Enhancement of the enantioselectivity in lipase-catalyzed kinetic resolutions of 3-phenyl-2H-azirine-2-methanol by lowering the temperature to -40 °C

Full text of DOI:10.1021/jo970581j

4906
J . Org. Chem. 1997, 62, 4906-4907
chemists are still being sought. One problem to be solved
in the lipase-catalyzed resolution is low enantioselectivity
for chiral primary alcohols, except the relatively success-
ful case for meso-compounds,8 where the methodologies
devised for secondary alcohols are not readily applicable
to primary ones.9 We now propose that a simple cooling
of the reaction system to -40 °C can be effective for
enhancement of the enantioselectivity of the enzymatic
reaction.
Azirine 110,11 adopted here seems to be a useful chiral
building block because the highly strained CdN double
bond accepts a variety of chemical transformations such
as a reduction to aziridine or an introduction of appropri-
ate nucleophiles diastereoselectively.11 Quite recently,
the asymmetric synthesis of azirine derivatives has been
the focus of several groups,12 because naturally-occurring
antibiotics containing the skeleton have been found. This
class of compounds involves (S)-azirinomycin13 and (R)-
(-)-dysidazirine,14 the latter of which has been recently
synthesized by developing an asymmetric synthetic
method of the azirine skeleton.12b The lipase-catalyzed
resolution of chiral azirine has not been reported so far.
Racemic azirine (()-1 was prepared from cinnamyl
alcohol by the reported method15 with some modifications
in 50% overall yield as shown in Scheme 1, which
involves bromination of cinnamyl alcohol, reaction with
NaN3 in DMSO, and subsequent dehydrobromination and
then thermolysis to (()-1. In the final thermocyclization
step, the reaction temperature should be carefully con-
trolled not to exceed 100 °C until the evolution of nitrogen
ceases. Chromatographic purification gave pale yellow
crystals (mp 57-58 °C), which are stable enough to be
stored in a refrigerator.
To begin, the conditions of the lipase-catalyzed trans-
esterification of (()-1 were optimized according to the
conventional method (Scheme 2). Thus, lipase PS was
found to be a suitable lipase after screening commercially
available lipases.16 The reaction with an equimolar
amount of vinyl acetate in diisopropyl ether at 30 °C was
En h a n cem en t of th e En a n tioselectivity in
Lip a se-Ca ta lyzed Kin etic Resolu tion s of
3-P h en yl-2H-a zir in e-2-m eth a n ol by
Low er in g th e Tem p er a tu r e to -40 °C
Takashi Sakai,* Isamu Kawabata, Tetsuo Kishimoto,
Tadashi Ema, and Masanori Utaka*
Department of Applied Chemistry, Faculty of Engineering,
Okayama University, Tsushima-naka, Okayama 700, J apan
Received April 1, 1997
We report here an efficient preparation of (S)-(+)-
phenyl-2H-azirine-2-methanol ((S)-(+)-1) and its acetate
((R)-(-)-2) by a lipase-catalyzed kinetic resolution carried
out preferentially at -40 °C in ether under unusual
conditions for enzyme. We disclosed that a lipase from
Pseudomonas cepacia (lipase PS) exerts its function at
such a very low temperature to markedly enhance the
enantioselectivity. In an enzymatic reaction, the enan-
tioselectivity in the kinetic resolution is temperature
dependent and obeys the following thermodynamic equa-
tion:1
ln E ) ∆∆Sq/R - ∆∆Hq/(RT)
(1)
Hence, lowering the temperature can increase the
enantioselectivity so far as the reaction is carried out
below the racemic temperature Tr.1 However, no report
on exemplification of the theory by the enzymatic reaction
below 0 °C is available2 so far, because an enzyme is
generally believed not to work effectively at such low
temperatures. We found that the lipase-catalyzed reac-
tion obeys the equation (eq 1) ranging from +30 to -50
°C. Recently, the chemoenzymatic synthesis has at-
tracted much attention because of the demand for an
environmentally-acceptable and total-cost-effective syn-
thetic method. In these aspects, the lipase-catalyzed
kinetic resolution3 has been widely utilized as a reliable
and readily available method for the resolution of racemic
alcohols and carboxylic acid esters. In order to increase
the enantioselectivity, a variety of methods, e.g., reaction
in appropriate organic media,3g,4 use of additive,5 choice
of acyl donor,6 and so on,7 have been invented, while new
and readily available methods for synthetic organic
(7) (a) Pressure effect: Kamat, S. V.; Beckman, E. J .; Russell, A. J .
