Tungsten-Promoted Intramolecular Alkoxycarbonylation for Synthesis of Complex Oxygenated Molecules

Abstract

Intramolecular alkoxycarbonylation of tungsten−propargyl compounds proceeds with excellent diastereoselectivities to form η3-δ- and -ε-lactones but for γ-lactones. With OSi(t-Bu)Me2 substituted for an α-hydroxy group, η3-γ-lactones are stereoselectively formed with syn stereoselection. An optically active tungsten η3-γ-lactone is prepared from d-(+)-xylose to illustrate the stereochemical effect of OSi(t-Bu)Me2. All these η3-γ-, -δ-, and -ε-lactones are converted to allyl anions that react in situ with aldehydes and ketones to produce various β-(hydroxylalkyl)-α-methylene-γ-lactones with good diastereoselectivity. This reaction is also applied to the synthesis of chiral α-methylene butyrolactones. Organic carbonyls add to the π-allyl groups of η3-γ- and -δ-lactones opposite the tungsten fragment, whereas additions occur from the metal side for η3-ε-lactones. The stereochemical courses of these reactions are discussed in detail. These two tungsten-promoted reactions efficiently effect stereoselective transformation of chloroalkynols to complex α-methylene-γ-lactones, which are useful materials for syntheses of trisubstituted 1,3-, 1,4-, and 1-5-diols.