STEREOCHEMISTRY OF PLANARCHIRAL COMPOUNDS .14. STATIC AND DYNAMIC STEREOCHEMISTRY OF 3,3'-DIMETHOXY-2,2'-BI(1,6-METHANO[10]ANNULENYL)

The title compound 6 was prepared from 3-methoxy-1,6-methano[10]annule ne (4) via lithiation and oxidative coupling of the intermediate 5 wit h copper(II)chloride. Three stereoisomers (two rotamers of the racemat e, 6a and 6b, and the meso-form 6c) were obtained and their configurat ions assigned both by H-1 NMR spectroscopy and by X-ray crystal struct ure analysis of 6a. Starting the reaction sequence from optically acti ve 2-bromo-1,6-methano[10]annulene, (-)-3, of known absolute chirality (S)p established the absolute stereochemistry of (+)-6a as (R)p(R)a(R )p and (R)p(S)a(R)p for the dextrorotatory rotamer 6b. 3-Methoxy-1,6-m ethanol[10]annulene (4) as well as 6a and 6b were easily resolved by e nantioselective chromatography of the racemic mixtures on cellulose tr iacetate (CTA) in ethanol. A rotational barrier of DELTAG# = 132 kJ . mol-1 between 6a and 6b was determined both by thermal equilibration a nd by CD-kinetics. Finally, also the meso-form 6c - because of its hig h rotational barrier (118 kJ) - could be resolved on CTA in its enanti omers ([alpha]D = 200-degrees in ethanol). From chiroptical comparison (CD) with 6a and 6b, resp., the chirality (R)p(S)a(S)p was deduced fo r (+)-6c.