Enzymatic lactonization stategy for enantioselective synthesis of a tetrahydrolipstatin synthon

Article abstract of DOI:10.1021/jo990370+

A novel lipase-catalyzed protocol has been formulated for the simultaneous enantiocontrol of three stereogenic centers in a flexible acyclic system. This involved a porcine pancreatic lipase (PPL)-catalyzed δ- lactonization of a racemic 3,5-dihydroxy-2-alkyl ester to produce the lactone with high enantioselectivity (92.8%). The product lactone and its analogues are useful synthons for the asymmetric synthesis of various bioactive compounds, which include the potential anti-obesity compound, tetrahydrolipstatin.

Full text of DOI:10.1021/jo990370+

VOLUME 64, NUMBER 22
OCTOBER 29, 1999
© Copyright 1999 by the American Chemical Society
Articles
En zym a tic La cton iza tion Sta tegy for En a n tioselective Syn th esis of
a Tetr a h yd r olip sta tin Syn th on
A. Sharma and S. Chattopadhyay*
Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
Received March 2, 1999
A novel lipase-catalyzed protocol has been formulated for the simultaneous enantiocontrol of three
stereogenic centers in a flexible acyclic system. This involved a porcine pancreatic lipase (PPL)-
catalyzed δ-lactonization of a racemic 3,5-dihydroxy-2-alkyl ester to produce the lactone with high
enantioselectivity (92.8%). The product lactone and its analogues are useful synthons for the
asymmetric synthesis of various bioactive compounds, which include the potential anti-obesity
compound, tetrahydrolipstatin.
Absolute acyclic stereocontrol especially for compounds
with nonrigid conformations poses challenges to asym-
metric synthesis. Earlier, lipase-catalyzed desymmetri-
zation (“meso-trick”) has produced encouraging results1a-c
to this end. This methodolgy was further extended to the
simultaneous differentiation of two enantiotopic groups
even for asymmetric molecules.2a,b This report describes
a lipase-catalyzed lactonization strategy for the enantio-
control of three stereogenic centers in one step. This is
in continuation of our earlier efforts on enzymatic mac-
rolactonization of some chemically fragile substrates.3a,b
Recently, we have embarked on the synthesis of some
bioactive 2-oxetanones because several of these consti-
tute4 promising candidates as antibiotics, immunostimu-
lators, and anti-obesity drugs. In addition, these com-
pounds are of contemporary significance due to their
utility as versatile synthons and monomers for chiral
biodegradable polymers.5 Besides the â-lactone moiety,
most of the pharmacologically important 2-oxetanones
contain two additional stereogenic centers viz. an alkyl
group and a hydroxy function at the C-2 and C-5
positions, respectively. This skeleton may be derived from
the progenitors such as the suitably protected 2-alkyl-
3,5-dihydroxy esters. Thus, for the synthesis of this class
of compounds, a lipase-catalyzed δ-lactonization seemed
appealing as this, in principle, opens up the possibility
of controlling absolute stereochemistry of three stereo-
genic centers simultaneously. We selected racemic 2-hexyl-
3,5-dihydroxy C16-ester 10 as the starting material for
this enzymatic method to synthesize lactone 11, a key
synthon for tetrahydrolipstatin, THL (I). Compound I is
well-known4 for its efficacy in sustained weight loss in
humans, its mode of action being the inhibition of
pancreatic lipase and cholesterol esterase by acylation
of their serine residues. Besides I, other related 2-oxet-
anones are also potentially useful in the treatment of
(1) (a) Bonini, C.; Racioppi, R.; Viggiani, L.; Righi, G.; Rossi, L.
Tetrahedron: Asymmetry 1993, 4, 793-805. (b) Chenevert, R.;
Courchesne, G. Tetrahedron: Asymmetry 1995, 6, 2093-2096. (c)
Bonini, C.; Racioppi, R.; Viggiani, L.; Righi, G.; Viggiani, L. J . Org.
Chem. 1993, 58, 802-803.
