Electrophilic Aromatic Deuteration of Lignans: Mostly Reliable but

Review pubs.acs.org/JAFC

Electrophilic Aromatic Deuteration of Lignans: Mostly Reliable but Occasionally Aberrant Selectivities Monika Pohjoispaä ̈ and Kristiina Waḧ al̈ a*̈ Laboratory of Organic Chemistry, Department of Chemistry, University of Helsinki, A. I. Virtasen aukio 1, P.O. Box 55, FIN-00014 University of Helsinki, Finland ABSTRACT: Lignans are a ubiquitous group of natural products of plant or mammalian origin. In the human diet, especially in fiber-rich foods, there are measurable amounts of lignans. Lignan intake is associated with a reduced risk of a range of chronic Western diseases, and in studying these compounds and their biological activity, authentic stable isotope labeled analogues are needed. This review summarizes the reported labeling methods and discusses the selectivity and reactivity in the electrophilic aromatic deuteration of lignans where recently a number of unexpected selectivities or nonselectivities have been encountered. KEYWORDS: stable isotope labeling, deuteration, lignan, reactivity, selectivity

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INTRODUCTION Lignans, ubiquitous secondary plant and mammalian metabolites, are included in our daily diet. Being fiber-related polyphenols,1 they are present in considerable concentrations in fiber-rich foods, such as whole grain products, cereals, and seeds and also in fruits, berries, vegetables, and legumes. The highest lignan concentrations have been found in oilseeds such as flaxseed,2 linseed, and sesame seed.3 Numerous studies have been conducted to evaluate the hypothesis that the consumption of a fiber-rich, low-fat diet is linked with a lower risk of breast cancer and other estrogenrelated diseases.4,5 It has been suggested that this may be related to the high intake of fiber-associated plant lignans that are converted to the mammalian lignan enterolactone (Figure 1, 1, R = R′ = 3-OH). In several case-control epidemiological

reported lignan compositions and concentrations in different studies are greatly dependent on the sample pretreatment applied and on the analytical method used, and variations are observable even when using the same sample preparation and analytical methods.15 Thus, it is possible that the contents of lignans and the conjugation patterns of individual lignans may vary within the same species depending on natural variations, such as genetic factors or growth conditions.2,16 It also seems that as the research is increasing, new different precursors are continually found. In many studies only a few lignans have been determined, presumably due to a lack of standards. Synthetic methods for stable isotopically labeled analogues are therefore required. In this paper we review the methods used to synthesize labeled lignans. The deuterium labeling methods utilizing the electrophilic aromatic H/D exchange reactions catalyzed by acids are discussed. We also expand on the observed and reported reactivity and selectivity of these reactions when used to obtain stable isotopically pure labeled compounds, suitable as internal standards. The main focus is on the butyrolactone and tetrahydrofuran type lignans, namely, lignano-9,9′-lactones 1 and 9,9′-epoxylignanes 2, respectively (IUPAC nomenclature) (Figure 1).17

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Figure 1. General structures of lignano-9,9′-lactones 1 and 9,9′epoxylignanes 2.17

STABLE ISOTOPE LABELED COMPOUNDS The interest in stable isotope labeled compounds has resulted from the wide applications of mass spectrometry as a specific detection tool in biomedical, pharmacological, and environmental analysis. In addition to metabolic18−20 and analytical studies,21−23 isotopically labeled analogues are required for reaction mechanistic studies24 and structural elucidation.25 The most common techniques for the determination of lignan contents are liquid chromatography (LC) and gas

studies, an inverse correlation between serum and urine enterolactone concentrations and breast cancer risk has been found, whereas in some other studies no correlation was found or the correlation between enterolactone and breast cancer risk remained unclear.4,6−8 However, in recent meta-analyses, combining results of several cohort and case-control studies, plant lignans have shown a possible association with reduced breast cancer risk in postmenopausal women.9,10 Additionally, a prospective nested case-control study indicates that lignan metabolites, especially enterolactone, are associated with a lower risk of type 2 diabetes.11 To estimate lignan intake in various populations and demonstrate the association between the lignan-containing food and risk of developing chronic diseases, lignan contents of foods in different countries have been quantified.2,12−14 The © XXXX American Chemical Society

Special Issue: 27th ICP and 8th Tannin Conference (Nagoya 2014) Received: January 13, 2015 Revised: March 26, 2015 Accepted: March 29, 2015

