CHIRAL 2-OXACYCLOPENTYLIDENE COMPLEXES VIA METAL-ASSISTED CYCLOISOMERIZATION OF CARBOHYDRATE-DERIVED BUTYNOLS

Re action of 2,3-di-O-isopropylidene-D-glyceraldehyde with allenylmagn esium bromide affords a mixture of diastereomeric butynols which under go cycloisomerization upon reaction with pentacarbonyl(tetrahydrofuran )chromium(0) and -tungsten(0) to yield the corresponding 2-oxacyclopen tylidene complexes. These chiral 2-oxacyclopentylidene complexes are m odified into their a-exo-methylene and -benzylidene derivatives, respe ctively. The absolute configurations of the diastereomeric (R)-1',3'-d ioxacyclopentyl)-2-oxacyclopentylidene) chromium(0) and -tungsten(0) c omplexes have been determined by single-crystal X-ray analysis of one of the two diastereomers in each case. First studies on the chromium a lpha-exo-alkylidene 2-oxacyclopentylidene complexes show good to very good diastereoselectivities in the Diels-Alder reaction with 2,3-dimet hylbutadiene.