10.1021/jo990370+ CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/08/1999

8060 J . Org. Chem., Vol. 64, No. 22, 1999
Sharma and Chattopadhyay
obesity, hyperlipaemea, atherosclerosis, and arterioscle-
rosis. The importance of the compounds, especially of
THL, understandably, has resulted in several elegant
syntheses.4,6a-c Most of these were based on a building-
up of the 2-alkyl â-lactone moiety on a preformed chiral
3-hydroxytetradecanal derivative, the preparation of
which itself involved several steps. The construction of
the â-lactone moiety was either accomplished via a
nonstereoselective methodology followed by separation
or an enantiocontrolled strategy often requiring expen-
sive reagents and/or poorly accessible chiral auxiliaries.
The present approach, on the other hand, is simple and
provides three stereogenic centers directly.
Thus, Zn-mediated allylation7 of dodecanal 1 with allyl
bromide gave the alcohol 2, which on silylation with
TBSCl to 3 and subsequent reductive ozonolysis8 gave
the aldehyde 4. The other required synthon 6 was
prepared by R-bromination9 of n-octanoic acid and sub-
sequent esterification. A Reformatsky reaction between
4 and 6 under ultrasonic irradiation gave compound 7
as a inseparable stereoisomeric mixture in modest yield.
Before proceeding for the subsequent enzymatic reaction,
it seemed prudent to fix the relative stereochemistry of
the stereogenic centers in 7. This would reduce the
number of possible diastereomers formed, thereby sim-
plifying the isolation process and also increasing the yield
of the desired stereoisomer. For this, compound 7 was
oxidized with pyridinium chlorochromate (PCC)10 to give
10, the resonance at δ 4.25 was assigned to the C-3 proton
on the basis of its mulitplicity pattern (dt). Since the
coupling constants of its doublet was 2.3 Hz, the relative
stereochemistry at C-2 and C-3 must be syn. Finally,
lactonization of 10 was attempted using PPL as the
catalyst in ether in the presence of molecular sieve 4 Å
powder. As mentioned previously, lipase-catalyzed lac-
tonization of 3,5-dihydroxy esters has been reported.2a,b
However, this was restricted to substrates with sterically
more demanding cyclic substituents. In contrast, the
designated substrate is highly flexible and also contains
an additional alkyl group at the C-2 center. This, we
thought, might pose a problem, as lipases are known13
to be sensitive to the presence of substitution R to the
reactive site of the substrate. It was gratifying to find
that the reaction proceeded smoothly, and after 48 h, 11
(Scheme 1) was obtained in 70% chemical yield (based
on conversion) along with recovered (68%) substrate.
HPLC analysis of the lactone 11 on Chiralcel OD
column with 20% 2-propanol/hexane (flow rate 1 mL/min)
revealed its ee to be 92.8%, the respective retention times
being 27.5 (major) and 24.6 min (minor). Compound 10,
being all syn, would possess either the 2S,3R,5R or
2R,3S,5S configuration. On the basis of the previous
analogy2a,b to the PPL-catalyzed δ-lactonization, the
lactone 11 was assigned the 2S,3R,5R configuration. A
formal synthesis of I from compound 11 can be easily
accomplished according to the reported procedure.6b
For confirmation of the configurational assignment,
compound 11 was dehydrated14 with POCl3 in pyridine
to give the olefinic lactone 12. This on alcoholysis with
1-butanol in the presence of Pseudomonas cepacia lipase
(PCL) as the catalyst gave 13, which on benzylation
followed by epoxidation15 with oxone and HIO4 cleavage
gave the known6a aldehyde 14. Comparison of its chirop-
tical data with those reported established its configura-
tion to be R. Thus, the configuration at the C-5 center of
its progenitor 11 would also be the same. Moreover, since
the relative configurations of the other stereogenic cen-
ters of 11 was predesigned, its absolute stereochemistry
would be 2S,3R,5R.
8, which on desilylation furnished the hydroxy ketone 9.