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DOI: 10.1021/acs.jafc.5b00214 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry

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chromatography (GC) with various detection systems. By far the predominant and most popular applications for analyzing lignans and other phytoestrogens are the (HP)LC-MS methods. In contrast to GC-MS, LC-MS has the advantage that it is not necessary to prepare volatile derivatives and that conjugated forms, such as the glycosides, can be analyzed as well.26−28 However, the phytoestrogen contents are usually very low in biological matrices, and these techniques remain largely qualitative unless authentic standards are available. Thus, isotopically and isomerically pure polydeuterated lignans and other phytoestrogens are needed as references for the quantitation of these compounds in biological samples. The use of structural analogues as internal standards, rather than authentic isotope labeled isotopologues (i.e., substances that differ only in isotopic composition, e.g., CH3OH, CD3OH, CH2DOH), is undesirable because the former will have different behaviors in sample preparation as well as different retention times and ionization properties compared with the analytes. An authentic stable isotope labeled analogue of a compound is essentially identical to the compound of interest except for mass. One of the most reliable and sensitive methods of quantification is based on the addition of a stable isotope analogue of the analyte of interest into a sample at a known concentration. The technique is called stable isotope dilution (SID).29 Because the analyte and its isotopologue possess almost identical chemical and physical properties, the isotopic ratios remain stable, as they are affected equally by variations in extraction, sample preparation, injection, and instrument parameters.30 In the ion chromatogram, the analyte and its labeled analogue usually have the same retention time. Thus, all of the losses during sample preparation can be corrected for and the analytes reliably identified and quantified.

Review

LABELS FOR LIGNANS

Apart from radioactive labels, the possible labels for lignans are 18 O, 13C, and D, the heavier stable isotopes of oxygen, carbon, and hydrogen, respectively. 18O-labeled compounds are used to study reaction mechanisms,34,35 but they could also be used as stable isotope labeled analogues for quantitative determinations. 16O/18O exchange is limited to compounds such as carboxylic acids that contain functional groups prone to facile oxygen exchange reactions.36 Oxygen labeling has been used also when reactive phenols, for example, resorcinol moieties, are present, but the labeling conditions required were quite harsh (180 °C for 16 h), the yields were low, and protecting groups were needed.37 To the best of our knowledge, no 18Olabeled lignans have been synthesized apart from those in our mechanistic studies.35 In the literature there are some examples of carbon-labeled lignan precursors38 that have been used to examine the biosynthesis and mechanisms of formation of lignans and neolignans.18,24,39 The nonradioactive isotope of carbon, 13C, might be considered as an optimal, general-purpose label because the isotope effects with 13C are small (compared with 18 O/16O and D/H), it can be used to label the fairly stable carbon backbone of lignans, and it cannot be exchanged for 12C under ordinary conditions. However, the stability advantage is offset by the necessity of total or partial synthesis using expensive 13C-labeled reagents or starting materials. Only one total synthesis of stable 13C-labeled lignan derivatives has been reported in the literature.40,41 After the first 13C label was introduced, this synthesis took 11 or 12 steps to obtain the final triply labeled target, and although the yields for most of the steps are very good, the overall yields were 0.3−2%.40,41 On the other hand, deuterium labeling provides by far the most important route to stable isotope labeled standards in organic analysis. This is a result from the wide distribution of hydrogen in the target material, the ease of H/D exchange in most cases, and the cheap and ready availability of deuterated solvents and reagents. Furthermore, depending on the functional groups of the target molecules, it is often possible to acquire the desired label without any synthesis by performing simple exchange reactions on the native molecules in deuterated solvents.36 The various deuteration reagents and solvents, such as D2O, CH3OD, DCl, D2SO4, and NaOD for deuterium exchange, and the reducing agents LiAlD4 and NaBD4, used in total synthesis approaches, are readily available and less costly than 18O- and 13C-labeled reagents. As a result of the isotope effects, deuterated analogues may have certain defects as internal standards. There may be a small separation of the deuterated internal standard and its endogenous protium form during chromatography.42 Ion fragmentation is sensitive to the kinetic isotope effect, but differences in ionization efficiency between labeled and unlabeled compounds are very small and become unmeasurable when the labeling site is remote from the charge center, even in the case of deuterium.43 Yet, structural analogues are even less representative of the endogenous compounds because, in addition to differences in retention time, the structural analogue can show different absorption loss.42 Thus, exchange− deuteration has remained the most important labeling method for obtaining stable isotope labeled compounds.

