Chemoenzymatic formal synthesis of (S)-(-)-phosphonotrixin

Article abstract of DOI:10.1016/j.tetasy.2004.04.046

Phosphonotrixin is a herbicidal antibiotic characterized by a unique structure bearing a C-P bond on an isoprene unit. The formal enantioselective synthesis of (S)-(-)-phosphonotrixin (ee=93%) in nine steps and 12% overall yield from 1,3-dihydroxyacetone

Full text of DOI:10.1016/j.tetasy.2004.04.046

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 1889–1892
Chemoenzymatic formal synthesis of (S)-(–)-phosphonotrixin
a,
Robert Chenevert, Mathieu Simard, Jerome Bergeron and Mohammed Dasser
a
a
b
*
^
ꢀ ^
aDꢀepartement de chimie, Facultꢀe des sciences et de gꢀenie, Universitꢀe Laval, Quꢀebec, Canada G1K 7P4
bOmegaChem, 8800, boul. de la Rive-Sud, Lꢀevis (QC), Canada G6V 9H1
Received 22 March 2004; accepted 9 April 2004
Abstract—Phosphonotrixin is a herbicidal antibiotic characterized by a unique structure bearing a C–P bond on an isoprene unit.
The formal enantioselective synthesis of (S)-(ꢀ)-phosphonotrixin (ee ¼ 93%) in nine steps and 12% overall yield from 1,3-dihy-
droxyacetone or in eight steps and 11% overall yield from 1,3-dichloroacetone is reported. The key reaction is the enzymatic
desymmetrization of 2-isopropenylpropane-1,2,3-triol.
ꢀ 2004 Elsevier Ltd. All rights reserved.
1. Introduction
O
O
OH
a
b
Phosphonotrixin 14 was first isolated from the micro-
organism Saccharotrix sp. ST-888 by Takahashi et al.1;2
in 1995. This secondary metabolite has attracted interest
primarily due to its antibiotic and herbicidal proper-
ties1;3 as well as its unique structure possessing a C–P
bond on an isoprene unit. Two syntheses of racemic
phosphonotrixin have been reported.4;5 The assignment
of the absolute configuration of natural (S)-(ꢀ)-phos-
phonotrixin is based on the synthesis of both enantio-
mers6;7 and the use of the Sharpless’ epoxidation
prediction rule.8 Herein we report an enantioselective
formal synthesis of (S)-(ꢀ)-phosphonotrixin via the
enzymatic desymmetrization of an achiral substrate.
HO
OH
TBSO
OTBS
TBSO
OTBS
1
2
3
4
c
O
OH
OH
OH
Cl
b
d
HO
Cl
Cl
Cl
5
6
Scheme 1. Reagents and conditions: (a) TBSCl, imidazole, DMAP,
DMF, 94%, Ref. 9; (b) (i) isopropenylmagnesium bromide, THF, 0–
20 ꢁC, (ii) 3 M HCl, 84–87%; (c) TBAF, THF, 81%; (d) 6 M NaOH,
50 ꢁC, 73%.
2. Results and discussion
fluoride (TBAF) afforded triol 4 in 81% yield. Com-
pound 4 was also prepared in two steps from 1,3-di-
chloroacetone 5 without any group protection. Grignard
reaction of isopropenylmagnesium bromide with 5 in
THF gave tertiary alcohol 6 in 84% yield. Treatment of
6 with aqueous sodium hydroxide led to triol 4 in 73%
yield. Next, we completed some screening experiments in
order to find hydrolases with the ability to distinguish
the enantiotopic groups of compound 4. Of the enzymes
and conditions studied, the esterification of 4 with vinyl
acetate as the acylation reagent and solvent in the
presence of porcine pancreatic lipase (PPL) gave the best
result (Scheme 2) and provided chiral monoester (S)-7 in
high yield (88%) and high enantiomeric excess (93%)
along with a small amount of the corresponding achiral
Retrosynthetic analysis of the phosphonotrixin structure
suggested that prochiral triol 4 might serve as a suitable
substrate for an enzyme-catalyzed desymmetrization.
