Tungsten-promoted intramolecular alkoxycarbonylation for synthesis of complex oxygenated molecules

Intramolecular alkoxycarbonylation of tungsten-propargyl compounds proceeds with excellent diastereoselectivities to form eta(3)-delta- and -epsilon-lactones but for gamma-lactones. With OSi(t-Bu)Me(2) substituted for an or-hydroxy group, eta(3)-gamma-lactones are stereoselectively formed with syn stereoselection. An optically active tungsten eta(3)-gamma-lactone is prepared from D-(+)-xylose to illustrate the stereochemical effect of OSi(t-Bu)Me(2). All these eta(3)-gamma-, -delta-, and -epsilon-lactones are converted to allyl anions that react in situ with aldehydes and ketones to produce various beta-(hydroxylalkyl)-alpha-methylene-gamma-lactones with good diastereoselectivity. This reaction is also applied to the synthesis of chiral alpha-methylene butyrolactones. Organic carbonyls add to the pi-allyl groups of eta(3)-gamma- and -delta-lactones opposite the tungsten fragment, whereas additions occur from the metal side for eta(3)-epsilon-lactones. The stereochemical courses of these reactions are discussed in detail. These two tungsten-promoted reactions efficiently effect stereoselective transformation of chloroalkynols to complex alpha-methylene-gamma-lactones, which are useful materials for syntheses of trisubstituted 1,3-, 1,4-, and 1-5-diols.