2h7 2 methylpropanoicacid

The following
isotopically labeled standards were synthesized as previously reported:
(¹³C₄)-butane-2,3-dione,^{29 (link)} (²H₂)-butanoic acid, (²H₂)-δ-decalactone and (²H₂)-δ-dodecalactone,^{30 (link)} (²H₃)-ethyl butanoate,^{31 (link)} (²H₃)-ethyl hexanoate and (²H₃)-hexanoic acid,^{32 (link)} (²H₂)-3-methylbutanal,^{33 (link)} (²H₂)-3-methylbutanol and (²H₂)-3-methylbutanoic
acid,^{34 (link)} (²H₃)-pentanoic
acid,^{35 (link)} (¹³C₂)-2-phenylethanol,^{36 (link)} (²H₃)-acetic acid, and
(¹³C₂)-2-phenylacetic acid were purchased from
Sigma-Aldrich Chemie, and (²H₇)-2-methylpropanoic
acid was from Merck.
Diethyl ether, sodium carbonate, sodium
chloride, and anhydrous sodium sulfate were purchased from Merck.
Liquid nitrogen was obtained from Linde (Munich, Germany). Diethyl
ether was freshly distilled prior to use. l-Norleucine and
4-methylumbelliferyl butanoate were obtained from Sigma-Aldrich.

The isotopically labeled internal standards used for quantitation
of the aroma compounds in the cheeses were synthesized as described
recently.¹² The following differently
labeled internal standards needed in the precursor studies were synthesized
according to the literature cited: (²H_10,11)-3-methyl-1-butanol^{22 (link)} and (²H₉)-3-methylbutanoic
acid.^{22 (link)} (²H_7–9)-3-Methylbutanal was synthesized from (²H_10,11)-3-methyl-1-butanol by oxidation with Dess-Martin periodinane.^{23 (link)}(²H₃)-Acetic acid
and (¹³C₂)-2-phenylacetic acid were purchased
from Sigma-Aldrich Chemie,
and (²H₇)-2-methylpropanoic acid was from Merck.
(¹³C₆)-l-Leucine was from Cambridge
Isotope Laboratories Inc. (Andover, MA, USA), and (²H₃)-2-Keto-4-methylpentanoic acid was purchased from Sigma-Aldrich
(Taufkirchen, Germany).