First synthesis of rac-(5-2H3)-α-CEHC, a labeled analogue of a major vitamin E metabolite

Article abstract of DOI:10.1021/jo048514u

Over the past few years, quantitative determination of α- and γ-CEHC, main urinary metabolites of vitamin E, has been becoming more and more important as a key parameter in assessing oxidative stress and supplementation of tocopherols. The use of deuterated analogues of these metabolites allows their accurate determination even in complex matrixes. While preparation of γ-CEHC-d₂ has already been described, here we report the first synthesis of α-CEHC-d₃ together with α-tocopheronolactone-d₃, its oxidation product.

Full text of DOI:10.1021/jo048514u

First Synthesis of rac-(5-2H3)-r-CEHC, a
Labeled Analogue of a Major Vitamin E
Metabolite
SCHEME 1. Preparation of Unlabeled r-CEHC
Francesco Mazzini,† Thomas Netscher,‡ and
Piero Salvadori*,†
Dipartimento di Chimica e Chimica Industriale,
Universita` di Pisa, Via Risorgimento 35, 56126 Pisa, Italy
psalva@dcci.unipi.it
Received August 24, 2004
hypothesis that the previously identified Simon metabo-
lites were artifactually produced by oxidation of R-CEHC
during the extraction procedure. Therefore, confirmation
of the authenticity and origin of the small amounts of
R-tocopheronolactone observed in human urines is im-
portant to understand the utility of this metabolite as
biomarkers of oxidative stress.6
In that regard, suitable analytical methods have to be
developed to avoid oxidation of the metabolites during
sample preparation necessary for the analysis. Recently,
Muller et al. reported the first direct analysis of conju-
gated vitamin E metabolites using ESI-MS-MS,7 showing
a number of advantages over previous methods. However,
they suggested the improvement of the method introduc-
ing a separation step (HPLC) prior to MS analysis
together with the strong requirement of the use of
vitamin E deuterated standards to allow more accurate
quantitations.
Here we describe the first synthesis of racemic R-CEHC-
d3 12 and its oxidation product, R-tocopheronolactone-d3
13, through the setting up of an efficient route to the key
building block trimethylhydroquinone-d3 11.
Unlabeled R-CEHC was prepared in a satisfactory way
by Kantoci et al.8 by condensing trimethylhydroquinone
(TMHQ) with γ-methyl-γ-vinylbutyrolactone (1) in the
presence of BF3‚Et2O.9 The lactone can be easily obtained
on a gram scale from inexpensive vinylmagnesium bro-
mide and ethyl levulinate (Scheme 1).
Abstract: Over the past few years, quantitative determi-
nation of R- and γ-CEHC, main urinary metabolites of
vitamin E, has been becoming more and more important as
a key parameter in assessing oxidative stress and supple-
mentation of tocopherols. The use of deuterated analogues
of these metabolites allows their accurate determination
even in complex matrixes. While preparation of γ-CEHC-d2
has already been described, here we report the first synthe-
sis of R-CEHC-d3 together with R-tocopheronolactone-d3, its
oxidation product.
Vitamin E is a generic term for tocopherols and
tocotrienols that exhibit the biological activity of R-toco-
pherol.1 Each group can occur in R-, â-, γ-, and δ-forms,
which differ in the number and position of methyl groups
on the chroman ring. Two of these homologues, R- and
γ-tocopherol, are the most important ones for human
nutrition, playing an essential role against oxidative
stress and in preventing some chronic degenerative
diseases.2
Over the past few years, there has been a growing and
renewed interest in determining urinary metabolites of
vitamin E because of their proposed relationship with
oxidative stress and vitamin E supply.3 Simon et al. were
the first who isolated vitamin E-derived compounds from
the urine of animals and humans supplied with large
doses of R-tocopherol.4 In these so-called Simon metabo-
lites, the chroman ring has been opened and oxidized to
the quinone structure (R-tocopheronolactone and R-toco-
pheronic acid). Recently, another R-tocopherol-related
compound has been found3 as the major urinary metabo-
lite present in human urine, the 2,5,7,8-tetramethyl-2-
(â-carboxyethyl)-6-hydroxychroman 2 (R-CEHC). Shortly
afterward, also the γ-homologue (γ-CEHC) was isolated
as a new natriuretic factor.5 The identification of R-CE-
HC, in which the chroman ring is intact, led to the
As far as deuterated analogues are concerned, however,
there are no reports regarding labeled R-CEHC and only
one paper describing the synthesis of the analogue
γ-CEHC-d2 by Swanson et al.10 The latter was prepared
through DDQ dehydrogenation of the saturated ring on
acetylated racemic γ-CEHC, followed by palladium cata-
lyzed deuteration, with a 25% yield over five steps based
on the starting dimethylhydroquinone.