The synthesis of triol 4 is described in Scheme 1. Readily
available 1,3-dihydroxyacetone 1 was selected as the
starting material. The bis-silyl ether 2 was prepared as
described in the literature.9 A Grignard reaction of
isopropenylmagnesium bromide with the dihydroxyac-
etone derivative 2 in THF provided tertiary alcohol 3 in
87% yield. Desilylation of 3 with tetrabutylammonium
* Corresponding author. E-mail: robert.chenevert@chm.ulaval.ca
0957-4166/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.04.046

1890
R. Ch^enevert et al. / Tetrahedron: Asymmetry 15 (2004) 1889–1892
3. Conclusion
OH
OH
OH
OH
OH
a
+
In conclusion, enzymatic desymmetrization of achiral
2-isopropenylpropane-1,2,3-triol 4 has provided the
corresponding (S)-monoacetate 7 in good yield (88%)
with high enantiomeric excess (93%). This product is a
key intermediate in the formal synthesis of (S)-(ꢀ)-
phosphonotrixin.
HO
AcO
OAc
AcO
(S)-7
4
8
Scheme 2. Reagents and conditions: (a) PPL, vinyl acetate, 88%.
diester 8 (7%). Desymmetrizations of 4 with lipases from
Candida antarctica, Pseudomonas sp., Pseudomonas
cepacia and Candida rugosa resulted in lower enantio-
meric excesses (ee <60%) or lower chemical yields (for-
mation of achiral diester 8). The enantiomeric
composition of 7 was measured by 19F NMR (376 MHz)
analysis of the corresponding (þ)-a-methoxy-a-trifluo-
romethyl-a-phenyl acetate (Mosher’s ester). The abso-
lute configuration of monoacetate (S)-7 was determined
by chemical correlation with 13, an advanced interme-
diate in Nakamura’s synthesis of phosphonotrixin 14.
4. Experimental
4.1. General
NMR spectra were recorded on Bruker AC300 or Var-
ian Inova AS400 spectrometers (300 and 400 MHz,
respectively). Infrared spectra were recorded on a
Bomem MB-100 spectrometer. Optical rotations were
measured using a JASCO DIP-360 digital polarimeter (c
as gram of compound per 100 mL). Flash column
chromatography was carried out using 40–63 lm (230–
400 mesh) silica gel. PPL (lipase, type II, crude, from
porcine pancreas, 23% protein, 61 units/mg of protein
using triacetin at pH 7.4) was purchased from Sigma
Chem. Co. Candida antarctica lipase (Chirazyme L-2)
was from Boehringer Mannheim.
Our initial goal was to transform the primary alcohol of
compound 7 into a good leaving group and introduce
the C–P bond by nucleophilic substitution. Tosylate 9
was easily prepared (Scheme 3) but the formation of 10
was unsuccessful with phosphorus nucleophiles under
various conditions. To avoid this difficulty, we finally
adopted the alternative reaction sequence illustrated in
Scheme 3.
Formation of the tosylate in the presence of a stronger
base, such as triethylamine, gave epoxide 11 in 71%. The
acetate group of 11 was selectively removed by trans-
esterification with ethanol in ether in the presence of
Candida antarctica lipase. Epoxy-alcohol 12 is unstable
to acidic media (e.g., silica gel, chloroform) and was
immediately transformed into phosphonate 13, whose
data matched that reported by Nakamura et al.6;7 thus
completing a formal synthesis of natural (S)-(ꢀ)-phos-
photrixin. This correlation confirmed the absolute con-
figuration of 13 to be (R), which requires that the
enzymatic desymmetrization of 4 leads to monoester 7
with an (S)-configuration at the newly created stereo-
genic centre.
4.2. 1,1-Bis-{[(tert-butyldimethylsilyl)oxy]methyl}-2-
methylprop-2-en-1-ol 3
To a solution of isopropenylmagnesium bromide in
anhydrous THF (25 mL) freshly prepared from magne-
sium powder (2.48 g, 102 mmol) and 2-bromopropene
(3.02 mL, 34 mmol) cooled to 0 ꢁC was added a solution
of 2 (4.35 g, 13.7 mmol) in THF (15 mL). The mixture
was stirred at room temperature for 1.5 h. The mixture
was cooled to 0 ꢁC and 3 M HCl (100 mL) then added.
