The first enzymatic desymmetrizations of prochiral phosphine oxides

Article abstract of DOI:10.1016/j.tetasy.2003.08.003

Prochiral bis(methoxycarbonylmethyl)phenylphosphine oxide was subjected to PLE-mediated hydrolysis to give a chiral monoacetate in 92% yield and 72% e.e. In turn, prochiral bis(hydroxymethyl)phenylphosphine oxide was desymmetrized, using either a lipase-catalyzed acetylation or hydrolysis of the corresponding diacetyl derivative, to give a chiral monoacetate in the yield up to 76% and with e.e.s up to 79%. Absolute configurations of the products were determined by means of chemical correlation.

Full text of DOI:10.1016/j.tetasy.2003.08.003

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 3379–3384
The first enzymatic desymmetrizations of prochiral phosphine
oxides
:
Piotr Kiełbasin´ski,* Remigiusz Zurawin´ski, Małgorzata Albrycht and Marian Mikołajczyk*
Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Department of Heteroorganic Chemistry,
90-363 Ło´dz´, Sienkiewicza 112, Poland
Received 17 July 2003; accepted 5 August 2003
Abstract—Prochiral bis(methoxycarbonylmethyl)phenylphosphine oxide was subjected to PLE-mediated hydrolysis to give a chiral
monoacetate in 92% yield and 72% e.e. In turn, prochiral bis(hydroxymethyl)phenylphosphine oxide was desymmetrized, using
either a lipase-catalyzed acetylation or hydrolysis of the corresponding diacetyl derivative, to give a chiral monoacetate in the yield
up to 76% and with e.e.s up to 79%. Absolute configurations of the products were determined by means of chemical correlation.
© 2003 Elsevier Ltd. All rights reserved.
1. Introduction
optically active, functionalized organophosphorus
derivatives. Herein we disclose the details of our investi-
gations on the enzymatic asymmetric synthesis of non-
racemic P-stereogenic phosphine oxides 2 and 5,
starting from the appropriate prochiral substrates: bis-
(methoxy-carbonylmethyl)phenylphosphine oxide 1 and
Chemoenzymatic methodologies have recently become
powerful tools for the synthesis of chiral, nonracemic
compounds.1 Of several enzymes used for such pur-
poses, hydrolases are of particular importance, since
most of them combine a high reaction stereoselectivity
with a broad substrate tolerance.2 Thus, it has recently
been shown that commercially available hydrolases
(esterases, lipases, proteases) are also capable of recog-
nizing heteroatom stereogenic centres which makes
them useful in the synthesis of chiral heteroorganic
compounds.3 As a result of our own studies on the use
of hydrolytic enzymes in the synthesis of chiral phos-
phorus compounds, we have so far reported the kinetic
resolution of racemic, P-stereogenic phosphinyl-
acetates,4 phosphonyl- and phosphorylacetates,5
hydroxymethanephosphinates, -phosphonates,6,7 and
phosphine oxides, including using ionic liquids as sol-
vents.8 In connection with our ongoing work focused
on a search for new P-stereogenic phosphorus-contain-
ing catalysts for asymmetric synthesis, we have become
interested in the preparation of certain new types of
bis(hydroxymethyl)phenylphosphine oxide
4 or its
diacetyl derivative 6.9 It should be stressed that such an
approach has never been used for the preparation of
optically active phosphorus derivatives, although there
were precedents created by the desymmetrization of
other prochiral heteroatom substrates, viz. sulfinyldi-
carboxylates,10
bis(hydroxymethyl)silanes11
and
bis(hydroxymethyl)germanes.12
2. Results and discussion
2.1. Desymmetrization of bis(methoxycarbonylmethyl)-
phenylphosphine oxide 1
Bis(methoxycarbonylmethyl)phenylphosphine oxide 1,
which was synthesized according to the procedure pre-
(1)
* Corresponding authors. Tel.: (+48-42) 6815832; fax: (+48-42) 6847126; e-mail: piokiel@bilbo.cbmm.lodz.pl; marmikol@bilbo.cbmm.lodz.pl
0957-4166/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2003.08.003

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P. Kiełbasin´ski et al. / Tetrahedron: Asymmetry 14 (2003) 3379–3384
viously described by us,13 was subjected to hydrolysis in
a phosphate buffer in the presence of several hydro-
lases, of which only pig liver esterase (PLE) proved to
be efficient. The expected product, viz. the monoacetate
2 was isolated after acidification and purification by
column chromatography in 92% yield and 72% e.e. (Eq.
