Site–site interactions in a polymer matrix: The effect of polymer backbone structure on the transformation of crosslinked resins with substituted hydrazines

Crosslinked chloromethylated polystyrene ( 1 ) and crosslinked copoly(styrene-p-nitro-phenylacrylate) ( 3 ) readily reacted with 1,1-dimethylhydrazine, but the course of the reaction was strongly dependent on the structure of the backbone. Monofunctionalization was observed with chloromethylated polystyrene ( 1 ) giving the 1,1,1-dimethylhydrazinium chloride derivative ( 2 ), while high degree of additional crosslinking took place with crosslinked copoly(styrene-p-nitrophenylacrylate) ( 3 ), and additional crosslinking was also observed in functionalization with N-aminopiperidine and N-aminomorpholine. The additional crosslinking suggested a higher backbone mobility in acrylate beads ( 3 ) compared to chloromethylated polystyrene ( 1 ). The type of transformation and the degree of additional crosslinking also depended on the starting crosslinking of copoly(styrene-p-nitrophenylacrylate) ( 3; 3a , 2% DVB; 3b , 4% DVB; 3c , 10% DVB). Replacement of p-nitrophenol groups in copoly(styrene-p-nitrophenylacrylate) ( 3 ) with hydrazino units resulted in enhanced swelling abilities of the hydrazine derivatives ( 4, 5, 6 ) in methanol, dimethylformamide, and chloroform, while formation of the hydrazinium chloride derivative ( 2 ) from chloromethylated polystyrene ( 1 ) caused enhancement of swelling in methanol but diminished it in toluene. The degree of crosslinking of copoly(styrene-p-nitrophenylacrylate) ( 3 ) also influenced the swelling abilities of 3 and its hydrazino derivatives, being higher with 2% cross-linked resins and lower with 4% and 10% crosslinked resins. © 1996 John Wiley & Sons, Inc.     

An improved synthetic approach to thieno[2,3-d]-1,2,3-thiadiazolecarboxylates via diazotization of aminothiophene derivatives

The common synthetic method for the synthesis of fused 1,2,3-thiadiazoles via diazotization is not generally applicable to aminothiophenes. A substantially improved experimental protocol for the preparation of the title compounds as a potential inducer of systemic acquired resistance in plants is reported based on a novel cyclization mechanism via a Huisgen-White type rearrangement.     

Stereoselective 1,3-Dipolar Cycloadditions to (S)-1-Benzoyl-3-(cyanomethylidene)-5-(methoxycarbonyl)pyrrolidin-2-one

(5S)-1-Benzoyl-3-[(E)-cyanomethylidene]-5-(methoxycarbonyl)pyrrolidin-2-one ( 5 ) was prepared in four steps from L -pyroglutamic acid ( 1 ). 1,3-Dipolar cycloadditions of diazomethane ( 6 ) and 2,4,6-trimethoxybenzonitrile oxide ( 7 ) gave substituted 1,2,7-triazaspiro[4.4]non-1-en-6-one 12 and 1-oxa-2,7-diazaspiro[4,4]non-1-en-6-one 13 in 38 and 20% de, respectively. On the other hand, reaction of 5 with N-phenylbenzonitrile imines 8 and 9 , generated in situ from the corresponding hydrazonoyl chlorides 10 and 11 , respectively, and Et₃N, furnished racemic pyrrolo[3,4-c]pyrazoles 14 and 15 in 61 and 56% de, respectively. Cycloaddition of nitrile oxide 7 , when performed in the presence of Et₃N, led to pyrrolo[3,4-d]isoxazole 16 in 85% de.     

Self-Assembly of Chiral Cyclohexanohemicucurbit[n]urils with Bis(Zn Porphyrin): Size,Shape, and Time-Dependent Binding

In order to investigate the ability of bis(zinc octaethylporphyrin) (bis–ZnOEP) to discriminate cyclohexanohemicucurbit[n]urils (cycHC[n]) of different shapes and sizes, the self-assembly of barrel-shaped chiral cycHC[n] with bis–ZnOEP was studied by various spectroscopic methods (absorption, fluorescence, circular dichroism (CD), and NMR). While the binding of 6-membered cycHC[6] induced a tweezer-like conformation followed by the formation of anti-form of bis–ZnOEP upon further addition of cycHC[6], the interaction of 8-membered cycHC[8] is more complex and proceeds through the featured syn-to-anti conformational change of bis–ZnOEP and further intermolecular self-assembly via multiple noncovalent associations between cycHC[8] and bis–ZnOEP. Whilst bis–porphyrins are known to be effective chemical sensors able to differentiate various guests based on their chirality via induced CD, their ability to sense small differences in the shape and size of relatively large macrocycles, such as chiral cycHC[6] and cycHC[8], is scarcely examined. Both studied complexes exhibited characteristic induced CD signals in the region of porphyrin absorption upon complexation.     

A Screening Protocol for Exploring Loop Length Requirements for the Formation of a Three Cytosine-Cytosine+ Base-Paired i-Motif

DNA sequences containing at least four runs of repetitive cytosines can fold into tetra-helical structures called i-Motifs (iMs). The interest in these DNA secondary structures is increasing due to their therapeutical and technological applications. Still, limited knowledge of their folding requirements is currently available. We developed a novel step-by-step pipeline for the systematic screening of putative iM-forming model sequences. Focusing on structures comprising only three cytosine-cytosine⁺ base pairs, we investigated what the minimal lengths of the loops required for formation of an intra-molecular iM are. Our data indicate that two and three nucleotides are required to connect the strands through the minor and majorgrooves of the iM, respectively. Additionally, they highlight an asymmetric behavior according to the distribution of the cytosines. Specifically, no sequence containing a single cytosine in the first and third run was able to fold into intra-molecular iMs with the same stability of those formed when the first and the third run comprise two cytosines. This knowledge represents a step forward toward the development of prediction tools for the proper identification of biologically functional iMs, as well as for the rational design of these secondary structures as technological devices.