The aqueous phase was extracted with ethyl acetate
(3 ꢁ 100 mL). The combined organic phases were
washed with brine (2 · 100 mL), dried over MgSO4 and
evaporated. The crude product was purified by flash
O
OH
OH
OH
OH
a
c
AcO
OTs
AcO
P O
OR
AcO
(S)-7
RO
(R)-9
10
b
O
O
O
OH
OH
d
e
AcO
(S)-11
HO
(R)-12
HO
BnO
P
O
OBn
HO
P O
HO
OH
(R)-13
(S)-14
phosphonothrixin
Scheme 3. Reagents and conditions: (a) TsCl, pyridine, CHCl3, 97%; (b) TsCl, Et3N, DMAP, CH2Cl2, 71%; (c) Candida antartica lipase, EtOH,
Et2O, 90%; (d) Ref. 6; (e) two steps, Ref. 6.

R. Ch^enevert et al. / Tetrahedron: Asymmetry 15 (2004) 1889–1892
1891
chromatography (100% hexane to hexane/ethyl acetate,
95:5) to yield 3 (4.94 g, 87%) as a colourless oil. IR (neat)
AcOEt/5% MeOH) to give 4 (538 mg, 73%) as a white
solid (analytical data as above).
1
3559, 2955–2858, 1471, 1256, 1096, 838 cmꢀ1; H NMR
(400 MHz, CDCl3): d 0.03 (s, 12H), 0.86 (s, 18H), 1.77
(dd, J ¼ 1.5 and 0.9 Hz, 3H), 3.53 (d, J ¼ 9.5 Hz, 2H),
3.65 (d, J ¼ 9.5 Hz, 2H), 4.89 (m, 1H), 4.96 (m, 1H). 13C
NMR (100 MHz, CDCl3): d ꢀ5.4, 18.4, 19.8, 26.0, 65.5,
76.4, 111.8, 146.3; HRMS (CI, NH3) calcd for
C18H41O3Si2 (MþH)þ: 361.2594. Found: 361.2588.
4.6. Enzymatic desymmetrization of 4
To a solution of 4 (500 mg, 3.8 mmol) in vinyl acetate
ꢁ
(50 mL) were added molecular sieves (4 A, 200 mg) and
porcine pancreatic lipase (PPL, 800 mg) and the mixture
stirred at rt. The progress of the reaction was monitored
by TLC. As the reaction proceeded, the amount of
diacetate in the reaction mixture increased before the
complete disappearance of the starting material. The
reaction was stopped when the trace of diacetate became
as visible as the trace of the starting diol (5 h). The
mixture was then filtered and concentrated. The crude
product was purified by flash chromatography (AcOEt/
hexane, 3:2 to pure AcOEt) to give monoacetate 7
(583 mg, 88%) and diacetate 8 (57.5 mg, 7%) as colour-
less oils.
4.3. 2-Isopropenylpropane-1,2,3-triol 4
To a solution of 3 (4.521 g, 12.5 mmol) in THF (30 mL)
was added a solution of tetrabutylammonium fluoride in
THF (1 M, 38 mL, 38 mmol). The mixture was stirred
overnight at room temperature. The solvent was evap-
orated and the crude product purified by flash chro-
matography (AcOEt/hexane, 9:1 to pure AcOEt) to
afford 4 (1.342 g, 81%) as a white solid. Mp 56 ꢁC; IR
1
(KBr) 3600–3100, 1645, 1449, 1084, 1048, 999 cmꢀ1; H
NMR (400 MHz, CDCl3): d 1.74 (dd, J1 ꢂ J2 ꢂ 1 Hz),
2.87 (br s, 1H), 3.62 (d, J ¼ 11.4 Hz, 2H), 3.70 (d,
J ¼ 11.4 Hz, 2H), 5.01 (m, 1H), 5.06 (m, 1H); 13C NMR
(100 MHz, CDCl3): d 19.6, 66.3, 77.9, 113.3, 145.0;
HRMS (CI, NH3) calcd for C6H16O3N (MþNH4)þ:
