Hetero Diels−Alder-Biocatalysis Approach for the Synthesis of (<i>S</i>)-3-[2-{(Methylsulfonyl) oxy}ethoxy]-4-(triphenylmethoxy)-1-butanol Methanesulfonate, a Key Intermediate for the Synthesis of the PKC Inhibitor LY333531¹

Abstract

A cost-effective and easily scaled-up process has been developed for the synthesis of (S)-3-[2-{(methylsulfonyl)oxy}ethoxy]-4-(triphenylmethoxy)-1-butanol methanesulfonate, a key intermediate used in the synthesis of a protein kinase C inhibitor drug through a combination of hetero Diels−Alder and biocatalytic reactions. The Diels−Alder reaction between ethyl glyoxylate and butadiene was used to make racemic 2-ethoxycarbonyl-3,6-dihydro-2H-pyran. Treatment of the racemic ester with Bacillus lentus protease resulted in the selective hydrolysis of the R-enantiomer and yielded S-2-ethoxycarbonyl-3,6-dihydro-2H-pyran in excellent optical purity, which was reduced to S-3,6-dihdro-2H-pyran-2-yl methanol. Tritylation of this alcohol, followed by reductive ozonolysis and mesylation afforded the product in 10−15% overall yield and with >99% ee and chemical purity. Details of the process development work done on each step are given.