A structural analysis of 2,4,6,8-tetraphenyl-3-aza-7-thiabicyclo[3.3.l]nonan-9-one was performed via 'H and '9 NMR spectroscopy and single-crystal X-ray diffraction. An unusually large deshielding of C(1,5) (bridgehead carbons) was detected in the 19C NMR spectrum and was rationalized on the basis of the deshielding influence of the phenyl rings attached to C(6,8) a to the sulfur atom. This bispidinone system is believed to exist as a chabboat in solution (the thianone ring as a chair and the piperidone ring as a boat) as judged from spectroscopic evidence and from X-ray diffraction analysis of a single crystal of the compound. The bispidinone crystallizes as the benzene solvate in the triclinic space group PI. The cell dimensions are a = 8.910 (2) A, b = 9.754 ( 2 ) A, and c = 19.342 (3) A, a = 92.09 ( 2 ) O , f l = 98.84 ( 2 ) O , y = 110.75 (2)O. The system is not perfectly symmetricaland some strain is apparently in the system as evidenced by the slightly abnormally long C-C bonds involving the bridgehead carbon atoms. The nitrogen atom is pyramidal with the attached hydrogen in a pseudoequatorial position and very likely with the lone pair of electrons in a position unfauorable for interaction with the carbonyl carbon although the piperidone ring is a boat form. The thianone ring is in a chair form and the C(6)4(23)4(28) and C (8)4(29)4(30) bond angles [C(6) and C(8) are a to the sulfur atom] are slightly enlarged, which suggests some strain around the sulfur atom from steric interactions of sulfur with the phenyl groups.Considerable interest in the stereodynamics and potential as useful synthons of bicyclo[3.3.l]nonan-9-one systems' with heteroatoms at the 3-and 7-positions2 prompts us to reveal our findings on the stereochemistry of 2,4,6,8-tetraphenyl-3-aza-7-thiabicyclo[3.3.l]nonan-9one. The emergence of such bicyclic systems (sometimes referred to as 3,7-diheterabispidinones) as good candidates to contain a chair-boat (CB) arrangement is recognized213 but examples based on unequivocal experimental evidence are very rare. One example of a dynamic CB e BC system is that predicted for 1.% Variable-temperature 'H and '3c NMR studies supported the postulate.2ca The title compound 2 might reasonably be expected to have a preferred form but heretofore no definitive evidence has been revealed to support this postulate. CB 1 NCH3 BC ~~ (1) (a) Peters, J. A. Synthesis 1979, 321. (b) Bhattacharjee, S. K.; Chacko, K. K. Tetrahedron 1979,36,1999. (c) Maetryukov, V. S.; Popih, M. V.; Dorofeeva, 0. V.; Golubinekii, A. V.; Vilkov, L. V.; Belikova, N.