The stereochemical assignment of acyclic polyols: a computational study of the NMR data of a library of stereopentad sequences from polyketide natural products

The reliable stereochemical assignment of flexible molecules, such as acyclic polypropionates is an enormously challenging task. This is illustrated by the NMR chemical shifts for a complete set of sixteen diastereomeric stereopentads whose experimental data is reported here for the first time. Although the experimental spectra are very similar to each other, analysis of the similarity between the shifts of different diastereoisomers reveals that some diastereoisomers are much more distinctive than others. In addition, the NMR shifts of the sixteen compounds have also been calculated using DFT GIAO calculations, and the use of our recently developed CP3 parameter for structure assignment is illustrated for these molecules. Even in cases where the experimental spectra are very similar, our CP3 parameter makes possible the correct assignment of pairs of diastereoisomers with high confidence.     

Effective separation and C-24 stereochemical assignment of epimeric 24-isopropenylcholesterols

Separation of epimeric 24-isopropenylcholesterols ($\text{1a}$) was effectively achieved by reversed-phase HPLC. The C-24 stereochemistry of these epimers was determined on the basis of chemical and spectroscopic evidence.     

Biosynthetic pathway of macrolactam polyketide antibiotic cremimycin

Biosynthetic origin of macrolactam polyketide antibiotic cremimycin was investigated by feeding experiments with [1-¹³C]acetate, [1,2-¹³C₂]acetate, [1-¹³C]propionate, succinate-d₄, and d-[6,6-²H₂]glucose. NMR analysis of the resultant isotope-enriched cremimycins showed distinctive incorporation patterns, which suggested that the aglycon of cremimycin was constructed from two propionates and eleven acetates. Thus, 3-oxononanoate was proposed as a potential polyketide intermediate, that is, aminated to be the unique nitrogen-containing moiety of cremymicin. Further, characteristic propionate biosynthetic pathway in the cremimycin-producing strain was also described.     

Structural and stereochemical revision of isocyanide and isothiocyanate amphilectenes from the Caribbean marine sponge Cribochalina sp.

The absolute stereochemistry of amphilectene metabolites from Cribochalina sp. has been revised by a detailed NMR spectroscopic study of the Mosher ester derivatives of a related alcohol. The relative stereochemistry of the previously described amphilectenes has been reinvestigated and reassigned on the basis of the X-ray structural analysis carried out on one of them. The structure of a new amphilectene metabolite, which is an isothiocyanato analogue is also presented.     

Restricted rotations and stereodynamics of aziridine-2-methanol derivatives

‘Gear-like’ rotations of simple C-C bonds have been observed in some aziridine methanol derivatives. These restricted rotations have been studied by dynamic and multinuclear magnetic resonance experiments, and the barrier for rotations of Csp³-Csp³ and Csp³-Csp² bonds have been calculated. The role of an intramolecular hydrogen bond on the stereodynamics has also been demonstrated.     

Total synthesis of cryptomoscatones D1 and D2: stereochemical assignment of cryptomoscatone D1

The first total synthesis and structural elucidation of cryptomoscatone D1, and a novel synthetic approach for cryptomoscatone D2 were achieved in 30% and 29% overall yield, respectively. The synthesis relied on the use of a key Mukaiyama aldol reaction followed by a diastereoselective carbonyl reduction that allowed the preparation of four cryptomoscatone isomers in a stereochemically divergent manner. Comparison of NMR data and CD curves of the synthetic stereoisomers and natural products confirmed the stereochemical nature of cryptomoscatone D2, and led to establishing the absolute configuration of cryptomoscatone D1.     

Spectroscopic evidence for the unusual stereochemical configuration of an endosome-specific lipid

At a glance: The stereochemical configuration of the diglycerophosphate backbone of the endosome-specific lipid bis(monoacylglycero)phosphate (BMP, see picture) was determined by (1)H?NMR spectroscopy. Enantiomeric discrimination was facilitated by introduction of D-camphor ketals as chiral shift reagents, and enantiopure synthetic BMP analogues were prepared as reference materials. Natural BMP exhibited the unusual sn-1,1' diglycerophosphate backbone.     

