To test the hypothesis of an inverse binding mode involving pro-S decarboxylation, we prepared a probe that carries a 13 C atom in the pro-R carboxylate (Scheme 1). While several isotope-labeled aryl malonate substrates have been reported to date, none of the syntheses were applicable to the corresponding alkenyl derivatives. To this end, we devised an auxiliary-based asymmetric route that was inspired by the work of Bixa et al. using a chiral imidazolidinone to enable stereoselective bis-alkylation of malonates. The chiral auxiliary 4, which is readily accessible in a single step from urea and (1R,2S)-(-)-ephedrine hydrochloride, was acylated with commercial 1-[ 13 C]-acetyl chloride (99 atom% 13 C) to give 5 in excellent yield. Deprotonation and quenching with methyl chloroformate furnished the 13 Clabeled malonate backbone 6. The first alkylation using ICH2CH2OTBS (obtained in 85% yield over two steps from 2-bromoethanol) as electrophile afforded 7 in 62% yield (over two steps) as a mixture of diastereomers (d.r. = 56:44). Subsequent deprotonation and methylation gave rise to the desired quaternary stereogenic center in excellent diastereoselectivity (d.r. > 99:1). The absolute configuration of 8 was unambiguously determined by single crystal X-ray diffractometry of an unlabeled sample that was prepared under identical conditions. Cleavage of the TBS protecting group under mild conditions using PPTS was crucial to avoid lactonization and afforded 9 in quantitative yield. Subsequent Grieco elimination was found to be superior among all tested protocols to install the vinyl group of 11 in 76% yield (over two steps). Methanolate-induced cleavage of the auxiliary and final saponification of the ester groups afforded the 13 C-labeled probe 2a in good overall yield.