Synthesis and Antioxidative Properties of 1,2,3,4-Tetrahydropyridine Derivatives with Different Substituents in 4-Position

Natural products are an excellent source of inspiration for the development of new drugs. Among them, betalains have been extensively studied for their antioxidant properties and potential application as natural food dyes. Herein, we describe the seven-step synthesis of new betalamic acid analogs without carboxy groups in the 2- and 6-position with an overall yield of ~70%. The Folin–Ciocalteu assay was used to determine the antioxidant properties of protected intermediate 21. Additionally, the five-step synthesis of betalamic acid analog 35 with three ester moieties was performed. Using NMR techniques, the stability of the obtained compounds towards oxygen was analyzed.

In the last fifty years, there has been a growing interest in betalains. With few exceptions, plants and fruits of the order Caryophyllales exhibit a range of colors from red/purple to orange/yellow, due to the presence of these hydrophilic pigments. Initially, betalains were classified as anthocyanins. However, it was later discovered that the main enzymes required for the formation of anthocyanins are not present in betalain-producing plants [8,9].
Betalains are nitrogen-containing water-soluble pigments. Their biosynthesis in plants starts with L-tyrosine (4), which is converted into L -3,4-dihydroxyphenylalanine L -DOPA (5). The enzyme tyrosinase was thought to be responsible for the hydroxylation of L -tyrosine [10]. Recently, it has been observed that cytochrome P-450 monooxygenases are also able to catalyze this reaction [11]. Through the action of the enzyme 4,5-DOPA-extradiol dioxygenase, L -DOPA is converted into 4,5-seco-DOPA (7). Spontaneous cyclization of 4,5-seco-DOPA leads to betalamic acid (9), the key intermediate in the biosynthesis of all betalains. Moreover, tyrosinase is also involved in the oxidation In the last fifty years, there has been a growing interest in betalains. With few exceptions, plants and fruits of the order Caryophyllales exhibit a range of colors from red/purple to orange/yellow, due to the presence of these hydrophilic pigments. Initially, betalains were classified as anthocyanins. However, it was later discovered that the main enzymes required for the formation of anthocyanins are not present in betalain-producing plants [8,9].
Betalains are nitrogen-containing water-soluble pigments. Their biosynthesis in plants starts with L-tyrosine (4), which is converted into L-3,4-dihydroxyphenylalanine L-DOPA (5). The enzyme tyrosinase was thought to be responsible for the hydroxylation of Betaxanthins are yellow, regardless of the amino acid or amine condensed with betalamic acid. Betaxanthins have a maximum absorption wavelength of 480 nm, while betacyanins show a maximum absorption wavelength of 536 nm. Additionally, a sugar moiety is linked to one of the phenolic OH moieties in betacyanin's cyclo-DOPA portion [10,12,13].
The majority of betalains employed in biological research are extracted directly from plants by solid-liquid extraction. Maceration of vegetables facilitates the diffusion of the substances. Additional cellular components are released after tissue breakdown, which makes further purification necessary. Although betalains are typically extracted with H 2 O, other solvents such as MeOH and EtOH are frequently added to aid the extraction process. Unfortunately, this approach requires longer extraction time, additional purification procedures, and provides limited yields. As a result, innovative extraction methods were used to increase the efficiency of the isolation process of betalains, such as diffusion extraction, ultrafiltration, reverse osmosis, and cryogenic freezing [25,[29][30][31][32]. Another significant issue encountered during the extraction and purification of these pigments is their chemical instability when exposed to oxygen, acids, bases, light, and heat. These parameters have a considerable impact on the extraction and purification procedures' efficiency [33]. Several strategies for increasing the stability of betalains have been implemented, most notably in the food industry [34].
