With -methoxylated amino acid derivatives in hand, we next focused on the synthesis of dhAA derivatives via elimination of the methoxy group. To access N-Boc-protected dhAAs, we tested different Brønsted-acid catalysts. For less sensitive enecarbamates, elimination can be catalyzed by NH4Cl (17 mol%) at 100-160 °C under solvent-free conditions. Due to the susceptibility of dhAA derivatives to polymerize, 12 we aimed at suppressing this path using a solution of 2. Molecular sieves, NH4Cl and AcOH showed no conversion of 2a upon refluxing in THF. The use of Amberlyst 15 ® or 1 M HCl resulted in polymerization at room temperature while Lewis-acids Ti(OiPr)4 or AlMe3 gave a complex product mixture. Base-induced elimination using KOtBu or DBU was successful but resulted in incomplete conversion even with 3 equiv. of DBU. Finally, we discovered pyridinium para-toluenesulfonate (PPTS) to be a superior catalyst for the elimination, giving a clean reaction with virtually no side-products in the case of methoxylated alanines 2a-2c (Scheme 4). The reactions proceeded in excellent yield without the need for chromatographic purification. A 70 mmol-scale elimination of 2a afforded 4a with virtually quantitative yield (see Supporting Information).