Five prolinol-based chiral auxilaries have been compared for the stereoselective synthesis of gamma-butyrolactones via the sequence of N-acylation, C(alpha)()-allylation, and iodolactonization under both solution-phase and solid-phase conditions. Comparisons of stereoselectivity of both the C(alpha)()-allylation and iodolactonization processes indicate that incorporation of a non-C(2)-symmetric auxiliary as a polymer cross-link gives results superior to those obtained either in solution or with other non-C(2)-symmetric auxiliaries and comparable to those observed using a polymer-supported pseudo-C(2)-symmetric auxiliary.
The first two polymer-supported versions of the Corey, Bakshi, and Shibata (CBS) catalyst have been prepared. Functional monomers based structurally upon the original B-methylated catalyst have been used to prepare catalytic polymers containing the CBS moiety bound both in a pendant fashion and in the form of a cross-link. Enantioselective reductions of two prochiral ketones have been carried out using the original catalyst in the solution phase as well as the two solid-state systems. While the pendant-bound system shows reduced stereoselectivity, the cross-linked version affords enantioselectivities almost identical to those of the solution-phase model.
Enantioselective syntheses
Enantioselective syntheses O 0031Oxazaborolidines as Functional Monomers: Ketone Reduction Using Polymer--Supported Corey, Bakshi, and Shibata Catalysts. -Two polymer-supported versions of the Corey-Bakshi-Shibata-oxazaborolidine catalyst are prepared and tested as solid-state catalysts in the reduction of ketones (I). While the pendant-bound catalyst (III) leads to reduced stereoselectivity, the crosslinked catalyst affords enantioselectivities almost identical to those of the solution-phase catalyst. -(PRICE, M. D.; SUI, J. K.; KURTH, M. J.; SCHORE*, N. E.; J. Org. Chem. 67 (2002) 23, 8086-8089; Dep. Chem., Univ. Calif., Davis, CA 95616, USA; Eng.) -Klein 16-022
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