Betalamic acid (9) is a critical intermediary in the formation of both kinds of betalains. Although two syntheses of this compound have already been reported [35][36][37], the first synthesis developed by Dreiding et al. [35,37] started with chelidamic acid (I). Hydrogenation of 19 in the presence of rhodium on activated alumina afforded an all-cis-configured piperidine derivative, which was converted into the dimethyl ester II upon treatment with methanol and HCl. Oxidation of the secondary alcohol led to the formation of piperidin-4-one III. To avoid overoxidation to the corresponding pyridine derivative, a polymeric carbodiimide was used for the Pfitzner-Moffatt oxidation and the transformation was carefully monitored. For the introduction of the side chain, a fully methyl-protected semicarbazide was employed as the Horner-Wittig reagent. This reagent led to the formation of hydrazone IV as a pure €-configured diastereomer (C=N bond). Dehydrogenation of IV with t-butyl hypochlorite and triethylamine (NEt 3 ) provided dihydropyridine V as a 7:3 mixture of (E)-and (Z)-configured diastereomers. In this case, (E) and (Z) configuration refers to the exocyclic C=C double bond, whereas the C=N double bond is still (E)-configured. Recrystallization from t-butanol provided the pure (E,E)-configured betalamic acid derivative (E,E)-23 (Scheme 1). Betaxanthins are yellow, regardless of the amino acid or amine condensed with betalamic acid. Betaxanthins have a maximum absorption wavelength of 480 nm, while betacyanins show a maximum absorption wavelength of 536 nm. Additionally, a sugar moiety is linked to one of the phenolic OH moieties in betacyanin's cyclo-DOPA portion semicarbazide was employed as the Horner-Wittig reagent. This reagent led to the formation of hydrazone IV as a pure €-configured diastereomer (C=N bond). Dehydrogenation of IV with t-butyl hypochlorite and triethylamine (NEt3) provided dihydropyridine V as a 7:3 mixture of (E)-and (Z)-configured diastereomers. In this case, (E) and (Z) configuration refers to the exocyclic C=C double bond, whereas the C=N double bond is still (E)-configured. Recrystallization from t-butanol provided the pure (E,E)-configured betalamic acid derivative (E,E)-23 (Scheme 1). Scheme 1. Synthesis of betalamic acid derivative 23 according to Dreiding et al. [35,37]. Reagents and reaction conditions were as follows: (a) 1. 5% Rh/Al2O3, H2O, H2 (4 atm In Scheme 2, the second strategy for the synthesis of betalamic acid (9), developed by Bϋchi et al. [36], is displayed. This approach started from benzylnorteleoidine VI obtained by Robinson-Schӧpf condensation. The first reaction includes the protection of the diol by formation of a cyclic ortho ester. Hydrogenolytic cleavage of the N-benzyl protective group provided the secondary amine VII. Reaction of the aminoketone VII with allyl magnesium chloride yielded the tertiary alcohol VIII with high diastereoselectivity. The Scheme 1. Synthesis of betalamic acid derivative 23 according to Dreiding et al. [35,37]. Reagents and reaction conditions were as follows: (a) 1 In Scheme 2, the second strategy for the synthesis of betalamic acid (9), developed by B semicarbazide was employed as the Horner-Wittig reagent. This reagent led to the formation of hydrazone IV as a pure €-configured diastereomer (C=N bond). Dehydrogenation of IV with t-butyl hypochlorite and triethylamine (NEt3) provided dihydropyridine V as a 7:3 mixture of (E)-and (Z)-configured diastereomers. In this case, (E) and (Z) configuration refers to the exocyclic C=C double bond, whereas the C=N double bond is still (E)-configured. Recrystallization from t-butanol provided the pure (E,E)-configured betalamic acid derivative (E,E)-23 (Scheme 1). Scheme 1. Synthesis of betalamic acid derivative 23 according to Dreiding et al. [35,37]. Reagents and reaction conditions were as follows: (a) 1. 