Enolate Additions to a Chiral 3-Hydroxypropionate 2,3-Dication Equivalent. Enantioselective Synthesis of β,δ-Dihydroxy Esters

The use of optically active dicarbonyl cyclopentadienyliron(vinyl ether) BF₄ salts, 3 and 4, as enantioselective 3-hydroxypropionate 2,3-dication equivalents is outlined. Complexes 3 and 4 are readily available by exchange etherification of 2, and these have been transformed to enantiomeric dicarbonyl (η-cyclopentadienyliron)(η²-1-methoxypropene) BF₄ 5 and ent-5. Complex 5 has been converted to the corresponding p-methoxybenzyloxy vinyl ether complex 7 by exchange etherification. Condensation of this salt with a number of terminal and nonterminal enolates yields adducts, which are then transformed by redox-promoted alkoxycarbonylation, followed by alcohol deprotection, to optically active 2-methyl-3-hydroxy-5 keto esters. 1,3-Reduction of these ketols can be effected to give either syn- or anti-1,3-diols and thence their related pentanolides.