Reactions of Cobalt-Complexed Acetylenic Aldehydes with Chiral (γ-Alkoxyallyl)boranes: Enantioselective Synthesis of 3,4-Dioxy 1,5-Enynes and Stereoselective Entry to Polyfunctional Building Blocks

Article abstract of DOI:10.1021/jo9619387

Toward the goal of developing enantioselective routes to polyfunctional building blocks using Co-based technology, the reactions of (Z)- and (E)-(γ-alkoxyallyl)diisopinocampheylboranes (8, 13) with acetylenic aldehydes and their -Co₂(CO)₆ complexes (10a-c) have been investigated. Whereas phenylpropynal reacts with (Z)-8 with poor efficiency (< 10percent yield) and moderate enantioselectivity, the corresponding Co derivatives 10 undergo high-yielding (65-76percent), diastereoselective (88- >95percent de), and enantioselective (>96percent ee) reactions, producings syn-(3,4-dioxy 1,5-enyne)Co₂(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)Co₂(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.