Total synthesis of (±)-phytochromobilin starting from two pyrrole derivatives

Article abstract of DOI:10.1055/s-1999-3105

(±)-Phytochromobilin was synthesized as an acid form by developing a convenient method for the preparation of A- and D-rings starting from a 2- tosylpyrrole derivative, followed by efficient construction of A/B- and C/D- ring components via Wittig-type coupling reaction of 5-tosylpyrrolinones with 2-formylpyrrole, and palladium catalyzed deprotection of allyl esters of propanoic acid side chains at C-8 and C-12.

Full text of DOI:10.1055/s-1999-3105

LETTER
901
Total Synthesis of (±)-Phytochromobilin Starting from Two Pyrrole
Derivatives
Takashi Kakiuchi, Hideki Kinoshita, Katsuhiko Inomata*
Department of Chemical Science, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa,
Ishikawa 920-1192, Japan
Fax +81(76)2645742; E-mail: inomata@cacheibm.s.kanazawa-u.ac.jp
Received 1 February 1999
Dedicated to Professor Albert Eschenmoser in recognition of his fundamental contributions to organic chemistry
bilin (2) and its derivatives using 4-(1-methoxyethyl)-3-
Abstract: (±)-Phytochromobilin was synthesized as an acid form
methyl-5-tosyl-1,5-dihydro-2H-pyrrol-2-one as a precur-
by developing a convenient method for the preparation of A- and D-
sor of the A-ring and 2-formylpyrrole 11.3c
rings starting from a 2-tosylpyrrole derivative, followed by efficient
construction of A/B- and C/D-ring components via Wittig-type cou-
pling reaction of 5-tosylpyrrolinones with 2-formylpyrrole, and pal-
ladium catalyzed deprotection of allyl esters of propanoic acid side
chains at C-8 and C-12.
Key words: phytochrome, tetrapyrrole, phytochromobilin, total
synthesis, transformation of pyrroles
Phytochrome is a chromoprotein concerned in a variety of
processes in higher plants such as growth, development,
and morphogenesis etc. The chromophore named phyto-
chromobilin (1) is a linear tetrapyrrole derivative and co-
valently bound to the apoprotein at A-ring. Recent
developments in gene technology have made it possible to
assemble the chromophores such as phytochromobilin (1)
and phycocyanobilin (2), the latter of which is a chro-
mophore of phycocyanin found in algal photosynthetic
systems, with the apoproteins obtained by the over-ex-
pression of the corresponding cDNA in bacteria and yeast.
The photophysical and photochemical properties of wild
type phytochrome are quite similar to those of the recon-
stituted phytochromes.1
On the other hand, though the total syntheses of dimethyl
ester derivatives of the chromophores 1 and 2 have been
reported by Gossauer and his co-workers,2 there had been
no report regarding the syntheses of their acid forms ap-
plicable to assemble with the apoproteins.
Scheme 1
If it is possible to construct the A/B- and C/D-rings com-
ponents of 1 starting from the same building blocks and
the same coupling way, respectively, it will provide a
much more efficient route for the total synthesis of 1.
Based on this strategy, we first prepared the A/B-ring
component (Scheme 2).
In 1998, we reported the total synthesis of 2 and its deriv-
atives bearing a photoreactive group at the D-ring in their
acid form for the structure/function analysis of phyto-
chrome.3 Also Gärtner and his co-workers reported the
syntheses of 1 and its isomer by the cleavage at C-10 of
biliverdin IXa dimethyl ester with the thiobarbiturate an-
ion.4
The aldehyde 5, readily available by addition of p-tolu-
enethiol to acrolein (4), was reacted with nitroethane to
give nitroalcohol 6, which was acetylated and cyclized
utilizing tosylmethyl isocyanide (TosMIC)5 in the pres-
ence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to af-
ford the pyrrole derivative 8 being regarded as a common
precursor of the A- and D-rings.6 The 5-tosylpyrrolinone
10 was obtained from 8 by bromination and subsequent
acid hydrolysis, then coupled with 11 using DBU and
PBu3 to afford 12 according to our previous method.7
In this paper, we wish to describe the novel total synthesis
of (±)-phytochromobilin (1) in its acid form starting from
4-methyl-3-[2-(tolythio)ethyl]-2-tosylpyrrole (8) as a
common precursor of the A- and D-rings and 2-
formylpyrrole 11 which is common to the B- and C-rings
via our Wittig-type coupling reaction (Scheme 1).
