Synthesis of oxygen heterocycles

Radical cyclization to triple bonds is used as the key step for the synthesis of oxygen heterocycles. This methodology can benefit from a Lewis acid, such as aluminum fns(2,6-diphenyl phenoxide) (ATPH), which forms a complex with the... 

Synthesis of Oxygen Heterocycles (Excluding Epoxides, Hemiacetals, Lactones and Acetals) as Pheromones... 

The synthesis of oxygen heterocycles in which cyclization onto a pendant alkyne is a key step has also been achieved. Reaction (7.36) shows an example of iodoacetal 29 cyclization at low temperature that afforded the expected furanic derivative in moderate Z selectivity [47]. A nice example of Lewis acid complexation which assists the radical cyclization is given by aluminium tris(2,6-diphenyl phenoxide) (ATPH) [48]. The (3-iodoether 30 can be com-plexed by 2 equiv of ATPH, which has a very important template effect, facilitating the subsequent radical intramolecular addition and orienting the (TMS)3SiH approach from one face. The result is the formation of cyclization products with Z selectivity and in quantitative yield (Reaction 7.37). 

In the 1,4-dinucleophile - 1,2-dielectrophile approach the most common situation is where heteroatoms function as nucleophiles and 1,2-dicarbonyl compounds, 1,2-dihalides or sulfonate esters, a-halocarbonyl compounds, etc., as the dielectrophile component. Representative examples are given in equations (76)-(79). Condensation of hydrazine and its derivatives with 1,4-dicarbonyl compounds is the common situation in the 1,2-dinucleophile- 1,4-dielectrophile approach, and the vast majority of pyridazine derivatives are prepared in this way (e.g equations 80 and 81). Sulfur 1,4-dielectrophiles have been used in related processes, but the method is not readily adaptable to the synthesis of oxygen heterocycles. 

By analogy with the synthesis of oxygen heterocycles, the zinc-ene-allene cychzation was applied to the synthesis of 2,3,5-trisubstituted pyrrolidines with moderate diastereo-selectivity (equation 170)199,200. 

The application of polyketone-derived anions to the synthesis of oxygen heterocycles has much to offer a comprehensive review of this area has been published (77T2159). 

A very few examples of the use of aryl-tin,21 -lead,21 -lithium43 and -magnesium21143,44 derivatives as the source of the aryl group in vinyl substitutions have been reported. Tin derivatives have been used with palladium dichloride bis(benzonitrile) and a copper(II) chloride reoxidant in a regioselective synthesis of oxygen heterocycles from unsaturated alcohols.43... 

The synthesis of oxygen-heterocycles by intermolecular reactions of a-TK with nucleophiles has received only limited attention. However, a-tosyloxy derivatives of several acetophenones have been prepared by brief exposure of neat HTIB/ketone mixtures to microwave (MW) radiation, and employed on mineral oxide surfaces for microwave-assisted syntheses of thiazoles and 2-aroylbenzofurans 40 (Scheme 11) (98JCS(P1)4093, 99JHC1565). 

Synthesis of oxygen heterocycles via alkynyltungsten compounds 01PAC265. 

Intramolecular addition of alcohols across alkenes was established as one of the first examples of this type, and represents a classic approach to the catalytic synthesis of oxygenated heterocycles [12]. A first reaction to this end was described in 1975, with the conversion of 2-allyl phenols 1 to their corresponding dihydrobenzofuranes 2. 

The first use of carbonyl ylides as dipoles in intramolecular cycloadditions has been published and has led to a stereoselective synthesis of oxygen heterocycles. On heating above 170 °C the ene-oxiran (70), readily available by a Darzens reaction, isomerized to the intramolecular cycloadducts (71) and (72). The reaction is most likely initiated by conrotatory opening of the oxiran ring to a carbonyl ylide, and the product then depends on the conformation of the alkenyloxy-group leading to trans-(71) or c/s-(72) annelation (Scheme 8). 

K.C. Majumdar, B. Chattopadhyay, P.K. Majhi, S.K. Chattopadhyay, S. Samanta, Recent development in palladium-mediated synthesis of oxygen heterocycles. Heterocycles 81 (2010) 517-584. 

S.A. Singh, S. Kabiraj, R.P. Khandare,S.P. Nalawade,K.B. Upar,S.V. Bhat,Amberlyst-15-catalyzed efficient cyclization of y-and 6-uiwaturated alcohols green synthesis of oxygen heterocycles, Synth. Commun. 40 (2010) 74-80. 

Scheme 5-208. A domino Wacker-Heck reaction for the synthesis of oxygen heterocycles. EWG Electron withdrawing group. ...

Meyer C, Cossy J. Synthesis of oxygenated heterocycles from cyclic allylsiloxanes using ring-closing olefin metathesis. Tetrahedron Lett. 1997 38 7861-7864. 

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