For the synthesis of deuterated R-CEHC we used a
different approch. In the preparation of labeled δ-toco-
* To whom correspondence should be addressed. Tel: +39 050
2219918273. Fax: +39 050 2219918260.
(5) Wechter, W. J.; Kantoci, D.; Murray, E. D.; D’Amico, D. C.; Jung,
M. E.; Wang, W. H. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 6002-6007.
(6) Pope, S. A. S.; Clayton, P. T.; Muller, D. P. R Arch. Biochem.
Biophys. 2000, 381, 8-15.
(7) Pope, S. A. S.; Burtin, G. E.; Clayton, P. T.; Madge, D. J.; Muller,
D. P. R. Free Radic. Biol. Med. 2002, 33, 807-817.
(8) Kantoci, D.; Wechter, W. J.; Murray, E. D.; Dewind, S. A.;
Borchardt, D.; Khan, S. I. J. Pharmacol.-Exp. Ther. 1997, 282, 648-
656.
† Dipartimento di Chimica e Chimica Industriale, Universita` di Pisa,
56126 Pisa, Italy.
‡ Research and Development, DSM Nutritional Products, P.O. Box
3255, CH-4002 Basel, Switzerland.
(1) IUPAC-IUB Eur. J. Biochem. 1982, 123, 473-475.
(2) Bramley, P. M.; Elmadfa, I.; Kafatos, A.; Kelly, F. J.; Manios,
Y.; Roxborough, H. E.; Schuch, W.; Sheehy, P. J. A.; Wagner, K.-H. J.
Sci. Food Agric. 2000, 80, 913-938.
(3) Schultz, M.; Leist, M.; Petrizka, M.; Gassmann, B.; Brigelius-
Flohe, R. Am. J. Clin. Nutr. 1995, 62, 1527S-1534S.
(4) Simon, E. J.; Eisengart, A.; Sundheim, L.; Milhoart, A. T. J. Biol.
Chem. 1956, 221, 807-817.
(9) Wechter, W. J.; Kantoci, D.; Murray, E. D.; D’Amico, D. C.; Jung,
M. E.; Wang, W.-H. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 6002-6007.
(10) Swanson, J. E.; Ben, R. N.; Burton, G. W.; Parker, R. S. J. Lipid
Res.1999, 40, 665-671.
10.1021/jo048514u CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/06/2004
J. Org. Chem. 2004, 69, 9303-9306
9303

SCHEME 2. Two-Step Reduction of 5-Benzyloxy-2-hydroxybenzoic Acid into
4-Benzyloxy-2-(2H3)-methylphenol
SCHEME 3. Synthesis of r-CEHC-d3 12 and Lactone 13a
a Key: (a) NaH, DMF, then MOMCl, 93%; (b) t-BuLi, hexane, ClCO2Et, 0 °C, 84%; (c) MgBr2, C6H6, rt, overnight, 91%; (d) KOH 2 M,
MeOH, 50 °C, 2.5 h, then HCl pH ) 4, quant; (e) (1) ClCO2Et, Et3N, 0 °C, 2 h, (2) NaBD4, 8 equiv, D2O/THF, 0 °C, 3 h, 89%; (f) pTsOH,
MeOH, 50 °C, 3 h, 90%; (g) (1) KOH 4 M, EtOH/H2O, (2) THF, HClconc, 86%; (h) (1) ClCO2Et, Et3N, 0 °C, 2 h, (2) NaBH4, 8 equiv, H2O/
THF, 0 °C, 3 h, 70%; (i) dioxane, BF3‚Et2O, 110 °C, 5 h, 79%; (j) MeOH/H2O, FeCl3‚6H2O, 82%.
