Tetrahydropyrans, ether transfer

Ley et al. previously demonstrated the synthetic value of 2-sulfonylpyrans for the synthesis of 2,6-cis and 2,6-tra 5-tetrahydropyran. These more complex systems resulting from the ether transfer contain oxidation and the 4-position common to many polyketides. Conversion of intermediate sulfone 96 into 4-methoxy-2,6-tra/Js-tetrahydropyran 97 was accomplished by direct ionization of the sulfone in the presence of aluminum trichloride and a nucleophile (Scheme 37.24). ... 

Kartika R, Taylor RE. Electtophrle-induced ether transfer an expedient route to 2-cyano-tetrahydropyrans. Heterocycles 2007 74 447 59. 

Intramolecular cyclization of the chiral oxime ether 993 in the presence of isopropyl iodide and triethylborane affords the 3,4,5-trisubstituted tetrahydropyran-2-one 994 in poor yield but with good diastereoselectivity (Equation 388) <2003JOG5618>. Similarly, a triethylborane-induced atom transfer radical cyclization of 3-butenyl 2-iodoacetate leads to 4-(iodomethyl)tetrahydropyran-2-one. Higher yields are achieved when conducting the reaction at lower concentrations (Equation 389) <2000JA11041 >. 

Attempts to measure kn for the following cyclic acetals and ethers were unsuccessful because of the too low value of kn (below 10 mole"" 1 s at 25 °Q 2-Trichk>ioinethyi l,3 x-olane, 1,3,5-trioxane, tetrahydropyran, and 1,4-dioxane. The non-planar and puckered structure of the last three compounds can hinder a sufficiently close approach to the central atom of the triphenylmethylium cation necessary for hydride transfer to proceed. ... 

A stirred soln. of 4-benzyloxybutanal and trimethylsilylketene in methylene chloride treated at 0° with 0.1 M BFj-etherate in the same solvent until t.l.c. indicated completion of reaction, 2 drops of water added, the organic phase transferred by pipette onto anhydrous Na2S04, solvent replaced by acetonitrile, treated with finely crushed KF 2H2O, and stirred vigorously for 20 min at room temp, to complete desilylation - intermediate P-lactone (Y 81%), treated with 2 eqs. BF3-etherate in methylene chloride at —20° - product (Y 69%). Also tetrahydropyran deriv. s. K.T. Mead, B. Samuel, Tetrahedron Letters 29, 6573-6 (1988). 

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