Surprising Structural Lability of a Cysteine‐ S ‐Conjugate Precursor of 4‐Methyl‐4‐sulfanylpentan‐2‐one, a Varietal Aroma in Wine of Vitis vinifera L. cv. Sauvignon Blanc

4‐Methyl‐4‐sulfanylpentan‐2‐one ( 1 ; 4MSP) provides a characteristic aroma compound of wines made from Vitis vinifera L. cv. Sauvignon blanc. 4MSP has a strong box‐tree odor with a very low perception threshold and is derived from the cysteinylated precursor S ‐(1,1‐dimethyl‐3‐oxobutyl)cysteine ( 4 ; P‐4MSP). P‐4MSP is transformed into 4MSP during alcoholic fermentation and is an excellent marker of varietal aroma potential. An improved synthesis of P‐4MSP as well as of its deuterium‐labeled analogue [D 6 ]‐P‐4MSP is described. Several analytical methods (NMR, IR, LSI‐MS, GC/MS, ESI‐MS n ) were combined to elucidate spontaneous reversible structural changes of P‐4MSP at different pH values. At low pH, P‐4MSP has a linear keto form. The keto–enol tautomerism was observed at neutral pH. At pH 8, the formation of N ‐substituted intramolecular hemiaminal was characterized by ESI‐MS and ESI‐MS n experiments. The hemiaminal loses H 2 O at high pH to produce a cycloimine, which is easily opened by acid hydrolysis. The keto–enol tautomerism explained the incorporation of only six D‐atoms during the preparation of the P‐4MSP deuterated standard even if [D 10 ]mesityl oxide was used. Derivatization conditions for GC/MS analysis strongly affected the ratio of the monosilylated intramolecular cyclic form and the disilylated linear form of P‐4MSP. The structural changes of P‐4MSP may have a considerable impact on the development of methods of measuring varietal aroma potential.