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 polyfunctional building blocks using Co-based technology, the reactions of(Z)- and (E)-(gamma-alkoxyallyl)diisopinocampheylboranes (8, 13) with acetylenic aldehydes and their -Co-2(CO)(6) complexes (10a-c) have been investigated. Whereas phenylpropynal reacts with (Z)-8 with poor efficiency (<10% yield) and moderate enantioselectivity, the corresponding Co derivatives 10 undergo high-yielding (65-76%), diastereoselective (88->95% de), and enantioselective (>96% ee) reactions, producing syn-(3,4-dioxy 1,5-enyne)Co-2(CO)(6) 11a-f. Similarly, the newly prepared (E)-boranes 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 phenylpropynal itself reacts inefficiently and with lower enantioselectivity. Oxidative demetalation of the complexed adducts affords the 3,4-dioxy 1,5-enynes 17 with complete maintenance of stereochemical integrity. Osmium-catalyzed dihydroxylation of a representative dioxy enyne, i.e., 17c, proceeds with high diastereoselectively (9:1), favoring addition anti to the allylic alkoxy group. Although epoxidation of enynediol 25 by t-BuOOH/VO(acac)(2) occurs stereoselectively, it is accompanied by epimerization of the diol unit.