TETRAHYDROFURAN DERIVATIVES FROM EPOXIDES VIA GROUP-TRANSFER CYCLIZATION OR REDUCTIVE RADICAL CYCLIZATION OF ORGANOTELLURIUM AND ORGANOSELENIUM INTERMEDIATES

Monosubstituted epoxides were regiospecifically ring-opened from the s terically least hindered side by benzenetellurolate and benzeneselenol ate reagents to afford aryl P-hydroxyalkyl tellurides and selenides, r espectively. These materials were O-allylated by treatment with allyli c bromides/sodium hydride in tetrahydrofuran and O-prop-2-ynylated whe n reacted with propargyl bromide/sodizum hydride. On photolysis in ben zene containing 40 mol% of hexabutylditin, the beta-(allyloxy)alkyl ar yl tellurides were found to undergo group transfer cyclization to affo rd 2-substituted 4-[(aryl-telluro)methyl]tetrahydrofurans (cis/trans=1 /3-1/10). The aryl beta-(prop-2-ynyloxy)alkyl tellurides similarly aff orded 2-substituted 4-[(aryltelluro)methylene]tetrahydrofurans with an E/Z-ratio close to unity. The beta-(allyloxy)alkyl aryl selenides and aryl beta-(prop-2-ynyloxy)alkyl selenides failed to undergo group tra nsfer cyclization. In the presence of tributyltin hydride and 2,2'-azo bisisobuty with ronitrile, the former compounds were found to undergo reductive radical cyclization in high yields to afford 2-substituted 4 -methyltetrahydrofurans (cis/trans=1/3-1/10). Aryl beta-(prop-2-ynylox y)alkyl selenides similarly afforded 2-substituted 4-methylenetetrahyd rofurans. 2-Alkoxy-2-(allyloxy)-ethyl phenyl selenides, prepared by al lyloxyselenenation of vinyl ethers, were found to undergo reductive ra dical cyclization to afford 2-alkoxy-4-methyltetrahydrofurans (cis/tra ns=1/3-1/4). The preference for formation of trans-2,4-disubstituted t etrahydrofurans in the group transfer and reductive radical cyclizatio ns was rationalized assuming a chairlike transition state with a prefe rred adoption of a pseudoequatorial position of the 2-substituent. By carrying out the reactions at lower temperatures (ambient or -45 degre es C), using triethylborane as an initiator, it was possible to furthe r increase the trans selectivity in the reductive cyclizations.