Elsevier

Tetrahedron

Volume 74, Issue 21, 24 May 2018, Pages 2567-2574
Tetrahedron

Synthesis of apocarotenoids by acyclic cross metathesis and characterization as substrates for human retinaldehyde dehydrogenases

https://doi.org/10.1016/j.tet.2018.03.050Get rights and content

Abstract

A new synthesis of three apocarotenoids, namely 14′-apo-β-carotenal, 12′-apo-β-carotenal and 10′-apo-β-carotenal, has been achieved that is based on the acyclic cross-metathesis of the hexaene derived from retinal and the corresponding partners. These compounds can be enzymatically converted to their carboxylic acids by the human aldehyde dehydrogenases involved in retinaldehyde oxidation. Their kinetic parameters suggest that these enzymes might play a role in the physiological metabolism of apocarotenoids.

Introduction

Carotenoids are a family of natural compounds synthesized by plants, microorganisms and some animals but not by humans.1 Carotenoids are partly responsible for the colour in nature and play a key role both in the photosynthesis process and the photoprotection of the producing organisms. They can be broadly divided into two classes of chemical compounds: carotenes (e.g., β,β-carotene and lycopene) and their oxygenated derivatives termed xanthophylls (e.g., lutein, zeaxanthin and cryptoxanthin). Both carotenes and xanthophylls exhibit relevant physiological functions, serving as antioxidants in lipophilic environments,2 a property that might contribute to the prevention of certain human diseases such as cardiovascular, ocular diseases and cancer.3,4

Within carotenoids, the term apocarotenoids is used to designate those with a backbone of less than 40 carbon atoms.5 Apocarotenoids are formed by the oxidative degradation of one or both termini of carotenoids, a process that is catalyzed by carotenoid cleavage enzymes. The oxidation products derived from dietary β,β-carotene 1 (Fig. 1) can be C20 all-trans-retinal 2 resulting from the central C15single bondC15′ cleavage catalyzed by BCO16,7 or non-symmetrical β-apocarotenoids obtained by eccentric cleavage catalyzed by BCO2.8,9 Carotenoid metabolism appears to be cell compartmentalized,10 as BCO1 is a cytosolic enzyme,11 while BCO2 has been associated with mitochondria.9 BCO2-mediated cleavage is the preferred pathway for xanthophylls.9

The metabolism of β,β-carotene 1 to all-trans-retinal 2 (Fig. 1) promoted by BCO1 is the first step in the production of the natural retinoids (including, but not restricted to, vitamin A, 11-cis-retinal and all-trans-retinoic acid 3) required for eliciting the various functions of these compounds in the human body. Thus, enzymes responsible for isomerization and changes in the oxidation state provide the cognate ligands for receptors implicated in vision, cell proliferation, cell differentiation, immunity and development.4 In particular, the oxidation of all-trans-retinal 2 promoted by aldehyde dehydrogenases (ALDHs)12 generate all-trans-retinoic acid 3, the natural hormone that binds to and activates the retinoic acid receptors (RARs),13 members of the nuclear receptor superfamily of ligand-inducible transcription factors.14

Apocarotenoids (such as 4–11, Fig. 1) have been detected in food and in the blood of animals,15 and the concentration of some of them are similar to that of all-trans-retinoic acid 3. Their biological functions, however, remain unclear.16,17 Recently, it has been proposed that the activity of BCO2-derived metabolites can protect against the damage induced by β,β-carotene 1. This compound is deemed partially responsible for oxidative stress in the mitochondria, a process that can trigger signaling pathways related to cell survival and proliferation.17 Moreover, some apocarotenoids, namely 10′-apo-β-carotenoic acid 10, 12′-apo-β-carotenoic acid 9,17 14′-apo-β-carotenoic acid 8, 14′-apo-β-carotenal 4, and 13′-apo-β-carotenone (not shown), have been described as low-affinity agonists potentially acting as endogenous antagonists of RARs,16,17 and also were found to regulate other functions, such as the placental lipoprotein biosynthesis.18

It is believed that β-apocarotenals can be a potential source of β-apocarotenoic acids, which can be considered as vinylogues of all-trans-retinoic acid 3, through the oxidation of the functional group. However the enzymes involved in these transformations have not been described. Logical candidates are aldehyde dehydrogenases (ALDHs),12 enzymes that transform aldehydes into carboxylic acids. Within the ALDH superfamily, ALDH1A1, ALDH1A2 and ALDH1A3 are closely related enzymatic forms that catalyze the oxidation of retinaldehydes.19 In order to shed some light on the metabolism and biological functions of β-apocarotenoids, here we present an efficient synthetic strategy for the preparation of these molecules and their biological characterization as substrates for human ALDHs.

