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Lactams 23. thermal cis trans isomerization of the 5 ethyl 2 oxo 4 piperidineacetic acid system effect of an n substituent

, : Lactams 23. thermal cis trans isomerization of the 5 ethyl 2 oxo 4 piperidineacetic acid system effect of an n substituent. Chemical and Pharmaceutical Bulletin (Tokyo) 33(3): 1062-1068

In order to investigate the effect of an N-substituent on thermal cis-trans isomerization of the 5-ethyl-2-oxo-4-piperidineacetic acid system, the N-alkyl analogs (.+-.)-1b,c and (.+-.)-2b,c and the N-(2-arylethyl) analogs (.sbd.)-1f, g, i and (+)-2f, g, i were separately heated neat at C, and the progress of their cis-trans isomerization reactions was followed by determining the isomer ratios in the reaction mixtures according to the previously reported 13C NMR spectroscopic method. In all cases the reaction comes to equilibrium within 28-130 min, when the cis and trans isomers exist in a ratio of 1:2. These results together with those obtained previously with the analogs (.+-.)-1a, d, e, (.sbd.)-1h, (.+-.)-2a, d, e and (+)-2h indicate that a higher and/or bulkier N-substituent tends to decrease the rate of cis-trans isomerization of the 5-ethyl-2-oxo-4-piperidineacetic acid system, which is a useful synthon for the synthesis of benzo-[a]quinolizidine-type Alangium alkaloids. Among the substrates used in the present equilibration study, (.+-.)-1b, c and (.+-.)-2b, c were prepared from the corresponding N-unsubstituted lactam esters (.+-.)-3 and (.+-.)-7 by the lactim ether method.


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