Regioselectivity growth reactions

Ionic liquids represent a unique class of reaction media for catalytic processes, and their application in catalysis has entered a period of exploding growth. The number of catalytic reactions involving ionic liquids continues to increase rapidly. These liquids offer promising solutions to the problems associated with conventional organic solvents the potential advantages may include enhanced reaction rates, improved chemo- and regioselectivities, and facile separation of products and catalyst recovery. 

The hexasubstituted benzenoid compound (145) is synthesized in one step by the condensation of 2 mol of the dienol silyl ether (146) with 1 mol of methyl orthoacetate and TiCU. The reaction is believed to proceed through the intermediate (147). Compound (147) can be considered as a potential 1,3,5-trielectrophile. The more reactive sites in (147) are at C-5 and at C-3 respectively. Condensation of (147) with a second mole of (146) gives therefore regioselectively the aromatic compound (145). By this method, the plant growth substance sclerin (148) can easily be prepared. 

Radical Cyclization Reactions. Since the first report of a catalytic tin system used in combination with sodium cyanoboro-hydride for trapping radicals from cyclization reactions (eq 53), its use has seen steady growth. Recent examples, including the one discussed previously where unexpected lactone reduction occurred (eq 52), have focused on regioselective (eq 54) and stereoselective (eq 55) cyclizations. It has been postulated that the role of NaBHsCN in these reactions is to maintain a low concentration of tin hydride and thus maintain the turnover of the catalytic cycle. 

Cyclothiazomycin is 1 of 76 structurally distinct actinomycete thio-peptide antibiotics, can inhibit bacterial protein synthesis, and prevent the growth of gram-positive bacteria [38]. In 2011, Deiters and cowor-kers reported their work on the synthesis of the pyridine core of cyclothiazomycin based on ruthenium-catalyzed [2 + 2 + 2] cyclotri-merization as the key step (Scheme 3.17) [39]. The electron-deficient nature of the thiazole-bearing nitrile enables ruthenium catalysis under mild reaction conditions with excellent yields. Complete chemo and regioselectivity in the construction of the trisubstituted pyridine core were achieved by applying a temporary silyl tether. 

Another example using the same methodology is found in the first enantiocontrolled total synthesis of meroterpe-noid brevione C, in which a regioselective 1-endo-trig mode of acyl radical cyclization (91 —> 92) is a key reaction step (Scheme 25.42). Breviones A—E were first isolated from Penicillium brevicompactum and inhibit etiolated wheat coleoptile growth. 

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