SynopsisInfrared spectra of polypeptides were measured in the region of 1800-400 cm-1. For the a-helical form, disordered form, and antiparallel-chain &form, amide V bands arising from N-H out-of-plane bending modes were observed at 610-620, around 650, and 700-705 cm-1, respectively, and amide V' bands arising from N-D oubof-plane bending modes were observed at 455-465, around 510, and 515-530 cm-1, respectively. These correlations are useful for conformation diagnoses, particularly for copolyamino-acids or proteins which are not oriented. The nature of low-frequency amide bands are discussed with reference to potential energy distributions calculated for the a-helical form and p form.

Als mildes Reduktionsmittel ist Chrom(II) in der Lage, zwischen Alkyliodiden, Alkylradikalen und Allylradikalen zu diskriminieren: Das Alkylradikal ist in Gegenwart von CrII so langlebig, daß es mit einem 1,3‐Dien reagieren kann, und das dabei entstehende Allylradikal bildet nach rascher Einelektronenreduktion eine Allylchrom(II)‐Spezies, die an einen Aldehyd unter Bildung des Drei‐Komponenten‐Produkts addiert [siehe z. B. Gl. (a)].

The reduction of carbon ± carbon unsaturated bonds by chromium(ii) has been observed since the 1920s, [1] and in some cases dianionic species have been postulated. [2] However, most of these reactions were conducted in protic solvents, and protonated anionic species were produced which cannot be employed for carbon ± carbon bond formation except in a few cases. [3,4] We report here that the a,b-unsaturated ketone 1 is reduced with chromium(ii) in an aprotic solvent to give the ketone a,b-dianion equivalent 2, which reacts with an aldehyde at the a position (aldol condensation) followed by intramolecular cyclopropanation to afford cis-2-(1-hydroxyalkyl)-substituted cyclopropanol 3 in a stereoselective manner [Eq. (1)].Treatment of enone 4 [see Eq.(2)] with CrCl 2 in DMF under strictly water-free conditions did not give cyclopropanol 5, but instead resulted in a complex mixture. [5] However, when water was added to the reaction mixture before addition of the enone, the cyclopropanol formation proceeded smoothly. [6] For example, treatment of enone 4 with a mixture of CrCl 2 (4 equiv) and D 2 O (2 equiv) in DMF at 25 8C for 4 h gave cyclopropanol 5 in 96 % yield, and deuterium was incorporated in 89 % content [Eq. (2)]. O Ph OH Ph D CrCl 2 , D 2 O DMF 25 °C, 4 h 5 96% 4 (D: 89 %, cis / trans = 1 / 1)To clarify the reactivity of the nucleophilic position, the reaction was conducted in the presence of an aldehyde under water-free conditions. A solution of enone 4 and nonanal in dry DMF was added at 0 8C to a blue-green solution of CrCl 2 in DMF, and the resulting mixture was stirred for 2 h at 0 8C to give diol 6 in 93 % yield (Scheme 1). The two isomers 6 a and 6 b (6 a/6 b 58/42) [7] were produced selectively out of four O Ph OH Ph OH n-C 8 H 17 O O C H 2 Ph n-C 8 H 17 O O Ph n-C 8 H 17 H CrCl 2 DMF 0 °C, 2 h n-C 8 H 17 CHO + 6 93% (6a / 6b = 58 / 42) 93% major minor 95% 4 7a 7b NOE a) a) Scheme 1. Sequential aldol condensation and cyclopropanol formation followed by acetalization. a) Me 2 C(OMe) 2 , cat. PPTS, acetone, 25 8C, 2 h.possible diastereomers, and the relative configuration of the cyclopropane rings was shown to be cis by quantitative transformation to the corresponding acetonides 7 a and 7 b with 2,2-dimethoxypropane and pyridinium p-toluenesulfonate (PPTS). The NOE experiments of the acetonides showed that the octyl group and the cyclopropane ring of the acetonide 7 a, derived from the major adduct 6 a, had a trans configuration of the acetonide ring. A possible mechanism which explains the configuration of the cyclopropane rings is shown in Scheme 2. One-electron reduction of an enone with chromium(ii) gives the enolate O R 2

No abstract