Five-membered rings, characteristics

The Se NMR chemical shifts of Se-N compounds cover a range of >1500 ppm and the value of the shift is characteristic of the local environment of the selenium atom. As a result, Se NMR spectra can be used to analyse the composition of a complex mixture of Se-N compounds. For example, Se NMR provides a convenient probe for analyzing the decomposition of selenium(IV) diimides RN=Se=NR e.g., R = Bu). By this method it was shown that the major decomposition products are the six-membered ring (SeN Bujs, the five-membered ring Sc3(N Bu)2 and fifteen-membered ring Sc9(N Bu)6 (Figure 3.2 and Section 6.3). 

There is obtained the hydroxytetronic acid L, the simplest analog of ascorbic acid containing the characteristic five-membered ring. Although this compound shows many of the reactions of an ascorbic acid it exhibits no antiscorbutic activity. If instead of using only ethyl benzoyl-... 

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. 

There is good evidence for the presence of a five-membered ring in D-araboascorbic acid (XXXIX). This analog of L-ascorbic acid, prepared by method 2 (page 87) from methyl 2-keto-D-gluconate shows the same chemical properties and the same absorption characteristics as L-ascorbic acid. Titration of XXXIX with diazomethane affords, as in the case of L-ascorbic acid, a 3-methyl derivative LXXXII which upon further action with diazomethane gives rise to the 2,3-dimethyl-D-arabo-ascorbic acid (LXXXIII)3 ... 

Both six-membered pyranose and five-membered furanose structures are encountered, a particular ring size usually being characteristic for any one sugar. Thus, although glucose has the potential to form both six-membered and five-membered rings, an aqueous solution consists almost completely of the six-membered hemiacetal form five-membered rings... 

T FIGURE 10-18 Arachidonic acid and some eicosanoid derivatives. (a) In response to hormonal signals, phospholipase A2 cleaves arachidonic acid-containing membrane phospholipids to release arachidonic acid (arachidonate at pH 7), the precursor to various eicosanoids. (b) These compounds include prostaglandins such as PCE, in which C-8 and C-12 of arachidonate are joined to form the characteristic five-membered ring. In thromboxane A2/ the C-8 and... 

A characteristic feature of a second type of [2.3.3]cyclazines is an odd number of peripheral heteroatoms contributing a pair of electrons to the 7r-system. The synthesis of cyclazines with a heteroatom in the five-membered ring (260) starting from 2,5-diaminoazoles (261) an ethyl 2-cyano-3-ethoxyacrylate has been described (Scheme 41) (76ACS(B)468). 

The basic differences in electron delocalization between the homoaromatic homotropenylium and homocyclopropenium ions and the bicyclo[3.1. Ojhexenyl cations result in fundamentally different reactions of these cations. As was noted earlier, the homotropenylium and homocyclopropenium ions undergo a characteristic ring-inversion process which interconverts the exo and endo substituents on the methylene bridge. With 61 and its derivatives no such reaction occurs. Rather, two different types of thermal isomerization occur. The first of these is the irreversible rearrangement to the cyclo-hexadienyl ions mentioned above. The second thermal isomerization involves a circum-ambulation of the methylene group around the periphery of the five-membered ring,37-143.,45.152. 

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). 

The position of the E.T. band depends in a predictable manner upon the extent of conjugation, the degree of substitution, etc., and may be calculated following rules which are analogous to those available for the prediction of absorption characteristics of conjugated dienes and which are set out in Table 3.9. The base values selected are 215 nm for an enone in an acyclic or six-membered ring system, or 202 nm for an enone system in a five-membered ring, or 207 nm for an a,/ -unsaturated aldehyde. 

Clear theoretical evidence is presented to show that the u-electron systems of benzenoid aromatic molecules are described well in terms of localized, non-orthogonal, singly-occupied orbitals. The characteristic properties of molecules such as benzene or naphthalene arise from a profoundly quantum mechanical phenomenon, namely the mode of coupling of the electron spins, rather than from any supposed delocalization of the orbitals. Other systems considered include azobenzenes, such as pyridine, five-membered rings, such as furan, and inorganic heterocycles, such as borazine ( inorganic benzene ). 

The characteristic hemispheric charge density of the La atom presents a contrast to the Sc feature in Sc C82. The potential for the La atom must be much shallower than for Sc along the fullerene cage. The six- and five-membered rings on the C82 surface can easily be seen. Based on these identifications of the six- and five-membered rings, it is concluded that the cage structure of Sc C82 also has C2V symmetry. 

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