Stereochemical Characteristics Research Articles

Transition-metal Complexes of Pyrrole Pigments. XIII. Reduction and Coordination Behaviors of the Cobalt(II) Complexes of 1,19-Disubstituted Tetradehydrocorrins as Demonstrated by the Presence of Various Bases

Abstract The interaction of various bases with the cobalt(II) complexes of 1,19-disubstituted tetradehydrocorrins (Co(II)–TDHC) was investigated. The parent cobalt(II) complexes were shown to undergo ready reduction at the nuclear cobalt(II) with the unsolvated hydroxide ion as well as with bulky triethylamine. A relatively large positive reduction potential of Co(II)–TDHC is responsible for these novel electron transfer reactions. Less hindered amines (B), on the other hand, failed to show such electron transfer reactions with the cobalt(II) complexes and Co(II)–TDHC became the pentacoordinated complex by taking up an amine base in the axial position: Co(II)–TDHC+B\ightleftharpoonsB–Co(II)–TDHC. For the interaction of 4-substituted pyridines with the cobalt(II) complex, the general behavior of the equilibrium constants was interpreted in terms of the amine basicity and the Hammett equation by referring to the corresponding behavior of the porphyrin complexes. The weaker coordination tendency of some aliphatic amines and hindered pyridines toward Co(II)–TDHC was attributed to their electronic and steric effects in the coordination process. The intrinsic correlation between electronic and stereochemical characteristics pertaining to the tetradehydrocorrin skeleton and the nature of nuclear cobalt(II) was discussed on the basis of these observations.

Dark and photochemical oxidation reaction of 1-ethoxy-2-ethylthioethene stereoisomers

1. We were the first to identify the oxidation product of 1-ethoxy-2-ethylthioethene as being the diethyl monothioacetal of glyoxal; it proved that the dark oxidation of 1-ethoxy-2-ethylthioethene with molecular oxygen to glyoxal diethyl monothioacetal is possible only for the cis-isomer. 2. A method was developed for the photochemical oxidation of the trans-isomer of 1-ethoxy-2-ethylthioethene, adsorbed on porous glass, to the diethyl monothioacetal. 3. Based on the kinetic and stereochemical characteristics of the reaction, a mechanism was proposed for the oxidation process, which includes the formation of an intermediate complex from two molecules of the oxidized substrate and one molecule of oxygen. 4. It was established by the EPR method that the O2− ion-radical is formed in the photochemical oxidation of trans-1-ethoxy-2-ethylthioethene, which takes part in the oxidation process.

Mass Spectra of Steroid Sapogenins

A study of the mass spectra of 15 natural and synthetic steroid sapogenins confirmed the general modes of fragmentation in the side chain and revealed additional features due to breakdown of ring D and to special structural and stereochemical characteristics.

Androgen structure and male sexual behavior in the castrated rat

The following androgens were injected sc (1 mg for 30 days) to groups of inexperienced castrated male rats: testosterone, androstenedione, androstenediol, dehydroepiandrosterone, 11 β-hydroxy-androstenedione, androsterone, dihydrotestosterone, androstanedione, androstanediol, and 5 β-androstanediol. The mating tests were performed every third day. Only testosterone, androstenedione, and androstenediol elicited the full mating pattern. Both 5 a and 5β reduced androgens were ineffective in stimulating clear sexual behavior. In contrast, two 5 a reduced androgens, androstanediol and dihydrotestosterone, were the most potent androgens to induce growth of the sex accessories. The results suggest that 5 a reduction does not play a role in the action of androgens on the brain structures related to the expression of sexual behavior in the male rat, and that the stereochemical characteristics of the androgen molecule required to elicit sexual behavior are different from those involved in androgenicity.

Nature of transition-metal cluster structures

The theory of cluster structure presented in this article, which is based on a fundamental classification of ligands as having weak or strong fields, has a sufficiently general character. The concepts developed permit reliable interpretation of the most important experimentally established patterns of cluster-compound structure and prediction of the energy and stereochemical characteristics of different cluster groupings. Thus, it is possible for the first time to explain why cluster structures are generally unstable for metals at the beginning of the transition series, while cluster structures for transition metals in the central groups are very stable only for lower compounds with weak-field ligands (halogens, carboxylic acids, etc.) and cluster structures occur for transition metals at the end of the period only for formally low-valence compounds with strong-field ligands (CO, C5H5, etc.). Moreover, it is possible to give a concise interpretation of the characteristic tendency toward formation of metal-metal bonds of different multiplicities (extending as far as quadruple dicentric M-M bonds) in chloride and carboxyl clusters and to explain the redominantly single σ-character of the M-M bonds in carbonyl clusters, where one of the main factors limiting formation of multiple M-M bonds is the destabilizing nature of the metal-metal π interaction. Finally, our concepts of the relative energy of the filled metallic molecular orbitals enable us to explain why the energy of two-electron M-M bonds in carbonyl clusters is materially reduced (by a factor of almost 2) in cornparison with that in chloride clusters and thus give us an understanding of the unusual strength of halide clusters (and clusters containing weakfield ligands in general) and the exceptional diversity of forms in which they exist.

A conformational analysis of the methyl ester of O-acetyl-L-lactyl-L-alanine and the methylamide of N-acetyl-L-alanyl-L-lactic acid

1. The most preferred extended conformations for Ac-L-Lac-L-Ala-OMe and Ac-L-Ala-L-Lac-NHMe in a polar medium are R-R, R-B, B-R, and B-B. Of the conformations with a hydrogen bond of the 1–4 type, the most stable are R-R and R-B. 2. There is an interrelationship between the conformational states of neighboring hydroxy and amino acid residues. 3. The stereochemical characteristics of the ester group are similar to those of the amide group.

Stereochemistry of heterogeneous and homogeneous hydrogenation of substituted bicyclo(2,2,1) heptenes

1. The stereochemistry of heterogeneous and homogeneous hydrogenation of p-bornenylanisole is mainly determined by the accessibility of the double bonds of these compounds. 2. The reduction with p-norbornenylanisole by diimide leads to the formation of only the endoisomer whereas the reduction of p-norbornenylanisole by diimide fails because of steric factors involved. 3. The reduction of p-norbornenylanisole and p-bornenylanisole by sodium in liquid ammonia leads to the formation of only the appropriate endo-isomers. An interpretation of the stereochemical characteristics of this reaction has been advanced.

Stereochemical characteristics of variotin as deduced from the nuclear Overhauser effect in the NMR spectra.

Stereochemical characteristics of variotin as deduced from the nuclear Overhauser effect in the NMR spectra.

Opium Alkaloids VI: Isolation of N-Methyl-14-O-desmethylepiporphyroxine

The papaverrubine alkaloids and their <i>N</i>-methyl derivatives are widely distributed in the genus <i>Papaver</i>. They have a number of interesting structural and stereochemical characteristics. A new alkaloid of this general class has now been isolated from opium. It has been characterized by means of NMR and mass spectrometry and by chemical conversions to known compounds.