11a,b
Its reduction with ZnBH4
furnished the desired all-
syn-diol ester 10, which was easily separated from the
minor amount of other isomers by column chromatogra-
phy. The syn relationship of its 1,3-diol function was
assigned on the basis of the 13C NMR resonances11b of
the C-3 and C-5 carbon atoms containing hydroxyl
groups. These appeared at δ 68.2 and 73.7 in contrast to
comparatively higher field signals for the anti-diols. The
assignment of the relative stereochemistry at C-2 and
C-3 was accomplished from the fact that J 2,3 (threo) >
1
J 2,3 (erythro)12 in the H NMR spectra of these types of
compounds. Generally, in the case of threo compounds,
the J 2,3 values for the C-3 protons are comparatively
larger (6-9 Hz), while the same for the erythro com-
pounds are 2-4 Hz. In the present case, for compound
In conclusion, an enzymatic protocol has been devel-
oped for one-step enantiocontrol of three asymmetric
centers. The resultant products amenable by the present
method are useful intermediates for biologically active
2-oxetanones.
(2) (a) Henkel, B.; Kunath, A.; Schick, H. Tetrahedron: Asymmetry
1993, 4, 153-156. (b) Bonini, C.; Pucci, P.; Viggiani, L. J . Org. Chem.
1991, 56, 4050-4052.
(3) (a) Pawar, A. S.; Chattopadhyay, S.; Chattopadhyay, A.; Mam-
dapur, V. R. J . Org. Chem. 1993, 58, 7535-7536. (b) Sankaranaray-
anan, S.; Sharma, A.; Chattopadhyay, S. J . Org. Chem. 1996, 61, 1814-
1816.
(4) Pommier, A.; Pons. J . M. Synthesis 1995, 729 and references cited
therein.
(5) J edlinski, Z.; Kurcok, P.; Kowalczuk, M.; Matuszowicz, A.;
Dubois, P.; J erome, R.; Kricheldorf, H. R. Macromolecules 1995, 28,
1276-1280.
(6) (a) Barbier, P.; Schneider, F.; Widmer, U. Helv. Chim. Acta 1987,
70, 1412-1418. (b) Pommier, A.; Pons, J . M. Synthesis 1993, 441-
459. (c) Yang, H. W.; Romo, D. J . Org. Chem. 1997, 62, 4-5.
(7) Petrier, C.; Luche, J . L. J . Org. Chem. 1985, 50, 910-912.
(8) Fieser, L.; Fieser, M. Reagents in organic synthesis; J . Wiley &
Sons: New York, 1980; Vol. 1, p 1238.
Exp er im en ta l Section :
Dod eca n a l (1). To a cooled (0 °C) and stirred suspension
of PCC (32.33 g, 0.15 mol) in CH2Cl2 (200 mL) was added
dodecan-1-ol (18.6 g, 0.1 mol) in one lot. After being stirred
for 3 h, when the reaction was complete (cf. TLC), the reaction
was quenched with anhydrous ether (200 mL). The mixture
was stirred vigorously and the supernatant passed through a
small pad (6 in.) of silica gel. The eluent was concentrated in
vacuo to obtain 1 (16.56 g, 90%), which was purified by column
chromatography (silica gel, 5% ether/hexane): IR 2720, 1730
cm-1 1H NMR δ 0.9 (dist. t, 3H), 1.29 (br. s, 18H), 2.1-2.3
;
(m, 2H), 9.78 (t, J ) 1.5 Hz, 1H).
(9) Furniss, B. S.; Hannaford, A. J .; Smith, P. W. G.; Tatchell, A. R.
Vogel’s textbook of practical organic chemistry, Vth ed.; Longmans
Group: London, 1989; p 510.
1-P en ta d ecen -4-ol (2). To a stirred mixture of 1 (10.0 g,
0.054 mol), allyl bromide (13.15 g, 0.11 mol), and Zn dust (5.0
(10) Corey, E. J .; Suggs, J . W. Tetrahedron Lett. 1975, 2647-2650.
(11) (a) Kathawate, F. G.; Prager, B.; Prasad, K.; Repic, O.; Shapiro,
M. J .; Stubler, R. S.; Widler, L. Helv. Chim. Acta 1986, 69, 803-805.
(b) Nakata, T.; Kuwabona, T.; Tani, Y.; Oishi, T. Tetrahedron Lett.