(1)).
shows that the (R) enantiomer of 1 should be pro-
duced, which is in agreement with the experiment and
establishes the applicability of the Jones model also to
these types of substrates.
2.2. Desymmetrization of bis(hydroxymethyl)phenyl
phosphine oxide 4 and bis(acetoxymethyl)phenyl-
phosphine oxide 6
The absolute configuration of the product was easily
determined by a simple chemical correlation shown in
Eq. (2).
The phosphine oxide 4 was synthesized according to
literature procedures,15,16 and the diacetyl derivative 6
(2)
(3)
When the monoacetate (+)-2 was refluxed in toluene,
decarboxylation took place to give the phosphine oxide
(+)-3 whose absolute configuration was known from the
literature to be (R).4 As the transformation did not
involve the phosphorus stereogenic centre, its absolute
configuration was not changed; hence (+)-2 has the (R)
configuration.
was obtained by acetylation of 4 with acetic anhydride
in the presence of Yb(OTf)3.17 Two procedures were
used to obtain the desired optically active (acet-
oxymethyl)(hydroxymethyl)phenylphosphine oxide 5: a
lipase-catalyzed acetylation of the prochiral substrate 4
(A) and a reverse reaction i.e. lipase-catalyzed hydroly-
sis of the corresponding prochiral diacetyl derivative 6
(B) (Eq. (3)).
Having established the stereoselectivity of the above
reaction, we decided to check whether the Jones model
of the PLE active site14 is also applicable to prochiral
phosphoryl substrates as it is for a prochiral sulfinyl
analog10b as well as for racemic phosphoryl deriva-
tives.4,5 Thus, according to this model the diacetate 1
should be located in the PLE active site as follows (Fig.
1). The ester group which undergoes hydrolysis should
be placed in the neighbourhood of serine, the second
ester group in the front polar pocket (PF), the phospho-
ryl oxygen atom in the back polar pocket (PB) and
finally the large organic substituent (phenyl) in the large
hydrophobic pocket (HL). This mode of location clearly
In the latter case the reaction was always performed in
an organic solvent saturated with a phosphate buffer
(pH 7.2). Several lipases proved to be suitable for these
transformations and gave 5 in reasonable yields and
with moderate to good ee. The results are summarized
in Table 1. It should be noted that the application of
the two reverse procedures enabled us to obtain both
enantiomerically enriched forms of 5. This is due to a
common feature of enzymes which exhibit the same
sense of stereoselectivity towards substrates, irrespective
of the direction of the reversible reaction they catalyze
(e.g. ester formation versus ester hydrolysis). Consider-
ing the influence of solvents and certain additives on
the yield and stereoselectivity of the reaction, no partic-
ular regularity could be seen. Thus, application of ionic
liquids instead of common organic solvents did not
exert a profound influence on the reaction, which was
in contrast to the kinetic resolution of racemic hydroxy-
methanephosphonates where the stereoselectivity was
markedly enhanced.8
Moreover, the yields of the product were usually lower,
most probably due to some difficulties in its isolation
from the highly polar medium. In turn, amines were
added with the hope of increasing enantioselectivity of
the lipase-mediated transformations.18 However, the
effect was generally opposite to expectations. The addi-
Figure 1. Preferred binding orientation of 1 in the active site
of PLE.

P. Kiełbasin´ski et al. / Tetrahedron: Asymmetry 14 (2003) 3379–3384
Table 1. Enzymatic desymmetrization of 4 and 6
3381
Enzyme/solvent
Method
Monoacetate 5
Yield (%)
[h]D (CHCl3)
E.e. (%)
Abs. conf.
