Ring contraction in schiff base macrocycles: The X-Ray crystal structure of an imidazolidine-containing macrocycle

Cyclocondensation of benzene-1,3-dicarbaldehyde and 3-azapentane-1,5-diaminein acetonitrile leads to isolation of an 18-membered imidazolidine-containing Schiff base macrocycle rather than the anticipated 24-membered tetraimine Schiff base macrocycle. This has occurred as a consequence of internal nucleophilic addition of the secondary amine functions across adjacent azomethine bonds. The X-ray crystal structure of the imidazolidine-containing Schiff base macrocycle is reported.     

The photoelectron spectra of the oxalyl halides (COX)2, [X = F,Cl and Br]

We have examined the HeI photoelectron spectra of the oxalyl halides (COX)₂, (X = F, Cl, and Br). We assume the presence of the trans-isomer onWe have used semi-empirical calculations (SCF MO), and Koopmans' Theorem, as aids in assigning the spectral bands.     

The use of residual dipolar coupling for conformational analysis of structurally related natural products

Determining the conformational preferences of molecules in solution remains a considerable challenge. Recently, the use of residual dipolar coupling (RDC) analysis has emerged as a key method to address this. Whilst to date the majority of the applications have focused on biomolecules including proteins and DNA, the use of RDCs for studying small molecules is gaining popularity. Having said that, the method continues to develop, and here, we describe an early case study of the quantification of conformer populations in small molecules using RDC analysis. Having been inspired to study conformational preferences by unexpected differences in the NMR spectra and the reactivity of related natural products, we showed that the use of more established techniques was unsatisfactory in explaining the experimental observations. The use of RDCs provided an improved understanding that, following use of methods to quantify conformer populations using RDCs, culminated in a rationalisation of the contrasting diastereoselectivities observed in a ketone reduction reaction. Copyright © 2015 John Wiley & Sons, Ltd.     

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Book reviews

     

The microwave spectrum and centrifugal distortion constants of difluoroborane,HBF2

The microwave spectra of two isotopic species of the unstable molecule difluoroborane have been measured between 8 and 36 GHz. Transitions have been measured up to J = 56 and K_a = 9 for H¹⁰BF₂, and up to J = 66 and K_a = 11 for H¹¹BF₂. Improved values for the rotational constants, boron nuclear quadrupole coupling constants, and quartic and sextic centrifugal distortion constants have been evaluated for both species.     

Thiocyanogen (SCN)2− preparation,ultraviolet photoelectron spectrum and structure

The unstable thiocyanogen molecule, (SCN)₂, has been investigated in the gas phase by ultraviolet photoelection spectroscopy. Semi-empincal and ab intio calculations are used to assist with the orbital assignments and to provide an assessment of the structure A disulphide structure, with a dihedral angle of 87.8° is indicated, similar to those of other well known S₂X₂ molecules.     

An improved protocol for the Prins desymmetrisation of cyclohexa-1,4-dienes

Improved conditions are reported for the Prins-pinacol rearrangement of cyclohexa-1,4-diene-derived acetals. The use of triflic acid gives particularly clean reaction, resulting in a mixture of regioisomers in an approximately 10:1 ratio. A tethered version of this reaction is also reported, giving a tricyclic compound with the same stereochemistry as the core of the cladiellin diterpenes.     

Stereochemical Assignment of the Protein–Protein Interaction Inhibitor JBIR‐22 by Total Synthesis

Recent reports have highlighted the biological activity associated with a subfamily of the tetramic acid class of natural products. Despite the fact that members of this subfamily act as protein–protein interaction inhibitors that are of relevance to proteasome assembly, no synthetic work has been reported. This may be due to the fact that this subfamily contains an unnatural 4,4‐disubstitued glutamic acid, the synthesis of which provides a key challenge. A highly stereoselective route to a masked form of this unnatural amino acid now enabled the synthesis of two of the possible diastereomers of JBIR‐22 and allowed the assignment of its relative and absolute stereochemistry.