150.1130. Found: 150.1125.
(S)-2-Hydroxy-2-(hydroxymethyl)-3-methylbut-3-enyl
acetate 7: ½aꢃ ¼ +17.3 (c 1.1, CHCl3); IR (neat) 3600–
;
1048 cmꢀ1 1H NMR (400 MHz, CDCl3): d 1.76 (dd,
J ¼ 1.5 and 0.8 Hz, 3H), 2,06 (s, 3H), 2.47 (br s, 1H),
3.58 (d, J ¼ 11.4 Hz, 1H), 3.61 (d, J ¼ 11.4 Hz,
1H), 4.18 (d, J ¼ 11.7 Hz, 1H), 4.24 (d, J ¼ 11.7 Hz,
1H), 5.02 (m, 1H), 5.1 (m, 1H); 13C NMR (100 MHz,
CDCl3): d 19.5, 21.0, 65.4, 67.0, 77.0, 113.7, 144.4, 171.7;
HRMS (CI, NH3) calcd for C8H15O4 (MþH)þ;
175.0970. Found: 175.0968.
25
D
3100, 2957–2890, 1727, 1646, 1450, 1378, 1242,
4.4. 1-Chloro-2-(chloromethyl)-3-methylbut-3-en-2-ol 6
To a solution of isopropenylmagnesium bromide in
THF (0.5 M, commercially available, 150 mL, 75 mmol)
cooled to 0 ꢁC under a dry atmosphere was added
dropwise a solution of 1,3-dichloroacetone 5 (3.147 g,
24.8 mmol) in THF (15 mL). The mixture was stirred at
room temperature for 45 min. The mixture was cooled
to 0 ꢁC and 3 M HCl (125 mL) then added. The aqueous
phase was extracted with ethyl acetate (3 · 100 mL). The
combined organic phases were washed with brine
(2 · 100 mL), dried over MgSO4 and evaporated. The
crude product was purified by flash chromatography
(100% hexane to hexane/ethyl acetate, 90:10) to give 6
(3.513 g, 84%) as an oil. IR (neat) 3548, 3483, 3095,
2-[(Acetyloxy)methyl]-2-hydroxy-3-methylbut-3-enyl
acetate 8: IR (neat) 3477, 3094, 2960, 1742, 1645, 1450,
1376, 1229, 1164, 1134, 1047, 911 cmꢀ1
;
1H NMR
(400 MHz, CDCl3): d 1.77 (dd, J ¼ 1.4 and 0.8 Hz, 3H),
2.06 (s, 6H), 4.16 (d, J ¼ 11.2 Hz, 2H), 4.19 (d,
J ¼ 11.2 Hz, 2H), 5.03 (m, 1H), 5.14 (m, 1H); 13C NMR
(100 MHz, CDCl3): d 19.5, 21.0, 66.9, 75.7, 114.3, 143.9,
171.2; HRMS (CI, NH3) calcd for C10H17O5 (MþH)þ:
217.1076. Found: 217.1080.
1
2966–2863, 1646 cmꢀ1; H NMR (400 MHz, CDCl3): d
4.7. (R)-2-Hydroxy-3-methyl-2-({[(4-methylphenyl)sulfon-
yl]oxy}methyl)but-3-enyl acetate 9
1.80 (s, 3H), 2.52 (br s, 1H), 3.75 (d, J ¼ 11.6 Hz, 2H),
3.68 (d, J ¼ 11.6 Hz, 2H), 5.12 (s, 1H), 5.19 (s, 1H); 13C
NMR (100 MHz, CDCl3): d 19.5, 49.2, 76.4, 115.6,
143.7; HRMS (EI, 70 eV) calcd for C6H10O1Cl2 (M)þ:
168.0109. Found: 168.0114.
To a solution of 7 (350 mg, 2 mmol) in anhydrous
chloroform (4 mL) were added anhydrous pyridine
(6 mL) and p-toluenesulfonyl chloride (1.00 g.
5.2 mmol). The solution was stirred overnight at room
temperature. Water was added (2 mL) and the reaction
mixture extracted with CH2Cl2 (100 mL). The organic
phase was washed with 1 M HCl (3 · 40 mL), saturated
aqueous NaHCO3 (2 · 40 mL) and brine (2 · 40 mL).