PFL/CHCl3
PFL/DPEa
A
B
B
A
A
B
A
A
A
A
B
B
50
50
40
57
30
40
76
40
35
60
18
25
+3.9
−3.4
−1.8
+1.8
0
−2.4
+0.7
+2.6
+0.6
+1.1
−3.3
−1.0
79
68
37
37
0
50
15
53
14
22
65
20
R
S
S
R
PFL/DPEa,b
PS/THFb
PS/THF/Et3N
AHS/DPEa
S
AK/CHCl3
AK/THF
PFL/BMIM·PF6
PFL/BMIM·PF6
PFL/BMIM·PF6
PFL/BMIM·PF6
R
R
R
R
S
b
a
a,b
S
Enzymes: PFL: lipase from Pseudomonas flourescens; AHS: lipase AHS (AMANO); AK: lipase AK (AMANO); PS: lipase PS (AMANO).
Solvents: DPE: diisopropyl ether; BMIM·PF6-ionic liquid: 1-butyl-3-methylimidazolium hexafluorophosphate.8
a The solvent saturated with a phosphate buffer.
b Pyridine added.
tion of triethylamine resulted in a complete loss of
stereoselectivity, with a concomitant decrease of the
yield (due to the preferential formation of the product
of exhaustive acetylation). The addition of pyridine, on
the one hand decreased the enantioselectivity of the
reaction performed in organic solvents, on the other
hand slightly improved the result of the hydrolysis
carried out in the ionic liquid.
To establish the absolute configuration of 5, the chemi-
cal correlation shown in Scheme 1 was carried out.
Thus, (−)-5 was transformed into the tosyl derivative
Scheme 1.

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P. Kiełbasin´ski et al. / Tetrahedron: Asymmetry 14 (2003) 3379–3384
(−)-7 and then into the iodomethylphosphine oxide (−)-8.
The latter was reduced using the conditions developed by
Bałczewski19 to give the corresponding hydroxy-
methylphosphine oxide (+)-9. So far, none of the trans-
formations involved the phosphorus stereogenic center
nor did the silylation procedure leading to (−)-10, which
was performed to protect the hydroxy group during the
ensuing transformations. Thus, (−)-10 was subjected to
an O-alkylation followed by a reduction with LiAlH4 and
a quench with borane. This reaction was carried out
according to a procedure described by Imamoto et al.20
who proved that it proceeds with inversion of configura-
tion at phosphorus. The levorotatory adduct 11, whose
absolute configuration is known to be (R),21 was
obtained by us, however, with a fairly low enantiomeric
excess equal to 7%. This is in contrast to very high
stereoselectivities of this reaction found for alkylaryl-
methylphosphine oxides.20 Nevertheless, knowing the
absolute configuration of the final product of the chem-
ical correlation, (R)-(−)-11, and assuming the inversion
of configuration at phosphorus in the last step, we were
able to assign the absolute configurations to all the
intermediary compounds and ultimately to the mono-
acetate 5, which is (S)-(−) (Scheme 1).
3.3. Enzymatic desymmetrization of bis(methoxycar-
bonylmethyl)phosphine oxide 1
To a stirred solution of 1 (100 mg, 0.37 mmol) in a
phosphate buffer (pH 7.2, 11 mL) pig liver esterase, PLE
(45 mL) was added and the mixture was stirred at 30°C.