J . Am. Chem. Soc. 1993, 115, 8845. (b) Immobilization on Florisil:
Yamada, H; Sugai, T.; Ohta, H.; Yoshikawa, S. Agric. Biol. Chem. 1990,
54, 1579.
(8) For example: (a) Yokomatsu, T.; Sato, M.; Shibuya, S. Tetrahe-
dron: Asymmetry 1996, 7, 2743. (b) Hirose, Y.; Kariya, K.; Sasaki, I.;
Kurono, Y.; Ebiike, H.; Achiwa, K. Tetrahedron Lett. 1992, 33, 7157.
(c) Fuji, K.; Kawabata, T.; Kiryu, Y.; Sugiura, Y. Tetrahedron Lett.
1990, 31, 6663.
* To whom correspondence should be addressed. E-mail:
tsakai@cc.okayama-u.ac.jp
(1) Review: Phillips, R. S. Trends Biotechnol. 1996, 14, 13 and
references cited therein. Tr is defined as ∆∆Hq/∆∆Sq, a temperature
at which there is no enantiomeric discrimination.
(2) (a) Optimization of the selectivity in a PLE-catalyzed hydrolysis
in aqueous methanol at -10 °C: Lam, L. K. P.; Hui, R. A. H. F.; J ones,
J . B. J . Org. Chem. 1986, 51, 2047. No theoretical discussion on the
temperature effect was made. (b) Increasing the temperature for high
enantioselectivity: Yasufuku, Y.; Ueji, S. Biotechnol. Lett. 1995, 17,
1311.
(3) For example: (a) J ones, J . B. Tetrahedron 1986, 42, 3351. (b)
Chen, C.-S.; Sih, C. J . Angew. Chem., Int. Ed. Engl. 1989, 28, 695. (c)
Klibanov, A. M. Acc. Chem. Res. 1990, 23, 114. (d) Wong, C.-H.;
Whitesides, G. M. Enzymes in Synthetic Organic Chemistry; Perga-
mon: Oxford, 1994. (e) Enzyme Catalysis in Organic Synthesis; Drauz,
K., Waldmann, H., Eds.; VCH: New York, 1994; Vol. 1. (f) Faber, K.
Biotransformations in Organic Chemistry; Springer-Verlag: Berlin,
1995. (g) Enzymatic Reactions in Organic Media; Koskinen, A. M. P.,
Klibanov, A. M., Eds.; Blackie Academic: Glasgow, 1996.
(4) For recent papers: (a) Ke, T.; Wescott, C. R.; Klibanov, A. M. J .
Am. Chem. Soc. 1996, 118, 3366. (b) Nakamura, K.; Kinoshita, M.;
Ohno, A. Tetrahedron 1995, 51, 8799.
(9) Weissfloch, A. N. E.; Kazlauskas, R. J . J . Org. Chem. 1995, 60,
6959.
(10) Chemical resolution with low ee by using brucine: Stegman,
W.; Uebelhart, P.; Heimgartner, H.; Schmid, H. Tetrahedron Lett. 1978,
3091. The absolute configuration is not given.
(11) Review: Padwa, A.; Woolhouse, A. D. In Comprehensive
Heterocyclic Chemistry; Lowowski, W., Ed.; Pergamon Press: New
York, 1984; Vol. 7, Chapter 5, p 47.
(12) (a) Bucher, C. B.; Linden, A.; Heimgartner, H. Helv. Chim. Acta
1995, 78, 935. (b) Davis, F. A.; Reddy, G. V.; Liu, H. J . Am. Chem.