5% Rh/Al2O3, H2O, H2 (4 atm In Scheme 2, the second strategy for the synthesis of betalamic acid (9), developed by Bϋchi et al. [36], is displayed. This approach started from benzylnorteleoidine VI obtained by Robinson-Schӧpf condensation. The first reaction includes the protection of the diol by formation of a cyclic ortho ester. Hydrogenolytic cleavage of the N-benzyl protective group provided the secondary amine VII. Reaction of the aminoketone VII with allyl magnesium chloride yielded the tertiary alcohol VIII with high diastereoselectivity. The chi et al. [36], is displayed. This approach started from benzylnorteleoidine VI obtained by Robinson-Schöpf condensation. The first reaction includes the protection of the diol by formation of a cyclic ortho ester. Hydrogenolytic cleavage of the N-benzyl protective group provided the secondary amine VII. Reaction of the aminoketone VII with allyl magnesium chloride yielded the tertiary alcohol VIII with high diastereoselectivity. The secondary amine VIII was then converted into O-benzoylhydroxylamine IX. First, amine IX was neutralized with K 2 CO 3 and reacted with dibenzoyl peroxide in DMF, leading to formation of the protected amine. Subsequently, acetylation of the alcohol provided O-benzoylhydroxylamine IX. Next, the ortho ester was cleaved with oxalic acid in water to obtain diol X. This latter compound was then oxidized with N-chlorosuccinimide (NCS) and dimethylsulfide to achieve the diketone XI. Ozonolysis of XI led to the formation of aldehyde XII. Treatment of XII with lead tetraacetate in benzene and methanol converted the diketone moiety into an unstable dicarboxylic acid, which, upon loss of HOAc and BzOH, yielded (±)-betalamic acid (9) as a mixture of (E)-and (Z)-configured diastereomers after silica gel chromatography [36].
Despite the fact that two methods for the synthesis of betalamic acid (9) and its derivatives have been reported in the literature, the majority of betalamic acid (9) is produced through extraction from pigments, followed by basic hydrolysis.
To investigate relationships between the chemical structure and biological properties of indicaxanthin derivatives in further detail, analogs 13 of 12 that lack the two carboxy moieties in positions C-2-and C-6 were considered first. Herein, we describe the design and synthesis of betalamic acid analog 13 that is devoid of carboxy groups in positions C-2 and C-6. Additionally, experiments were conducted to synthesize the betalamic acid derivative 14 in a simpler and more cost-effective manner and to evaluate its reactivity toward oxygen (Scheme 3).
benzoylhydroxylamine IX. Next, the ortho ester was cleaved with oxalic acid in wat obtain diol X. This latter compound was then oxidized with N-chlorosuccinimide (N and dimethylsulfide to achieve the diketone XI. Ozonolysis of XI led to the formatio aldehyde XII. Treatment of XII with lead tetraacetate in benzene and methanol conve the diketone moiety into an unstable dicarboxylic acid, which, upon loss of HOAc BzOH, yielded (±)-betalamic acid (9) as a mixture of (E)-and (Z)-configured diastereo after silica gel chromatography [36]. Despite the fact that two methods for the synthesis of betalamic acid (9) and it rivatives have been reported in the literature, the majority of betalamic acid (9) is duced through extraction from pigments, followed by basic hydrolysis.
To investigate relationships between the chemical structure and biological prope of indicaxanthin derivatives in further detail, analogs 13 of 12 that lack the two car moieties in positions C-2-and C-6 were considered first. Herein, we describe the de and synthesis of betalamic acid analog 13 that is devoid of carboxy groups in position 2 and C-6. Additionally, experiments were conducted to synthesize the betalamic derivative 14 in a simpler and more cost-effective manner and to evaluate its react toward oxygen (Scheme 3).  In Scheme 2, the second strategy for the s Bϋchi et al. [36], is displayed. This approach st by Robinson-Schӧpf condensation. The first re formation of a cyclic ortho ester. Hydrogeno group provided the secondary amine VII. R magnesium chloride yielded the tertiary alco chi et al. [36]. Reagents and reaction conditions were as follows: (a) 1. CF 3 COOH, DMF, (MeO) 3  Despite the fact that two methods for the synthesis of betalamic acid (9) and its derivatives have been reported in the literature, the majority of betalamic acid (9) is produced through extraction from pigments, followed by basic hydrolysis.
