Lewis acid promoted stereoselective carbon-carbon bond formation of 3-formyl-.DELTA.2-isoxazolines
摘要:
4,5-Disubstituted 3-formyl-DELTA2-isoxazolines undergo the aldol, allylation, and carbonyl ene reactions in the presence of appropriate Lewis acid to give the adducts with an effective 1,3-asymmetric induction. The stereoselectivity of the reaction mainly depends on the nature of the Lewis acid and the relative configuration of the ring. It is remarkable that both diastereomers can be readily prepared stereoselectively. For example, TiCl4 promotes the 1,3-syn-selective aldol reaction over 93/7 of selectivity, while the 1,3-anti adducts are prepared by the reaction catalyzed by BF3.OEt2. This difference in stereoselectivity is to be attributed to the preferable conformation of isoxazoline-Lewis acid complex intermediates, which depends on the nature of Lewis acid. Without the 4-substituent of isoxazolines the selectivity is not observed. The 5-substituent is too far from the formyl carbon to influence the face differentiation of the formyl group. Subsequent treatment of the adducts with LiAlH4 affords 2-amino 1,4-diol derivatives. The protective group of the hydroxyl group on the C(3) side chain is crucial for the stereoselectivity of the reduction. An almost complete diastereoselectivity of the relative configuration at four contiguous stereogenic centers is readily achieved by the reduction of the adducts protected by O-methoxymethyl (O-MOM). Consequently, the present strategy provides a facile method for the preparation of the compounds containing a sequence of several stereocenters.
Diastereoselective Ene Reaction of 3-Formyl-Δ<sup>2</sup>-isoxazolines
作者:Akio Kamimura、Akinori Yamamoto
DOI:10.1246/cl.1990.1991
日期:1990.11
The ene reaction of 3-formyl-Δ2-isoxazolines proceeds smoothly in the presence of appropriate Lewis acid. An efficient 1,3-asymmetric induction takes place to give syn- and anti-homoallyl alcohols in a stereoselective way.
An experimental and theoretical study on stereoselective addition to 3-formyl-Δ2-isoxazolines. Part 1. 1,3-anti-selectivity induced by BF3·OEt2
作者:Akio Kamimura、Akikazu Kakehi、Kenzi Hori
DOI:10.1016/s0040-4020(01)87239-2
日期:1993.8
Experimental and theoretical investigations of the anti-selective nucleophilic addition induced by BF3 to 3-formyl-Δ2-isoxazolines are described. Abinitiomolecularorbital (MO) calculations show that the 3-formyl-Δ2-isoxazoline-BF3 complex prefers s-trans conformation to s-cis conformation. Nucleophiles attack the stable s-trans conformer from the opposite side of C(4) substituent, giving anti-adducts
Diastereoselective Cyclocondensation Reaction of 3-Formyl-2-isoxazolines
作者:Akio Kamimura、Akikazu Kakehi
DOI:10.1246/cl.1992.1133
日期:1992.7
The cyclocondensationreaction of 3-formyl-2-isoxazolines proceeds smoothly under the Lewis acid catalyzed conditions to give dihydropyran derivatives in moderate to high yields. Both diastereoisomers of the dihydropyrans, syn and anti, are prepared stereoselectively in the presence of an appropriate Lewis acid and solvent.