主催: 第58回 天然有機化合物討論会
Acrolein, a highly toxic a,b-unsaturated aldehyde, exists both as ubiquitous pollutants in environment (e.g. in tobacco smoke or exhaust gas) and as endogenous metabolites generated by cells through enzymatic oxidation of polyamines or through reactive oxygen species (ROS)-mediated lipid peroxidation. Acrolein, which is sometimes generated on millimolar scale in cells under oxidative stress, has been reported to be more toxic than ROS molecules (e.g. H2O2, •OH). Considering that acrolein has been used as longstanding key biomarker in numerous oxidative stress-related diseases including cancer and Alzheimer’s, consequently detection of acrolein level in biosystems is becoming of significant importance for defining pathogenesis of the disease and to provide information for the therapeutic and diagnostic remedies.
The conventional analytical method for detection of acrolein, e.g. HPLC analysis after derivatization with 3-aminophenol under harsh reaction conditions, is not suitable for high-throughput assay and frequently provide poor selectivity when other aldehydes are present. Furthermore, while notable, the use of antibodies that can recognize 3-formyl- 3,4-dehydropiperidine (FDP), an acrolein-lysine adducts, is not only costly but also requires procedures that are time-consuming. More critically, the formation of FDP is quite slow, and thus, this method suffers from lack of detection sensitivity in a time-dependent manner. Consequently, developing new analytical tools for acrolein detection that are straightforward, cost-effective, selective, and preferably feasible in live cells remains a highly essential pursuit in the therapeutic treatment of oxidative stress related diseases. Herein we would like to demonstrate a simple but robust method for detecting and imaging acrolein generated by cells in the context of oxidative stress processes or introduced via environmental exposure.
1. Discovery of 1,3-dipolar cycloaddition between phenyl azide and acrolein
Recent advances in Huisgen 1,3-dipolar cyclo- addition between azide and terminal acetylene has led to extensive application in the fields of chemical biology and organic functional materials. The reaction may be accele- rated in the presence of Cu(I) catalyst or by placing the acetylene group within a strained ring. Aside from these “click reactions”, we serendipitously uncovered that phenyl azide can participate in similar 1,3-dipolar cycloaddition with acrolein to produce triazoline and triazole derivatives.
Reaction of phenyl azide with 10 equiv. of acrolein present in THF at millimolar level smoothly gave the heterocyclic products (Fig. 1-i). The reaction is generally complete within 30 minutes at room temperature in the absence of catalyst. After silica gel column chromatography, the products were identified to be 4-formyl-1,2,3-triazoline 2 and 4-formyl-1,2,3-triazole 3 (Fig. 1-i). Interestingly, in the case of 4-formyl-1,2,3-triazoline 2, the double bond isomerized at conjugated position of C4-aldehyde, and decomposition was not observed even over an incubation period of several days. Thus, the thermally labile triazoline cycloadduct was stabilized by this isomerization. Significantly, the 1,3-dipolar cycloaddition between phenyl azide and acrolein is highly chemoselective for acrolein. Under the same conditions, no discernable products were found when phenyl azide was reacted with a- or b- substituted acrolein (e.g. methacrolein, crotonaldehyde, trans-2- octenal) and activated olefins serving as model of lipid metabolites (Fig. 1-ii). Surprisingly, to the best of our knowledge, such inherent reactivity of acrolein towards phenyl
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