DYNAMIC GEARING AND RESIDUAL STEREOISOMERISM IN LABELED BIS(9-TRIPTYCYL)METHANE AND RELATED MOLECULES - SYNTHESIS AND STEREOCHEMISTRY OF BIS(2,3-DIMETHYL-9-TRIPTYCYL)METHANE, BIS(2,3-DIMETHYL-9-TRIPTYCYL)CARBINOL, AND BIS(1,4-DIMETHYL-9-TRIPTYCYL)METHANE