Configuration intramolecular carbolithiation

An interesting chain elongation of 4-oxo-l-tributylstannylbutyl carbamate 83 by ethynyl A,A-diisopropyl carbamate (82) to the dicarbamate 84 and the subsequent intramolecular carbolithiation to form the alkenyllithium 86 has been reported very recently (equation 18). 86 was trapped by electrophiles to yield—as expected—both epimers 87 and 88 in essentially equal amounts. From an X-ray analysis of a derivative, the shown absolute configuration and the ( )-stereochemistiy at the double bond was concluded. Thus, the attack at the triple bond here took place in the anti fashion, which is quite unusual. We suggest that a lithium cation in 85, captured by the second carbamoyl group, catalyses the anti addition. ... 

It is presumed that the initial step for the intermolecular as well as intramolecular carbolithiation is an energetically favorable coordination of the lithium atom with the n-system, which serves to establish the geometry of the system prior to addition. The chiral benzylic organolithium compound 45, obtained after the carbolithiation step, reacted diastereoselectively with a number of electrophiles, yielding a formal inversion of the configuration. 

Wittig rearrangements [66],but also to perform intramolecular carbolithi-ations on a terminal C=C bond, or an internal C=C bond bearing an allylic leaving group [55d, 67] (Scheme 40). A trans-2,3-disubstituted tetrahydrofuran is formed with a total retention of configuration. 

Advantage has been taken of the low-temperature conditions (—105°C, THF) required for intramolecular carbolithiation of vinyl sulfides (75), in order to smdy the stereochemistry of the process with regard to the new lithium-bearing stereogenic center, which remains configurationally stable. Formation of the new carbon-carbon bond is stereospecific, whereas formation of the new carbon-lithium bond is non-stereospecific. Since (Z)-(75) and ( )-(75) give rise to different ratios of the diastereoisomeric products (76b) and (77b), formed via (76a) and (77a), the kinetically controlled (76b) (77b) ratios (87 13 and 40 60, respectively) are believed to reflect the stereochemistry of the corresponding carbolithiation process. 

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