NEW METHODOLOGY FOR THE SYNTHESIS OF FURANS, PYRROLIDINES, AND 1,3-POLYOLS USING ALLYL(CYCLOPENTADIENYL)IRON(II) DICARBONYL COMPLEXES

Synthetic methodology is reported for the preparation of 1,3-polyols, tetrahydrofurans, and pyrrolidines based on the Lewis acid-promoted re action of allyl(cyclopentadienyl)iron dicarbonyl with aldehydes, keton es, and imines. The initial products of these additions are zwitterion ic iron-olefin pi-complexes, which can be further utilized for the con struction of either acyclic (polyol) or heterocyclic (furan, pyrrolidi ne) structures. Methoxide and primary alkoxides add to the internal al kene center of the pi-complex in a regiospecific manner to give (beta- alkoxyalkyl)iron complexes, which upon stirring in methanolic eerie am monium nitrate solution and subsequent reduction with lithium borohydr ide give 1,3,5-triol derivatives. Alternatively, treatment of the zwit terionic ct-complex with potassium tert-butoxide in CH2Cl2 leads to fo rmation of the furan cycloadduct by intramolecular addition of the hom oallylic alkoxide to the cationic pi-complex. The [3 + 2]-cycloadditio n reaction of the allyliron complex with the carbonyl compound can als o be executed, without isolation of the intermediate iron-olefin pi-co mplex, using a Lewis acid catalyst. Similarly, N-tosylimines can be em ployed in the [3 + 2]-process for the synthesis of pyrrolidine esters. Of the catalysts examined, ZnCl2 gives the best results for [3 + 2]-r eactions involving aldehydes or N-tosylimines, while TiCl4 is preferre d for the ketone cycloadditions. The allyliron-carbonyl [3 + 2]-annula tions can be effected in intramolecular fashion to provide multicyclic fused furan rings in a single synthetic operation.