Tetrahydrofuran Derivatives from Epoxides via Group Transfer
Cyclization or Reductive Radical Cyclization of Organotellurium
and Organoselenium Intermediates
posted on 1997-01-10, 00:00authored byLars Engman, Vijay Gupta
Monosubstituted epoxides were regiospecifically ring-opened from
the sterically least hindered side
by benzenetellurolate and benzeneselenolate reagents to afford aryl
β-hydroxyalkyl tellurides and
selenides, respectively. These materials were O-allylated by
treatment with allylic bromides/sodium
hydride in tetrahydrofuran and O-prop-2-ynylated when reacted with
propargyl bromide/sodium
hydride. On photolysis in benzene containing 40 mol % of
hexabutylditin, the β-(allyloxy)alkyl
aryl tellurides were found to undergo group transfer cyclization to
afford 2-substituted 4-[(aryltelluro)methyl]tetrahydrofurans
(cis/trans = 1/3−1/10). The aryl
β-(prop-2-ynyloxy)alkyl tellurides
similarly afforded 2-substituted
4-[(aryltelluro)methylene]tetrahydrofurans with an
E/Z-ratio close
to unity. The β-(allyloxy)alkyl aryl selenides and aryl
β-(prop-2-ynyloxy)alkyl selenides failed to
undergo group transfer cyclization. In the presence of tributyltin
hydride and 2,2‘-azobisisobutyronitrile, 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
β-(prop-2-ynyloxy)alkyl selenides similarly afforded 2-substituted
4-methylenetetrahydrofurans.
2-Alkoxy-2-(allyloxy)ethyl phenyl selenides, prepared by allyloxyselenenation of vinyl
ethers, were found to undergo
reductive radical cyclization to afford
2-alkoxy-4-methyltetrahydrofurans (cis/trans =
1/3−1/4). The
preference for formation of trans-2,4-disubstituted
tetrahydrofurans in the group transfer and
reductive radical cyclizations was rationalized assuming a chairlike
transition state with a preferred
adoption of a pseudoequatorial position of the 2-substituent. By
carrying out the reactions at lower
temperatures (ambient or −45 °C), using triethylborane as an
initiator, it was possible to further
increase the trans selectivity in the reductive
cyclizations.