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REQUIREMENTS FOR THE INTERNAL STANDARDS In quantitation methods, internal standards are used to take into account all of the losses during the process and the behavior of the compounds during the measurement. A stable, chemically, isomerically, and isotopically pure isotopologue of the analyte is the optimal standard. As the physicochemical properties of an isotope labeled analogue are very similar to those of the analyte, no separation takes place during the process: the analyte and its labeled analogue have the same retention times in chromatography but different m/z values in mass spectrometry. Labeled compounds that are used as internal standards in quantitative analytical techniques such as GC-MS and LC-MS must fulfill certain important criteria. First, the compounds and the labels must remain stable during the entire sample preparation procedure and analysis. Second, there cannot be any unlabeled species present in the standard. Third, the labeled analogue and the analyte should have a difference of at least 3 units, or more if the analyte contains elements other than C, H, O, and N, in the M+ m/z value to avoid overlap with the internal standard.31 On the other hand, it has been suggested that for internal standards an optimal number of deuteriums would be from three to five, as more deuteriums in the molecule may cause chromatographic separation between the standard and the analyte.32,33 However, no chromatographic separation of a deuterated compound from the corresponding unlabeled compound has been reported for lignans. B


Review


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D2-enterolactone and D2- and D4-enterodiol,48,49 utilizing the method based on Pelter et al.’s synthesis of transdibenzylbutyrolactones via tandem conjugate addition.50 The Michael addition−alkylation method was modified by using D2butenolide D2-3 as starting material to produce D2-enterolactone D2-4 (Figure 5). Deuterium-labeled enterodiol was available in two forms: D2-5, either by reducing unlabeled enterolactone 4 with LiAlD4 or by reducing labeled enterolactone D2-4 with LiAlH4, and D4-5 by reducing the labeled lactone D2-4 with LiAlD4. Neidigh et al. synthesized monodeutero-labeled podophyllotoxin precursors from podorhizone by using NaBD4 as a reducing agent.51 A two-step synthesis52 of symmetrically substituted butyrolactone lignans was applied to prepare hexadeuterated benzyl protected derivatives of matairesinol 6 (Figure 6).53 The first step is the formation of a fulgenic acid 7 by the Stobbe condensation of 4-benzyloxy-3-methoxybenzaldehyde 8 and dimethyl succinate 9. The second, deuteration step is a ruthenium-catalyzed ring closure with simultaneous deuteration, giving the benzyl protected D6-matairesinol derivatives trans-D6-6 and cis-D6-6 in a 10:1 ratio (the yield of the reaction was not mentioned). Labeling via H/D Exchange of Aromatic Protons. All of the other syntheses for deuterium-labeled lignans are based on H/D exchange within the complete molecular framework. In the case of lignans, the exchangeable protons are in the aromatic rings, and the benzylic or α-carbonyl sites remain unaffected. The first efforts to deuterate aromatic compounds were in 1934, three years after the discovery of the heavy hydrogen isotope.54 Horiuti and Polanyi heated benzene in 3% heavy water in the presence of a nickel catalyst.55 Later in the same year Ingold et al. introduced deuterium into the aromatic nucleus by means of ordinary electrophilic reagents, namely, concentrated aqueous sulfuric acid (without heterogeneous catalysis).56 They also proposed that the mechanism of the exchange is an electrophilic aromatic substitution57 and proved the theory with comparison of the efficiencies of some acidic and basic deuterating agents and the influence of some aromatic substituents.58,59 In lignans the aromatic substituents usually are hydroxy and/or methoxy groups.60 When an electrophilic substitution reaction, such as deuteration, is performed, the existing substituents determine which position the new group will take and whether the reaction will be slower or faster than that with benzene. Both hydroxy and methoxy are ortho−para directing and activating groups.61 The first deuterium exchange within a lignan skeleton was performed with DBr (PBr3 in D2O, Figure 7).62 The protons of phenolic groups were replaced with deuterons by D2O treatment before the deuteration procedure, and the predeuterated enterolactone 10 was labeled with deuterium by exchange in DBr−D2O to give the hexadeuterated enterolactone D6-4. Aromatic protons that are ortho or para to the directing and activating phenolic OH groups were exchanged, whereas the protons that are meta to OH remained unaffected. The H/D treatment was repeated to ascertain complete deuteration and >90% isotopic purity. After final exchange, the reaction products were treated with a large excess of H2O or ethanol to restore the protic hydroxy groups. Deuterated phosphoric acid D3PO4 was used to synthesize a D6 derivative of matairesinol 11 (Figure 8).21 Matairesinol 11 was first predeuterated with CH3COOD, made from freshly

