The Mcpba Oxidation of Alkyl Trimethylsilyl Ketene Acetals

Abstract

Recent interest in developing methods for the synthesis of α-hydroxy esters has focused on both the direct oxygenation of ester enolates¹ and the treatment of ester enolates with MoOPh (MoO₅·Py·HMPA).² Based upon our previous observations that enol silyl ethers³ and ketene bis(trimethylsilyl) acetals⁴ react with m-chloroperbenzoic acid (MCPBA) to afford high yields of the corresponding α-hydroxy carbonyl compounds we have attempted to extend the scope of this type of oxidation to include alkyl trimethylsilyl ketene acetals 1. We would like to report here that the treatment of 1 with MCPBA followed by treatment of the crude reaction mixture with triethylammonium fluoride leads to the high yield production of α-hydroxy esters 2. Since 1 are easily obtained from a variety of esters by using the method reported by Ainsworth⁵ the current procedure would appear to be quite general for the production of 2. Some representative examples are given in the Table. In the cases cited yields are uniformly high and compare favorably with those reported from other oxidation procedures. Choice of solvent is crucial for the success of the oxidation. With methylene chloride, protonation of 1 becomes the major path and workup leads to the isolation of esters only. With hexane as solvent, oxidation competes effectively and good yields of Lresult. Attempts to extend the oxidation procedure to include 3 were not successful. Using either methylene chloride or hexane as solvent the sole reaction products were the lactones corresponding to 2. In these instances protonation of & apparently occurs to the exclusion of oxidation. It should also be noted that we did not observe carbon-carbon bond cleavage when Awere treated with MCPBA. The production of Lalong with Ahas been reported by Vedejs in the MCPBA oxidation of le.²