1982, 23, 1015-1016.
(12) House, H. O.; Crumrine, D. S.; Teranishi, A. Y.; Olmstead, H.
D. J . Am. Chem. Soc. 1973, 95, 3310-3324.
(13) Drauz, K.; Waldmann H. Enzyme Catalysis In Organic Syn-
thesis; VCH Verlagsgesellschaft mbH, VCH Publishers Inc.: Weinheim
and New York, 1995.
(14) Oda, M.; Yamamuro, A.; Watabe, T. Chem. Lett. 1979, 1427-
1430.
(15) Corey, P. F.; Ward, F. E. J . Org. Chem. 1986, 51, 1925-1926.

Enantioselective Synthesis of a TetrahydrolipstatinSynthon
Sch em e 1a
J . Org. Chem., Vol. 64, No. 22, 1999 8061
a
(i) Allyl bromide/Zn/THF/aqueous NH4Cl; (ii) TBSCl/NaH/THF; (iii) O3/CH2Cl2/-78 °C; Ph3P; (iv) POCl3/Br2; (v) MeOH/H+; (vi) Zn/
PhH/ultrasonic irradiation; (vii) PCC/NaOAc/CH2Cl2; (viii) TBAF/THF/-78 °C; (ix) ZnBH4/ether; (x) PPL/ether/molecular sieves 4 Å; (xi)
POCl3/py; (xii) PCL/n-BuOH/CH2Cl2; (xiii) NaH/THF/BnBr/Bu4NI; Oxone/acetone/pH 7.4 buffer; HIO4/THF-H2O.
g) in THF (50 mL) was dropwise added aqueous saturated NH4-
Cl (2.0 mL) at ambient temperature. After ∼5 min, a vigorous
reaction set in. After being stirred for 1 h, the mixture was
extracted with ether and the organic extract washed with
water and brine and dried. Removal of solvent and subsequent
column chromatography (silica gel, 0-10% EtOAc/hexane)
subjected to column chromatography (silica gel, 0-5% EtOAc/
hexane) to furnish pure 4 (10.4 g, 81%): IR 2710, 1720, 1470
1
cm-1; H NMR δ 0.1 (s, 6H), 0.8-0.96 (s and t merged, 12H),
1.29 (br s, 20H), 2.2-2.4 (m, 2H), 3.7-3.9 (m, 1H), 9.9 (t, J )
1.5 Hz, 1H).
Meth yl 2-Br om oocta n oa te (6). To a stirred mixture of
1-octanoic acid (10.0 g, 0.069 mol) and dry Br2 (3.92 mL, 0.076
mol) was added PCl3 (0.12 mL). The mixture was heated to
65-70 °C when evolution of HBr started. After ∼4 h, the
temperature of the reaction mixture was raised to 100 °C and
heating continued for 1 h. The reaction flask was assembled
for a downward distillation, and the product, 5 (8.69 g, 56%),
afforded pure 2 (11.35 g, 92%): IR 3380, 1640, 990, 910 cm-1
;
1H NMR δ 0.9 (dist t, 3H), 1.1-1.5 (m containing a br. s at δ
1.32, 20H), 2.1-2.3 (m, 2H), 2.84 (br s, D2O exchangeable, 1H),
3.5-3.7 (m, 1H), 4.8-6.2 (m, 3H). Anal. Calcd for C15H30O:
C, 79.58; H, 13.36. Found: C, 79.81; H, 13.26.