CHARACTERIZATION OF THE DEUTERATED COMPOUNDS Isotopic purity is an important feature of an isotope-labeled compound. However, in the literature and in the catalogs of commercial suppliers there are no established practices on how to define or measure the isotopic purity, isotope distribution, or isotope content. In this paper, the isotope distribution is determined by NMR and the isotopic purity is indicated as the percentage of the most abundant D species, determined from the molecular ion region in EI mass spectra by comparison with those of the undeuterated compounds. The number of deuterium atoms in a product as well as the isotopic purity is determinable by MS. The isotopic purities of deuterated compounds may be determined from the ion clusters in the molecular ion region in the EI or ESI mass spectra by comparison with those of undeuterated compounds. For the native butyrolactone and tetrahydrofuran type lignans the M − 1 and M − 2 peaks usually are very small (2−5% of the intensity of the molecular ion peak) or totally absent (Figure 2).44 For flavonoids, which may have intense M − 1 and M − 2 fragments, an equation to determine the isotopic purity is reported.45

Figure 2. Molecular ion regions of an unlabeled butyrolactone type lignan (M = 358) and its stable deuterium-labeled D3 analogue (M = 361).44

The locations of the deuterium atoms can be established from 1H and 13C NMR studies. In the 13C NMR spectrum, the carbon atom bearing a deuterium atom produces a lowintensity triplet (instead of the relatively intensive aromatic C− H singlet) because the spin of deuterium is 1 (Figure 3). The 1 H NMR spectra of deuterium-labeled compounds are the same as for unlabeled compounds, with the exception that the aromatic deuterons cannot be seen and the coupling patterns of the protons attached to the adjacent carbons are simplified (Figure 4).

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DEUTERATED LIGNANS Introducing Deuteriums via Reduction. In the literature there are some reports of the introduction of deuterium into the lignan skeleton via reduction. The first reported synthesis for deuterium-labeled lignans was a multistep total synthesis for C


Review


Figure 3. 13C NMR spectra of D3- and D9-labeled 3′-hydroxylignano-9,9′-lactones (in CDCl3).44,46,47 The C−H singlets for deuterated sites are replaced by low-intensity C−D triplets (expanded).

DClO4 (68% in D2O) in THF/CDCl3 at room temperature in only 4 h.51 An effective method using the deuterated phosphoric acid− boron trifluoride complex D3PO4·BF3/D2O was developed in our group initially to synthesize a stable deuterium-labeled isoflavone genistein63 and, later, also other related compounds.64−66 Originally tetradeuterogenistein was synthesized by refluxing predeuterated genistein 12 in CF3COOD for 2 days to obtain D4-genistein [6,8,3′,5′-D4]-12 in 75% yield and >90% isotopic purity (“a” in Figure 9).21 However, it was found later that the most highly activated 6- and 8-site labels were unstable during the ID-GC-MS-SIM analytical procedure and were lost to some extent, making the [6,8,3′,5′-D4]-12

distilled acetic anhydride by slow addition to heavy water and stirring the solution under argon for 2 h. The phenolic protons were exchanged with deuteriums by dissolving matairesinol in labeled acetic acid, leaving it to stand for several hours and finally evaporating the solvent. The labeled phosphoric acid D3PO4 was prepared by adding D2O to P2O5. Predeuterated matairesinol was added to deuterated phosphoric acid under argon, and the reaction mixture was stirred at 80 °C for 3 days. Deuteration and workup were repeated three times. The isotopic purity was estimated from the mass spectrum to be 95% for D6-11 and 5% for the D5 derivative. Neidigh et al. prepared a 99% isotopic purity. No exchange was observed at the αcarbonyl or benzylic sites.47 The use of dielectric microwave heating has been increasingly utilized in organic synthesis since the pioneering works of Gedye69 and Giguere70 in 1986. Despite being commonly employed in laboratory work, microwave techniques had not been reported in the synthesis or labeling of lignans until very recently.71−73 It was found that the use of 35% DCl in D2O and the ionic liquid 1-butyl-3-methylimidazolium chloride, [bmim]Cl, as a cosolvent under microwave irradiation is a fast and high-yielding deuteration method for polyphenolic compounds including lignans.71,74 The reaction times were shortened significantly (from several days to 40 min). The selectivities were not different from those observed under conventional heating.

