Heterosubstituted Alkyltins

It was established very early that both oxygen and nitrogen atoms considerably increased the stability of a a arbanion [332] so that a-heterosubshtuted organo-lithiums have gained considerable importance as normal synthetic intermediates. 

A great deal of knowledge was accumulated about their mode of generation, their stability and their reactivity with various electrophiles [333]. This explains the amount of literature on the use of a-heterosubshtuted organohns in hn-to-lithium transmetallahon [334]. Another striking point of the process is the complete retention of configuration at the carbanion. 

Since Still introduced the use of a-alkoxyorganostannanes as precursors of a-alk-oxyorganolithium by hn-lithium exchange [335], this chemistry was exploited in a variety of applicahons including the stereocontrolled synthesis of complex natural molecules, such as Zoanthamine [336] and Aspidospermin [337] alkaloids and (+)-Taxusin [338]. 

The transient alkyloxymethyllithium can react intermolecularly with electrophilic subtrates as illustrated by the diastereoselective addition to aldehydes of an a-alkoxyorganolithium prepared from a chiral derivahve of tributylstaimylmetha- 

The progress has been much slower with acyclic systems. However, the trans-metallation of N-benzyl-N-Cbz- [350], N-Boc-N-methyl- [351], and N-Boc-N-tertbu-tylthiomethyl- [352] protected aminotins provided the corresponding amino-lithium derivatives that may be trapped with electrophiles with complete retention of configuration. This chemistry was applied to the preparation of /3-aminoalco-hols, N-methyl-/3-aminoalcohols and N-methyl-/3-amino acids. It can be noted that the preparation of enantio-enriched a- or /3-amino carbanions can be achieved starting from linear racemic precursors when the tin-to-lithium exchange is done in the presence of (-)-sparteine [353]. 

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