General method for synthesizing pyranoid glycals. A new route to allal and gulal derivatives.

[reaction: see text] Pyranoid glycals of all configurations can be obtained from pentoses through an olefination-cyclization-elimination sequence. The elimination can be carried out with excellent yields under radical conditions or by using common reductive reagents such as Zn/Cu, TiCl(4)/LiAlH(4), or lithium naphthalenide. The proposed method is appropriate for the synthesis of glycals with allo or gulo configurations because the cyclization step is more efficient for these substrates.