Preparation and structural properties of 7,8-dioxa[6]helicenes and 7a,14c-dihydro-7,8-dioxa[6]helicenes.

The potentially chiral 7,8-dioxa[6]helicenes 1-1c have been prepared by oxidation of their precursors the 7a,14c-dihydro-7,8-dioxa[6]helicenes 3. The crystal structure determination of 3b cis-7a,14c-dihydro-3,12-dibromo-7,8-dioxa[6]helicene unambiguously confirms the cis configuration of the 7a,-14c hydrogens in compounds 3 as previously implied from NMR measurements and also shows that 3b crystallizes in a chiral conformation in the solid state. Selective deuteration of the sterically crowded 1,14 positions of 7,8-dioxa[6]helicene 1 influenced the crystal structure. The deuterium labeled compound D2-1 exhibits a disordered structure, whereas 1 had been found to crystallize in a complex structure which can be described as an analogous partly ordered modulated superstructure. When dehydrogenation of compound 3 to obtain compound 1 was attempted, harsh synthetic conditions gave the unexpected halogenated compounds 5-chloro-7,8-dioxa[6]helicene 1c and cis-7a,14c-dibromo-7,8-dioxa[6]helicene 3c. Compounds 1d and 3b were identified by solving their crystal structure.