THE ALDOL REACTION FOR 2,3-ETA-2-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 stere oselective aldol reactions with various aldehydes in the presence of a Lewis acid (Sn(OTf)(2) or BF3.OEt(2)) to give 4-acetylated-4,5-dihydr ofuran complexes where the aldehyde carbonyl group has been incorporat ed into the dihydrofuran nucleus, A detailed analysis of the substitut ion pattern and stereochemistry of these products reveals a reaction s equence involving an aldol reaction at C(4) of the eta(2)-furan follow ed by nucleophilic displacement at C(2) of the furan oxygen by the ald ol-derived alkoxide. When the parent furan complex is subjected to oth erwise identical reaction conditions, this rearrangement does not occu r, Instead, two diastereomers of a novel bicyclic diacetal are formed.