Conformation of hindered piperidines: Spectroscopic evidence for contribution of boat conformations

Abstract

High resolution 1 H and 13 C NMR resonance assignments and conformational assignments were carried out for four t(3)-benzyl-r(2),c(6)-bis(aryl)piperidin-4-ones 1-4 and their four N-nitroso-t(3)-benzyl-r(2),c(6)-bis(aryl)piperidin-4-ones 5-8. In addition to conventional 1D NMR methods, 2D shiftcorrelated NMR techniques ( 1 H-1 H COSY and 1 H-13 C COSY) were used for signal assignments. At room temperature the t(3)-benzyl-r(2),c(6)-bis(aryl)piperidin-4-ones 1-4 exist in only one isomeric form whereas their N-nitroso derivatives 5-8 exist in two isomeric forms. The preferred conformations of both the isomeric forms of nitrosamines were determined by comparison of the spectral data with those of the corresponding parent amines 1-4 and with the aid of substituent parameters. The results indicate the presence of an equilibrium mixture of boat forms B 1 and B 2 for Z isomers of 5-8. For the E isomers of 5-8, boat form B 1 is predicted to be the major conformer. The piperidin-4-ones 1-4 exist in normal chair conformations with equatorial orientations of all the substituents.

Acid-responsive nanospheres from an asparagine-derived amphiphile

“…The major product was assigned the diequatorial 2,4- cis -isomer ( cis -ALA-11; Fig. 2A) based on spectroscopic evidence 23 and prior precedent for the relative stereochemistry of aldehyde-asparagine cyclocondensates, 19,24 which have been ascribed to the expected thermodynamically preferred 1,3-diequatorial orientation of larger substituents in the ring closure step 25b . The equatorial assignment of the 4-CO 2 − group was unequivocally established by vicinal spin-spin coupling constants ( 3 J 4,5α ≈ 12 Hz, and 3 J 4,5β ≈ 5 Hz), in the 1 H NMR spectrum (Fig.…”

Conformational Studies of Some N-Acyl-3- Ethylpiperidin-4-Ones

“…The expected coupling constant values i.e., J 2,3 value around 10 Hz but the actual value is around 4 Hz in N-acyl-3-alkyl derivatives 1-4. Based on literature survey 14 the boat form B4 is having higher energy. So B4 is also ruled out.…”

“…A mixture of boat forms (B1and B2) is obtained for some substituted nitrosopiperidones (Z isomer). E isomer exhibited boat form B1 be the major conformer 14 . At present we discussed conformational studies of substituted N-acyl-3-ethyl-difuryl-piperidones.…”

Synthesis, characterisation, stereochemistry and dynamic NMR studies of N-nitroso and N-formyl-t-3-isopropyl-r-2,c-6-bis(4-methoxyphenyl)piperidin-4-ones

“…This method is based on the observation that the carbons syn to -X=Y functions, in compounds containing N-X=Y functions such as N-NO, N-COR , etc., are always more shielded than the anti [7][8][9][10][11][12][13][14][15][16][17][18]. This method is based on the observation that the carbons syn to -X=Y functions, in compounds containing N-X=Y functions such as N-NO, N-COR , etc., are always more shielded than the anti [7][8][9][10][11][12][13][14][15][16][17][18].…”