Elucidating the Formation Pathway of the Off-Flavor Compound 6-Propylbenzofuran-7-ol

Abstract

In a previous work, we identified 6-propylbenzofuran-7-ol as an off-flavor compound formed from ascorbic acid and (E)-hex-2-enal in a test apple beverage. In this study, we elucidate the pathway by which 6-propylbenzofuran-7-ol formed. Isotope labeling studies revealed that the propyl group of 6-propylbenzofuran-7-ol derives from (E)-hex-2-enal and that 6-propylbenzofuran-7-ol contains carbons 2-6 of ascorbic acid. Two compounds, namely, 2,3-dihydro-6-propylbenzofuran-3,7-diol and 3-(2-furoyl)hexanal, were identified as byproducts of a model reaction of ascorbic acid and (E)-hex-2-enal. Each of these compounds was dissolved in an aqueous solution of citric acid and stored at 60 °C for 1 week. After storage, 6-propylbenzofuran-7-ol was detected from a solution of 2,3-dihydro-6-propylbenzofuran-3,7-diol, but not from 3-(2-furoyl)hexanal. 6-Propylbenzofuran-7-ol was formed by isolating tricyclic hemiacetal lactone derived from the Michael addition of ascorbic acid to (E)-hex-2-enal, mixing the tricyclic hemiacetal lactone with the aqueous solution of citric acid, and applying heat. This confirmed that 6-propylbenzofuran-7-ol was formed via the Michael adduct.

Factors Influencing the Formation of Medicinal Off-Flavor from Ascorbic Acid and α,β-Unsaturated Aldehydes

“…In a previous work, 2,3-dihydro-6-propylbenzofuran-3,7-diol ( 2 ) and 3-(2-furoyl)­hexanal ( 3 ) were identified other than 6-propylbenzofuran-7-ol ( 1 ) from the reaction of ( E )-hex-2-enal with ascorbic acid. As it turned out, there were two routes by which these compounds formed (Figure ) . Given the strong likelihood that different conditions would also change the amounts of all three products, that is, benzofuranol 1 , diol 2 , and aldehyde 3 via this multistep mechanism, we decided to quantitate all three compounds in the model solutions by HPLC.…”

“…The main constituents responsible for the formation of the off-flavor compound in the test apple beverage turned out to be ascorbic acid and ( E )-hex-2-enal. Once this was determined, the possible reaction pathway of 6-propylbenzofuran-7-ol ( 1 ) was elucidated by investigating with isotope-labeled precursors and by isolating the reaction intermediates and byproducts . The investigations revealed that the pathway involved two routes and gave two products other than 6-propylbenzofuran-7-ol ( 1 ), namely, 2,3-dihydro-6-propylbenzofuran-3,7-diol ( 2 ) and 3-(2-furoyl)­hexanal ( 3 ) (Figure ).…”

“…Possible reaction mechanism for the formation of 6-propylbenzofuran-7-ol ( 1 ) from ascorbic acid and ( E )-hex-2-enal …”

“…The compounds identified as products and intermediates of the reaction of ascorbic acid and ( E )-hex-2-enal. 6-Propylbenzofuran-7-ol ( 1 ), 2,3-dihydroxy-6-propylbenzofuran-3,7-diol ( 2 ), 3-(2-furoyl)­hexanal ( 3 ), 1,3,7-trioxa-8-oxo-5,9,12-trihydroxy-10-propyltricyclo-[4.3.2.0 2,6 .0 2,9 ]-dodecane ( 8 ) . Identification procedure: (a) NMR ( 1 H, 13 C, HMQC, and HMBC), (b) HRMS, (c) FT-IR, (d) NMR ( 1 H, 13 C), (e) GC–MS.…”