pherol, we set up and optimized a two steps reduction
procedure that afforded the synthesis of 4-benzyloxy-2-
(2H3)-methylphenol11 in very good yield from 5-benzyloxy-
2-hydroxybenzoic acid. According to this protocol, the
starting hydroxybenzoic acid was quantitatively con-
verted with ethyl chloroformate into a bis-ethoxycarbonyl
derivative. Treatment with NaBD4 in D2O/THF solution
directly afforded the labeled 2-methyl phenol in 89% yield
(Scheme 2).
Considering this excellent result, we looked for an
effective procedure for the preparation of 2,5-dihydroxy-
3,4-dimethylbenzoic acid 5, which should then be subse-
quently reduced applying the same protocol, affording the
deuterated hydroquinone 11 (Scheme 3).
For the introduction of a carboxylic group into 2,3-
dimethylhydroquinone (DMHQ) we planned to use a
metalation reaction. For this purpose, as the use of CO2
as electrophile gave unsatisfying results both by direct
and bromine-lithium exchange metalation, we prepared
the ethyl ester 4.12 Subsequent saponification and depro-
tection provided the desired acid 5 in 86% yield. High
selectivity for ring over benzylic metalation was achieved
using t-BuLi as metalating agent in hexane at 0 °C.13
Hence, carbonate 6 was obtained in 70% yield from 5 by
applying the reduction protocol mentioned above.11,14
However, removing the ethyl formate group to obtain
7 was tougher than expected. Applying strong acidic
conditions (TFA and HCl) in a H2O/THF mixture to 6,
as well as mild saponification by LiOH in MeOH, gave
poor results (25% and 55%, respectively). Heating the
carbonate at 50 °C in methanolic H2SO4 (10% v/v) gave
slightly better but still unsatisfactory results (62% at
best).
To avoid carbonate formation, we tried to selectively
remove the MOM group of 4 ortho to the ester moiety,
thus leaving the other phenolic moiety protected by the
MOM group during the reduction step. Substrates con-
taining two MOM groups can be differentiated if one is
ortho to a chelating group, like a carbonyl group. Such
selective deprotection has been achieved on several
substrates using montmorillonite K 1015 and the I2/MeOH
(12) Hammond, M. L.; Kopka, I. E.; Zambias, R. A.; Caldwell, C.
G.; Boger, J.; Baker, F.; Bach, T.; Luell, S.; MacIntyre, D. E. J. Med.
Chem. 1989, 32, 1006-1020.
(11) Mazzini, F.; Mandoli, A.; Salvadori, P.; Netscher, Th.; Rosenau,
Th. Approaches to the preparation of 4-benzyloxy-2-(R,R,R-2H3)-
methylphenol, building block for labeled δ-tocopherol, and a new R,R,R-
5-D3-R-tocopherol synthesis. Eur. J. Org. Chem., accepted for publi-
cation.
(13) Winkle, M. R.; Ronald, R. C. J. Org. Chem. 1982, 47, 2101-
2108.
(14) Minami, N.; Kijima, S. Chem. Pharm. Bull. 1979, 27, 816-820.
9304 J. Org. Chem., Vol. 69, No. 26, 2004

system.16 However, we decided to follow the procedure
of Haraldsson regarding the selective cleavage of aro-
matic benzyl ethers ortho to a carbonyl group with
MgBr2.17 Stirring a mixture of MgBr2 and 4 in benzene
overnight at room temperature proved indeed to be very
effective, allowing the recovery of desired 8 in 91% yield.
