We then investigated this elimination for the -substituted methoxylated products 2e-2j. Stereoisomers could be separated by chromatography and configurations were determined by H NOESY (see Supporting Information). Aliphatic derivatives like Boc-Leu-OMe 4h and Boc-Glu-OMe 4j could be synthesized in good yield but moderate E/Z-selectivity. Whereas formation of the (E)-isomer of 4h was preferred due to the bulky isopropyl residue, Glu derivative 4j showed moderate (Z)-selectivity. In contrast, conjugated products like Cbz-Phe-OMe (4f) and Boc-Asp-OMe (4i) were accessed with high (E)-selectivity. In case of Boc-Phe-OMe rapid polymerization was observed, probably due to deprotection of the conjugated enamine. Even milder acids like HNEt3Cl resulted in polymerization. Base-mediated elimination using DBU resulted in product decomposition starting at about 56% conversion (GC-MS analysis) thus demonstrating the lability of the substrate.