Five-membered ring systems derivatives

Hernadez Daranas et al. have explored the conformations of five-membered ring systems (derivatives of furan), by NMR spectroscopy, molecular dynamic methods (MD) and DFT calculation of J(H,H), J(C.H) and V(C,H) couplings. Couplings were calculated as the functions of pseudorotational phase angles, and compared with the experimental ones. Such an approach allowed identifying ring conformations. 

Ring expansion of five- to six-membered rings such as oxazole —> pyridine derivatives via a Diels-Alder reaction is a well-established procedure. However, the conversion of a six-membered heterocycle into a five-membered ring system has not been exploited to any great extent, and those systems that have been studied usually involve a cationic species. 

Five Membered Ring Systems Pyrroles and Benzo Derivatives... 

Fig. 3.10 Recognition of the DNA minor groove with benzimidazole derivatives, (a) Structure of the dimeric core for Py-benzimi-dazole (Bi), Py-hydroxybenzimidazole (Hz) and Py-imidazopyridine (Ip) (left) in comparison with the five-membered ring system (right). H-bonding surfaces along the recogni-...

The enormous literature of five-membered ring systems containing sulfur primarily describes the synthesis, properties and chemistry of thiophene and its derivatives . 

From the method of preparation of the analogs of L-ascorbic acid and from their close resemblance to L-ascorbic acid in respect to chemical properties and absorption data, all these derivatives were assumed by analogy to have the characteristic unsaturated five-membered ring system already proved to be present in L-ascorbic acid. In two cases, namely that of D-gluco- (XV) and D-araboascorbic acid (XXXIX) the assumption has proved correct. 

A molecular structure, similar to that of hexasilylated benzene derivative 229, was obtained from tetracyclic hexasilylbenzene 230 by Kira, Sakurai and coworkers, where the six silicon centres are incorporated in three five-membered ring systems (Scheme 80) °. In dimetalated compound 231, two lithium centres, coordinated by a quinuclidine ligand each, are capping the phenyl ring plane from both sides in the solid state. Moreover, it could be found that compound 231 has a thermally accessible triplet state, investigated by temperature-dependent ESR spectroscopy °°. 

The characteristic feature of the aforementioned oxazaborolidine catalyst system consists of a-sulfonamide carboxylic acid ligand for boron reagent, where the five-membered ring system seems to be the major structural feature for the active catalyst. Accordingly, tartaric acid-derived chiral (acyloxy)borane (CAB) complexes can also catalyze the asymmetric Diels-Alder reaction of a,P-unsaturated aldehydes with a high level of asymmetric induction [10] (Eq. 8A.4). Similarly, a chiral tartrate-derived dioxaborolidine has been introduced as a catalyst for enantioselective Diels-Alder reaction of 2-bromoacrolein [11] (Eq. 8A.5). 

II. Five-Membered Ring Systems and Their Benzo Derivatives. 218... 

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