To investigate relationships between the chemical structure and biological properties of indicaxanthin derivatives in further detail, analogs 13 of 12 that lack the two carboxy moieties in positions C-2-and C-6 were considered first. Herein, we describe the design and synthesis of betalamic acid analog 13 that is devoid of carboxy groups in positions C-2 and C-6. Additionally, experiments were conducted to synthesize the betalamic acid derivative 14 in a simpler and more cost-effective manner and to evaluate its reactivity toward oxygen (Scheme 3).

Chemistry
The plan for the synthesis of 13, the analog of betalamic acid without carboxy groups in positions C-2-and C-6, is outlined in Scheme 4.

Chemistry
The plan for the synthesis of 13, the analog of betalamic acid without carboxy groups in positions C-2-and C-6, is outlined in Scheme 4. We planned to synthesize 13 from the α,β-unsaturated ester 15 that bears a Boc-protective group at the piperidine ring. At first, the ester must be reduced to afford an aldehyde and finally, the Boc-protective group must be removed. The α,β-unsaturated ester 15 can be obtained by a Wittig reaction of α-bromoketone 17 and the subsequent β-elimination of 16. The α-bromoketone should be available by α-bromination of an appropriate piperidone derivative, e.g., 18.
The synthesis started with piperidine 19 (Scheme 5), which was protected with (Boc)2O to afford Boc-protected piperidone 18. In order to introduce a double bond in po- We planned to synthesize 13 from the α,β-unsaturated ester 15 that bears a Bocprotective group at the piperidine ring. At first, the ester must be reduced to afford an aldehyde and finally, the Boc-protective group must be removed. The α,β-unsaturated ester 15 can be obtained by a Wittig reaction of α-bromoketone 17 and the subsequent β-elimination of 16. The α-bromoketone should be available by α-bromination of an appropriate piperidone derivative, e.g., 18.
The synthesis started with piperidine 19 (Scheme 5), which was protected with (Boc) 2 O to afford Boc-protected piperidone 18. In order to introduce a double bond in positions C-5 and C-6 of the piperidine ring, piperidone 18 was brominated in the α-position using Br 2 and AlCl 3 to generate the α-bromoketone 17 in a 46% yield [38]. The conjugated double bond system is a characteristic feature of the class of betalains. Thus, the first double bond was introduced by a Wittig reaction of the α-bromoketone 17 with Ph 3 P=CHCO 2 Et to give the α,β-unsaturated ester 16 in a 95% yield [39]. Although the formation of (E)/(Z)diastereomers was expected, the 1 H and 13 C NMR spectra reveal only one set of signals, indicating a single diastereomer, presumably (E)-16. LiBr and Li 2 CO 3 were used to induce dehydrobromination (β-elimination), resulting in the formation of completely conjugated compound 15, which was isolated in a 88% yield [40]. The 1 H NMR spectrum of 15 reveals two distinct sets of signals, indicating the presence of (E)-and (Z)-configured esters 15 in the ratio 9:1. Since diastereomeric (E)-and (Z)-configured esters 15 could not be separated by flash column chromatography, the mixture was used to prepare the aldehyde 21. According to the first theory, aldehyde 21 should be obtained directly by the reduction of the ester 15 with DIBAL-H. However, even at −78 • C in toluene, only the primary alcohol 20 was formed and isolated in a 94% yield. Alternatively, the primary alcohol 20 was synthesized by the reduction of the ester 15 with LiAlH 4 . Several methods have been reported in the literature for the oxidation of primary alcohols to aldehydes [41]. A method with broad applicability and high yields is the Dess-Martin periodinane (DMP) oxidation method. Unexpectedly, the oxidation of allyl alcohol 20 with DMP resulted in low yields of the product, which was difficult to purify. Therefore, the alcohol 20 was oxidized via radical oxidation with TEMPO [41] and CuCl to provide the aldehyde 21 in a 76% yield. To obtain the aldehyde 13 as an analog of betalamic acid (9), the Boc-protective group of 21 was