The organic phase was dried over MgSO4 and evapo-
rated. The crude product was purified by flash chro-
4.5. 2-Isopropenylpropane-1,2,3-triol 4 from 6
A solution of 6 (943 mg, 5.58 mmol) in 6 M NaOH
(50 mL) was stirred at 50 ꢁC for 66 h. The solution was
cooled to 0 ꢁC and neutralized with concentrated HCl.
The solution was filtered and evaporated. Residual
water was eliminated by azeotropic evapora-
tion with benzene (4 · 50 mL). The residue was dissolved
in ethyl acetate (150 mL) and the organic phase dried
over MgSO4 and evaporated. The crude product was
purified by flash chromatography (100% AcOEt to 95%
matography (hexane/ethyl acetate, 2:1) to give 9
25
D
(neat) 3491, 2958–2923, 1743, 1646, 1598, 1451, 1362,
(640 mg, 97%) as an oil: ½aꢃ ¼ )2.3 (c 0.81, CHCl3); IR
1
1234, 1177, 1097, 1048, 985 cmꢀ1; H NMR (300 MHz,
CDCl3): d 1.72 (s, 3H), 2.03 (s, 3H), 2.45 (s, 3H), 2.60 (br
s, 1H), 4,03–4.21 (m, 4H), 5.03 (m, 1H), 5.09 (m, 1H),

1892
R. Ch^enevert et al. / Tetrahedron: Asymmetry 15 (2004) 1889–1892
7.35 (d, J ¼ 7.0 Hz, 2H), 7.78 (d, J ¼ 7 Hz, 2H). 13C
NMR (100 MHz, CDCl3): d 19.3, 20.9, 21.9, 66.5, 71.8,
115.0, 128.2, 130.2, 132.6, 142.9, 145.4, 171.1; HRMS
(CI, NH3) calcd for C15H21O6S (MþH)þ: 329.1059.
Found: 329.1052.
3.52 (d, J ¼ 2.4 Hz, 1H), 3.68 (d, J ¼ 2.4 Hz, 1H), 4.84
(m, 1H), 5.07 (m, 1H); 13C NMR (100 MHz, C6D6): d
17.5, 49.0, 59.7, 60.9, 111.8, 140.7.
4.10. Dibenzyl (R)-2-hydroxy-2-(hydroxymethyl)-3-
methylbut-3-enylphosphonate 13
4.8. (S)-(2-Isopropenyloxiran-2-yl)methyl acetate 11
To
a
solution of dibenzyl phosphite (0.46 mL,
To a solution of monoacetate 7 (100 mg, 0.57 mmol) in
anhydrous CH2Cl2 (10 mL) were added 4-(N,N-di-
methylamino)pyridine (13 mg, 0.11 mmol), triethylamine
(1.5 mL, 10 mmol) and p-toluenesulfonyl chloride
(130 mg, 0.68 mmol). The solution was stirred at room
temperature for 72 h. The volatiles were evaporated,
water (10 mL) added and the mixture extracted with
ether (3 · 30 mL). The organic layer was washed with
1 M HCl (3 · 20 mL), saturated NaHCO3 (2 · 20 mL),
brine (2 · 20 mL), dried over MgSO4 and activated
2.09 mmol) in anhydrous THF (15 mL) cooled to ꢀ78 ꢁC
was added dropwise a solution of isopropylmagnesium
chloride in THF (2 M, 0.46 mL, 2.09 mmol). After stir-
ring for 1 h, a solution of epoxy-alcohol 12 (78.8 mg,
0.69 mmol) in anhydrous THF (2 mL) was added and
the solution stirred for 2 h at ꢀ78 ꢁC. The reaction
mixture was allowed to warm to 0 ꢁC at which time
water (10 mL) was added dropwise. The mixture was
extracted with ethyl acetate (3 · 20 mL). The organic
layer was washed with 1 M HCl (3 · 20 mL), saturated
