Synthesis of vinyl- and alkynylcyclopentanetetraols by SmI2/Pd(0)-promoted carbohydrate ring-contraction

A variety of vinyl- or alkynyl-substituted polyhydroxylated cyclopentanes and cyclobutanes are prepared in enantiomerically pure form from appropriate carbohydrate precursors, in a direct one-step ring-contraction procedure promoted by SmI2 and catalytic Pd(O). This reaction is thought to proceed through intermediate ring-opened allyl- or allenylsamarium complexes that undergo ring-closure by intramolecular carbonyl addition. A predominant trans relationship is found between vinyl (or alkynyl) and hydroxyl groups at the two newly created stereogenic centers, with good to excellent levels of stereoselectivity being observed in the formation of homopropargyl cyclopentanol products. Under appropriate conditions, preparatively useful yields are realized of stereoisomers not directly available using alternative methodology.