Conformational analysis—III

A theoretical and NMR lanthanide-induced shift (LIS) investigation of the conformations of lactams

Molecular mechanics (MM) with MMFF94 and MMX force fields and ab initio (RHF/6-31G*,RHF/6-311G**, and B3LYP/6-311G**) calculations are used with lanthanide-induced shift (LIS) to investigate the conformations of N-methyl-2-pyrrolidone 1, N-methyl-2-piperidone 2, ε-caprolactam 3, γ-valerolactam (1,5-dimethyl-2-pyrrolidone) 4, 2-azetidinone 5, 4-methyl azetidinone 6, 4-phenyl azetidinone 7, and N-methyl-4-phenyl azetidinone 8. The Yb(fod)₃ paramagnetic induced shifts of all the ¹ H and ¹³ C nuclei are measured and the corresponding diamagnetic complexation shifts obtained by the addition of Lu(fod)₃ . The complexation model (two-, three-, or four-site) used depends on the relative rates of the processes involved. The amide inversion is the same order as that of the 5- and 6-membered lactam rings and much faster than the lanthanide complexation and the inversion of the 7-membered ring. Both MM and ab initio calculations give an envelope conformation for 1 with C-4 out of the ring plane in agreement with the LIS analysis. For the piperidone ring of 2, the half-chair is calculated as the most stable form. The LIS analysis confirms this but cannot exclude a small amount (<2%) of the boat conformation. For 3, the LIS analysis gives a minimum for 90:10% chair to boat conformation, and 4 exists in two envelope conformations with the C₅ -Me ps-eq and ps-ax in an eq/ax ratio of 94:6%. In 2-azetidinone 5, the ab initio calculations gave both ring and nitrogen planar, but the MMFF94 calculations give a butterfly ring and pyramidal nitrogen. The LIS analysis for 5 gave good agreement (Rcryst 0.46%) for the MMFF94 geometry with endo NH but the planar ab initio geometries worse agreement (Rcryst = 1.1%). For 4-methyl-2-azetidinone 6, the MMFF94 geometry gave good agreement (Rcryst 0.96%) with two butterfly conformations with axial and equatorial methyl groups in 1:1 ratio. All the planar geometries gave worse agreement (Rcryst >1.5%). In 4-phenyl azetidinone 7, the MMFF94 geometry with 60% of the axial conformer gave Rcryst 1.2% but the other geometries Rcryst >1.5%. In contrast the N-methyl-4-phenyl-2-azetidinone 8 gave good agreement for all the geometries. The butterfly conformation gave Rcryst 1.1% for 80% of the axial conformer and the planar geometries Rcryst 0.98%. The LIS results confirm the ab initio and MM optimised geometries, but the conformer energies at times differ from the calculated values. They also differ considerably from the corresponding values for the lactones studied previously, and possible reasons for this are discussed.

N-Glycosyl-thiophene-2-carboxamides: synthesis, structure and effects on the growth of diverse cell types

A range of N-glycosyl-thiophene-2-carboxamides, including a 6H-thieno[2,3-c]pyridin-7-one and a bivalent compound, have been synthesised and assayed for their effects on DNA synthesis in bovine aortic endothelial cells or on the growth of synoviocytes. Per-O-acetylated analogues of the glycoconjugates were significantly more effective inhibitors when compared to their corresponding non-acetylated analogues, indicating that the lower potency observed for hydroxylated derivatives is due to less efficient transport of these compounds across the cell membrane. Thiophene-2-carboxamide was inactive as an inhibitor of bFGF induced proliferation, confirming the requirement of the carbohydrate residue for the observed biological properties. Glucose, mannose, galactose and 2-amino-2-deoxy-glucose analogues were active as were a variety of substituted thiophene derivatives; the 6H-thieno[2,3-c]pyridin-7-one conjugate was inactive. Conformational analysis of the title compounds was investigated. X-ray crystal structural analysis of four N-glucosyl-thiophene-2-carboxamides showed that the pyranose rings adopted the expected 4C1 conformations and that Z-anti structures were predominant (H1-C1-N-H anomeric torsion angle varied from -168.2 degrees to -175.0 degrees ) and that the carbonyl oxygen and sulfur of the thiophene adopted an s-cis conformation in three of the isomers. In a crystal structure of a 3-alkynyl derivative, the hydrogen atom of the NH group was directed toward the acetylene group. The distance between the hydrogen atom and acetylene carbons and angles between nitrogen, hydrogen and carbon atoms were consistent with hydrogen bonding and this was supported by IR and NMR spectroscopic studies. The geometries of thiophene-2-carboxamides were explored by density functional theory (DFT) and Møller-Plesset (MP2) calculations and the s-cis conformer of thiophene-2-carboxamide was found to be more stable than its s-trans isomer by 0.83 kcal mol(-1). The s-cis conformer of 3-ethynyl-thiophene-2-carboxamide was 5.32 kcal mol(-1) more stable than the s-trans isomer. The larger stabilisation for the s-cis conformer in the 3-alkynyl derivatives is explained to be due to a moderate hydrogen bonding interaction between the alkyne and NH group.