Figure 7. Reagents and conditions: (a) D2O; (b) (i) PBr3, D2O, reflux, 5 h; (ii) H2O.62

Figure 8. Reagents and conditions: (a) CH3COOD; (b) (i) D3PO4, 80 °C, 9 days in total, (ii) H2O.21

for deuteration of the lignan matairesinol 17 to good effect,21 could exchange only a third of the ring A protons and none from the B ring. However, the deuterated phosphoric acid− boron trifluoride complex D3PO4·BF3/D2O was active enough to exchange all of the aromatic ring protons (“b” in Figure 9).63 F


Review


Figure 9. Reagents and conditions: (a) CF3COOD;21 (b) (i) D2O/acetone, (ii) D3PO4·BF3/D2O, 55 °C, 3 days, (iii) H2O/EtOH; (c) 1% CH3COCl in MeOH, reflux, 30 min.63

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SELECTIVITIES IN ELECTROPHILIC AROMATIC DEUTERATION Relative Ease of H/D Exchange at Various Aromatic Positions. When D 3 PO 4 ·BF 3 /D 2 O was used for the isoflavones daidzein and dihydrodaidzein, the degree of deuteration could be modulated by the reaction temperature.64,66 In the case of isoflavonoids64 and flavones,65 raising the temperature to 55 °C does not seem to diminish the yields, but the lignan framework is found to be more fragile.44 Hence, the deuteration reactions were carried out at room temperature. It is also interesting that within the lignan skeleton, all of the aromatic sites undergo exchange at room temperature (Figure 10),47,68 whereas in the case of the isoflavone daidzein, only three positions were deuterated at room temperature in 3 days. Repeating the reaction deuterated the fourth position, and raising the temperature to 55 °C deuterated the fifth position.64 Deuteration using the highly acidic D3PO4·BF3/D2O also exchanges protons to deuteriums at the less reactive aromatic sites, giving, for example, a fully aryl deuterated derivative of 3′hydroxylignano-9,9′-lactone 13 (D9-13, Figure 11).47 The other reported acidic methods, however, work in a more conventional way. For example, DCl in D2O exchanges protons of 13 at the substituted B-ring only and at the activated sites ortho and para to the hydroxy group (Figure 11).46 As the deuteration reagent D3PO4·BF3/D2O was found to be very efficient in labeling isoflavones and especially lignans, exchanging protons for deuterium even at the unactivated aromatic positions of the lignan skeleton (see D8-4 in Figure 10 and D9-13 in Figure 11), several new deuterium-labeled

Figure 10. Reagents and conditions: (a) (i) D2O/acetone, (ii) D3PO4· BF3/D2O, room temperature, 20 h; (iii) H2O.67,68

Lignans have been shown to be susceptible to heating.44 The method using DCl/D2O together with microwave heating works well for lignans even at lower temperatures (40 °C) and in some cases also without an ionic liquid solvent. Several tetrahydrofuran and butyrolactone type lignans were deuterated.46

Figure 11. Deuteration of 3-hydroxylignano-9,9′-lactone 13.46,47 * The deuteration reaction was repeated with fresh reagent until the isotopic purity was >85%. G


Review


Figure 12. Synthesis of stable deuterated lignano-9,9′-lactones. Reagents and conditions: (a) (i) D3PO4·BF3/D2O (freshly prepared), room temperature; (ii) H2O; (b) CH3COCl, MeOH, reflux.47

Figure 13. Deuteration of dehydroxycubebin 14 and brassilignan 15. Reagents and conditions: (a) (i) 35% DCl in D2O, [bmim]Cl, MW, 40 °C; (ii) H2O.46

Figure 14. Mechanism of H/D exchange. R is for the rest of the lignan molecule.

