Five-membered ring systems synthesis

The synthesis of these five-membered ring systems has classically been discussed in terms of the ring system formed. In recent years many synthetic procedures have been classified in terms of the bonds being formed and the position and nature of the heteroatoms involved. Both methods have their advantages, and also their drawbacks. 

Phosphorus heterocyclic compounds, 1, 493-538 five-membered ring systems, 1, 513-523 nomenclature, 1, 496 six-membered ring systems, 1, 497-513 Photoaromatization oxirenes from, 7, 125-126 Photobleaching chromenes in, 3, 880 Photochemical reactions heterocyclic compound synthesis from, 5, 159 reviews, 1, 56 heterocyclic compounds reviews, 1, 71, 72... 

The enormous literature of five-membered ring systems containing sulfur primarily describes the synthesis, properties and chemistry of thiophene and its derivatives279. 

Hydrosilylation of 1,6-dienes accompanied by cyclization giving a five-membered ring system is emerging as a potential route to the synthesis of functionalized carbocycles.81,81a,81b 82 As its asymmetric version, diallylmalonates 86 were treated with trialkylsilane in the presence of a cationic palladium catalyst 88, which is coordinated with a chiral pyridine-oxazoline ligand. As the cyclization-hydrosilylation products, //ww-disubstituted cyclopentanes 87 were obtained with high diastereoselectivity (>95%), whose enantioselectivity ranged between 87% and 90% (Scheme 25).83 83a... 

Cyclization of 1,6-enynes. Pd(OAc)2, particularly when complexed with phosphine ligands, can effect cyclization of 1,6-enynes to a five-membered ring system. This cyclization, l- 2, provides a key step in a synthesis of the sesquiterpene sterepolide (3). ... 

In the second area of interest, the high reactivity of compounds that stems from inherent ring strain in five-membered ring systems has been exploited in the synthesis of mixed diesters of phosphoric acid. These, for example (15), may be obtained by... 

The synthesis of five-membered ring systems can be achieved by a formal [4+1]- or a [2 + 2 + 1]-cycloaddition process essentially depending on the point of view with which one chooses to consider the reaction. Here we discuss the progress which has been made with iron complexes as catalysts for such transformations. 

Tin hydride-mediated radical cyclizations onto a C-0 double bond, coupled with subsequent oxidation, can be applied to ketone synthesis. Fraiser-Reid and co-workers demonstrated that the radical cyclization onto an aldehyde carbonyl group is particularly useful for the construction of cyclohexanol derivatives [35]. Examples shown in Scheme 4-17 well feature the cyclization. As Beckwith s kinetic work predicts (Scheme 4-18) [36], when this method is applied to a five-membered ring system, the rapid ring opening hinders the formation of cyclopen-tanols. Accordingly, only cyclohexanols can be reliably prepared using this approach. 

Cycloaddition reactions involving diradicals, diradicaloids or zwitterions (diyls) have been successfully used for the selective synthesis of five-membered ring systems which are not otherwise readily accessible. 

Intermolecular cycloadditions have only been modestly successful for the synthesis of five-membered ring systems. 

The intermediate in the synthesis of aminoethylethanolamine-derived amphoterics is a heterocyclic imidazoline with an unsaturated five-membered ring system containing an amine and an imine group (Figure 15.13). 

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