Saponification afforded acid 9 quantitatively. Much to our
delight, treatment of 9 with ethyl chloroformate, followed
by reduction of the corresponding bis-ethoxycarbonyl
derivative with excess NaBD4 in D2O/THF solution and
p-TsOH-catalyzed deprotection, delivered 11 in an overall
yield of 80% from 8. Condensation of 11 with 1 in the
presence of BF3‚Et2O8 eventually afforded the desired
R-CEHC-d3 12 in 79% yield with an excellent 97.5%
isotopic purity. Moreover, R-tocopheronolactone-d3 13 was
easily prepared by FeCl3‚6H2O oxidation without any loss
of the level of deuteration, as confirmed by GC-MS
analysis (Scheme 3).
In summary, the first synthesis of R-CEHC-d3 has been
developed through an efficient way for the preparation
of the key building block 11, in 57% yield over seven steps
and 97.5% isotope purity. In particular, the mild selective
deprotection of 4 by MgBr2 and the two-step reduction
of salicylic acid 9 to the corresponding 2-CD3-phenol 10
resulted quite effective and promising for further devel-
opment. Results from a study evaluating the scope of this
unusually easy reduction sequence will be reported in due
course. Labeled hydroquinone 11 can be also used in the
preparation of other labeled tocopherol derivatives and
metabolites.
allowed to warm to room temperature and stirred for 1 h. After
cooling again to 0 °C, a solution of MOMCl (19.7 mL, 260 mmol,
2.6 equiv) in DMF (20 mL) was added dropwise, and the
resulting mixture was stirred overnight at rt. Complete conver-
sion of the starting phenol was confirmed by TLC (Hex/EtOAc
6:1). After workup in Et2O, the residue obtained was filtered
through a plug of silica gel (Hex/EtOAc 6:1) to give 3 (21.02 g,
1
yield 93%) as pale yellow oil. H NMR (CDCl3/TMS): δ 2.21 (s,
6 H), 3.52 (s, 6 H), 5.10 (s, 4 H), 6.92 (s, 2 H). 13C NMR (CDCl3):
δ 12.3, 55.9, 95.4, 112.5, 127.6, 150.3.
Ethyl 2,5-Bis(methoxymethoxy)-3,4-dimethylbenzoate
(4). to a solution of 3 (15.7 g, 69.5 mmol) in dry hexane (100
mL) was added dropwise a solution of t-BuLi (45 mL, 76.4 mmol,
1.1 equiv) in pentane at 0 °C, and the resulting mixture was
stirred for 3 h at 0 °C. Then dry hexane (40 mL) and ClCO2Et
(60 mL, 625.5 mmol, 9 equiv) were added in one portion, and
the reaction was stirred at 0 °C for an additional 3 h. Then the
reaction was quenched with 1 M NaOH (40 mL). After workup
in Et2O, the residue obtained was purified by column chroma-
tography (Hex/EtOAc 8:1), affording 4 (17.4 g, 84% yield) as a
yellow semisolid liquid. 1H NMR (CDCl3/TMS): δ 1.35 (t, J )
6.8 Hz, 3 H), 2.18 (s, 3 H), 2.24 (s, 3 H), 3.45 (s, 3 H), 3.55 (s, 3
H), 4.31 (q, J ) 6.8 Hz, 2 H), 4.95 (s, 2 H), 5.14 (s, 2 H), 7.33 (s,1
H). 13C NMR (CDCl3): δ 12.8, 13.3, 14.2, 56.0, 57.5, 60.8, 95.0,
101.2, 113.8, 122.2, 132.5, 132.8, 150.4, 151.0, 166.1.