NaHCO3 (2 · 20 mL), brine, dried over MgSO4 and
concentrated. Flash chromatography (hexane/ethyl
acetate, 4:1 to pure ethyl acetate) afforded 13 (102 mg,
charcoal and evaporated to give 11 (64 mg, 71%) as a
25
D
1743, 1652, 1450, 1368, 1333, 1231, 1145, 1088, 1051,
yellow oil. ½aꢃ ¼ +35.8 (c 0.94, CHCl3); IR (neat) 2977,
977 cmꢀ1
;
1H NMR (400 MHz, CDCl3): d 1.76 (dd,
40%) as a colourless oil. ½aꢃ ¼ )7.1 (c 1.00, CHCl3);
D
23
D
J1 ꢂ J2 ꢂ 1 Hz, 3H), 2.05 (s, 3H), 2.73 (d, J ¼ 5.3 Hz,
1H), 2.86 (d, J ¼ 5.3 Hz, 1H), 4.04 (d, J ¼ 12.3 Hz, 1H),
4.48 (d, J ¼ 12.3 Hz, 1H), 5.01 (m, 1H), 5.10 (m, 1H);
13C NMR (75 MHz, CDCl3): d 18.9, 20.7, 51.6, 58.9,
64.9, 114.2, 140.6, 170.6; HRMS (CI, NH3) calcd for
C8H13O3 (MþH)þ: 157.0864. Found: 157.0868.
lit.6 ½aꢃ ¼ ꢀ7.2 (c 1.00, CHCl3). Analytical data were
consistent with published results.6;7
References and notes
1. Takahashi, E.; Kimura, T.; Nakamura, K.; Arahira, M.;
Iida, M. J. Antibiot. 1995, 48, 1124–1129.
2. Kimura, T.; Nakamura, K.; Takahashi, E. J. Antibiot.
1995, 48, 1130–1133.
3. Lange, B. M.; Ketchum, R. E. B.; Croteau, R. B. Plant
Physiol. 2001, 127, 305–314.
4. Fields, S. C. Tetrahedron Lett. 1998, 39, 6621–6624.
5. Nakamura, K.; Kimura, T.; Kanno, H.; Takahashi, E.
J. Antibiot. 1995, 48, 1134–1137.
6. Nakamura, K.; Yamamura, S. Tetrahedron Lett. 1997, 38,
437–438.
7. Nakamura, K.; Kimura, T.; Takahashi, E. Phosphorus,
Sulfur and Silicon and the Related Elements 1999, 144–146,
613–616.
8. Johnson, R. A.; Sharpless, K. B. Asymmetric Methods of
Epoxidation. In Comprehensive Organic Synthesis; Trost, B.
M., Fleming, I., Ley, S. V., Eds.; Pergamon: New York,
1991; Vol. 7, p 389.
4.9. (R)-(2-Isopropenyloxiran-2-yl)methanol 12
To a solution of 11 (200 mg, 1.75 mmol) in diethyl ether
(15 mL) were added Candida antarctica lipase (500 mg,
total weight enzyme on carrier, 500 U/mg dry carrier)
and dry ethanol (0.5 mL), and the reaction mixture
stirred at room temperature. The reaction was moni-
tored by thin layer chromatography and stopped when
all the starting materials were consumed (72 h). The
reaction mixture was filtered and evaporated to give 12
(180 mg, 90%) as a colourless oil. Product 12 was
unstable on silica gel or in acidic media (e.g., CDCl3)
and was used in the next step immediately after work-
25
D
23
D
up. ½aꢃ ¼ +62.4 (c 1.37, C6H6); lit.6 ½aꢃ ¼ +55.6
(c 1.00, CHCl3); IR (neat) 3432, 2970, 2925, 2865, 1634,
1604, 1453, 1363, 1247, 1172, 1092, 976, 906 cmꢀ1; H
1
NMR (400 MHz, C6D6): d 1.54 (dd, J1 ꢂ J2 ꢂ 1 Hz,
9. Sodeaka, M.; Yamada, H.; Shibasaki, M. J. Am. Chem.
Soc. 1990, 112, 4906–4911.
3H), 2.33 (d, J ¼ 5.7 Hz, 1H), 2.66 (d, J ¼ 5.7 Hz, 1H),