4-ter t-Bu tyld im eth ylsilyloxyp en ta d ec-1-en e (3). To a
stirred solution of hexane-washed NaH (2.34 g, 0.049 mol, 50%
suspension in oil) in THF (50 mL) was added dropwise
compound 2 (10.0 g, 0.044 mol). After the initial evolution of
H2 subsided, the mixture was refluxed for 1 h. It was brought
to room temperature, and TBSCl (8.0 g, 0.053 mol) was added
to it. Stirring was continued for 12 h at room temperature,
the mixture was treated with ice-cold water, and the organic
layer was separated. The aqueous portion was extracted with
ether, and the combined organic extract was washed with
water and brine and dried. Solvent removal followed by column
chromatography of the residue gave pure 3 (12.87 g, 86%): IR
collected as the distillate: IR 3700-3500, 1720, 1730 cm-1
;
1H NMR δ 0.9 (dist t, 3H), 1.3 (br s, 8H), 1.8-2.5 (m, 2H),
4.22 (t, J ) 6 Hz, 1H), 9.4 (br s, D2O exchangeable, 1H).
A solution of the above acid (8.69 g) in MeOH (100 mL)
containing two to three drops of H2SO4 was refluxed for 4 h.
Most of the solvent was removed in vacuo, and water was
added to the residue, which was extracted in ether. The ether
layer was washed with H2O and brine and dried. Removal of
solvent followed by distillation gave pure 6 (8.1 g, 88%): bp
1
85 °C/3 mm; IR 1760, 1470 cm-1; H NMR δ 0.9 (dist t, 3H),
1.26 (s, 8H), 2.2-2.4 (m, 2H), 3.8 (s, 3H), 4.2 (t, J ) 6 Hz, 1H).
7-Ca r bom et h oxy-8-h yd r oxy-10-ter t-b u t yld im et h ylsil-
yloxyh en eicosa n e (7). A mixture of the aldehyde 4 (3.42 g,
0.01 mol), the bromoester 6 (3.56 g, 0.015 mol), and freshly
prepared Zn wool (2.0 g) in benzene (30 mL) was sonicated
for 2 h. The mixture was carefully poured into cold aqueous 2
N HCl and extracted with EtOAc. The organic portion was
washed with water and brine and dried. After concentration,
the crude product was chromatographed over silica gel using
0-5% EtOAc/hexane to get 7 (2.25 g, 45%) as a diastereomeric
1
1640, 990, 910 cm-1; H NMR δ 0.1 (s, 6H), 0.8-0.95 (s and t
merged, 12H), 1.32 (br s, 20H), 2.2-2.5 (m, 2H), 3.5-3.7 (m,
1H), 4.8-6.2 (m, 3H). Anal. Calcd for C21H44OSi: C, 74.04; H,
13.03. Found: C, 74.14; H, 13.20.
3-ter t-Bu tyld im eth ylsilyloxytetr a d eca n a l (4). Ozone
was bubbled through a solution of 3 (12.8 g, 0.038 mol) in CH2-
Cl2 (50 mL) until no unreacted starting material remained (cf.
TLC, 2 h). The mixture was purged with N2 to remove the
excess O3 and cooled to 0 °C, triphenylphosphine (29.96 g,
0.114 mol) was added, and the mixture was stirred for 40 h.
The mixture was concentrated in vacuo, the solid residue
extracted with hexane, and the hexane layer concentrated and
mixture: IR 3440, 1740 cm-1 1H NMR δ 0.1 (s, 6H), 0.84-
;
0.95 (s and t merged, 15H), 1.1-1.6 (m with a br. s at δ 1.29,
32H), 2.27-2.35 (m, partially D2O exchangeable, 2H), 3.7-

8062 J . Org. Chem., Vol. 64, No. 22, 1999
Sharma and Chattopadhyay
4.2 (m containing a s at δ 3.84, 5H). Anal. Calcd for C29H60O4-
Si: C, 69.54; H, 12.08. Found: C, 69.77; H, 12.24.
