Substitution oxygen-substituted alkyllithiums

Oxygen-substituted alkyllithium derivatives of type 2 were the first chiral car-banions which were accessible in enantiopure form and proved to be configurationally stable, at least at temperatmes below -40°C [Eq. (1)] Still and Sreeku-mar cleaved (J )-l-(benzyloxymethoxy)-l-tributylstannylpropane (1) with n-butylUthiiun and trapped the intermediate lithium compound 2 [1] by dimethyl sulfate to give the BOM ether of (S)-2-butanol (3) [2,3]. 

The addition reactions of alkyllithium-lithium bromide complexes to a-trimethylsilyl vinyl sulfones that have as a chiral auxiliary a y-mono-thioacetal moiety derived from ( + )-camphor are highly diastereoselective. A transition state that involves chelation of the organolithium reagent to the oxygen of the thioacetal moiety has been invoked. The adducts are readily converted via hydrolysis, to chiral a-substituted aldehydes22. 

In pyridine-A-oxide, 2-proton acidity is considerably enhanced by the inductive effect of the oxide and by the complexing capability of the lone pair on oxygen with lithium. Hence, 2-lithiation and sometimes 2,6-dilithiation with alkyllithium and lithium dialkylamide bases is feasible. In the case of ring substituted pyridine-A-oxides 498, fair to good yields of... 

Synthetically useful precursors of oxygen-substituted carbenes (see Houben-Weyl Vol. E19b. pp 1628-1682) are diazirines (photolysis or thermolysis), a-halogen ethers (base treatment), a-dihalogen ethers (treatment with alkyllithium compounds), and stable carbene complexes of the Fischer type (thermolysis). Only the mechanism-based stereoselectivity and the simple diastcreoselectivity of their reactions with alkenes have been studied to date. 

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