REACTIONS OF COBALT-COMPLEXED ACETYLENIC ALDEHYDES WITH CHIRAL (GAMMA-ALKOXYALLYL)BORANES - ENANTIOSELECTIVE SYNTHESIS OF 3,4-DIOXY 1,5-ENYNES AND STEREOSELECTIVE - ENTRY TO POLYFUNCTIONAL BUILDING-BLOCKS

Toward the goal of developing enantioselective routes to polyfunctiona l building blocks using Co-based technology, the reactions of(Z)- and (E)-(gamma-alkoxyallyl)diisopinocampheylboranes (8, 13) with acetyleni c aldehydes and their -Co-2(CO)(6) complexes (10a-c) have been investi gated. Whereas phenylpropynal reacts with (Z)-8 with poor efficiency ( <10% yield) and moderate enantioselectivity, the corresponding Co deri vatives 10 undergo high-yielding (65-76%), diastereoselective (88->95% de), and enantioselective (>96% ee) reactions, producing syn-(3,4-dio xy 1,5-enyne)Co-2(CO)(6) 11a-f. Similarly, the newly prepared (E)-bora nes 13 also react efficienctly with the complexed aldehydes, affording the corresponding (anti-3,4-dioxy 1,5-enyne)Co-2(CO)(6) complexes 15a -c with virtually complete diastereo- and enantioselectivity, while ph enylpropynal itself reacts inefficiently and with lower enantioselecti vity. Oxidative demetalation of the complexed adducts affords the 3,4- dioxy 1,5-enynes 17 with complete maintenance of stereochemical integr ity. Osmium-catalyzed dihydroxylation of a representative dioxy enyne, i.e., 17c, proceeds with high diastereoselectively (9:1), favoring ad dition anti to the allylic alkoxy group. Although epoxidation of enyne diol 25 by t-BuOOH/VO(acac)(2) occurs stereoselectively, it is accompa nied by epimerization of the diol unit.