removed. Unfortunately, removing the Boc-protective group under typical conditions with F 3 CCO 2 H did not result in the desired aldehyde 13. Several methods were investigated to remove the Boc-protective group from 21 to achieve 13. In the end, a rather unusual method, i.e., heating the Boc-protected compound 21 in a mixture of water and dioxane under neutral conditions [42], was successful. Due to the instability of the secondary amine 13, the isolated yield of 13 was rather low. In particular, condensation and polymerization reactions, as well as oxidation processes, were observed during the purification process. Despite the instability, 1 H and 13 C NMR spectra could be recorded to identify and characterize 13. In addition to betalamic acid analog 13, 1,2,3,4-tetrahydropyridine derivatives 22 and 23 were designed and synthesized (Scheme 6). The reactivity of these 1,2,3,4-tetrahydropyridines 22 and 23 and further analogs towards oxygen should be investigated. The key intermediate for the synthesis of 22 and 23 is 4-methylenepiperidine 24, which can be obtained by double allylation of iminodiacetic acid diester 25 with dichloride 26, as reported by Einhorn et al. [43]. Transformation of the methylene moiety of 24 into a ketone and subsequent introduction of a double bond in the ring result in the formation of 23. The α,β-unsaturated ester 22 can prepared by an additional Wittig reaction of a ketone intermediate. In addition to betalamic acid analog 13, 1,2,3,4-tetrahydropyridine derivatives 22 and 23 were designed and synthesized (Scheme 6). The reactivity of these 1,2,3,4-tetrahydropyridines 22 and 23 and further analogs towards oxygen should be investigated. The key intermediate for the synthesis of 22 and 23 is 4-methylenepiperidine 24, which can be obtained by double allylation of iminodiacetic acid diester 25 with dichloride 26, as reported by Einhorn et al. [43]. Transformation of the methylene moiety of 24 into a ketone and subsequent introduction of a double bond in the ring result in the formation of 23. The α,β-unsaturated ester 22 can prepared by an additional Wittig reaction of a ketone intermediate. For the synthesis of methylenepiperidine 24, the diester 25 and the diiodide 29 were prepared (Scheme 7). The diester HCl salt 28 was obtained by esterification of iminodiacetic acid (27) with SOCl2 in refluxing ethanol. The secondary amine of 27 was protected with Boc2O to afford the carbamate 25 in a 76% yield. The diester 25 was initially treated with dichloride 26, which, however, did not lead to the desired 4-methylenepiperidine 24. For the synthesis of methylenepiperidine 24, the diester 25 and the diiodide 29 were prepared (Scheme 7). The diester HCl salt 28 was obtained by esterification of iminodiacetic acid (27) with SOCl 2 in refluxing ethanol. The secondary amine of 27 was protected with Boc 2 O to afford the carbamate 25 in a 76% yield. The diester 25 was initially treated with dichloride 26, which, however, did not lead to the desired 4-methylenepiperidine 24. To obtain the desired methylenepiperidine 24, the more reactive diiodide 29 should be employed instead of the dichloride 26. Allyl diiodide 29 was freshly prepared by a Finkelstein reaction of commercially available 3-chloro-2-(chloromethyl)prop-1-ene (26) with NaI in acetone [44]. After a reaction time of 16 h in refluxing acetone, the diiodide 29 was obtained in a 99% yield. To obtain the desired methylenepiperidine 24, the more reactive diiodide 29 should be employed instead of the dichloride 26. Allyl diiodide 29 was freshly prepared by a Finkelstein reaction of commercially available 3-chloro-2-(chloromethyl)prop-1-ene (26) with NaI in acetone [44]. After a reaction time of 16 h in refluxing acetone, the diiodide 29 was obtained in a 99% yield. For the double allylation of diester 25, LDA was generated in situ from n-BuLi and i-Pr2NH. Deprotonation of diester 25 with freshly prepared LDA and subsequent treatment with diiodide 29 provided the methylenepiperidine 24 in a 77% yield. The IR and 1 H NMR spectra of piperidine 24 