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Paper | Regular issue | Vol 53, No. 9, 2000, pp.1891-1903
Published online, 1st January, 1970
DOI: 10.3987/COM-00-8895
Studies on Pyrimidine-annulated Heterocycles: Synthesis and Dynamic Properties of [n](2,4)Pyridinophanes (n = 11, 9, 8, 6)

Hiroyuki Yamamoto, Hidekazu Takeda, and Makoto Nitta*

*Department of Chemistry, School of Science and Engineering, Materials Research Laboratory for Bioscience and Photonics, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan

Abstract

A new short synthesis of the 1,3-dimethyluracil-annulated [n](2,4)pyridinophane ring system (n = 11, 9, 8, 6) (5,7-polymethylene-substituted pyrido[2,3-d]pyrimidine-2,4(1H,3H)-diones) consists of allowing 6-amino-1,3-dimethyluracil to react with cyclic α,β-unsaturated ketones. Structural studies of the compounds prepared were based on spectroscopic measurements and theoretical calculations. The 1H and 13C NMR spectra at various temperatures showed dynamic behavior for the oligomethylene chain of [6](2,4)pyridinophane derivative (9d). The energy barriers ΔGc of the bridge flipping is ca. 20 kcalmol-1 (Tc, 150 °C). The lower-energy process of the oligomethylene in 9d is the pseudorotation with ΔGc being 10.1 kcalmol-1 (Tc, -30 °C). Two stable conformers of the hexamethylene bridge of annulated [6](2,4)pyridinophane (9d) were determined for the first time by low-temperature NMR spectroscopy and theoretical calculations. Furthermore, the first example of rearrangement observed in the dehydrogenation reaction of a 3,4-dihydro[5](2,4)pyridinophane derivative in the presence of DDQ suggested the large strain of the [5](2,4)pyridinophane ring system.