A biomimetic chromanol cyclization leading to α-tocopherol

Article abstract of DOI:10.1002/anie.200503123

(Figure Presented) Vitamin E synthesis: Covalently linked dipeptides serve as readily removable and recyclable chiral auxiliaries in the diastereoselective cyclization of phytylhydroquinones. Depending on the chirality of the handle, (2R)- or (2S)-α-tocop

Full text of DOI:10.1002/anie.200503123

Communications
Vitamin E
DOI: 10.1002/anie.200503123
A Biomimetic Chromanol Cyclization Leading to
a-Tocopherol**
Christian Grütter, Emma Alonso, Antoinette Chougnet,
and Wolf-D. Woggon*
The diastereoselective formation of the chromanol ring is one
of the most challenging reactions during the biosynthesis of
tocopherols in photosynthetic organisms. We discovered the
enzyme that catalyzes this transformation in the cyanobac-
teria Anabaena variabilis[1] and subsequently determined its
substrate specificity[2] and recently cloned the tocopherol
cyclase from the related Anabaena sp. into E. coli.[3] The
enzymatic reaction mechanism was determined by isotopic
labeling experiments,[4] which revealed that the reaction
proceeds by si protonation of the double bond of 1 followed
by re attack of the phenolic oxygen to yield g-tocopherol (2;
Scheme 1).
Two further observations suggest a concerted nonsyn-
chronous cyclization, during which positive charge develops
at the carbon atom that is trapped by the phenol (see 3,
Scheme 2): 1) the tetrahydroisoquinolinium terpenoid 4 as a
transition-state analogue is an excellent inhibitor (IC50 =
1.4 nm) of tocopherol cyclase,[5] and 2) the epoxide of 1
cyclizes under acidic conditions yielding two compounds, the
five-membered-ring “Baldwin” product and the six-mem-
bered ring analogue of the enzymatic product (4:6 ratio).[3,4]
[*] C. Grütter, Dr. E. Alonso, Prof. Dr. A. Chougnet,
Prof. Dr. W.-D. Woggon
Department of Chemistry
University of Basel
St. Johanns-Ring 19, 4056 Basel (Switzerland)
Fax: (+41)61-267-1109
E-mail: wolf-d.woggon@unibas.ch
[**] We thank DSM, F. Hoffmann–La Roche, and the Swiss National
Science Foundation for generous financial support, and Dr. Daniel
Häussinger for NMR spectra.
Supporting information for this article, including spectroscopic data
of 5, 6, 13, 18, and 22-pTsOH, is available on the WWW under
http://www.angewandte.org or from the author.
1126
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1126 –1130

