The highly stereoselective synthesis of 3-methyl-l-halogenoallenes, 3-tert-butyl-l-halogenoallenes, and 1,3di-iert-butylallene is described. The R absolute configuration is assigned to the levorotatory halogenoallenes by relating them to alienes of known configurations. The configurational assignments are in agreement with (vacuum) circular dichroism data for the tert-butylallenes. The obtained results are used to clarify the hitherto existing confusion in the literature concerning absolute configurations of chloroand bromoallenes. Further, the signs and magnitudes of the ligand specific parameters for the halogens in the chirality functions approach, used to predict optical rotations of alienes, are discussed. The literature parameter for chlorine is revised and adequate numerical values for those of bromine and iodine are proposed.Recently, a synthetic route to optically active 3phenyl-1-halogenoallenes 1 and 21-halogeno steroidal alienes 2 of high enantiomeric purity has been described.2,3The present investigation demonstrates the utility of the method to obtain optically active 3-alkyl-1-halogenoallenes, viz., 3-tert-butyl-l-halogenoallenes 3a-c and 3-methyl-lhalogenoallenes 4b and 4c. These compounds, together with 1,3-di-tert-butylallene 3d, are very interesting from a theoretical point of view, particularly in relation to theoretical treatments of optical rotations of alienes.4,5
A number of dithioesters R-C( = S)SR'(R =alkyl, aryl, 2-thienyl; R = alkyl, allyl or propargyl) can be obtained in good yields by reaction of RMgX (X = CI or Br) in tetrahydrofuran with carbon disulfide, followed by alkylation with alkyl iodides, allyl bromide or propargyl bromide.
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