s‐Cis and s‐trans isomerism in acylproline analogs

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Abstract
N‐Acetyl‐2,3‐dehydroproline, N‐acetyl‐5‐oxo‐L‐proline, N‐acrylyl‐L‐proline, N‐acetyl‐L‐azetidine‐2‐carboxylic acid and N‐acetyl‐D,L‐pipecolic acid have been examined in 2H2O by 1H and 13Cn.m.r. for the purpose of finding s‐cis or s‐trans locked acylprolines. Conformationally locked acylprolines could be incorporated into proline‐containing peptide hormones such as angiotensin and thyroliberin in order to determine the rotational state of the peptide bond to proline in the hormone receptor complex. The populations of trans and cis rotational isomers were determined as a function of p2 H in order to assign the trans and cis isomers and to compare the populations in all the acylprolines at neutral p2 H, where the cis isomer is normally present. Proton spectra were also recorded at from 7° to 75° in order to qualitatively determine the exchange rate between the isomers. The majority of these analogs exhibit a cis‐trans isomerization similar to that of N‐acetyl‐L‐proline in the ratio of trans to cis rotational isomer found at neutral p2 H (about 1:1), the temperature dependence of the population ratio (none), and the coalescence temperature for proton resonances (greater than 75°). However, N‐acetyl‐5‐oxo‐L‐proline was found to be greater than 98% s‐trans at neutral pH, compared to 50% s‐trans in N‐acetyl‐L proline, and therefore a good candidate for synthesis of an s‐trans locked peptide hormone. N‐Acetyl‐2,3‐dehydroproline rapidly exchanges between s‐cis and s‐trans in contrast to all other proline analogs examined and exhibits coalescence of the β‐proton cis and trans resonances at 45°. Titration with the shift reagent Pr+++ was employed to confirm the assignments of the cis and trans methyl resonances of all of the N‐acetyl compounds except N‐acetyl‐5‐oxo‐L‐proline.