Recently we reported a new and efficient construction of
the A/B-ring component for the syntheses of phycocyano-
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902
T. Kakiuchi et al.
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The 2-(tolylthio)ethyl group of 12 was converted to a vi-
nyl group by heating the corresponding sulfoxide ob-
tained by oxidation with mCPBA to afford 13. Compound
13 was then reduced with aluminum amalgam to 14,8 fol-
lowed by acid treatment to result in the formation of A/B-
ring component 15.9 The transformation of 13 to 15
through the reduced intermediate 14 is of interest in con-
nection with the mostly speculative pathway for the trans-
formation of biliverdin IXa to
1
through
“dihydrobiliverdin.”10
On the other hand, we previously reported a novel syn-
thetic method of C/D-ring component of phytochromobi-
lin dimethyl ester utilizing 4-methyl-5-tosyl-3-(2-
tosylethyl)pyrrolinone.7c However, it required a strong
base and high temperature at the stage to introduce a vinyl
group to the D-ring by elimination of the tosyl group.
Therefore, 2-tosylpyrrole 8 was at first transformed to 17
by tosyl rearrangement of the sulfoxide 16 obtained by
oxidation of 8 (Scheme 3).11 Then the sulfoxide 17 was re-
duced by (COCl)2/NaI in CH3CN to the sulfide 18.12 We
then tried to hydrolyze the a-brominated pyrrole 19 to the
5-tosylpyrrolinone 20 under acidic conditions as previ-
ously reported,7a but the yield was disappointingly poor.
Many attempts to improve the yield of the desired 20 by
hydrolysis were unfruitful. Ultimately, it was found that
20 as a precursor of D-ring was available in high yield by
treating the corresponding sulfoxide of 19 with NaI in
TFA under anhydrous conditions for a short time. A prob-
able reaction mechanism is shown in Scheme 4.
Scheme 2
a TolSH (0.83 eq) in THF/H2O (2/1, v/v) at rt, 3 h; 5 98% (based on
TolSH). b 1 M KOH/MeOH (0.1 eq) in EtNO2 (20 eq); 6 93%. c Ac2O
(1.4 eq), DMAP (0.04 eq) in THF at rt, 10 min; 7 quant. d TosMIC (1.0
e
eq), DBU (2.2 eq) in CH3CN at -40 °C - rt, overnight at rt; 8 96%.
PhMe3N+Br3- (1.1 eq) in CH2Cl2 at 0 °C, 10 min; 9 98%. f TFA/H2O
(5/1, v/v) at rt, 4 h; 10 63%. g (i) THF solution of DBU (1.5 eq) was
added dropwise over a period of 30 min to the mixed solution of 11
(1.0 eq), 10 (1.2 eq), PBu3 (2.0 eq) in THF at 0 °C - rt, 21 h at rt. (ii)
cat. I2 in CH2Cl2 at rt, 2 d; 12 80%. h (i) mCPBA (1.0 eq) in CH2Cl2 at
rt, 5 min. (ii) xylene, reflux, 2 h; 13 95%. i (i) Al(Hg) (3.0 eq) in THF/
H2O (10/1, v/v) at rt, 1 h. (ii) TsOH·H2O (1.0 eq) in CH2Cl2 at rt, 2
min; 15 75%.
Scheme 4
The 5-tosylpyrrolinone 20 thus obtained was coupled with
the 2-formylpyrrole 11 using PBu3 in the presence of
DBU to afford 21, which was transformed to C/D-ring
component 2313 via successive treatments, namely, oxida-
tion to the sulfoxide 22, formylation with methyl orthofor-
mate under acidic conditions accompanied by
decarboxylation, and elimination of p-toluenesulfinic acid
to form a vinyl group.
