3-lithio-5-t-butyl-l,2-oxathian 2,2-dioxide (195) apparently exerted a strong preference for the equatorial orientation, since treatment of 5-t-butyl-1,2-oxathian 2,2-dioxide with butyl-lithium at — 78 and then with methyl iodide, acetone, or deuterium oxide gave respectively (196), (197), or (198) it was assumed that the lithium was replaced with retention of configuration. The marked equatorial preference of the lithium was strikingly demonstrated by the conversion of cw-5-t-butyl-3-methyl-... 

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