Open-chain derivatives,— FIVE MEMBERED RINGS

With cyclic materials in which the CO groups form part of a five-membered ring, the influence of ring strain induces a shift to higher frequencies, similar to that found in cyclic ketones and lactones with five-membered rings. Succinic anhydride [ 1 ], for example, absorbs at 1865 cm" and 1782 cm". In cyclic materials also, conjugation lowers these values to some extent, so that differentiation between an a 3-unsaturated cychc anhydride and a saturated open-chain anhydride is not always possible on the basis of the carbonyl absorptions alone. Typical values for unsaturated cyclic products are 1845 cm" and 1775 cm" for phthalic anhydride [4], and 1848 cm" and 1790 cm" for maleic anhydride [4]. Naphthalene-1 2-dicarboxylic anhydride and some substituted derivatives [7] absorb at 1848—1845 cm" and 1783—1779 cm". In the case of cyclic six-membered ring anhydrides condensed to aromatic systems a much smaller separation... 

Although brominated derivatives of the five-membered heterocycles may be prepared by reactions of the co-ordinated ligands, these may then undergo further reactions with nucleophiles. As an example, the nucleophilic displacement of bromide from 8.15 by sulfide has been used to form new macrocyclic systems (Fig. 8-12). The palladium probably serves a dual function in this reaction. First, it organises the open-chain ligand such that the two reactive sites are held in proximity, so allowing the intramolecular formation of the sulfide and, second, it may activate the pyrrolic ring to nucleophilic displacement of bromide. 

In five-membered a-azido N-heterocycles, the effect of the azole ring is completely in favor of the azide form. All azidoazoles which are known have spectroscopic properties typical of azido derivatives. However, an azidomethine unit as part of the thiazole and 1,3,4-thiadiazole rings may exist in either the cyclized or open-chain form under normal conditions both forms are usually detected (77AHC(2l)323, 75BSB1189). 

If use is made of the five-membered cyclic phosphorus derivatives, the geometry of which is more suitable than that of the triphenylphosphine, formation of the phos-phorane intermediate is more favored and the yield of olefin increases. Deoxygenation with KSeCN in mildly alkaline medium can similarly occur by heteroatom exchange retention is observed as a consequence of double inversion. Open-chain compounds react with good yields, but cyclic oxiranes yields vary considerably with the number of ring atoms, due to steric hindrance. Alkali metal 0,0-diethyl-phosphorotellurates are very useful reagents for the conversion of terminal oxiranes, Eq. 104 but the reaction does work for other types. In the case of oxiranes formally derived from acyclic olefins, the (Z) compounds react more readily than the (E)-oxiranes. ... 

Hydroxyphthalide derivatives have been formed from open-chain precursors via an enaminoketone derived with ammonia in which process either terminal keto system appears open to reaction. Prior formation of the five-membered lactone ring apparently occurs (ref.59). 

In contrast to most open-chain dipoles (see p. 56), the five-membered meso-ionic compounds (328)—(330) react with various diphenylcyclopropenylidene derivatives (231), (331)—(334) by initial 1,2-addition extrusion, and ring expansion (Scheme 47). 

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