The Aldol Reaction for 2,3-η2-Furan Complexes of Osmium(II): Cyclization across C(2) and C(4) To Form a New Heterocycle

The pentaammineosmium(II) complex of 5-methylfuran (2) undergoes stereoselective aldol

reactions with various aldehydes in the presence of a Lewis acid (Sn(OTf)2 or BF3OEt2) to

give 4-acetylated-4,5-dihydrofuran complexes where the aldehyde carbonyl group has been

incorporated into the dihydrofuran nucleus. A detailed analysis of the substitution pattern

and stereochemistry of these products reveals a reaction sequence involvingan aldol reaction

at C(4) of the ?/2-furan followed by nucleophilic displacement at C(2) of the furan oxygen by

the aldol-derived alkoxide. When the parent furan complex is subjected to otherwise identical

reaction conditions, this rearrangement does not occur. Instead, two diastereomers of a

novel bicyclic diacetal are formed.