Cyclization Ueno-Stork

Substituted tetrahydrofuran from radical cyclization of 6-bromoolefin. 

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Scheme 4 Ueno-Stork cyclizations mediated by Cp2Zr(H)Cl...

Oshima has demonstrated that the Schwartz reagent Cp2Zr(H)Cl can act as an excellent replacement for Bu3SnH for Ueno-Stork cyclizations (Scheme 4) [17]. 

Scheme 1-56. Ueno-Stork cyclizations affording, after oxidation, the diols 79 and 80.

SCHEME 25.22. The Ueno-Stork cyclization synthesis of (+)-eldanolide. 

Scheme 11.18 Auxiliary control in an Ueno-Stork-type 5-exo-trig cyclization in the synthesis of an enantiomerically pure lactone 60 [77]. a)90 10 mixture of diastereomers. b)>99% ee.

Exo radical cyclizations of bromoacetals that contain allylic chiral centers are often referred to as Ueno-Stork reactions [50]. It was recently discovered that stereochemistry in these cyclizations could also be controlled by the acetal center, as shown in Eq. (13.39) [51]. A chair-like transition state (125) is envisaged where the alkene occupies a pseudoequatorial position leading to predominantly cis product 126. Yields of approximately 70% and cis trans ratios of up to 98 2 are obtained. 

Syntheses of y-lactones using the Ueno-Stork reaction of radical cyclization of haloacetals 04S1903. 

Corminboeuf, Renaud and Schiesser explored the applicability of various quantum methods to the diastereoselective radical cyclization of several bromoacetals during the Ueno-Stork reaction (Scheme 10) [22]. It is interesting to note the performance of the various methods. All of the methods employed correctly predict the anomeric affect (transition states 61, 62 were calculated to lie some 20 kJ mol above 59, 60) and the observed cis stereochemistry [23]. However, only a handful of... 

The Ueno-Stork reaction is a radical cyclization of the a-haloacetal of an aUylic alcohol that provides substituted tetrahydrofuran and y-lactone derivatives. The chiral center of the aUylic moiety can control the stereochemical outcome, and the stereoselectivity may also be affected by the conformational anomeric effect. The Ueno-Stork radical cyclization has been applied in the synthesis of (-l-)-eldanolide (Scheme 25.22)," the pheromone of the male African sugarcane stem borer Eldana saccharina, in which bromoacetal 49 evolves to 50. The stereochemical outcome of these reactions is rationalized by a chair-like transition state, in which the adopted conformation maximizes the anomeric effect of the acetal center. 

Salom-Roig XJ, Denes F, Renaud R Radical cyclization of haloacetals the Ueno-Stork reaction synthesis 2004 (12) 1903-1928. 

Villar F, Equey O, Renaud P. Desymmetrization of l,4-dien-3-ols and related compounds via Ueno-Stork radical cyclizations. Org. Lett. 2000 2(8) 1061-1064. 

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