Preparation of 3 Kinds of trans-3-Methyl-γ-Lactones Chiral Flavors

Chiral compounds play an important role in flavor research, among which 3-methyl-γ-lactones with two chiral centers are a key class of spices, mainly used to prepare peach, coconut, vanilla and other flavors. Because of its water solubility, it is also widely used in alcohols and beverages as a food additive. The previous synthetic methods often had defects such as difficult acquisition of starting materials, complicated operation, and poor versatility. The process involved obtaining optically active trans-3-methyl-octanolide, nonanolide, and decanolactone was explored. (E)-3-alkenoic acids starting from the corresponding aliphatic aldehydes and malonic acid were synthesized through Knoevenagel condensation. Subsequently, their methyl esters underwent Sharpless asymmetric dihydroxylation to introduce chiral centers, followed by elimination and conjugate addition steps to yield the desired target compounds. The final structures of all products were confirmed by NMR and GC-MS, and the enantiomeric excess (ee) values of the final products were determined by chiral GC. The enantiomeric excess (ee) values for (3R, 4S)-trans-3-methyl-γ-octanolides and (3S, 4R)-trans-3-methyl-γ-octanolides were 84% and 92%, respectively. For (3R, 4S)-trans-3-methyl-γ-nonanolides and (3S, 4R)-trans-3-methyl-γ-nonanolides, the ee values were 87% and 95%, and for (3R, 4S)-trans-3-methyl-γ-decanolides and (3S, 4R)-trans-3-methyl-γ-decanolides, the values were 83% and 96%. Notably, each enantiomer exhibited distinct odor characteristics. All presented different degrees of coconut aroma, sweet aroma, and burnt aroma. The aroma characteristics of the enantiomers of the three lactone trans-configuration products prepared in this paper all showed significant differences.