The design and synthesis of peptide mimetics of thyrotropin-releasing hormone (TRH) in which the peptide backbone is entirely replaced by a cyclohexane framework are described. The cis-1,3,5-trisubstituted ring was expected to permit key pharmacophoric groups to adopt conformations consistent with proposed bioactive conformations of the peptide. Compounds were synthesized by a stereoselective synthesis starting from L-glutamic acid. In a behavioral model of cognition in which TRH is active, the mimetics are potent, active compounds, exhibiting oral activity. One analog (26, (1S,3R,5(2S),5S)-5-[[5-oxo-1-(phenylmethyl)-2-pyrrolidinyl]-methyl]-5- [(1H-imidazol-5-yl)methyl]cyclohexaneacetamide) was radiolabeled for binding studies and evaluated in other binding assays and pharmacological tests. Competition binding of 26 vs [3H]MeTRH to rat brain slices suggests a two-site model for ligand binding with IC50's of 1 microM and 3 mM. Direct binding of [3H]-26 shows a biphasic curve with IC50's of 80 and 49 microM, respectively. Further studies would be needed to establish a link between the novel binding site(s) and the behavioral activity of 26 and TRH analogs.

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A ' 4, 4 included in an exhaustive study by Kaiser and co-workers6' and by Zirkle and co-workers6b on the effect of chemical structure upon RIA0 inhibition. Nevertheless, we wish to describe our chemical work and pharmacological findings in this field.The synthesis of 2-phenoxycyclopropylamine was accomplished by the series of reactions shown in Scheme I. The reaction between phenyl vinyl ether (1) and ethyl diazoacetate (2) in the presence of copper was employed by Julia' to obtain ethyl 2-phenoxycyclopropanecarboxylate (3) ; he hydrolyzed the ester to the corresponding acid, n1.p. 112". Since he described the one acid rather than the two possible diastereorneric acids, one might assume that the synthesis produced only one ester. Later, Looker and BraunE employed the same reactants but without copper and under slightly different experimental conditions, to prepare the ester 3. They stated that no attempt was made to obtain the pure diastereomeric ethyl esters. However, upon hydrolysis they were able to demonstrate the presence of the two acids, ie., trans-2-phenoxycyclopropanecarboxylic acid, m.p. 113-113.7', and the cis isomer, m.p. 135-137'. They therefore concluded that the latter acid was not obtained by Julia, and that the trans acid was his sole reaction product. In our hands, the reaction between 1 and 2 in the presence of copper produced the ester 3, which proved to be a mixture of the cis and trans esters of 3. The isomers were separated by redistillation through a spinning-band column and characterized. Thus, we were able to obtain a fraction which contained 87.3y0 of one ester and 99.4% of the other, based upon V.P.C. analyses. The interrelationship of the two ester forms was established when it was found that the 99.4y0 ester, which had a longer retention time could be epimerized by refluxing with sodium ethoxide in absolute ethanol into 95.1% of the other ester with the shorter retention time. We designated the ester with the shorter retention time as trans and the ester with the longer retention time as cis.Additional evidence to confirm these designations was obtained by hydrolyzing each ester separately to its (6) (a) (7) M. Julia, Bull. soc chzm. France, 181 (1956). (8) J. H. Looker and L. L. Braun, J . Org. Chem., 23, 933 (1958).corresponding acid. Thus, the cis ester yielded the cis acid 9, m.p. 13fj-138". The trans ester gave the trans acid 10, m.p. 111-113". These respective melting points are in good agreement with those previously reported.8It will be noted that the synthesis favored the formation of the thermodynamically more stable trans form by a ratio of approximately 3 : 1 over the cis form. A study of the Dreiding models indicated that phenyl vinyl ether and carbethoxycarbene, during reaction, develop a considerable crowding effert in an attcinpt to form the cis isoiner. In the formation of the tmns product, the two bulky groups do not interfere with each other. In retrospect, this relationship in yield of the cis and trans isomers, can be detected by careful comparison of the i...