2,5-Dihydroxy-3,4-dimethylbenzoic Acid (5). To a solution
of 4 (2.7 g, 9 mmol) in THF (15 mL) was added a solution of
KOH 4 M (10 mL) in H2O/EtOH 1:1, and the mixture was stirred
overnight. Complete saponification of starting 4 was confirmed
by TLC (Hex/EtOAc 6:100). EtOH was evaporated, and HClconc
was added until the mixture was made acid. After workup in
Et2O, the residue was dissolved in a THF/H2O/HClconc 12:4:2 (18
mL) mixture and heated at 50 °C for 2 h to complete the
deprotection sequence. Workup in Et2O after THF evaporation
afforded a residue that was filtered through a plug of silica gel
(Hex/EtOAc/AcOH 5:1:0.01) to give 5 (1.43 g, yield 86%) as white-
pale yellow solid. Mp: 208-211 °C. 1H NMR (CDCl3 + CD3OD/
TMS): δ 2.07 (s, 3 H), 2.11 (s, 3 H), 5.81 (bs, 2 H), 7.09 (s,1 H),
10.62 (bs, 1 H). 13C NMR (CDCl3 + CD3OD/): δ 11.4, 12.6, 108.0,
111.3, 127.5, 133.5, 146.3, 156.3, 167.2.
Experimental Section
General Methods. All reactions were performed under inert
atmosphere (argon or N2). All melting points are uncorrected.
NMR spectra were obtained at 200 MHz (1H), 50.3 MHz (13C),
and 46 MHz (2H) using Me4Si, CHCl3, or CDCl3 as internal
standards. Chemical shifts are expressed on the δ scale (ppm).
Deuteration level was determined by GC-MS by microSIS mode,
monitoring from m/z 422 to m/z 428 and from m/z 276 to m/z
282 for 12 (TMS derivative) and 13, respectively, using a DB-5
capillary column (30 m × 0.25 mm, 0.25 µm film thickness).
Operating conditions: injector temperature 280 °C; oven pro-
gram temperature 250 °C, increased at 30°/min to 300 °C, and
held for 7 min at 300 °C; transfer line temperature 295 °C; ion
trap temperature 250 °C; emission current 10 µA; isolation
window 3 amu. Hexane and 1,4-dioxane were distilled over CaH2
and Na, respectively, before use. DMF employed was HPLC
grade, kept over molecular sieves (4 Å). NaBD4 (98 atom % D)
and D2O (99.8 atom % D) were purchased from Aldrich. All other
commercial reagents were used without further purification.
Column chromatography was performed on silica gel 60 (70-
230 mesh). TLC was performed on silica gel Macherey-Nagel
Alugram Sil G/UV254 (0.20 mm). Workup involved addition of
water and three extractions into the solvent specified. The
organic extracts were combined, washed with water until the
aqueous phase was neutral, dried over Na2SO4, filtered, and then
concentrated on a rotary evaporator under vacuum. The residue
was further dried to constant weight under high vacuum. All
yields given refer to isolated yields.
Ethyl 2-Hydroxy-5-methoxymethoxy-3,4-dimethylben-
zoate (8). To a suspension of MgBr2 (1.03 g, 5.6 mmol) in
benzene (25 mL) was added a solution of 4 (1.2 g, 4 mmol) in
benzene (10 mL), and the mixture was stirred overnight, the
suspension color changing from white to yellow. After addition
of a saturated aqueous NH4Cl solution (10 mL), the mixture was
stirred for 30 min. After workup in EtOAc, the crude mixture
was purified by column chromatography (Hex/EtOAc 4:1), af-
fording 8 (930 mg, 91% yield) as pale yellow solid. Mp: 64-66
1
°C. H NMR (CDCl3/TMS): δ 1.39 (t, J ) 7.4 Hz, 3 H), 2.17 (s,
3 H), 2.20 (s, 3 H), 3.48 (s, 3 H), 4.36 (q, J ) 7.4 Hz, 2 H), 5.12
(s, 2 H), 7.32 (s, 1 H), 10.91 (s, 1 H). 13C NMR (CDCl3): δ 11.2,
12.5, 13.8, 55.6, 60.7, 95.0, 108.3, 111.0, 125.6, 135.4, 147.0,
154.9, 170.0.
