Tin/lithium permutation

Oxiranyllithiums are generated from activated oxiranes by deprotonation or by tin/lithium permutation. Frequently oxiranyllithium/2-lithiooxyethylidene equilibria are invoked for the ensuing transformations. However, evaluated in the light of recent investigations, they are are mostly carbenoid- rather than carbene-mediated. 

To complete the picture we have now to inquire about the stereochemical fate of the a-metalated center. The teams of Ted Cohen, Reinhard Bruckner, and Takeshi Nakai have systematically explored this aspect and have provided detailed insight. ° 1-Allyloxy- and l-benzyloxy-4-terr-butylcyclohexyllithium bearing the metal in the axial position were found to afford 1-allyl- and l-benzyl-4-tert-butylcyclohexanol wherein the hydroxyl group occupies the axial position. Inversion of the metalated center occurs predominantly also with l-(R)-(benzyloxy)propyllithium (361), generated by tin/lithium permutation (Scheme 1-280).In contrast, (5)-l-(trimethylsilyloxy)hexyllithium (362) affords (5)-1-trimethylsilyl-1-hexanol almost exclusively (Scheme 1-280). This retention suggests the intermediacy of a lithium (trimethylsilicata)oxirane. 

From a synthesis-oriented point of view, the rearrangement of 2-(alkenyloxy)-methyllithiums is particularly attractive. Obviously such intermediates cannot be produced by direct metalation of the corresponding ether at the unactivated methyl group. One has to resort instead to a reductive scission of carbon-selenium bonds or to the extremely smooth tin/lithium permutation. The latter approach was exploited in a short synthesis of the Cig Cecropia juvenile hormone starting from the readily available dihydroxyacetal 369 (Scheme 1-287). " As with other a,P-dialkylallyl migrations too, the product is obtained with high (Z/ ) stereoselectivities (>95 5). " ... 

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