In 1-arylhydantoins an o-methyl group is effectively larger than an o-chloro substituent in restricting rotation about the aryl-C-N bond, whereas in 3-arylhydantoins and 3-aryl-2-thiohydantoins the reverse order of sizes is observed. This difference is attributed to repulsion between the chlorine atom and a carbonyl oxygen atom in the torsional transition state of the 3-aryl compounds.Dans les aryl-1 hydantoines, un groupe nikthyle en ortho est effectivernent plus volurnineux qu'un substituant chloro en ortho lorsqu'il s'agit d'empkcher la rotation du groupe aryle autour du lien C-N; toutefois dans les aryl-3 hydantoines et les aryl-3 thio-2 hydantoines la grosseur relative des groupes est renversee. Cette difference est attribuee a la repulsion entre I'atorne de chlore et I'atorne d'oxygtne du carbonyle dans l'ttat de transition de torsion.[Traduit par le journal]Can. J . Chem.. 51. 3635 (1973) It has conimonly been observed ( 1 , 2) that a methyl group exerts a steric effect greater than that of a chlorine atom, for example, in restricting internal rotation in hindered biphenyls (1). This order is consistent with the relative size of these groups as determined by X-ray crystallographic measurements of van der Waals radii (3).In an n.m.r. study of restricted rotation about aryl-C-N bonds in 1-arylhydantoins we have found that the same order of relative sizes applies. The free energies of activation for rotation about the C-N bond joining the aryl and heterocyclic moieties are 0.5-1.5 kcal/mol higher for the o-methyl compounds than for the o-chloro compounds (Table 1).In contrast, the free energies of activation for rotation of the 3-arylhydantoins and the 3-aryl-2-thiohydantoins (Table 2) indicate a reversal in the relative steric effects of the o-methvl and the o-chloro substituents on the aryl group. The free energy barriers to rotation about the aryl-C-N bonds are 0.9-2. I kcal/mol higher for the chloro than for the methyl compounds. This reversal in the relative influences of the o-chloro and the o-methyl substituents is attributed to the nature of the interactions within the niolecules in the transition states for internal rotation. 'Revision received July 12, 1973. The rate constants for the internal rotation process in the I-aryl-and the 3-arylhydantoins were obtained by complete n.m.r. line shape analysis of the temperature dependent 5-methyl or o-tolyl-methyl signals over a range of temperatures, using an iterative fitting procedure. The linear least-squares method was used to fit the Arrhenius equation to the data. Except in the cases of l a and b, the free energies of activation Can. J. Chem. Downloaded from www.nrcresearchpress.com by 52.38.122.124 on 05/09/18For personal use only.

Acid hydrolysis of ristocetin A yields a number of amixo acids of unusual structure. One set of diastereoisomeric pairs has a molecular weight of 362 and an empirical formula of C1711sN207. Mass spectral and nuclear magnetic resonance (NMR) evidence suggest a diphenyl ether with hydroxyl and a-amino acid [-CH-(NH2)COOH] groups on one ring, and methyl, hydroxyl and a-amino acid groups on the other ring. Coupling in the NMR and nuclear Overhauser effect experiments favor certain substitution patterns, which are shown. Another set of diastereoisomers seems to be a lower homolog of the previous set, without the aromatic methyl group.Ristocetins (1) A and B have antibacterial properties (2), a molecular weight of several thousand, and yield 10 and 7 identified monosaccharide units, respectively, on mild acid hydrolysis (3). The aglycone is a polypeptide yielding amino acids of novel, but unknown, structure on strong acid hydrolysis; it has been recognized, however, that phenolic aamino acids are present. The mode of antibacterial action of the ristocetins has been studied extensively (4). The present communication describes information about the structure of the aglycone of ristocetin A.By hydrolysis of 1 mM ristocetin with 6 N HCl in a degassed sealed tube, followed by analysis with an amino acid analyzer (5), we have found five distinct amino acids (I, II, III, IV, V) giving positive ninhydrin tests; the elution of ninhydrin-active material runs parallel to increases in the ultraviolet absorption at 260 nm, so that separation of the five can be followed conveniently by spectrophotometry (Uviscan and recorder). The sample of hydrolyzate was placed on a 150-cm column containing Aminex Q-150S resin at 30°. Upon elution with pH 3.25 buffer at about 60 ml/hr, I came off at 5 hr 40 min, II at 6 hr 30 min and III at 7 hr 50 min.After a change to pH 4.25 buffer, IV came off in 3 hr 40 min and V. at 4 hr 30 min. Hydrolysis of 400 mg of ristocetin yielded a total, after separation and desalting, of about 75 mg of amino acids I-V.We found that acid II is converted to a mixture of I and II, II predominating, and IV is converted to a 50-50 mixture of IV and V by heating with HCl; each pair appears to consist of two diastereoisomers, with almost identical NMR spectra for each pair. Acid III is obtained in very small amounts.Evidence for the presence of Ar(H2N)CHCOOH groups comes from the ninhydrin reaction, ester formation, the NMR spectra, the formation of t-BOC and thiol-t-BOC derivatives with di-t-butyl dicarbonate and its sulfur analog (6), as well as mass spectra. Phenolic groups are shown by a bathochromic shift of the wavelength of maximal absorption in base, by ease of oxidation with Tollens reagent, by empirical formulas requiring aromatic rings, and by NMR. The empirical formula of IV (and V) is C17H18N207, based on peaks at m/e for the bis-t-BOC bis-ethyl ester, 1, of 618

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