Axial chirality in xanthene-4, 5-dicarboxamides: 1,9-stereocontrol mediated by remote interactions between conformationally constrained amide groups

Tertiary 9,9-dimethylxanthene-4,5-dicarboxamides have a C-2-symmetric, axia lly chiral ground state due to remote interactions between the conformation ally constrained tertiary amide substituents. Double ortholithiation-alkyla tion blocks amide rotation, and the 3,6-dialkylated products are chiral, ex isting as a pair of enantiomeric C-2-symmetric atropisomers. Double lateral lithiation-electrophilic quench introduces a pair of 1,9-related stereogen ic centres-each under the control of one of the conformationally constraine d amides-with complete diastereoselectivity. (C) 2000 Elsevier Science Ltd. All rights reserved.