Stereoselective synthesis of substituted cyclic ethers and amines by acid-catalyzed cyclization of vinylsilanes bearing a hydroxy or amino group

摘要

This account summarizes our recent study on the silicon-directed cyclization of vinylsilanes bearing a heteroatom nucleophile (OH, NHZ) via beta-silylcarbenium ion intermediates. In the presence of an acid catalyst, 5-silyl-4-penten-1-ols (1) are cyclized to 2-(silylmethyl)tetrahydrofurans (2) by a stereospecific syn addition of the hydroxy group. (Z)-Vinylsilanes I are more reactive toward cyclization than their E-isomers. Introduction of a substituent to the methylene tether of (Z)-1 enables the stereoselective synthesis of 2,n-disubstituted tetrahydrofurans (n = 3-5). This cyclization also provides a new route to 1,3-dioxanes using herniacetals prepared from (Z)-4-silyl-3-buten-1-ols (13) and chloral. In contrast to the results with alpha-unsubstituted vinylsilanes 1, the acid-catalyzed cyclization of (Z)-5-alkyl-5-silyl-4-penten-1-ols [(Z)-15] gives 2-alkyl-3-silyltetrahydropyrans (16) with high trans-selectivity, while the 1,2-silyl-migrative cyclization of (E)-15 proceeds with low cis-selectivity. Both geometrical isomers of 4-alkyl-4-silyl-3-buten-lols (18) also undergo the stereospecific cyclization to afford 2-alkyl-3-silyltetrahydrofurans (19) with high diastereoselectivity. The 1,2-silyl-migrative cyclization is applicable to the stereoselective synthesis of trisubstituted tetrahydropyrans and tetrahydrofurans. The acid-catalyzed reactions of 4-silyl-4-nonen-1-ols (25) and 3-benzyldimethylsilyl-3-octen-1-ol (26) form tetrahydropyrans 16 and tetrahydrofuran 19c by a stereospecific endo-cyclization. Like a hydroxy group, amino groups protected by an electron-withdrawing group can intramolecularly add to vinylsilanes with the aid of an acid catalyst. This cyclization is valuable for the stereoselective synthesis of pyrrolidines and piperidines. The silylated products, obtained by the above cyclizations, can be converted to the corresponding alcohols by oxidative cleavage of the Si-C bond. [References: 92]