Scheme 3
a mCPBA (1.0 eq) in CH2Cl2 at 0 °C, 10 min; 16 97%. b TsNa (0.1 eq)
in CHCl3/TFA (9/1, v/v) at 25 °C, 48 h; 17 42% (34% of 16 was re-
covered). c (COCl)2 (1.2 eq), NaI (2.5 eq) in CH3CN at 0 °C, 10 min;
18 92%. d PhMe3N+Br3- (1.1 eq) in CH2Cl2 at 0 °C, 10 min; 19 quant.
e (i) mCPBA (1.0 eq) in CH2Cl2 at rt, 5 min. (ii) NaI (5 eq) in TFA at
Thus, the A/B- and C/D-ring components (15 and 23)
were in hand. They were coupled in methanol using conc.
sulfuric acid as a catalyst after removing the t-butoxycar-
bonyl group of 15 with TFA to afford the desired diallyl
ester of (±)-phytochromobilin (3), whose structure was
fully characterized by spectroscopic methods (Scheme
5).14
f
rt, 10 min; 20 90%. (i) THF solution of DBU (1.5 eq) was added
dropwise over a period of 30 min to the mixed solution of 11 (1.0 eq),
20 (1.2 eq), PBu3 (2.0 eq) in THF at 0 °C - rt, 8 h at rt. (ii) cat. I2 in
CH2Cl2 at rt, overnight; 21 77%. g mCPBA (1.0 eq) in CH2Cl2 at 0 °C,
1 min; 22 95%. h (i) TFA at rt, 30 min, then CH(OMe)3 at rt, 1 h. (ii)
xylene, reflux, 1 h; 23 62%.
Synlett 1999, S1, 901–904 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Total Synthesis of (±)-Phytochromobilin
903
(7) (a) Kinoshita, H.; Hayashi, Y.; Murata, Y.; Inomata, K. Chem.
Lett. 1993, 1437. (b) Kinoshita, K.; Ngwe, H.; Kohori, K.;
Inomata, K. Chem. Lett. 1993, 1441. (c) Kohori, K.;
Hashimoto, M.; Kinoshita, H.; Inomata, K. Bull. Chem. Soc.
Jpn. 1994, 67, 3088.
(8) Gossauer, A.; Blacha-Puller, M. Liebigs Ann. Chem. 1981,
1492. Structure of 14 was confirmed by 1H NMR spectrum.
(9) Gossauer, A.; Nydegger, F.; Benedikt, E.; Köst, H.-P. Helv.
Chim. Acta 1989, 72, 518.
15: mp 112.0-113.5 °C (from cyclohexane/hexane); IR (KBr)
3369, 3169, 2979, 1732, 1699, 1681, 1640, 1442, 1364, 1318,
1278, 1254, 1180, 1156, 1127, 1053, 989, 934, 772, 713 cm-1;
1H NMR (CDCl3) d = 1.39 (d, J = 7.32 Hz, 3H), 1.54 (s, 9H),
1.85 (d, J = 7.32 Hz, 3H), 1.98 (s, 3H), 2.54 (t, J = 8.17 Hz,
2H), 3.00 (t, J = 8.17 Hz, 2H), 3.24 (brq, J = 7.56 Hz, 1H),
4.59 (d, J = 5.85 Hz, 2H), 5.23 (dd, J = 1.22, 10.09 Hz, 1H),
5.30 (dd, J = 1.22, 17.20 Hz, 1H), 5.70 (s, 1H), 5.92 (ddt,
J = 10.09, 17.20, 5.85 Hz, 1H), 6.18 (dq, J = 2.20, 7.32 Hz,
1H), 8.58 (s, 1H), 9.05 (s, 1H) ppm; Found: C, 67.13; H, 7.61;
N, 6.36%. Calcd for C24H32N2O5: C, 67.27; H, 7.53; N, 6.54%.
(10) Falk, H. The Chemistry of Linear Oligopyrroles and Bile
Pigment; Springer: Wien-New York, 1989; p 36.