demonstrate the successful synthesis of the piperidine ring. A band at 1655 cm −1 in the IR spectrum originates from the C=C stretching vibration. Two sets of signals can be found in the 1 H NMR spectrum, as illustrated by two singlets for the protons of the exocyclic methylene moiety (R2C=CH2) at 4.83 and 4.92 ppm and two singlets for the Boc group at 1.42 and 1.47 ppm. These signal pairs confirm the formation of trans-and cis-configured diastereomers trans-24 and cis-24, which are present in the ratio 9:1. Lemieux-Johnson oxidation using catalytic amounts of OsO4 and an excess of For the double allylation of diester 25, LDA was generated in situ from n-BuLi and i-Pr 2 NH. Deprotonation of diester 25 with freshly prepared LDA and subsequent treatment with diiodide 29 provided the methylenepiperidine 24 in a 77% yield. The IR and 1 H NMR spectra of piperidine 24 demonstrate the successful synthesis of the piperidine ring. A band at 1655 cm −1 in the IR spectrum originates from the C=C stretching vibration. Two sets of signals can be found in the 1 H NMR spectrum, as illustrated by two singlets for the protons of the exocyclic methylene moiety (R 2 C=CH 2 ) at 4.83 and 4.92 ppm and two singlets for the Boc group at 1.42 and 1.47 ppm. These signal pairs confirm the formation of transand cis-configured diastereomers trans-24 and cis-24, which are present in the ratio 9:1. Lemieux-Johnson oxidation using catalytic amounts of OsO 4 and an excess of NaIO 4 transformed the 4-methylenepiperidine 24 into piperidinone 30 [45]. Despite the fact that compound 24 was used as a mixture of diastereomers, only one diastereomer could be observed for compound 30. The subsequent Wittig reaction of ketone 30 provided the α,β-unsaturated ester 31, which shows an even higher structural similarity to betalamic acid than methylenepiperidine 24 and piperidinone 30 (Scheme 8).
For the double allylation of diester 25, LDA was generated in situ from n-BuLi and i-Pr2NH. Deprotonation of diester 25 with freshly prepared LDA and subsequent treatment with diiodide 29 provided the methylenepiperidine 24 in a 77% yield. The IR and 1 H NMR spectra of piperidine 24 demonstrate the successful synthesis of the piperidine ring. A band at 1655 cm −1 in the IR spectrum originates from the C=C stretching vibration. Two sets of signals can be found in the 1 H NMR spectrum, as illustrated by two singlets for the protons of the exocyclic methylene moiety (R2C=CH2) at 4.83 and 4.92 ppm and two singlets for the Boc group at 1.42 and 1.47 ppm. These signal pairs confirm the formation of trans-and cis-configured diastereomers trans-24 and cis-24, which are present in the ratio 9:1. Lemieux-Johnson oxidation using catalytic amounts of OsO4 and an excess of NaIO4 transformed the 4-methylenepiperidine 24 into piperidinone 30 [45]. Despite the fact that compound 24 was used as a mixture of diastereomers, only one diastereomer could be observed for compound 30. The subsequent Wittig reaction of ketone 30 provided the α,β-unsaturated ester 31, which shows an even higher structural similarity to betalamic acid than methylenepiperidine 24 and piperidinone 30 (Scheme 8). Since the piperidines 24, 30, and 31 do not contain a halogen atom for elimination, another method for the introduction of a double bond into the piperidine ring was required. For this purpose, the Boc-protective group was removed, yielding the secondary amines 32, 33 and 34. The secondary amines 32-34 were reacted with in situ prepared t-BuOCl followed by base-induced HCl elimination, according to the method reported by Zhong et al. [46] (Scheme 9). For compound 32, isolation of the expected product 35 was not possible due to the fast oxidation to its pyridine form 38, isolated in a 6% yield. For compound 33, the formation of the conjugate system was successful, leading to the desired product 36 in a 39% yield. For this product, we did not observe the formation of the pyridine form 39. With compound 34, the conjugate derivative 37 was obtained. Although, a slow conversion to the pyridine form 40 was observed.