The pH was maintained by a continuous addition of 0.2
M aqueous NaOH using an automatic titrator. When the
appropriate amount of NaOH·aq (1.85 mL) was dropped
in (after ca. 5 days), the reaction solution was acidified
with diluted sulfuric acid to pH 2.1. Then methanol was
added (100 mL) and, after 0.5 h, the resulted cloudy
solution was filtered through Celite. The filtrate was
evaporated and the residue was purified by column
chromatography, using chloroform–methanol (2:1) as
solvent, to give 2 (87 mg, 92%), [h]2D0=+3.9 (c 1.1,
MeOH), e.e.=72%. 31P NMR (CDCl3): l=34.66. H
1
NMR (CDCl3): l=3.58 (s, 3H), 3.25–3.75 (m, 4H),
7.31–7.81 (m, 5H), 10.38 (s, 1H). 13C NMR (CDCl3):
l=36.96 (d, J=32.6), 38.23 (d, J=33.3), 52.33, 128.5,
128.77, 130.42, 130.6, 130.84, 132.6, 166.64 (d, J=3.6),
169.34.
3.4. Enzymatic acetylation of bis(hydroxymethyl)phenyl-
phosphine oxide 4—general procedure
A mixture of 4 (1 mmol), a lipase (10–20 mg) and vinyl
acetate (1 mL) in an appropriate solvent (10 mL) was
stirred at room temperature. In some cases a few drops
of an amine were added (see Table 1). As the substrate
4 was hardly soluble in any organic solvent, the reaction
mixture was at the beginning usually cloudy and became
more clear when the reaction proceeded (a suspension of
the enzyme remained). The reaction was monitored by
31P NMR and stopped at the optimal conversion: usu-
ally, in addition to the expected monoacetyl product 5,
the bisacetyl derivative 6 was also formed. The average
reaction time exceeded 10 days. The enzyme was filtered
off and the solvent was removed under vacuum. The
residue was purified by preparative TLC (acet-
one:hexane, 2:1) to give optically active 5. 31P NMR
3. Experimental
3.1. General
The enzymes were purchased from AMANO or
FLUKA. NMR spectra were recorded on Bruker instru-
ments at 200 MHz for 1H and 81 MHz for 31P, with C6D6
or CDCl3 as solvents. The enantiomeric excess (e.e.)
values of all the products were determined by 1H NMR,
using (−)-(S) or (+)-(R)-t-butylphenylphosphinothioic
acid as a chiral solvating agent.22 Optical rotations were
measured on a Perkin–Elmer 241 MC photopolarimeter.
Column chromatography was carried out using Merck
60 silica gel. TLC was performed on Merck 60 F254 silica
gel plates.
1
(CDCl3): l=33.8. H NMR (CDCl3): l=2.1 (s, 3H),
4.17–4.33 (m, 2H), 4.59–4.79 (2×AB, 2H), 7.51–7.85 (m,
5H). 13C NMR (CDCl3): l=20.29, 58.93 (d, J=81.0),
58.50 (d, J=80.0), 126.59, 128.54, 128.77, 130.74, 130.91,
132.68, 170.01 (d, J=6.7). MS (EI): m/z 229 (M+H).
HRMS calcd for C10H14PO4 (M+H) 229.0626; found
229.0629.
3.2. Synthesis of bis(acetoxymethyl)phenylphosphine
oxide 6
Bis(hydroxymethyl)phenylphosphine oxide 4 (2 g, 10.7
mmol) was dissolved in hot dichloromethane (100 mL)
and ytterbium triflate, Yb(OTf)3 (0.66 g, 1.07 mmol) was
added followed by acetic anhydride (3.28 g, 32.2 mmol).
The resulting suspension was stirred overnight at room
temperature and the conversion was followed by TLC
(acetone:petroleum ether, 2:1). When the reaction was
completed, the dichloromethane solution was washed
with diluted sodium bicarbonate, dried over magnesium
sulfate and, after evaporation, purified by column chro-
matography using acetone:petroleum ether (2:1) as elu-
ent. Yield: 2.5 g (86%). 31P NMR (CDCl3): l=29.24. 1H
NMR (CDCl3): l=2.11 (s, 6H), 4.60–4.70 (2×AB, 4H),
7.5–7.9 (m, 5H). 13C NMR (CDCl3): l=20.10, 58.78 (d,
J=83.25), 125.69, 128.63, 128.86, 130.71, 130.84, 133.08,
169.58 (d, J=7.0). MS (EI): m/z 271 (M+H). HRMS
calcd for C12H16PO5 (M+H), 271.0737; found 271.0735.