Scheme 5
a 15 (2.0 eq), TFA at rt, 20 min, then TFA was evaporated and MeOH
solution of 23 (1.0 eq) was added, cat. conc. H2SO4 at rt, 1 h; 3 80%.
b cat. Pd(PPh3)4, morpholine (4.0 eq) in THF at rt, 1 h; 1 not yet opti-
mized.16
(11) Kohori, K.; Kinoshita, H.; Inomata, K. Chem. Lett. 1995, 799.
(12) Olah, G. A.; Malhotra, R.; Narang, S. C. Synthesis 1979, 58.
Direct bromination of the sulfoxide 17 produced not only the
desired product, but also the reduced compound 19 in low
yield.
The allyl ester groups could be deprotected15 according to
the method described in the previous paper to obtain the
acid form of phytochromobilin (1).16 However, 1 was ex-
tremely unstable compared to phycocyanobilin (2) pre-
pared previously.3,4
(13) 23: mp 171.5-173.0 °C (from CHCl3/AcOEt); IR (KBr) 3330,
3166, 3016, 2920, 2860, 1726, 1704, 1661, 1594, 1543, 1507,
1450, 1415, 1387, 1340, 1258, 1169, 1094, 986, 927, 793, 712
cm-1; 1H NMR (CDCl3) d = 2.10 (s, 3H), 2.11 (s, 3H), 2.61 (t,
J = 7.61 Hz, 2H), 3.06 (t, J = 7.61 Hz, 2H), 4.58 (d, J = 7.87
Hz, 2H), 5.21 (dd, J = 1.33, 10.36 Hz, 1H), 5.29 (dd, J = 1.33,
17.06 Hz, 1H), 5.42 (dd, J = 1.74, 11.60 Hz, 1H), 5.90 (ddt,
J = 10.36, 17.06, 7.87 Hz, 1H), 5.93 (s, 1H), 6.16 (dd,
J = 1.74, 17.43 Hz, 1H), 6.47 (dd, J = 11.60, 17.43 Hz, 1H),
9.66 (s, 1H), 10.67 (s, 1H), 11.01 (s, 1H) ppm; Found: C,
67.41; H, 6.37; N, 7.63%. Calcd for C20H22O4N2: C, 67.78; H,
6.26; N, 7.90%; HRMS (EI): (M+), Found: m/z 354.1583.
Calcd for C20H22O4N2:354.1580.
(14) 3: mp 177.5-179.0 °C (decomp.) (from CHCl3/hexane); IR
(KBr) 3337, 3088, 3009, 2914, 1733, 1675, 1610, 1590, 1449,
1416, 1375, 1279, 1243, 1223, 1166, 1096, 1052, 981, 929,
803, 748, 697 cm-1; 1H NMR (CDCl3) d = 1.33 (d, J = 7.44 Hz,
3H), 1.88 (d, J = 7.32 Hz, 3H), 2.05 (s, 3H), 2.14 (s, 3H), 2.21
(s, 3H), 2.58 (t, J = 7.56 Hz, 4H), 2.91 (t, J = 7.56 Hz, 2H),
2.95 (t, J = 7.56 Hz, 2H), 3.11 (brq, J = 7.44 Hz, 1H), 4.58 (m,
4H), 5.22 (dm, J = ca.10.3 Hz, 2H), 5.28 (dm, J = 17.08 Hz,
1H), 5.29 (dm, J = 17.08 Hz, 1H), 5.40 (dd, J = 2.20, 11.53
Hz, 1H), 5.83 (s, 1H), 5.84-5.95 (m, 1H), 6.09 (s, 1H), 6.22
(dd, J = 2.20, 17.63 Hz, 1H), 6.39 (dq, J = 2.19, 7.32 Hz, 1H),
6.52 (dd, J = 11.53, 17.63 Hz, 1H), 6.65 (s, 1H), 7.26 (s,
ca.2H) ppm; UV/Vis (MeOH) lmax 372 (e = 50,000), 631
(e = 16,000) nm; HRMS (FAB): (M++1), Found: m/z
665.3328. Calcd for C39H45O6 N4:665.3339.
As described above, phytochromobilin (1) was success-
fully prepared starting from two pyrrole derivatives, 8 and
11, employing novel synthetic reactions, i.e., 1) rearrange-
ment of the tosyl group of 2-tosylpyrroles to the 5-posi-
tion under acidic conditions, 2) efficient transformation of
a-bromopyrroles to the corresponding pyrrolinones under
anhydrous acidic conditions, 3) Wittig-type coupling re-
action between 5-tosylpyrrolinones and 2-formylpyrrole,
followed by reductive transformation to the A/B-ring
component, 4) protection and deprotection of propanoic
acid side chains via allyl esters.
