In spite of the fact that several flavors and fragrances are obtained either by chemical synthesis or by extraction from plants, the application of biocatalysis for the sake of a safe and productive pathway by more sustainable chemical processes is the main alternative. The current study focuses on the synthesis of three flavor esters, namely cis‐3‐hexen‐1‐yl acetate, 2‐phenylethyl acetate, and methyl phenylacetate, via transesterification and esterification of the natural corresponding alcohols or acids. The impact of optimizing variables that influence this lipase‐catalyzed synthesis, such as enzyme formulations, solvent‐free media, and acetylating agents, is crucial to achieving higher conversions. The enzymatic transesterification using Novozym 435 afforded cis‐3‐hexen‐1‐yl, and 2‐phenylethyl acetates with high yields (>90%) in green solvents. Similar results were obtained in solvent‐free system, which is more economic for the scaling up of this synthesis. In the case of esterification reactions, the removal of water, formed as a by‐product, with the use of Aquasorb enhanced the conversions as in the case of methyl phenylacetate attained with a conversion of 89% in the presence of Novozym 435. However, this effect was not observed in the larger scale reaction (with 0.85 mol of cis‐3‐hexen‐1‐ol). Instead, the efficient strategy of gradual addition of acetic acid has proven to significantly improve the yield of cis‐3‐hexen‐1‐yl acetate (from 2% up to about 80% in the case of the preparative reaction with 100 mL substrate). Box‐Behnken analysis was also performed to identify the lowest amount of acetylating agent and the shorter time to obtain the highest conversion ratio. This analysis showed that a triacetin/alcohol molar ratio of 1 and 1.75 can be sufficient to obtain a conversion >90% and up to 95%, for cis‐3‐hexen‐1‐yl acetate and 2‐phenylethyl acetate, respectively, similarly to what is obtained with higher triacetin/alcohol molar ratios and comparable reaction time.

Phenolic glycosides occur naturally in many plants and as such are often present in the human diet. Their isolation from natural sources is usually laborious due to their presence in complex matrices. Their chemical and enzymatic syntheses have been found complex, time-consuming, and costly, yielding only small amounts of glycosylated products. In quest of a convenient biocatalytic route to structurally complex phenolic glycosides, we discovered that the rutinosidase from Aspergillus niger not only efficiently converts hydroxylated aromatic acids (e. g. coumaric and ferulic acids) into the respective phenolic rutinosides, but surprisingly also catalyzes the formation of the respective glycosyl esters. We report here the results of a systematic study presenting the unique synthesis of naturally occurring glycosyl esters and phenolic glycosides accomplished by glycosidase catalysis. A panel of aromatic acids was tested as glycosyl acceptors and the crucial structural features required for the formation of glycosyl esters were identified. In the light of the present structure-activity relationship study, a plausible reaction mechanism was proposed. All the products were fully structurally characterized by NMR and MS. Scheme 2. Glycosylation of (E)-p-coumaric acid (3). Scheme 3. Proposed mechanism of rutinosylation of hydroxycinnamic acids at the carboxy moiety.

Biocatalytic approaches have been investigated in order to isolate the enantiomers of Wieland–Miescher ketone (1) and of its alcoholic derivatives (cis‐2 and trans‐3). Specifically, two enzymes from our in‐house metagenomic collection of oxidoreductases, IS2‐SDR and Dm7α‐HSDH, catalyzed the kinetic resolution of the starting racemic ketone 1 or its complete conversion into two diastereomeric products, respectively. Moreover, the kinetic resolution of the racemic cis‐alcohol (2) was very efficiently obtained (E≅2.000) by lipase PS catalyzed acetylation in dry acetone. All the products were isolated with ee≥95 %. Simple chemical elaborations of some of them allowed to isolate the missing enantiomers.