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Preparation for erythro-2-Hydroxy-succinic Acid Derivatives from Malic Esters

As a contribution to the much discussed diastereoselective synthesis of enantiomers of open chain compounds, > 90% erythro-selective branching of malic esters by alkylation of the doubly deprotonated derivative 2 (alkoxide-enolate) with methyl, allyl, and benzyl halides in THF at −78° ([RIGHTWARDS ARROW] 3aa, 3ba, 3bb, 3bc, Table 1) is described. A second alkylation ([RIGHTWARDS ARROW] 4) and addition of 2 to acetone ([RIGHTWARDS ARROW] 5) are also possible. Cyclization of 2 to the enantiomerically pure trans-epoxides 6 is achieved by treatment with iodine. Cuprate opening of 6b furnishes the same product 3ba obtained from the methylation of 2b, establishing the configurational assignment.