Angewandte
Chemie
First, the target prolinyl phytylhydroquinone derivatives 5
and 6 were prepared from the monoprotected hydroquinone 7
by addition of the Mannich reagents 8 or 9 and followed by
protecting group manipulations (Scheme 3). Exploratory
experiments with 5 and 6 in the presence of either Lewis or
Brønsted acids gave cyclized 10 and 11, respectively, which
were subsequently converted into the a-tocopheryl campha-
nate 12 (Scheme 4). The diastereomeric excess (de) w as
determined by HPLC, and a maximum value of 35–40% de
was obtained in favor of the S configuration at C2. As the
acidity of the chiral unit had no profound effect on the
reaction rate, yield, and de value, it was concluded that
systems 5 and 6 were too flexible and the distances between
the acids and the double bonds were too large.
Scheme 1. Chromanol ring closure catalyzed by tocopherol cyclase.
The structure of the enzyme–substrate (ES) complex of 1 is
tentatively drawn in a conformation that positions the double
bond in between the phenolic oxygen atom and an acid group
from the protein.[3] It appears that the enzyme enforces this
activated conformation in the enzyme–substrate complex;
computer modeling studies showthat 1, in solution or in the
gas phase, adopts a conformation in which the phytyl side
chain is bent down such that the double bond is 6–7 Š apart
from the phenol.
Herein, we report on a new biomimetic approach that
leads to enantiomerically enriched a-tocopherol. (For earlier
syntheses of tocopherols[6,7] and for two recent reports
regarding access to the chromanol unit of tocopherols, see
references [8,9] and references cited therein.) Our concept
comprises 1) the modification of structure 1 by attaching a
chiral acid at C3, b) restriction of the conformational freedom
of this acid through derivatization of the phenolic OH group
at C4 using a bulky substitutent, 3) the choice of a suitable
chiral acid, and 4) removal/recovery of the chiral unit after
cyclization and determination of the product on an a-
tocopherol derivative.
Accordingly, we thought a more-rigid acidic spacer would
be advantageous to accomplish higher diasteromeric excesses
and hence we prepared compound 13 (Scheme 5). Cyclization
of 13 in the presence of pTsOH indeed confirmed the idea, as
12 was obtained with a de value of 80% and (2S)-14 was
obtained as the dominant diastereoisomer (80% overall
yield). In contrast to 13, the monoesters 15 and 16 (Figure 1)
were less efficient, yielding a maximum diastereomeric excess
of 47%, which indicates cooperativity of the two aspartate
COOH groups in the presence of pTsOH. When the aspartate
residue of 13 was replaced by serine or threonine to give 17
and 18, respectively, the Lewis acid SnCl4 was the additive of
choice rather than pTsOH. For example, with (À)-(S,S)-18
and SnCl4 a de value of 77% for the final product 12 (80%
overall yield) was observed; that is, the same value was
observed, within experimental error, as that for the cycliza-
tion of 13 with pTsOH.
The proposed mechanism of this reaction (Figure 2)
resembles the formalism described by Yamamoto and co-
workers[10,11] for a binol/SnCl4 system and apparently belongs
to the category of “Lewis acid assisted, Brønsted acid
Scheme 2. The reaction mechanism of ring closure catalyzed by tocopherol cyclase. ES=enzyme–substrate; EP=enzyme–product.
Angew. Chem. Int. Ed. 2006, 45, 1126 –1130 ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1127

Communications
Scheme 3. Synthesis of cyclization precursors 5 and 6. Reagents and conditions: a) 1) (S)-proline, HCHO (35% in water), 408C, 10 min; 2) 8 (in
MeOH), 7, 408C, 8 h, 87%; b) (Me)3SiCHN2, MeOH, room temperature, 30 min, 98%; c) (À)-camphanoyl chloride, DMAP, CH2Cl2, room
temperature, 2 h, 96%; d) LiI, EtOAc, 708C, under N2, 13 h , 70%; e) 1n HCl, THF, room temperature, 1 h, 95%; f) 9 added to 7 in MeOH, 508C,
2 days, 80%; g) (À)-camphanoyl chloride, DMAP, CH2Cl2, room temperature, 1 h, 98%; h) MeOTf, DMAP, room temperature, 4 h, 83%; i) LiI,
EtOAc, 408C, 16 h, 65%; j) 1n HCl, THF, room temperature, 1 h, 89%. THP=tetrahydropyranyl; DMAP=(4-dimethyl)aminopyridine;
Tf =trifluoromethanesulfonyl.
supported reactions” (see compound 19; binol = 1,1’-bi-2-
naphthol).
(+)-(R,R)-20, cyclization of the latter occurred in the
presence of pTsOH to yield the natural (2R,4’R,8’R)-a-
tocopherol (21) as the dominant diastereoisomer (70% de) of
the reaction (Scheme 6).[12]
When the S-configured amino acids and the (À)-campha-
nate in 13 were replaced by their enantiomers to give
The superior results with the proline–aspartate phytyl-
hydroquinone 13 relative to the monoesters 15 and 16 suggest
cooperativity of the two “free” COOH units. Furthermore, as
more than one equivalent of pTsOH is required for cycliza-
tion a Brønsted acid assisted, Brønsted acid supported
reaction may be operative.[10] The structure of the reactive
supramolecular assembly could be tentatively assigned from
1H NMR spectroscopy of the precursor (À)-(R,R)-22 in
CH2Cl2 at 58C. In the absence of pTsOH a slightly resolved
NMR spectrum was obtained that showed the presence of two
conformations in 6:4 ratio. After addition of two equivalents
of pTsOH the system converged to one conformation that
displayed a well-resolved signal pattern. From these spectra a
significant NOE between a benzylic proton (at C7, in green)
and the proton at the stereogenic center of the proline moiety
(at C2’, in green) was observed which indicated that the
proline--aspartate unit of 22 lies belowthe double bond
(Scheme 7), ready to add the proton from the si face in an
enzyme-like fashion.
Interestingly a further significant NOE was detected
between the meta-positioned hydrogen atom of pTsOH (in
red) and the allylic methyl group of the phytyl side chain.
These results suggest that the second pTsOH molecule forms
the fairly stable complex 22-pTsOH through hydrogen
Scheme 4. Chromanol cyclization of 5 and 6. Reagents and conditions:
a) pTsOH (2.5 equiv), propylene carbonate, 508C, 17 h, 70%, 38% de;
b) AlCl3 (0.7 equiv), CH2Cl2, room temperature, 2 days, 65%, 35% de;
c) Pd/C (15–20%), HCOOH, MeOH, H2 (85 bar, room temperature)
2 days, 92%. pTsOH=p-toluenesulfonic acid.
=
bonding of the two S O bonds to aspartate COOH groups
to enhance the acidity of pTsOH. Accordingly, the proton
required to initiate cyclization appears to be delivered from
the second pTsOH moiety.
1128
www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1126 –1130