Soc. 1995, 117, 3651. (c) Gentilucci, L.; Grijzen, Y.; Thijs, L.; Zwanen-
burg, B. Tetrahedron Lett. 1995, 36, 4665. (d) Verstappen, M. M. H.;
Ariaans, G. J . A.; Zwanenburg, B. J . Am. Chem. Soc. 1996, 118, 8491.
(13) (a) Stapley, E. O.; Hendlin, D.; J ackson, M.; Miller, A. K. J .
Antibiot. 1971, 24, 42. (b) Miller, T. W.; Tristram, E. W.; Wolf, F. J . J .
Antibiot. 1971, 24, 48.
(14) (a) Molinski, T. F.; Ireland, C. M. J . Org. Chem. 1988, 53, 2103.
(b) Salomon, C. E.; Williams, D. H.; Faulkner, D. J . J . Nat. Prod. 1995,
58, 1463.
(15) (a) Hortmann, A. G.; Robertson, D. A.; Gillard, B. K. J . Org.
Chem. 1972, 37, 322. (b) Padwa, A.; Rasmussen, J . K.; Tremper, A. J .
Am. Chem. Soc. 1976, 98, 2605.
(5) (a) Itoh, T.; Takagi, Y.; Murakami, T.; Hiyama, Y.; Tsukube, H.
J . Org. Chem. 1996, 61, 2158. (b) Gao, Z.-W.; Sih, C. J . J . Am. Chem.
Soc. 1989, 111, 6836.
(6) Ema, T.; Maeno, S.; Takaya, Y.; Sakai, T.; Utaka, M. J . Org.
Chem. 1996, 61, 8610.
(16) Lipases showing the E values > 3 (origin, E value, fast-reacting
enantiomer): CHIRAZYME L-2 (Candida antarctica, 8, R),CHIRA-
ZYME L-7 (porcine pancreas, 5, R), lipase AK (Pseudomonas fluore-
scens, 4, R).
S0022-3263(97)00581-1 CCC: $14.00 © 1997 American Chemical Society

Communications
J . Org. Chem., Vol. 62, No. 15, 1997 4907
Ta ble 1. Tem p er a tu r e Mod u la tion in th e
Lip a se-Ca ta lyzed Resolu tion a
Sch em e 1
% yieldb (% eec)
T
lipase
time
×103c/h/
entry (°C) (mg) (S)-(+)-1 (R)-(-)-2 (h)
cd
lipase mg Ee
1
2
3
4
5
6
7
8
9
10
11
12
13
30
30
20
0
0
50
200
50
50 (59)
44 (76)
45 (90)
54 (65)
42 (83)
57 (68)
50 (77)
49 (78)
60 (54)
66 (39)
62 (46)
60 (62)
55 (61)
41 (80)
45 (77)
55 (72)
45 (88)
49 (85)
43 (87)
43 (86)
42 (90)
37 (94)
22 (97)
31 (97)
38 (96)
37 (94)
2.0 0.42
0.5 0.50
4.3 0.56
6.0 0.43
3.0 0.49
1.4 0.44
4.0 0.47
4.5 0.46
7.5 0.37
8.0 0.29
4.3 0.32
8.5 0.39
6.0 0.39
4.2
5.0
2.6
1.4
1.6
1.6
0.59
0.51
0.25
0.18
0.19
0.11
0.041
17
18
19
31
31
28
37
46
54
84
99
86
64
50
100
200
200
200
200
200
400
400
0
-10
-20
-30
-40
-40
-50
Sch em e 2
-60 1600
a
Conditions: (()-1 (50 mg, 0.34 mmol), vinyl acetate (29 mg,
0.34 mmol), dry Et2O (5 mL). Isolated yield. c Determined by
b
Sch em e 3a
HPLC analysis using a Chiralcel OB-H column for 1 and Chiralcel
d
OD-H for 2, respectively. Conversion calculated from c ) ee (1)/
(ee (1) + ee (2)) according to ref 17. e Reference 17.
