Methylenation, aldonolactones

The addition of organometallic reagents to the carbonyl group of conveniently substituted aldonolactones constitutes a viable chain-extension method. The reaction leads to the formation of hemiacetals of glyculoses, 1-methylene sugars, and C-glycosyl compounds, which are precursors of, or occur as subunits of, a variety of natural products. 

Carbene-mediated methylenation of aldonolactones provides a direct route to 1-methylene sugars, which may be used as intermediates for the preparation of furanoid or pyranoid C-glycosyl compounds, or chiral precursors for the synthesis of natural products. 

The Tebbe reagent [//-chloro-bis(cyclopentadienylXdimethylalumin-ium)-/r-methylenetitanium] in its pure, crystalline (70,71) or crude (72) forms has been employed for the methylenation of aldonolactones. Thus, D-ribono- 1,4-lactone derivatives 71a and 71b reacted with Tebbe s reagent to give (70,71) the exo-methylene compounds 72a and 72b. 

Wilcox, C S, Long, G W, Sub, H S, A new approach to C-glycoside congeners metal carbene-mediated methylenation of aldonolactones. Tetrahedron Lett., 25, 395-398, 1984. 

A paper describing the methylenation of aldonolactones using dicyclopentadienyldimethyl titanium as a more stable alternative to the Tebbe reagent has been presented (Scheme 9). Also described is the use of the zinc-silver/graphite reagent for effecting a Reformatsky-like reaction on aldonolactones with ethyl bromoacetate... 

Petasis reagent was also effective for the methylenation of aldonolactones, 96" and 98."" Petasis olefmation of unsymmetrical oxalates and oxalate monoesters or monoamides 100 under microwave-assisted, provides pyruvate-based enol ethers and enamines 101 in higher yields... 

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