ice-cold water (0.5 L water/100 mg lignan).44 Thus, when the deuteration had reached a satisfactory level, that is, the isotopic purity was >85%, the remaining labile deuterium labels were selectively back exchanged to hydrogens in refluxing 0.5−1% CH3COCl in methanol (“b” in Figure 12). This approach of deuteration with D3PO4·BF3/D2O and subsequently removal of the labile deuteriums in CH3COCl/MeOH was very efficient in synthesizing labeled lignans possessing three to nine deuteriums in the aromatic rings. The deuterated lignans were isomerically and isotopically pure (>85% isotopic purity), and the yields were good (81−97%).47 These deuterated standards have been successfully used in quantitating lignan contents in food samples.76 Aberrant Selectivity in Deuteration of Methylenedioxy Substituted Compounds. The deuteration of several butyrolactone and tetrahydrofuran type lignans with DCl in D2O gave the expected behavior, exchanging protons to deuteriums at all of the activated positions (see D3-13 in

lignanolactones were synthesized to examine the influence of the substitution pattern on the deuteration order.47,75 The exchange order of hydrogens, that is, the relative ease of H/D exchange at the various aromatic positions, was determined by following the progress of the deuteration reaction by 1H NMR. In addition, electrostatic potential (ESP) charges were calculated to study the relative reactivities of the aromatic protons. The observed reactivities were compared with the calculated reactivities and found to be in good agreement.47 The very inactive 5-position in the 3-hydroxyphenyl moiety (see 5′ in Figure 12) was tardy in reacting and in some cases needed several repetitions with fresh deuteration reagent. On the other hand, the presence of the two metasubstituted hydroxy or methoxy groups in ring A highly activated the aromatic sites 2, 4, and 6 (Figure 12). Because of the strong activation at these sites (2, 4, and 6), back exchange from deuterium to hydrogen occurred to some extent during quenching of the deuteration reaction into a large volume of H



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Figure 11). However, dehydroxycubebin 14 gave unexpected results (Figure 13).46 The deuteration was only partial, giving a mixture of di-, tri-, and tetradeuterated products D2−4-14. In contrast, brassilignan 15, a close analogue of dehydroxycubebin, was successfully hexadeuterated with >97% isotopic purity. As mentioned above, the mechanism of the H/D exchange is an ordinary electrophilic aromatic substitution,57 where hydroxy and alkoxy groups are ortho−para directing and activating. The methylenedioxy substituent, however, causes aberrant selectivity. In the model compound 1,3-benzodioxole (1,2-methylenedioxybenzene, Figure 14: R = H) the positions meta/para to the methylenedioxy substituent were fully deuterated, whereas the ortho positions were only partially deuterated.46 The higher para-selectivity of 1,3-benzodioxole in electrophilic aromatic substitution, in comparison to its cyclohomologues or veratrole, has been observed and rationalized by a distortion of the aromatic ring when annelated to a small ring and by quasiaromatic nature of the heterocyclic ring.77 The deuteration mechanism takes place in two steps: first, the electrophile attacks, giving rise to positively charged resonance stabilized intermediate, and the leaving group departs in the second step (Figure 14). Simultaneous attack and departure mechanisms are not known.61 With regard to the mechanism, computational studies suggested another, conformation-dependent factor responsible for the deviant behavior preventing the complete deuteration of 14.46 The calculations imply that under the reaction conditions, the tetrahydrofuran type lignans prefer π-stacked, so-called sandwich, conformations, suggesting that the first reaction step, that is, addition of the deuterium, occurs from the solventexposed face of the molecule where the steric pressure is weaker, and the second step, that is, departure of the protium, occurs from the internal face. The first step is possible for both structures 14 and 15; however, the second step of the electrophilic aromatic substitution is sterically prevented for one of the dehydroxycubebin 14 conformers. Presumably, due to interactions with the rigid methylenedioxy substituent, there is increased coplanarity between the aromatic rings of 14, compared to 15, which has two freely rotating methoxy groups.46 The butyrolactone and tetrahydrofuran type lignan structures undergo electrophilic aromatic substitution in deuteration reactions under acidic conditions. The degree of reactivity and selectivity of the deuteration is generally straightforward and predictable, enabling isotopically pure and stable polylabeled analogues to be prepared. However, some aberrant behavior is detected. The deuteration reagent D3PO4·BF3/D2O has the remarkable ability to exchange protons even at the unactivated aromatic positions, completely inaccessible with the other reported methods. Additionally, methylenedioxy substituents may cause unexpected deficiencies in the degree and orientation of atomatic deuteration, particularly in polycyclic aromatics.

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Review

REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*(K.W.) E-mail: kristiina.wahala@helsinki.fi. Phone: +358 9 191 50356. Notes

The authors declare no competing financial interest. I


Review

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