Angewandte
Chemie
Figure 2. Proline–serine and proline–threonine cyclization precursors
16 and 17, respectively.
Scheme 5. Synthesis of the proline–aspartate phytylhydroquinone 13,
and its cyclization to the chromanol. Reagents and conditions:
a) 1) HATU (1.4 equiv), DIPEA (6 equiv), di-Fm-aspartate
(3 equiv),CH2Cl2, room temperature, 2 h; 2) 20% Et2NH in CH2Cl2,
1 h, room temperature; iii) KHSO4, 59%; b) pTsOH (2 equiv), CH2Cl2,
room temperature, 2 days, 85%; c) Pd/C (15–20%), HCOOH, MeOH,
H2 (85 bar, room temperature) 2 days, 92%. HATU=N,N,N,N-
tetramethyluronium hexafluorophosphate; DIPEA=diisopropylethyl-
amine; Fm=9-fluorenylmethyl.
Scheme 6. Cyclization of (+)-(R,R)-20 to give natural a-tocopherol.
and concomitant attack of the phenolic oxygen atom. Similar
results are obtained with a proline–threonine moiety in the
presence of SnCl4. To the bets of our knowledge, this is the
first biomimetic cyclization that leads to the chromane
structure of tocopherols with high diastereoselectivity.
Figure 1. Aspartate monoesters for cyclization.
Experimental Section
General procedure for the acid-supported cyclization of phytylhy-
droquinones: Anhydrous p-toluenesulfonic acid (0.0715 mmol,
2 equiv; 1.23 mL of a 0.058m solution (10 mgmLÀ1) in CH3CN) was
In conclusion, through attachment of a proline–aspartate
unit to a phytylhydroquinone an enzyme-like conformation of
the cyclization precursor is created that allows for diastereo-
face-selective preferential protonation of the double bond
added to
a
round-bottomed flask containing 13 (30.0 mg,
0.0358 mmol) in CH2Cl2 (40 mL) at room temperature under argon,
Angew. Chem. Int. Ed. 2006, 45, 1126 –1130
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1129