2-Hydroxy-5-methoxymethoxy-3,4-dimethylbenzoic Acid
(9). Compound 8 (920 mg, 3.62 mmol) was dissolved in a
methanolic solution of KOH (13 mL, 2M). The resulting solution
was heated at 50 °C for 2.5 h, and then MeOH was evaporated
and the pH adjusted to 4 with 1 M HCl. Workup in EtOAc gave
pure 9 (820 mg, ∼100% yield), without further purification, as
a white-pale yellow solid. Mp: 142-145 °C. 1H NMR (CDCl3/
TMS): δ 2.14 (s, 3 H), 2.18 (s, 3 H), 3.47 (s, 3 H), 5.10 (s, 2 H),
5.76 (bs, 2 H), 7.38 (s, 1 H). 13C NMR (CDCl3): δ 12.6, 13.9, 57.1,
96.8, 111.9, 114.0, 126.7, 136.2, 148.6, 156.4, 175.1.
4-Methoxymethoxy-(2-2H3)-5,6-trimethylphenol (10). To
a solution of 9 (800 mg, 3.54 mmol) and Et3N (1.2 mL, 8.6 mmol,
2.4 equiv) in THF (50 mL) at 0 °C was added ethyl chloroformate
(0.82 mL, 8.6 mmol, 2.4 equiv), and the mixture was stirred for
3 h at 0 °C. The resulting white precipitate was filtered off and
washed with THF (30 mL). The combined filtrates were con-
centrated to small volume, rediluted with THF (15 mL), and
slowly added to a solution of NaBD4 (1.19 g, 28.5 mmol, 8 equiv)
in D2O (10 mL) and THF (5 mL) at 0 °C. After 3 h at 0 °C, the
white suspension was allowed to rise to rt and stirred overnight.
1,4-Bis(methoxymethoxy)-2,3-dimethylbenzene (3). To a
suspension of dry NaH (6.2 g, 260 mmol, 2.6 equiv) in DMF (40
mL) was added dropwise a solution of 2,3-dimethylhydroquinone
(13.8 g, 100 mmol) in DMF (50 mL) at 0 °C. The mixture was
(15) Deville, J. P.; Behar, V. J. Org. Chem. 2001, 66, 4097-4098.
(16) Keith, J. M. Tetrahedron Lett. 2004, 45, 2739-2742.
(17) Haraldsson, G. G.; Baldwin, J. E. Tetrahedron 1997, 53, 215-
224.
J. Org. Chem, Vol. 69, No. 26, 2004 9305

TLC (Hex/EtOAc/AcOH 4:1:0.005) showed complete conversion
of the bis-carbonate intermediate. The pH of the mixture was
then adjusted to 6 with 1 M HCl and AcOH and THF evaporated.
After workup in Et2O, the residue was purified by column
chromatography (Hex/EtOAc 4:1), affording 10 (630 mg, 89%
EtOAc. After workup in EtOAc, the crude brownish product
obtained was purified by column chromatography (Hex/EtOAc/
AcOH 3:5:0.01) to give 12 as a beige powder (1.44 g, 79%). Mp:
176-178 °C. 1H NMR (CD3OD/TMS): δ 1.18 (s, 3 H), 1.72-1.93
(m, 4 H), 2.04 (s, 3 H), 2.11 (s, 3 H), 2.39-2.47 (m, 2 H), 2.57 (t,
J ) 4.4 Hz, 2 H), 4.96 (s, 2 H). 2H NMR (CH3OH): δ 2.14 (s).
13C NMR (CD3OD/TMS): δ 11.2 (m), 12.1, 12.9, 21.6, 23.7, 29.5,
32.7, 35.5, 74.5, 118.0, 121.8, 123.1, 124.4, 146.1, 146.3, 177.7.
APCI-MS (in MeOH), m/z (amu): positive ion mode, 282 (M +
H)+, 299 (M + NH4)+, 304 (M + Na)+. Anal. Calcd for C16H19D3O4
(281.2): C, 68.30; H, 8.95. Found: C, 68.20; H, 8.87; 97.5%
isotope purity by GC-MS (TMS derivative).
