Chelate-controlled Mukaiyama reactions with chiral β-formyl esters
作者:Hubert Angert、Thomas Kunz、Hans-Ulrich Reissig
DOI:10.1016/0040-4020(92)80019-c
日期:1992.7
Additions of silyl enol ethers, 3, 6, and 8 to chiralβ-formylesters 1 in the presence of TiCl4 provide trans-γ-lactones 4, 7, and 9 in high yield and with good to excellent diastereofacial selectivity. This high trans-preference is due to effective chelate-control involving seven-membered ring 1-TiCl4 complexes.
Ester groups as effective ligands in chelate-controlled additions of cuprates and Grignard reagents to chiral .beta.-formyl esters to form .gamma.-lactones.
Addition of cuprates to chiral methyl beta-formyl carboxylates 1a-1d provided gamma-lactones 2-7 in excellent trans-selectivity. The high diastereofacial selectivity was only obtained employing diethyl ether as solvent while tetrahydrofuran gave inferior results. Similar solvent effects were observed in the additions of various Grignard reagents to 1a, which afforded gamma-lactones 2,3,12, and 13 in moderate trans-selectivity. The best solvent for these reactions was dichloromethane. The 1,3-induction of cuprate additions was studied by using aldehydes 8a-8c. The results obtained were interpreted in terms of chelate-controlled additions with formation of seven-membered ring chelates which involve both carbonyl functions of aldehyde 1 or 8. The function of ester groups as effective ligands of lithium or MgX cations may also be of importance for other stereoselective reactions employing organometallic reagents.