立体化学教学中不对称催化科研成果的融入

在基础有机化学教学中引入不对称催化的概念和科研成果,可将立体化学融入贯穿到有机化学课程的章节教学中,丰富立体化学的教学内容。通过对知识点的串联解析使学生了解不对称催化反应,从而逐步加深对立体化学的理解。组织相关研究方向的学术专题报告和科研实践,不仅有助于巩固相关理论知识,还可以让学生在知识熏陶和实验实践中培养自身的学术素养和科研能力,提高学习热情和积极性。     

Structural assignment of organotin hydrides containing the oxazoline ligand

A series of triorganotin hydrides and diorganotin dihydrides containing the optically active 2-(4-isopropyl-2-oxazolinyl)-5-phenyl ligand have been characterized by means of the multinuclear low-temperature NMR investigations, the results of which are discussed. In the corresponding organotin hydrides values of the ¹J(¹H-^117/119Sn) couplings appeared to be temperature dependent, supporting an axial/equatorial position of the hydrogen attached to the tin.     

Dynamic chirality: keen selection in the face of stereochemical diversity in mechanically bonded compounds

The template-directed syntheses, employing bisparaphenylene-[34]crown-10 (BPP34C10), 1,5-dinaphthoparaphenylene-[36]crown-10 (1/5NPPP36C10), and 1,5-dinaphtho-[38]crown-10 (1/5DNP38C10) as templates, of three [2]catenanes, whereby one of the two bipyridinium units in cyclobis(paraquat-p-phenylene) is replaced by a bipicolinium unit, are described. The crude reaction mixtures comprising the [2]catenanes all contain slightly more of the homologous [3]catenanes, wherein a "dimeric" octacationic cyclophane has the crown ether macrocycles encircling the alternating bipyridinium units with the bipicolinium units completely unfettered. X-ray crystallography, performed on all three [2]catenanes and two of the three [3]catenanes reveals co-conformational and stereochemical preferences that are stark and pronounced. Both the [3]catenanes crystallize as mixtures of diastereoisomers on account of the axial chirality associated with the picolinium units in the solid state. Dynamic (1)H NMR spectroscopy is employed to probe in solution the relative energy barriers for rotations by the phenylene and pyridinium rings in the tetracationic cyclophane component of the [2]catenanes. Where there are co-conformational changes that are stereochemically "allowed", crown ether circumrotation and rocking processes are also investigated for the relative rates of their occurrence. The outcome is one whereby the three [2]catenanes containing BPP34C10, 1/5NPPP36C10, and 1/5DNP38C10 exist as one major enantiomeric pair of diastereoisomers amongst two, four, and eight diastereoisomeric pairs of enantiomers, respectively. The diastereoisomerism is a consequence of the presence of axial chirality together with helical and/or planar chirality in the same interlocked molecule. These [2]catenanes constitute a rich reserve of new stereochemical types that might be tapped for their switching and mechanical properties.     

Applications of chiral allenylzinc additions and Noyori asymmetric reductions to an enantioselective synthesis of a C3-C13 precursor of the polyketide phosphatase inhibitor cytostatin

A 12-step synthesis of a C3-C13 precursor of the protein phosphatase inhibitor cytostatin is described. Key stereocenters were introduced by a chiral allenylzinc addition and Noyori asymmetric-transfer hydrogenation.     

A Mild and Convenient Oxidation of Aryl Nitriles to Aryl Amides by Aqueous Sodium Perborate

A series of benzonitriles has been reacted with sodium perborate in water and a cosolvent to produce the corresponding amides in good yield. The procedure is a mild and convenient alternative for the synthesis of aryl amides.     

A highly efficient, asymmetric synthesis of blastidic acid: the β-amino acid component of the antibiotic, (+)-blasticidin S

A new approach for the asymmetric synthesis of blastidic acid, the β-amino acid component of the antibiotic, (+)-blasticidin S has been achieved in 11 steps and 30% overall yield. The key transformations involve a one-pot Swern/Wittig reaction sequence to prepare the carbon backbone followed by a stereoselective conjugate addition to introduce the β-amino functionality.     

Qualitative and Quantitative Analysis of Amygdalin Using NMR Spectroscopy

Abstract

NMR analytical procedures are described for the qualitative and quantitative determination of amygdalin. The spectrum is very specific for amygdalin, particularly the sugar region. The optical nature may be conveniently ascertained from the chemical shifts of the aglycone methine hydrogens and the relative optical purity determined by comparing the corresponding peak areas. Satisfactory analyses of amygdalin-containing tablets have been carried out with a precision of 1.2% and accuracy of less than 1%.