Antioxidant Activity and Stability
Due to the instability of aldehyde 13, we decided to evaluate the total antioxidant activity (TAC) of the protected aldehyde 21. For this purpose, the Folin-Ciocalteu assay was employed [47]. This method can be classified among the protocols used to evaluate the TAC in the electron transfer (ET) group [48]. Reduction of the oxidant leads to a change in its properties, such as light absorption or fluorescence, which are measured using spectroscopy techniques [49]. In the Folin-Ciocalteu assay, a molybdotungstophosphate heteropolyanion (3H 2 O-P 2 O 5 -14WO 3 -4MoO 3 -10H 2 O) is used for the oxidation of phenolic compounds in basic solution (carbonate buffer). The reduction leads to a colored product with an absorption maximum (λ max ) at 765 nm. The molibdenum center in the complex is reduced from Mo(VI) to Mo(V) by an e − donated from the antioxidant, leading to a blue solution [49].
Unfortunately, during the test of the protected aldehyde 21 in the Folin-Ciocalteu assay, a change in the color of the solution could not be recorded as reduction of the molybdenum complex did not take place. All information were provided in Supplementary Materials (Page S2).
Since the piperidines 24, 30, and 31 do not contain a halogen atom for elimination, another method for the introduction of a double bond into the piperidine ring was required. For this purpose, the Boc-protective group was removed, yielding the secondary amines 32, 33 and 34. The secondary amines 32-34 were reacted with in situ prepared t-BuOCl followed by base-induced HCl elimination, according to the method reported by Zhong et al. [46] (Scheme 9). For compound 32, isolation of the expected product 35 was not possible due to the fast oxidation to its pyridine form 38, isolated in a 6% yield. For compound 33, the formation of the conjugate system was successful, leading to the desired product 36 in a 39% yield. For this product, we did not observe the formation of the pyridine form 39. With compound 34, the conjugate derivative 37 was obtained. Although, a slow conversion to the pyridine form 40 was observed.

Antioxidant Activity and Stability
Due to the instability of aldehyde 13, we decided to evaluate the total antioxidant activity (TAC) of the protected aldehyde 21. For this purpose, the Folin-Ciocalteu assay was employed [47]. This method can be classified among the protocols used to evaluate the TAC in the electron transfer (ET) group [48]. Reduction of the oxidant leads to a change in its properties, such as light absorption or fluorescence, which are measured using spectroscopy techniques [49]. In the Folin-Ciocalteu assay, a molybdotungstophosphate heteropolyanion (3H2O-P2O5-14WO3-4MoO3-10H2O) is used for the oxidation of phenolic compounds in basic solution (carbonate buffer). The reduction leads to a colored product with an absorption maximum (λmax) at 765 nm. The molibdenum center in the complex is reduced from Mo(VI) to Mo(V) by an edonated from the antioxidant, leading to a blue solution [49].
Unfortunately, during the test of the protected aldehyde 21 in the Folin-Ciocalteu assay, a change in the color of the solution could not be recorded as reduction of the molybdenum complex did not take place. All information were provided in Supplementary Materials (Page S2).