At the beginning of our studies aimed at mapping the
active site of FAS-TE in conjunction with structural analy-
1
5
sis, we wondered what role the position of the â-lactone
moiety and N-formylleucine ester along the hydrocarbon
backbone might have on the inhibitory activity toward FAS-
TE. It is intriguing and perhaps not surprising that the
â-lactone carbonyl carbon is located at C16′ in orlistat and
that the end product of FAS is the C16 carboxylic acid,
palmitic acid (Figure 2). In previous studies, we found that
Figure 1. Structures of â-lactone-containing natural products and
derivatives.
occurring derivative of orlistat with 2-fold higher potency
toward pancreatic lipase.7
We now report the development of several strategies for
the systematic study of orlistat congeners for exploration of
structure-activity relationships with respect to the thioesterase
8
domain of FAS using the TMAL process as a key step. The
utility of these strategies is demonstrated by enantioselective
syntheses of tetrahydrolipstatin (THL), valilactone, and a
2-carbon transposed THL derivative. The antagonistic activ-
ity of the title compounds toward FAS-TE was measured as
inhibition of turnover of a fluorogenic substrate by a
recombinant form of the enzyme. A comparison of selectiv-
ity toward various hydrolyases in cell lysates for the
transposed orlistat derivative vs orlistat and ebelactone A
by activity-based profiling is also reported.
Figure 2. Structural comparison of palmitic acid (the end product
of FAS), orlistat, and ebelactones A and B. Design of a 2-carbon
transposed derivative related to ebelactones.
9
Numerous total syntheses of orlistat have been reported
owing to its interest as an antiobesity agent and also as a
showcase for new methods in polyketide synthesis. The
majority of strategies has relied on late-stage lactonization
to access the â-lactone moiety.10 Alternative strategies to
synthesize this pharmacophore are quite scarce and include
the ebelactones are also antagonists of FAS; however, these
agents were not as potent nor as selective for FAS in cell
2
a
lysates as orlistat. It is noteworthy that in the ebelactones
the â-lactone carbonyl carbon is located at C14′ rather than
at C16′. To test whether the reduced activity was a conse-
quence of the transposed â-lactone in ebelactone vs orlistat
(C16′fC14′), we synthesized orlistat derivative 3, in which
both functionalities are shifted by two carbons (Figure 2).
The synthesis of (-)-tetrahydrolipstatin began with a
11
[
2+2] cycloadditions of ketenes and aldehydes, a bromo-
12
lactonization strategy, and one example employing the
TMAL process.1 There are two syntheses of valilactone
reported to date.
3
1
4
16
TMAL reaction between known aldehyde 4 and thiopyridyl
7
ketene acetal 5 which delivered the desired â-lactone as an
(
7) (a) Yang, H. W.; Romo, D. J. Org. Chem. 1997, 62, 4. (b) Yang, H.
W.; Zhao, C.; Romo, D. Tetrahedron 1997, 53, 16471.
8) Purohit, V. C.; Richardson, R. D.; Smith, J. W.; Romo, D. J. Org.
Chem. 2006, 71, 4549.
8
:1 mixture of anti/syn diastereomers with complete trans
(
(12) Bodkin, J. A.; Humphries, J.; McLeod, M. D. Tetrahedron Lett.
(
9) Details of this assay have been described; see ref 8.
2003, 40, 2869.
(10) (a) Barbier, P.; Schneider, F. J. Org. Chem. 1988, 53, 1218. (b)
(13) While our work was in progress, the TMAL process developed in
our laboratory was employed in a synthesis of orlistat; see: Yin, J.; Yang,
X. B.; Chen, Z. X.; Zhang, Y. H. Chin. Chem. Lett. 2005, 16, 1448.
(14) (a) Bates, R. W.; Fernandez-Moro, R.; Ley, S. V. Tetrahedron Lett.
1991, 32, 2651-2654. (b) Bates, R. W.; Fernandez-Moro, R.; Ley, S. V.
Tetrahedron 1991, 47, 9929.
(15) For crystallographic studies of the thioesterase domain of FAS,
see: (a) Chakravarty, B.; Gu, Z.; Chirala, S. S.; Wakil, S. J.; Quiocho, F.
A. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 15567. Mammalian FAS at 4.5
A resolution: (b) Maier, T.; Jenni, S.; Ban, N. Science 2006, 311, 1258.
Fungal FAS at 5.0 A resolution: (c) Jenni, S.; Leibundgut, M.; Maier, T.;
Ban, N. Science 2006, 311, 1263.
Fleming, I.; Lawrence, N. J. Tetrahedron Lett. 1990, 31, 3645. (c) Chadha,
N. K.; Batcho, A. D.; Tang, P. C.; Courtney, L. F.; Cook, C. M.; Wovkulich,
P. M.; Uskokovic, M. R. J. Org. Chem. 1991, 56, 4714. (d) Hanessian, S.;
Tehim, A.; Chen, P. J. Org. Chem. 1993, 58, 7768. (e) Giese, B.; Roth, M.
J. Braz. Chem. Soc. 1996, 7, 243. (f) Dirat, O.; Kouklovsky, C.; Langlois,
Y. Org. Lett. 1999, 1, 753. (g) Paterson, I.; Doughty, V. A. Tetrahedron
Lett. 1999, 40, 393. (h) Ghosh, A. K.; Liu, C. Chem. Commun. 1999, 1743.
(
i) Ghosh, A. K.; Fidanze, S. Org. Lett. 2000, 2, 2405. (j) Thadani, A. N.;
Batey, R. A. Tetrahedron Lett. 2003, 44, 8051. (k) Yadav, J. S.; Rao, K.
V.; Reddy, M. S.; Prasad, A. R. Tetrahedron Lett. 2006, 47, 4393. (l) Yadav,
J. S.; Reddy, M. S.; Prasad, A. R. Tetrahedron Lett. 2006, in press.
(
11) (a) Pons, J.-M.; Kocienski, P. Tetrahedron Lett. 1989, 30, 1833-
(16) Aldehydes 4 and 7 are available in four steps from ethylacetoacetate
involving (a) alkylation, (b) Noyori reduction, (c) alcohol silyl protection,
and (d) ester half-reduction. See Supporting Information for details.
1
1
836. (b) Pommier, A.; Pons, J.-M.; Kocienski, P.; Wong, L. Synthesis 1994,
294.
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