Synthesis and Stereochemistry of Allenes. 2. Absolute Configurations of 1-Halogeno-3,4,4-trimethylpenta-1,2-dienes and 3,4,4-Trimethylpent-1-yn-3-ol

Article abstract of DOI:10.1021/jo00384a028

The absolute configuration of (+)-t-Bu(Me)C(OH)C<*>CH ((+)-2a) has been reassigned S.This is shown by chemical correlation to its constitutional isomer (S)-t-Bu(H)C(OH)C<*>CMe, applying organocopper (I) reactions of known anti stereochemistry.Reaction of LiCuX2 with esters of (+)-2a gives (-)-t-Bu(Me)C=C=C(H)X (X = Cl, 1a; X = Br, 1b).The R configuration is attributed to these levorotatory halogenoallenes, on the basis of the known anti stereochemistry of the 1,3-substitution and the S configuration of (+)-2a.These new assignments lift a considerable confusion in the literature concerning absolute configuration of 1a and 2a.Implications regarding (semi)empirical rules, relating the sign of the optical rotation of a chiral allene to its configuration, are discussed.