3.5. Enzymatic hydrolysis of bis(acetoxymethyl)-
phenylphosphine oxide 6—general procedure
A mixture of 6 (1 mmol) and a lipase (ca. 10 mg) was
stirred in a solvent (10 mL) saturated with a phosphate
buffer (pH 7.2). In some cases a few drops of pyridine
were added (Table 1). The reaction was monitored by 31
P
NMR. After completion of the hydrolysis (which usually
resulted in the concomitant formation of certain amounts
of the bis-hydroxy derivative 4) magnesium sulfate was
added to remove water. The precipitates were filtered off,
the solvents were removed under vacuum and the residue
was purified by preparative TLC (conditions as above)
to give pure 5.

P. Kiełbasin´ski et al. / Tetrahedron: Asymmetry 14 (2003) 3379–3384
3383
3.6. Chemical correlation and absolute configuration of 5
added and stirring was continued for the next 5 h. After
this time, a BH3–THF complex (2 M solution in THF,
0.15 mL) was added and the mixture was stirred for 15
min. Water was added and the aqueous layer was
extracted with ethyl acetate. The organic layers were
dried over MgSO4 and the solvents evaporated under
vacuum. The residue was purified by preparative TLC
(ethyl acetate:hexane, 1:3) to give pure 11 (15.7 mg,
45%). [h]2D01=−0.5 (c 1.57, CHCl3). 31P NMR (CDCl3):
l=11.42. H NMR (CDCl3): l=−0.2–0.96 (br.q, 3H),
1.65 (d, J=10.46, 3H), 1.85 (br.s, 1H), 4.08 (s, 2H),
7.43–7.80 (m, 5H). MS (CI): m/z 167 (M−H), 137
(M−CH2OH). HRMS calcd for C8H13BOP (M−H)
167.0792; found 167.0798.
3.6.1. Synthesis of (R)-(−)-7. To a solution of 5 (60 mg,
0.26 mmol, ([h]D=−2.39, 48% e.e.) in dichloromethane
(4 mL) were added tosyl chloride (56 mg, 0.29 mmol)
and triethylamine (35 mg, 0.35 mmol) and the mixture
was stirred overnight. Then, the solution was washed
with water, dried with magnesium sulfate and the sol-
vent was removed under vacuum. The residue was
chromatographed on
a
preparative plate (acet-
one:hexane, 2:1) to give pure 7 (71 mg, 71%), [h]2D0=
−14.55 (c 1.12, CHCl3). 31P NMR (CDCl3): l=27.76.
1H NMR (CDCl3): l=2.06 (s, 3H), 2.42 (s, 3H), 4.63–
4.74 (4×AB, 4H), 7.29–7.81 (m, 9H). MS (CI): m/z 383
(M+H).
3.6.2. Synthesis of (R)-(−)-8. A mixture of 7 (70 mg,
0.18 mmol), sodium iodide (110 mg, 0.73 mmol) in
acetone (10mL) was stirred at room temperature for 4
days (TLC control: CHCl3–methanol, 15:1). Acetone
was removed under vacuum, the residue was dissolved
in chloroform and washed with water and an aqueous
solution of sodium thiosulfate. The chloroform solution
was dried and evaporated to dryness. The crude
product was purified by preparative TLC (CHCl3–
methanol, 15:1) to give pure 8 (89.5 mg, 92%), [h]2D0=
−10.1 (c 1.39, CHCl3). 31P NMR (CDCl3): l=29.64. 1H
NMR (CDCl3): l=2.13 (s, 3H), 3.29–3.51 (2×AB,
2H),4.66–4.84 (2×AB, 2H), 7.48–7.85 (m, 5H). MS (CI):
m/z 339 (M+H).
Acknowledgements
Financial support by the State Committee of Scientific
Research (KBN), Grant No 3 T09A 085 18, is grate-
fully acknowledged.
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