being stirred at the same temperature for 12 h, the mixture
was quenched with aqueous saturated NH4Cl, the organic
layer separated, and the aqueous portion extracted with
EtOAc. The combined organic extract was washed with water
and brine and dried. Removal of solvent followed by column
chromatography (silica gel, 0-15% EtOAc/hexane) of the
residue afforded the all-syn-diol derivative 10 (0.54 g, 70%):
7-Ca r bom eth oxy-8-oxo-10-ter t-bu tyld im eth ylsilyloxy-
h en eicosa n e (8). To a cooled (0 °C) and stirred mixture of
compound 7 (1.6 g, 3.2 mmol) and NaOAc (0.1 g) in CH2Cl2
(25 mL) was added PCC (1.4 g, 6.4 mmol) in one lot. Usual
workup10 and isolation gave 8 (1.14 g, 72%): IR 1740, 1720,
1460 cm-1; 1H NMR δ 0.1 (s, 6H), 0.88-0.98 (s and t merged,
15H), 1.32 (br s, 30H), 2.2-2.5 (m, 2H), 3.48 (t, J ) 7 Hz, 1H),
3.78 (s, 3H), 4.0-4.2 (m, 1H).
7-Ca r bom eth oxy-8-oxo-10-h yd r oxyh en eicosa n e (9). To
a cooled (-78 °C) and stirred solution of 8 (1.1 g, 2.2 mmol) in
THF (20 mL) was added TBAF (3.0 mL, 1 M in THF) and
stirring continued until the completion of the reaction (cf. TLC,
∼6 h). The mixture was poured into ice-cold water, the organic
layer separated, and the aqueous portion extracted with
EtOAc. The combined organic extract was washed with water
and brine, dried, and concentrated. The residue obtained was
subjected to column chromatography (silica gel, 0-15% EtOAc/
hexane) to furnish pure 9 (0.574 g, 68%): mp 34 °C (lit.6a mp
35-36 °C); IR 3440, 1730, 1710 cm-1; 1H NMR δ 0.87 (dist. t,
6H), 1.26 (br. s, 26H), 1.5-2.0 (m, partially D2O exchangeable,
5H), 2.5-2.8 (m, 2H), 3.41 (t, J ) 7 Hz, 1H), 3.74 (s, 3H), 4.0-
4.7 (m, 1H).
1
IR 3380, 1740, 1060 cm-1; H NMR δ 0.87 (dist. t, 3H), 0.93
(dist. t, 3H), 1.2-1.4 (m, containing a br s at δ 1.33, 30H), 1.7
(br s, D2O exchangeable, 2H), 2.2-2.5 (m, 3H), 3.68 (s, 3H),
4.25 (dt, J ) 4.2 Hz, 2.3 Hz, 1H), 4.3-4.4 (m, 1H); 13C NMR
11.0, 14.1, 14.7, 14.9, 22.7, 23.0, 23.8, 28.9, 29.7, 30.4, 31.9,
38.8, 68.2, 73.7, 167.8. Anal. Calcd for C23H46O4: C, 71.45; H,
11.99. Found: C, 71.70; H, 12.15.
(2S,3R,5R)-2-n -Hexyl-3-h ydr oxyh exa d eca n -5-olid e (11).
A mixture of 10 (0.6 g, 1.6 mmol) and PPL (0.5 g) in ether (25
mL) was stirred for 48 h. It was filtered to get rid of the solid
enzyme particles, and the organic extract was concentrated
in vacuo to obtain a liquid that on preparative chromatography
(silica gel, 10% EtOAc/hexane) afforded pure 11 (0.224 g, 41%)
as well as unreacted 10. 11: [R]22 +25 (c 1.06, CHCl3); IR
D
3380, 1730 cm-1
;
1H NMR δ 0.88 (dist. t, 6H), 1.2-1.6 (m
containing a br. s at δ 1.29, 30H), 1.8-2.0 (m, 2H), 2.1-2.5
(m, 1H), 4.1-4.3 (m, 1H), 4.35-4.45 (m, 1H). Anal. Calcd for
(7S′,8R′,10R′)-7-Ca r b om et h oxy-8,10-d ih yd r oxyh en e-
icosa n e (10). To a stirred and cooled (-20 °C) solution of 9
(0.768 g, 2.0 mmol) in anhydrous ether (10 mL) was added an
ethereal solution of ZnBH4 (1.0 mL, 1.0 M, 1.0 mmol). After
C
22H42O3: C, 74.52; H, 11.94. Found: C, 74.68; H, 11.89.
J O990370+