β-Ureido acids and dihydrouracils—VII

A conformational study of the adducts of 2′-deoxythymidine and 2,2,6,6-tetramethyl-1,4-piperidone-N-oxyl by 1H and 13C nuclear magnetic resonance

“…Evidence points to a distorted half-chair conformation for 5,6-saturated pyrimidines in which C5 and C6 lie on opposite sides of the mean plane defined by Nl--C2-N3--C4 (7,(17)(18)(19). Of the molecules considered here, crystallographic data are available only for 5HS, which shows a half-chair with the 5-hydroxyl and 5-methyl groups in pseudo-axial and pseudo-equatorial positions, respectively (7).…”

Erratum: Assisted hydrolysis of cis-2-(3-phenylthioureido)cyclopentane-carbonitrile in alkaline solution. Solvent dependent switch from hydrolysis to rearrangement of the iminothiooxopyrimidine intermediate

“…12 Hz) in the 1 H NMR (6). We proved this independently by double resonance experiments 2 which showed J 4a,7a to be 6.5 Hz, which fell within the usual range for cis coupling in dihydrouracils (7). The 7a-H signal splittings due to the neighbouring methylene hydrogens were considerably smaller than those for 4a-H, indicating that the NH group is pseudoaxial in the presumably half-chair conformation of the cyclopentane ring.…”

“…Noteworthy is the strong bias of cis-2 for conformation A (Scheme 2) suggested by the bandwidths of the signals for protons 4a and 7a in DMSO: 22.2 and 11.5 Hz respectively. 7a-H is axial in the pyrimidine ring thus J HNH will be 0-1 Hz (7) and pseudoequatorial in the cyclopentane ring so that the sum of 11.5 Hz has to be made up of three gauche couplings. The signals for the 4a-and 7a-protons in the imino tautomers of compound 5 were better resolved than those for the iminopyrimidine 2.…”

“…At pH 9.18 cis-7 hydrolyzes with an observed rate of 0.0015 s -1 7. Thin layer chromatography showed general retention of configuration.…”

β-Ureido acids and dihydrouracils. Part 15. Effect of allylic strain on ring opening of 1,6-disubstituted dihydrouracils

“…The nitrogen-substi tuted derivatives (9) and (10) are, however, 0.5 pK units weaker acids, demonstrating the direct field effect of an axial carboxylate group. Sterically the carboxy-group is similar to phenyl but has a smaller conformational energy in cyclohe~anes.~~ The rate decrease brought about by an N-methyl group in (9) compared with (8) in klk3/kbl is greater than that observed with the phenyl derivatives (6) and (7) which is due to a larger drop in kl. Since the combined steric and polar effect of an axial carboxylate group most probably strongly reduces k , this could be attributed to a greater decrease in the population of the equatorial conformation.…”