Investigation on the reconstituted chromoproteins using
synthesized racemic 1 and other phycobilins prepared so
far is in progress for the structure/function analysis of
phytochrome.
References and Notes
(1) Schmidt, P.; Westphal, U.; Worm, S.; Braslavsky, S. E.;
Gärtner, W.; Schaffner, K. J. Photochem. Photobiol. B :
Biology 1996, 34, 73 and references cited therein.
(2) (a) Gossauer, A.; Hinze, R.-P. J. Org. Chem. 1978, 43, 283.
(b) Weller, J.-P.; Gossauer, A. Chem. Ber. 1980, 113, 1603.
(3) (a) Masukawa, T.; Kato, H.; Kakiuchi, T.; Jayasundera, K. P.;
Kinoshita, H.; Inomata, K. Chem. Lett. 1998, 455.
(b) Kakiuchi, T.; Kato, H.; Jayasundera, K. P.; Higashi, T.;
Watabe, K.; Sawamoto, D.; Kinoshita, H.; Inomata, K. Chem.
Lett. 1998, 1001. (c) Jayasundera, K. P.; Kinoshita, H.;
Inomata, K. Chem. Lett. 1998, 1227.
(4) Lindner, I.; Knipp, B.; Braslavsky, S. E.; Gärtner, W.;
Schaffner, K. Angew. Chem., Int. Ed. Engl. 1998, 37, 1843.
(5) Hoogenboon, B. E.; Oldenziel, O. H.; van Leusen, A. M. Org.
Synth. 1988, Coll. Vol. VI, 987.
(6) Barton, D. H. R.; Kervagoret, J.; Zard, S. Z. Tetrahedron
1990, 46, 7587.
(15) Kunz, H.; Unverzagt, C. Angew. Chem., Int. Ed. Engl. 1984,
23, 436.
(16) To a mixed solution of 3 (31 mg, 0.0467 mmol) and Pd(PPh3)4
(11 mg, 0.00952 mmol) in THF (5 ml) was added a 0.1 M THF
solution (1.87 ml) of morpholine (0.187 mmol) in the dark
under nitrogen atmosphere at room temperature. After stirring
for 1 h at room temperature, the solvent was evaporated. The
resulting residue was separated by a silica gel column
chromatography (CHCl3/MeOH/AcOH = 200/15/1). The last
green fraction was evaporated and dissolved again in CHCl3
containing 1% MeOH. After a repetition of the back-
extraction with 1/15 M phosphate buffer solution (pH 7.8;
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KH2PO4/Na2HPO4;10 ml x 4), the combined aqueous extracts
were acidified by addition of 1 M acetate buffer solution (pH
4.8; NaOAc/AcOH) and extracted with CHCl3 containing 1%
MeOH (10 ml x 3). The solid residue obtained by evaporation
of organic solvent was recrystallized from CHCl3 (containing
1% MeOH)/hexane.
temperature in the dark under nitrogen atmosphere). UV/Vis
(MeOH) lmax 372 (e; not determined), 636 (e; not
determined) nm, e372/e636 = 3.20; HRMS (FAB) (M++1),
Found: m/z 585.2725. Calcd for C33 H37O6N4:585.2713.
1: 2 mg (a purple solid; 7%; not yet optimized. Though
improvement of isolation and purification of unstable
(especially during separation) 1 is now in progress, it seems to
be rather stable after recrystallization upon storage at low
Article Identifier:
1437-2096,E;1999,0,S1,0901,0904,ftx,en;W05199ST.pdf
Synlett 1999, S1, 901–904 ISSN 0936-5214 © Thieme Stuttgart · New York