Synthetic methods ring contraction

Other examples of ring contractions are given in Table 10. They fall into three main classes. The total loss of one or two (or sometimes more) ring members from the heterocycle, concerted with or followed by formation of a new ring, is a versatile synthetic method. Loss of N2, CO, C02, S, SO, S02, H2C = CH2, etc. is common. 

This section will not be concerned with the detailed description of the synthetic methods leading to the appropriate precursors we will limit our attention to the crucial step of the synthesis of corrinoids, i.e. the formation of the tetrapyrrolic ring. The corrinoid macrocycle has been synthesized following two different procedures the first one involves the cyclization of a proper linear precursor, while the second involves ring contraction of a porphyrinoid structure. 

In summary, the method described here provides an efficient and convenient route to a variety of a-diazo ketones including unsaturated derivatives that were not previously available by diazo transfer. a-Diazo ketones serve as key intermediates in a number of important synthetic methods including the Arndt-Eistert homologation, the photo-Woltf ring contraction strategy, and the carbenoid-mediated cyclopropanation reaction. We anticipate that improved access to a-diazo ketones will serve to enhance the utility of these valuable synthetic strategies. 

Three highly useful synthetic transformations are presented in this section the synthesis of isoflavones from chalcones, the synthesis of a-arylalkanones fmm arylalkenes, and the synthesis of a-arylalkanoic acids from aryl ketones. Two others are potentially useful methods, but are not as yet widely used the preparation of a-branched carboxylic acids from a ynes, and the ring expansion and ring contraction of cyclic alkenes and ketones. 

Such synthetic approaches can only be valid if the other heterocycles are readily available, or if their transformations lead to imidazoles difficult to make by other means. It is certainly important to be able to aromatize imidazolines since a number of ring-synthetic procedures lead to reduced imidazoles. 4-Aminoisoxazoles are sources of a-acylaminoenaminones which cyciize with bases to give 4-acylimidazoles. Oxazole-imidazole conversion has largely historical importance, but it is also implicated in some ring-synthetic procedures (e.g. the Bredereck method, see Chapter 5). Transformations of benzofuroxans into 2-substituted benzimidazole iV-oxides have some synthetic importance. Few, if any, ring contractions appear to have major application. 

An account of a method for the homologation of methyl esters, catalysed by a ruthenium-cobalt bimetallic system, may have some future synthetic potential. Six- and seven-membered heterocyclic enamino-esters (74) undergo ring-contraction to give five- and six-membered azacyclic esters (75) in 89-92% yield (Scheme 40). The reactions of dianions derived from carboxylic acids and esters have been reviewed. ... 

The classical Fischer indole synthesis has persisted as a commonly used procedure due to its working simplicity. However, this method suffers some drawbacks, such as the need for strong acids, poor selectivity with unsymmetrical ketones, and poor functional group tolerance. Huang et al. developed an alternative synthetic strategy for the synthesis of unprotected indoles a tandem ortho-C-H activation and annulation by using triazenyl arenes and alkynes. This reaction involves a ring contraction by a 1,2-Rh shift and an N=N insertion (Eq. (5.12))... 

The general synthetic methods for [2.2] CPs have been already estabhshed. Most of them employ the ring contraction reactions of the 2,11-diheteroatom-substituted [3.3]CPs. Photochemical ehmination of sulfur or selenium atoms from [3.3] CP-2,11-disulfide or diselenide and flash vacuum pyrolysis of sulfur dioxide obtained by the oxidation of the corresponding sulfides are the most general methods. 2,11-Diaza[3.3]CPs are converted to the corresponding [2.2]CPs via their nitroso derivatives by reductive elimination of nitrogen. " Photoextrusion reactions of COj from cychc diesters can also be appHed to the synthesis of [2.2]paracyclophanes (PCPs)"- and [2.2]heterophanes, as described below. "... 

The synthetic pathways leading to tetradehydrocorrins and isobacteriochlorins are very similar and it is just by fine variations of the reaction conditions that the preparation is driven towards specific tetrapyrrolic rings. The linear precursors have been synthesized using the sulfide contraction method (also indicated by other authors as sulfur extrusion ). The corrin skeleton is formed by alkaline hydrolysis of the cyano protecting group present at the 19 position and subsequent acid catalyzed coupling of pyrroles A and D, as described in Fig. 26. 

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