Communications
[4] A. Stocker, T. Netscher, A. Rüttimann, R. K. Müller, H.
Schneider, L. J. Todaro, G. Derungs, W.-D. Woggon, Helv.
Chim. Acta 1994, 77, 1721.
[5] R. Manetsch, L. Zheng, M. T. Reymond, W.-D. Woggon, J.-L.
Reymond, Chem. Eur. J. 2004, 10, 2487.
[6] T. Netscher, Chimia 1996, 50, 563.
[7] T. Netscher, M. Scalone, R. Schmid in Asymmetric Catalysis on
an Industrial Scale (Eds.: H. U. Blaser, E. Schmidt), Wiley-VCH,
Weinheim, 2004, p. 71.
[8] L. F. Tietze, K. M. Sommer, J. Zinngrebe, F. Stecker, Angew.
Chem. 2005, 117, 262; Angew. Chem. Int. Ed. 2005, 44, 257.
[9] B. M. Trost, H. C. Shen, L. Dong, J.-P. Surivet, C. Sylvain, J. Am.
Chem. Soc. 2004, 126, 11966.
[10] H. Yamamoto, K. Futatsugi, Angew. Chem. 2005, 117, 1958;
Angew. Chem. Int. Ed. 2005, 44, 1924.
[11] M. Uyanik, H. Ishibashi, K. Ishihara, H. Yamamoto, Org. Lett.
2005, 7, 1601.
[12] The diastereo-face-selectivity depends on the configurations of
the dipeptide, whereas the influence of the camphanoyl unit is
negligible. The use of (À)-camphanate is purely for convenience,
as tocopherols that are epimeric at C2 can be separated by
HPLC and hence the de of the cyclization can be easily
determined (C. Grütter, E. Alonso, A. Chougnet, W.-D.
Woggon, unpublished results).
Scheme 7. Proposed mechanism for the diastereoselective protonation of a
phytylhydroquinone. Colored protons show significant NOE interactions (see text
for details).
and the solution was stirred for 48 h at this temperature. Saturated
NaHCO3 solution (10 mL) was added, and the mixture was extracted
with CH2Cl2 (3 ” 10 mL). The organic layers were combined, dried
(Na2SO4), and filtered. Evaporation of the solvents provided the
crude product, which was taken up in CH2Cl2/MeOH (4:1) and
filtered through celite to give 14 as a colorless solid (28 mg, 93%).
This material was used directly in the reduction step.
General procedure for reductive debenzylation: Formic acid
(99%; 15 mL) and palladium hydroxide (6 mg) on activated charcoal
(15–20% Pd, moistened with water ꢀ 50%) were added to a solution
of 14 (3.0 mg, 3.58 mmol) in a mixture of MeOH (2.5 mL) and CH2Cl2
(0.1 mL). The reaction was stirred at room temperature under a
hydrogen pressure of 85 bar for 2 days. The reaction mixture was
filtered through a cotton plug, quenched with a saturated solution of
NaHCO3, and extracted with CH2Cl2 (2 ” 10 mL). The combined
organic layers were dried (Na2SO4) and filtered, and the solvents were
removed under vacuum. The resulting crude product was filtered, first
through celite and then through silica gel (CH2Cl2/MeOH 95:5) to
give 12 (2.3 mg, 92%). The diastereomeric excess (de) was deter-
mined by HPLC (DAICEL Chiralpac AD-H, 1% 2-propanol in
heptane; 0.5 mLminÀ1); retention time: 28.9 min for (À)-(S,R,R)-12
and 30.9 min for (À)-(R,R,R)-12, ratio 9:1). Correlations were made
by admixing the camphanate of authentic all-(R)-a-tocopherol.
Received: September 2, 2005
Keywords: biomimetic synthesis · cyclization ·
.
diastereoselectivity · enzyme catalysis · peptides
[1] A. Stocker, A. Rüttimann, W.-D. Woggon, Helv. Chim. Acta
1993, 76, 1729.
[2] A. Stocker, H. Fretz, H. Frick, A. Rüttimann, W.-D. Woggon,
Bioorg. Med. Chem. 1996, 4, 1129.
[3] M. Christen, A. Chougnet, R. Manetsch, J. Chapelat, B. Christen
U. Jenal, W.-D. Woggon, unpublished results.
1130
www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1126 –1130