1
yield) as a white solid. Mp: 92-93 °C. H NMR (CDCl3/TMS):
δ 2.18 (s, 6 H), 3.53 (s, 2 H), 5.03 (s, 1 H), 5.11 (s, 2 H), 6.73 (s,1
H). 2H NMR (CHCl3): δ 2.19 (s). 13C NMR (CDCl3): δ 12.1, 13.7
(m), 55.8, 95.7, 115.1, 120.7, 123.7, 124.9, 147.1, 148.6. APCI-
MS (in MeOH), m/z (amu): negative ion mode, 198 (M - H+).
(2-2H3)-5,6-Trimethylhydroquinone (11). A solution of 10
(590 mg, 2.96 mmol) and catalytic p-TsOH in MeOH (25 mL)
was heated at 50 °C for 3 h. TLC (Hex/EtOAc 6:1) confirmed
complete conversion of 10. After evaporation of the solvent, the
residue was purified by column chromatography (Hex/EtOAc
6:1), affording 11 (480 mg, 90% yield) as a white solid. Mp: 178-
181 °C. 1H NMR (CD3OD/TMS): δ 2.02 (s, 3 H), 2.06 (s, 3 H),
3.77 (s, 2 H), 6.35 (s,1 H). 2H NMR (CH3OH): δ 2.44 (s). 13C
NMR (CD3OD): δ 11.6, 12.1, 13.6 (m), 113.9, 121.1, 121.8, 124.5,
146.3, 147.5. APCI-MS (in MeOH), m/z (amu): negative ion
mode, 154 (M - H)-. Anal. Calcd for C9H9D3O2 (155.2): C, 69.65;
H, 9.74. Found: C, 69.43; H, 9.93; 97.5% isotope purity by GC-
MS spectrometry.
4-Methyl-6-[(32H3)-5,6-trimethylbenzochinoyl]-4-hexano-
lide (13). Oxidation of 12 (500 mg, 1.78 mmol) by solution of
FeCl3‚6H2O in MeOH/H2O 1:1, following the procedure reported
by Kantoci et al.,8 afforded 13 (420 mg, 82% yield) as yellow solid.
1
Mp: 66-68 °C. H NMR, (CDCl3/TMS): δ 1.39 (s, 3 H), 1.62-
1.69 (m, 2 H), 1.92 (s, 6 H), 1.96-2.28 (m, 2 H), 2.46-2.67 (m,
2
4 H). H NMR (CH3OH): δ 1.95 (s, ArCD3). 13C NMR (CD3OD/
TMS): δ 11.9 (m), 12.1, 12.2, 21.1, 25.3, 28.8, 32.4, 39.1, 85.9,
140.2, 140.5, 142.8, 153.4, 176.2, 186.8, 187.2. APCI-MS (in
MeOH), m/z (amu): positive ion mode, 280 (M + H)+, 297 (M +
NH4)+, 302 (M + Na)+. Anal. Calcd for C16H17D3O4 (279.2): C,
68.79; H, 8.30. Found: C, 68.59; H, 8.15; 97.5% isotope purity
by GC-MS.
(5-2H3)-2,7,8-Tetramethyl-2-(2′-carboxyethyl)-6-hydroxy-
chroman (R-CEHC-d3) (12). A dry dioxane (10 mL) solution
of lactone 2 (1.22 g, 9.63 mmol), prepared following the procedure
reported by Wechter et al.,9 was added over 4 h to a solution of
11 (990 mg, 6.42 mmol) and BF3‚Et2O (0.6 mL, 5 mmol) in dry
dioxane (8 mL) at 110 °C while stirring. After an additional 1 h
at 110 °C, the reaction mixture was cooled to room temperature,
the solvent was evaporated, and the residue was dissolved in
Acknowledgment. The University of Pisa and
MIUR (project “Stereoselezione in Sintesi Organica”) are
ackowledged for the financial support.
JO048514U
9306 J. Org. Chem., Vol. 69, No. 26, 2004