Zanoni et al.
JOCNote
SCHEME 3. General Application of CAL-B-Mediated
Enzymatic Hydrolysis of a Few Labile Natural Product (NP)
Methyl Esters
This scenario was even more intriguing considering the
two phytoprostanes 14 and 15 which, indeed, occur in nature
as free carboxylic acids. It is, thus, quite surprising that the
“total synthesis” of phytoprostanes 14 and 15 has been claimed
by all authors achieving, instead, only the methyl ester pre-
2
0
cursors, possibly due to difficulties encountered in the final
hydrolytic step. Actually, the biological activity of the esters
may be quite different from the free acids, due to likely inter-
actions of the carboxylic group with biomolecules.
According to our novel procedure, the two phytoprostanes
4 and 15 were obtained, for the first time, from correspond-
1
ing methyl esters in gratifying 90 and 87% isolated yields,
1
13
respectively. The H and C NMR spectra were consistent
2
1
with literature.
In summary, we have described a very efficient route to the
potent PPARγ partial agonist (E)-12-nitrooctadec-12-enoic
acid 4, requiring only four steps and proceeding in 50%
overall yield. This new synthesis of compound 4 was about
1
0 times more efficient than that reported in literature, which
4
proceeded in eight steps and in 4.8% overall yield. A one-
pot Henry-retro-Claisen ring cleavage and a high yield,
buffer free, CAL-B-mediated enzymatic methyl ester clea-
vage are the salient features of our approach to 4. Extending
this extremely mild hydrolysis procedure to other methyl
esters, we obtained phytoprostanes 14 and 15 for the first
time as the naturally occurring free acids. The merits of the
new enzymatic procedure are especially valuable with labile
substrates under standard hydrolytic conditions.
hydrolytic conditions but even to a neutral medium. In the
event, after a few preliminary experiments, exposure of ester
9
to CAL-B (9/CAL-B 5:1 w/w) in MTBE, in the presence of
H O (50 equiv), afforded, after 18 h at 35 °C, the desired acid
2
4
in 90% isolated yield as a single E-stereomer (NMR
spectra). MTBE was revealed to be the solvent of choice
for the reaction of ester 9 since in dichloromethane conver-
sion to 4 was very low (40%), while in THF, acid 4 was
formed in merely 30% isolated yield, contaminated by an
unidentified side product.
Experimental Section
Methyl 13-Hydroxy-12-nitrooctadecanoate (7). To a stirred
solution of K CO (605 mg, 4.4 mmol) in H O (7.5 mL) were
2 3 2
Carboxylic acids are usually protected as methyl esters,
thanks to the easy formation, little steric hindrance, and
added, in the order, hexanal (0.8 mL, 6.6 mmol) followed by
2-nitrocyclododecanone (500 mg, 2.2 mmol). The reaction was
1
7
clear NMR spectra of these derivatives. Thus, to prove its
reliability, our buffer-free procedure was employed to cleave
a methyl ester group in the last step of the synthesis of a few
2
stirred at 35 °C for 18 h, then diluted with H O (30 mL), and
quenched at rt with 3 M HCl (1.5 mL, 4.62 mmol). The solution
was extracted with EtOAc (3ꢀ10 mL), and the combined organic
layers were dried on anhydrous Na SO and concentrated under
1
8
2
4
labile natural products.
In the event, prostaglandin-E (10), isoprostane-A (11),
vacuum. The crude product was dissolved in CH
and treated with an ethereal solution of CH
pale yellow color persisted. The mixture was concentrated under
vacuum to give a crude product which was purified on silica gel.
