SYNTHESIS AND STEREOCHEMICAL ASSIGNMENT OF HETEROCYCLES DERIVED FROM (2S-ASTERISK,2'S-ASTERISK,4R-ASTERISK,4'R-ASTERISK,6S-ASTERISK,6'S-ASTERISK)-4,4',6,6'-TETRAMETHYLPERHYDRO-2,2'-BIPYRIMIDINE

(2S*,2'S*,4R*,4'R*,6S*,6'S*)-4,4',6,6'-Tetramethylperhydro-2,2'-bipyrimidine (1) reacts with excess formaldehyde in MeOH to give predominantly the tetracycle (1R*,3S*,5R*,7S*,8bS*,8cS*)-1,3,5,7-tetramethylperhydro-3a,4a,7a,8a-tetraazacyclopentano[def]fluorene (2) and in Et2O to give exclusively tetracycle (1R*,3S*,5S*,7R*,8bS*,8cS*)-1,3,5,7-tetramethylperhydro-3a,4a,7a,8a-tetraazacyclopentano[def]fluorene (3). With less formaldehyde, compound 1 yields an unusual 1:5 mixture of trans and cis tricycles, (1R*,3S*,4aS*,4bS*,6S*,8R*)-1,3,6,8-tetramethylperhydro-4,5,8a,9a-tetraazafluorene (5) and (1R*,3S*,4aS*,4bS*,6R*,8S*)-1,3,6,8-tetramethylperhydro-4,5,8a,9a-tetraazafluorene (6), respectively, in MeOH solvent but gives almost exclusively the trans tricycle 5 in Et2O. The structure of tetracycle 2 is supported by X-ray crystallographic data. Compounds 3 and 6 represent new structural types, and they appear to be conformationally stable. A mechanistic scheme for the formation of the tricycles and tetracycles which is consistent with the observed stereochemical changes is proposed.