Five-Membered Ring System

Ring stacking, see Organolithium compounds, ring stacking Ring systems five-membered... 

Rhodium carboxylates have been found to be effective catalysts for intramolecular C—H insertion reactions of a-diazo ketones and esters.215 In flexible systems, five-membered rings are formed in preference to six-membered ones. Insertion into methine hydrogen is preferred to a methylene hydrogen. Intramolecular insertion can be competitive with intramolecular addition. Product ratios can to some extent be controlled by the specific rhodium catalyst that is used.216 In the example shown, insertion is the exclusive reaction with Rh2(02CC4F9)4, whereas only addition occurs with Rh2(caprolactamate)4, which indicates that the more electrophilic carbenoids favor insertion. 

A more widely applied approach to modifying the terpyridine system so as to reduce the field has been to replace one or more of the pyridine rings with five-membered rings. The effects of replacement are much more pronounced for the central ring, because of the steric and electronic features mentioned above, and the [Fe N6]2+ derivative of l,3-bis(pyridin-2-yl)pyra-zole 47, for example, is high spin [64]. This effect is revealed further by the drastically different properties of the [Fe N6]2+ derivatives of the isomeric tridentate ligands 2,4-bis(pyridin-2-yl)thiazole 48 (Dq(Ni2+) 1125 cm-1) and 6-(thiazol-2-yl)-2,2/-bipyridine 49 (Dq(Ni2+) 1230 cm-1). 

As already stated, DIOP led the way for a number of ligand systems that were built on a carbon framework containing stereogenic centers. Some of these ligands followed closely on the heels of DIOP, such as ChiraPhos (16) where the chelate ring is five-membered [82, 83]. Even one stereogenic center in the backbone, as in ProPhos (17), provides reasonable selectivity [83, 84]. The main problem with these systems is that of slow reactions. 

Hall has drawn attention to the conformational problems associated with bicyclic fused ring systems, particularly where one or both of the rings is five-membered. Several n.m.r. studies of such systems (and similar systems with more rings) have been described subsequently, and these studies are discussed below. 

The recyclization mechanism circumvents the requirement for an energetically unfavourable pseudorotation step. Where recyclization is entropically favoured, owing to steric constraints, the rate of such a process may be competitive with that for direct nucleophilic substitution on phosphorus in a five-membered ring. In five-membered ring systems in which exocyclic cleavage is observed at an accelerated rate, the involvement of a recyclization process must be considered, especially if pseudorotational processes are energetically disfavoured. 

Not all carbohydrates exist in equilibrium with six-membered hemiacetal rings in several instances the ring is five membered. (Even glucose exists, to a small extent, in equilibrium with five-membered hemiacetal rings.) Because of this variation, a system of nomenclature has been introduced to allow designation of the ring size. 

The common structural feature of steroids is a system of four fused rings. The A, B, and C rings are six membered, and the D ring is five membered, usually all fused in a trans manner. 

Illranose ring A five-membered sugar ring system eontaining an oxygen atom. 

Structures m-1,7-1 and 3 are consistent with the pmr spectrum. Structure m-1, 7-3 is of the basic form III and have two twist, six-membered chelate rings and two, eclipsed five-membered chelate rings. Eclipsed five-membered chelate rings are not common in metal chelate systems. The complex Ni(cyclam)Cl2 is directly analogous to Ni(CTH) and its structure is identical with structures m-1,7-1 or 2 minus (the methyl groups). Therefore, structure m-1,7-1 is assigned to the Ni isomer. 

C-nor-D-homo- or [14(13 2)-abeo] systems. The latter, also referred to as Solanum alkaloids, share a common ABCD steroid skeleton, i.e., six-membered C-rings and five-membered D-rings. The Veratrum alkaloids are further divided into three subgroups (see Figure 1) the jervanine, the veratramine, and the cevanine. The Solanum alkaloids on the other hand, are further divided into the solanidine and verazine subgroups (see Figure 1). 

Three-membered Ring Systems Four-membered Ring Systems Five tnembered Ring Systems Part I Thiophens and their Selenium and Tellurium Analogues Part II Systems containing Nitrogen and Sulphur,... 

Rhodium carboxylates have been found to be effective catalysts for intramolecular C—H insertion reactions of a-diazoketones and esters. In flexible systems, five-membered rings are formed in preference to six-membered ones. Insertion into a methine carbon-hydrogen bond is preferred to insertion at a... 

The ligand effect seems to depend on the substrates. Treatment of the prostaglandin precursor 73 with Pd(Ph3P)4 produces only the 0-allylated product 74. The use of dppe effects a [1,3] rearrangement to produce the cyclopen ta-none 75(55]. Usually a five-membered ring, rather than seven-membered, is predominantly formed. The exceptionally exclusive formation of seven-membered ring compound 77 from 76 is explained by the inductive effect of an oxygen adjacent to the allyl system in the intermediate complex[56]. 

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