2
Cl
2
(25 mL)
2
2
2
N at 0 °C until a
2
preclavulone-A (12), isoprostane-E2t (13), phytoprostanes-
B type I (14) and type II (15), prostaglandin-F (16), and
1
2R
E)-10-nitrooleic acid 2 were produced as free acids in high
(
isolated yields upon treatment of the corresponding methyl
Elution with 9:1 hexane-EtOAc (R
f
= 0.30) gave 7 (554 mg
1
7
0%) as a colorless oil: H NMR (300 MHz, CDCl ) δ 0.90 (3H,
3
esters with CAL-B (5:1 ratio, w/w) in MTBE-H O (50 equiv)
br t, J=6.8 Hz), 1.25-1.85 (26H, m), 2.00-2.20 (1H, m), 2.30
(2H, t, J=7.4 Hz), 3.65 (3H, s), 3.85 (0.5H, m), 4.05 (0.5H m),
2
for 18 h at 35 °C (Scheme 3). Compared with existing
approaches to 10, 13, and 2, this method clearly performed
1
4.50 (1H, m); C NMR (75 MHz, CDCl
3
3
) δ 174.4 (s), 92.9 (d),
1
9a
9
2.3 (d), 72.4 (d), 72.0 (d), 51.4 (q), 34.1 (t), 33.5 (t), 33.1 (t), 31.5
much better, affording PGE (10) in 95% yield vs 60%,
2
1
isoPGE (13) in 92% yield vs 50%, and nitrooleic acid (2)
9b
(t), 31.4 (t), 30.4 (t), 29.3 (t), 29.1 (t), 29.1 (t), 28.9 (t), 28.9 (t),
27.9 (t), 25.9 (t), 25.6 (t), 25.3 (t), 24.9 (t), 22.5 (t), 13.9 (q); IR
2
t
7
in 98% yield vs 42%.
-
1
(
CH
HRMS C19
E)-Methyl 12-Nitrooctadec-12-enoate (9). Trifluoroacetic
anhydride (0.305 mL, 2.19 mmol) was added to a solution of
nitro-alcohol 7 (750 mg, 2.09 mmol) in dry CH Cl (30 mL)
2
Cl
2
) 3448, 2928, 2856, 1739, 1549, 1438, 1364, 1173 cm
;
H37NO
5
calcd 359.2672, found 359.2683.
(
17) (a) Greene, T. W.; Wuts, P. G. M. Protecting Groups, 3rd ed.; Georg
(
Thieme Verlag: Stuttgart, 2005. (b) Kocienski, P. J. Protecting Groups in
Organic Synthesis, 4th ed.; Wiley and Sons: New York, 2006.
(18) For the use of CAL-B-buffer system for methyl ester deprotection, see:
2
2
Barbayianni, E.; Fotakopoulou, I.; Schmidt, M.; Constantinou-Kokotou, V.;
Bornscheuer, U. T.; Kokotos, G. J. Org. Chem. 2005, 70, 8730–8733. and
references therein. To the best of our knowledge, there is only one report of
lipase-mediated hydrolysis of a methyl ester, namely, methyl oleate, using water
and tri-n-butyl phosphate as organic solvent: Charton, E.; Macrae, A. R.
Enzyme Microb. Technol. 1993, 15, 489. Lye et al., to validate the use of CAL-B
for enzymatic resolution in ionic liquid, performed an experiment in buffer-free
conditions, namely, without pH control: Roberts, N. J.; Seago, A.; Carey, J. S.;
Freer, R.; Preston, C.; Lye, G. J. Green Chem. 2004, 6, 475.
under Ar, followed by the addition of Et N (0.62 mL, 4.4 mmol)
3
dropwise at 0 °C. The reaction was allowed to reach room
temperature and after 4 h was quenched with aqueous saturated
4 2 2
NH Cl (10 mL). The mixture was diluted with CH Cl (50 mL),
(20) (a) V ꢀa zquez-Romero, A.; C ꢀa rdenas, L.; Blasi, E.; Verdaguer, X.;
Riera, A. Org. Lett. 2009, 11, 3104 and references therein. (b) Perlikowska,
W.; Mikolajczyk, M. Synthesis 2009, 2715.
(21) Thoma, I.; Krischke, M.; Loeffler, C.; Mueller, M. J. Chem. Phys.
Lipids 2004, 128, 135.
(
19) (a) Sih, C. J.; Heather, J. B.; Sod, R.; Price, P.; Peruzzotti, G.; Lee,
H. L. F.; Lee, S. S. J. Am. Chem. Soc. 1975, 97, 865. (b) Rodrıguez, A. R.;
Spur, B. W. Tetrahedron Lett. 2002, 43, 9249.
´
J. Org. Chem. Vol. 75, No. 23, 2010 8313