a
Key: (i) TBS-Cl, imidazole, THF (34%); (ii) LiAlH4, Et2O (51%);
(iii) TsCl, Et3N, CHCl3 (58%).
found to reach to a moderate conversion within a few
hours, giving the E value17 of 11 at best. Next, screening
of the solvents18 revealed that ether is the best choice,
raising the E value to 17. In addition, vinyl acetate
proved to give the highest selectivity among the several
types of the acylating agents6 examined. In spite of these
efforts, the E value still remained around 17 and no other
conventional means seemed to be applicable. The abso-
lute configuration of (+)-110 thus obtained was deter-
mined to be S by correlation to authentic N-tosyl aziri-
dine (S,S)-(+)-319 as shown in Scheme 3.
F igu r e 1. Temperature effect on the enantioselectivity.
Then, we examined the temperature effect on the
enantioselectivity in the prospects mentioned above. The
reaction of (()-1 was carried out with lipase PS and vinyl
acetate in ether at the temperature ranging from 30 to
-60 °C as shown in Table 1. As the reaction system was
cooled, the reaction rate was decreased as expected
(convn h-1 lipase mg-1), and thus, the amount of lipase
was increased from 50 mg to 100, 200, and then 1600
mg at -60 °C so as to finish the reaction within a
moderate time. The amount of the lipase, however, has
no significant influence on the E value as examined at
30, 0, and -40 °C, respectively. Noteworthily, as the
temperature was lowered the E values were markedly
increased and reached up to 99 (at -40 °C), sufficient
for practical use. Further cooling to -60 °C, however,
began to lose the efficiency, decreasing the E value to
64.20 The results listed in Table 1 are plotted in Figure
1 to give a straight line as a function of ln E and 1/T
ranging from 30 to -50 °C. The parameters, ∆∆Hq and
∆∆Sq, calculated according to the theoretical equation (eq
1) are -3.0 kcal mol-1 and -4.3 cal deg-1 mol-1, respec-
tively, and the racemic temperature Tr calculated is 425
°C.
These results suggest that the enantioselectivity of this
lipase-catalyzed kinetic resolution is temperature-predic-
tive even at such very low temperatures. Thus, lowering
the temperature increases the E value, favoring the
(R)-enantiomer in this case. The thermostability of the
lipase in organic solvent3g,4 enabled the reaction not only
at higher temperatures21 but also at such low tempera-
tures. This temperature modulation will be a readily and
generally applicable method to improve the enantiose-
lectivity with theoretical prediction in the lipase-cata-
lyzed resolution and should be a reliable method espe-
cially for primary alcohols. We are now actively
investigating how to generalize the low-temperature
modulating method by adopting to a variety of lipases
and alcohols and how to utilize the chiral azirines (S)-
(+)-1 and (R)-(-)-2 in organic synthesis.
Ack n ow led gm en t. This work was partially sup-
ported by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science and Culture of
J apan. We are grateful to Amano Pharmaceutical Co.,
Ltd. and Boehringer Mannheim GmbH for kindly sup-
plying the lipases and to the SC-NMR laboratory of
Okayama University for NMR measurements.
(17) Chen, C.-S.; Fujimoto, Y.; Girdaukas, G.; Sih, C. J . J . Am. Chem.
Soc. 1982, 104, 7294.
(18) Organic solvent (E value): THF (15), acetone (15), AcOEt (15),
CH3CN (14), toluene (11), i-Pr2O (11), benzene (9), cyclohexane (9),
n-hexane (7).
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and spectral data for all compounds, including copies
1
of H and 13C NMR spectra (18 pages).
(19) Fujii, N.; Nakai, K.; Habashita, H.; Hotta, Y.; Tamamura, H.;
Otaka, A.; Ibuka, T. Chem. Pharm. Bull. 1994, 42, 2241.
(20) When the temperature was decreased below -40 °C, the lipase
might begin to lose the conformational flexibility essential for the
catalytic activity.
J O970581J
(21) Lipase exhibits a high catalytic activity on heating at 100 °C:
Zaks, A.; Klibanov, A. M. Science 1984, 224, 1249.