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
1997; American Chemical Society; Volume: 62; Issue: 6 Linguagem: Inglês
10.1021/jo9619387
ISSN1520-6904
AutoresPothapragada S. K. Prabhakar Ganesh, Kenneth M. Nicholas,
Tópico(s)Alkaloids: synthesis and pharmacology
ResumoToward 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 ( 95% de), and enantioselective (>96% ee) reactions, producing 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.
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