ChemInform Abstract: Crystal Growth,Crystal Structure of New Polymorphic Modification, β‐Bi2B8O15 and Thermal Expansion of α‐Bi2B8O15.

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Preparation of Protected β2‐ and β3‐Homocysteine, β2‐ and β3‐Homohistidine,and β2‐Homoserine for Solid‐Phase Syntheses

The Ser, Cys, and His side chains play decisive roles in the syntheses, structures, and functions of proteins and enzymes. For our structural and biomedical investigations of β‐peptides consisting of amino acids with proteinogenic side chains, we needed to have reliable preparative access to the title compounds. The two β³‐homoamino acid derivatives were obtained by Arndt–Eistert methodology from Boc‐His(Ts)‐OH and Fmoc‐Cys(PMB)‐OH (Schemes 2–4), with the side‐chain functional groups' reactivities requiring special precautions. The β²‐homoamino acids were prepared with the help of the chiral oxazolidinone auxiliary DIOZ by diastereoselective aldol additions of suitable Ti‐enolates to formaldehyde (generated in situ from trioxane) and subsequent functional‐group manipulations. These include OH→O^tBu etherification (for β²hSer; Schemes 5 and 6), OH→STrt replacement (for β²hCys; Scheme 7), and CH₂OH→CH₂N₃→CH₂NH₂ transformations (for β²hHis; Schemes 9–11). Including protection/deprotection/re‐protection reactions, it takes up to ten steps to obtain the enantiomerically pure target compounds from commercial precursors. Unsuccessful approaches, pitfalls, and optimization procedures are also discussed. The final products and the intermediate compounds are fully characterized by retention times (t_R), melting points, optical rotations, HPLC on chiral columns, IR, ¹H‐ and ¹³C‐NMR spectroscopy, mass spectrometry, elemental analyses, and (in some cases) by X‐ray crystal‐structure analysis.     

13‐cis‐β,β‐Carotene and 15‐cis‐β,β‐carotene

13‐cis‐β,β‐Carotene, C₄₀H₅₆, crystallizes with a complete molecule in the asymmetric unit, whereas 15‐cis‐β,β‐carotene, also C₄₀H₅₆, has twofold symmetry about an axis through the central bond of the polyene chain. The polyene methyl groups are arranged on one side of the polyene chains for each molecule and the 6‐s‐cisβ end groups, with the cyclohexene rings in half‐chair conformations, are twisted out of the planes of the polyene chains by angles ranging from 41.37 (17) to 52.2 (4)°. The molecules in each structure pack so that the arms of one occupy the cleft of the next, and there is significant π–π stacking of the almost‐parallel polyene chains of the 15‐cis isomer, which approach at distances of 3.319 (1)–3.591 (1) Å.     

Rhodium‐Catalyzed Asymmetric Arylation of β,γ‐Unsaturated α‐Ketoamides for the Construction of Nonracemic γ,γ‐Diarylcarbonyl Compounds

A highly regio‐ and enantioselective rhodium‐catalyzed 1,4‐addition of arylboronic acids to β,γ‐unsaturated α‐ketoamides using a simple new chiral sulfinylphosphine ligand is described. This transformation provides an attractive approach to construct chiral nonracemic γ,γ‐diarylsubstituted carbonyl compounds, as exemplified in the concise syntheses of sertraline and tetrahydroquinoline‐2‐carboxylamide.     

5β,6β‐Epoxy‐17‐oxoandrostan‐3β‐yl acetate and 5β,6β‐epoxy‐20‐oxopregnan‐3β‐yl acetate

In the title compounds, C₂₁H₃₀O₄, (I), and C₂₃H₃₄O₄, (II), respectively, which are valuable intermediates in the synthesis of important steroid derivatives, rings A and B are cis‐(5β,10β)‐fused. The two molecules have similar conformations of rings A, B and C. The presence of the 5β,6β‐epoxide group induces a significant twist of the steroid nucleus and a strong flattening of the B ring. The different C17 substituents result in different conformations for ring D. Cohesion of the molecular packing is achieved in both compounds only by weak intermolecular interactions. The geometries of the molecules in the crystalline environment are compared with those of the free molecules as given by ab initio Roothan Hartree–Fock calculations. We show in this work that quantum mechanical ab initio methods reproduce well the details of the conformation of these molecules, including a large twist of the steroid nucleus. The calculated twist values are comparable, but are larger than the observed values, indicating a possible small effect of the crystal packing on the twist angles.     

The nearly constant loss,Johari‐Goldstein β‐relaxation,and α‐relaxation of 1,4‐polybutadiene

From high‐resolution dielectric spectroscopy measurements on 1,4‐polybutadiene (1,4‐PB), we show that in addition to the structural α‐relaxation and higher frequency secondary relaxations in the spectra, a nearly constant loss (NCL) is observed at shorter times/lower temperatures. The properties of this NCL are compared to those of another chemically similar polymer, 1,4‐polyisoprene. The secondary relaxations in 1,4‐PB include the well‐known Johari‐Goldstein (JG) β‐relaxation and two other higher‐frequency peaks. One of these, referred to as the γ‐relaxation, falls between the JG‐relaxation and the NCL. Seen previously by others, this γ‐relaxation in 1,4‐PB is not the JG‐process and bears no relation to the glass transition. At very low temperatures (<15 K), we confirm the existence of a very fast secondary relaxation, having a weak dielectric strength and an almost temperature‐invariant relaxation time. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 342–348, 2007     

5-雄烯-3β, 7α, 17β-三醇和5-雄烯-3β, 7β, 17β-三醇的合成

以5-雄烯二醇为原料,用微生物转化的方法合成了两个重要的神经甾体5-雄烯-3β, 7α, 17β-三醇和5-雄烯-3β, 7β, 17β-三醇。所用菌种总枝毛霉为我们自己筛选,并首次应用于5-雄烯-3β, 7α, 17β-三醇和5-雄烯-3β, 7β, 17β-三醇的合成中。