SYNTHESIS OF 2,5-FURANOCYCLES THROUGH INTRAANNULAR CYCLIZATION OF MACROCYCLIC ALLENONES

A new approach to 2,5-bridged furanocyclic compounds was demonstrated for rings of 12-14 members. Accordingly, allenylstannyl aldehydes 1.16, 2.11, 3.12a, 3.12b, 4.12, 5.15, and 6.12, upon treatment with BF3.OEt2 at -78-degrees-C, smoothly cyclized to the homopropargylic alcohols 1.17, 2.12,3.13a, 3.13b, 4.13,5.16, and 6.13 in 87-94% yield. Oxidation and basic isomerization afforded the allenones 1.19, 2.14, 3.15a, 3.15b, 4.14, 5.18, and 6.15 in high yield. Intraannular cyclization to the furanocycles 1.21, 2.15, 3.16a, 3.16b, 4.16, 5.19, and 6.16 was effected with catalytic AgNO3 and CaCO3 in aqueous acetone. Furanocycles 1.21, 2.15, 3.16a, and 3.16b, with an appropriately disposed transannular (Z)-double bond, underwent facile intramolecular Diels-Alder cyclization in over 90% yield. The 12-membered furanocycles 4.16 and 5.19 with a transannular (E) double bond did not cyclize but instead were oxidized by the AgNO3 catalyst to macrocyclic enediones 4.17 and 5.20. These unusual furan reactions are presumably facilitated by ring strain (furan bending) in accord with molecular mechanics calculations.