Parthiban P, Balasubramanian S, Aridoss G, Kabilan S
Department of Chemistry, Annamalai University, Annamalainagar 608002, Tamil Nadu, India.
Spectrochim Acta A Mol Biomol Spectrosc. 2008 Jun;70(1):11-24. doi: 10.1016/j.saa.2007.07.059. Epub 2007 Aug 15.
Variously substituted 2,6-diarylpiperidin-4-one O-benzyloximes were synthesized by the direct condensation of the corresponding 2,6-diarylpiperidin-4-ones with O-benzylhydroxylamine hydrochloride. All the synthesized compounds are characterized by IR, Mass and NMR spectral studies. NMR spectral assignments are made unambiguously by their one-dimensional (1H NMR and 13C NMR) and two-dimensional (1H-1H COSY, NOESY, HSQC and HMBC) NMR spectra. All the synthesized compounds are resulted as single isomer, i.e., exclusively E isomer (9-14). The conformational preference of 2,6-diarylpiperidin-4-one oxime ethers with and without alkyl substituents at C-3 and C-5 has also been discussed using the spectral studies. The observed chemical shifts and coupling constants suggest that compounds 8-13 adopt normal chair conformation with equatorial orientation of all the substituents while compound 14 contributes significant boat conformation along with the predominant chair conformation in solution. The effect of oximination on ring carbons, their associated protons, alkyl substituents and ipso carbons are studied. Every proton in the piperidone ring of the oxime ether is observed as distinct signal due to oximination. The order of chemical shift magnitude in compound 8 is H-2a>H-6a>H-5e>H-3e>H-3a>H-5a. For 9-12, the order is H-6a>H-5e>H-2a>H-3a>H-5a, for 13, H-6a>H-2a>H-5e>H-3a>H-5a and for 14, the order is H-2a>H-6a>H-5e>H-3a>H-5a while the 13C chemical shift magnitude for 8-14 due to oximination is C-2>C-6>C-3>C-5.
通过相应的2,6 - 二芳基哌啶 - 4 - 酮与O - 苄基羟胺盐酸盐直接缩合，合成了各种取代的2,6 - 二芳基哌啶 - 4 - 酮O - 苄基肟。所有合成的化合物均通过红外光谱、质谱和核磁共振光谱研究进行表征。通过一维（1H NMR和13C NMR）和二维（1H - 1H COSY、NOESY、HSQC和HMBC）核磁共振光谱明确进行核磁共振光谱归属。所有合成的化合物均以单一异构体形式得到，即仅为E异构体（9 - 14）。还使用光谱研究讨论了在C - 3和C - 5处有或没有烷基取代基的2,6 - 二芳基哌啶 - 4 - 酮肟醚的构象偏好。观察到的化学位移和耦合常数表明，化合物8 - 13采用所有取代基均为赤道取向的正常椅式构象，而化合物14在溶液中除了主要的椅式构象外，还呈现出显著的船式构象。研究了肟化对环碳、其相连质子、烷基取代基和本位碳的影响。由于肟化作用，肟醚哌啶酮环中的每个质子都表现为独特的信号。化合物8中化学位移大小顺序为H - 2a>H - 6a>H - 5e>H - 3e>H - 3a>H - 5a。对于9 - 12，顺序为H - 6a>H - 5e>H - 2a>H - 3a>H - 5a，对于13，顺序为H - 6a>H - 2a>H - 5e>H - 3a>H - 5a，对于14，顺序为H - 2a>H - 6a>H - 5e>H - 3a>H - 5a，而由于肟化作用，8 - 14的13C化学位移大小顺序为C - 2>C - 6>C - 3>C - 5。