Carotenoids have been traditionally synthesized following two different strategies.4,20 The first comprises carbonyl condensation reactions with heteroatom-stabilized carbanions, such as Wittig, Horner–Wadsworth–Emmons and Julia reactions, which form Csp2 = Csp2 bonds.21 The second is based on the formation of Csp2–Csp2 bonds by palladium-catalyzed cross-coupling reactions22 (primarily Negishi, Stille, and Suzuki reactions).23 In both cases, appropriate functionalization of the intermediates for the key reaction is required. Recently, the olefin metathesis reaction24, 25, 26 has been established as one of the most general and widely applicable synthetic methods for Csp2 = Csp2 bond formation. Despite the numerous applications of olefin metathesis reactions in the synthesis of natural products,24 its use in the preparation of conjugated polyene chains has been somehow limited because of concerns about the control of site-selectivity, stereoselectivity, and the stability of polyenes to the reaction conditions. The first application on the synthesis of retinoids27 and apocarotenoids starting from carotenoids was described by Wojtkielewicz and coworkers, although the products were obtained in very low yields.28,29 We30,31 and others32 have also contributed to this field with the synthesis of symmetrical and non-symmetrical carotenoids by dimerization and cross-metathesis processes.33

We considered that β-apocarotenoids could also be prepared by cross metathesis of appropriate precursors.33 We envisioned the synthesis of β-apocarotenoids with different lengths of the polyene chain by acyclic cross-metathesis33 starting from a common precursor, the already known hexaene 1230 and the complementary component functionalized as ester.30 More specifically, we undertook the synthesis of the three β-apocarotenoids (10′-apo-β-carotenal 4, 12′-apo-β-carotenal 5 and 14′-apo-β-carotenal 6; 8′-apo-β-carotenal 7 is a commercial compound) that could potentially by formed by eccentric cleavage of β,β-carotene 1 at the C9’ = C10′, C11’ = C12′ and C13’ = C14′ double bonds.4 Only the products of cleavage at the C9’ = C10′ bond have been characterized in mice.7 However, since β-apocarotenoids can potentially be formed from β,β-carotene 1 by non-enzymatic autoxidation processes, their availability by synthesis can provide useful tools for biochemical research. A general method for oxidation of β,β-carotene 1 with a mixture of KMnO4 and H2O2, that provided a mixture of three apo-carotenoids (8’-, 10′- and 12′-apo), was reported half a century ago.34

Section snippets

Synthesis of 14′-apo-β-carotenal 4

The synthesis of 14′-apo-β-carotenal 4 started with the cross metathesis33 of previously described hexaene 1230 and commercial butyl (E)-but-2-enoate 13. Four different ruthenium catalysts were tested with toluene as solvent based on the results described by Wojtkielewicz and coworkers29 and special attention was paid to the reaction time in order to avoid degradation of the formed polyenes (Scheme 1). The use of Neolyst® as catalyst35 was discouraging as the starting material was fully

Conclusion

We have developed a new stereoselective synthesis of three apocarotenoids (14′-apo-β-carotenal 4, 12′-apo-β-carotenal 5 and 10′-apo-β-carotenal 6) and demonstrated that the olefin metathesis protocol is a valid synthetic method for accessing these conjugated polyenes. Whereas the acyclic cross-metathesis of the common required hexaene was efficiently performed with the shorter enoate and dienoate partners, the chemoselectivity with the longer trienoates is poor due to the presence of several

Experimental section

General experimental procedures. See E.S.I.

Acknowledgments

This work was supported by funds from the Spanish MINECO (SAF2016-77620-R-FEDER, BFU2011-24276 and BIO2016-78057), Xunta de Galicia (Consolidación GRC ED431C 29017/61 from DXPCTSUG; ED-431G/02-FEDER "Unha maneira de facer Europa" to CINBIO, a Galician research center 2016–2019). INBIOMED-FEDER “Unha maneira de facer Europa”). We are indebted to Samuel Gallego and Pablo Fernández for preliminary results in this project and the Centro de Apoio Científico-Tecnolóxico á Investigación (C.A.C.T.I.)

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