REGIOSELECTIVE AND DIASTEREOSELECTIVE ALLENYLATION OF ALDEHYDES IN AQUEOUS-MEDIA - TOTAL SYNTHESIS OF (-GONIOFUFURONE())

The regio- and diastereoselectivities of metal-mediated allenylation o f carbonyl compounds were investigated in aqueous media. Different met al mediators showed varied regioselectivities on product formation dur ing propargylation-allenylation reactions of carbonyl compounds with s imple propargyl bromide. Under the standard reaction conditions, the u se of indium provided the highest regioselectivity, with a preference of formation of the homopropargyl alcohol. The use of tin and bismuth as the metal mediator provided slightly lower selectivities with the s ame preference. The use of zinc and cadmium as the mediators further l owered the product selectivity. The reactions of an aliphatic aldehyde with simple propargyl bromide showed a lower selectivity than the rea ction of an aromatic aldehyde in most cases, except for the use of tin or zinc (where comparable selectivities were observed). On the other hand, the reaction of terminal-substituted propargyl bromides with ald ehydes mediated by indium showed a high regioselectivity in forming al lenylation products. The indium-mediated allenylation of carbonyl comp ounds bearing an alpha-hydroxyl group also proceeded with a high diast ereoselectivity, forming syn-diols predominantly in aqueous ethanol. T he high diastereoselectivity in allenylation of alpha-hydroxyl-substit uted aldehydes was attributed to the chelation effect exhibited by the alpha-hydroxyl substitutent. Through the use of this highly diastereo selective allenylation, (+)-goniofufurone was synthesized from D-glucu rono-6,3-lactone.