C O M M U N I C A T I O N S
Figure 2. Structures of thermally relaxed (GROMACS 4.07) SsADH-10 (from 1R37) to which has been docked (Autodock Vina, left to right): (i) (S)-
flurbiprofenal, (ii) (S)-naproxenal, (iii) (S)-ketoprofenal, and (iv) (S)-fenoprofenal (Zn ligation sphere: H68, C38, C154, and substrate carbonyl).
Buchwald-Hartwig-type8 conditions, followed by reduction to the
aldehyde (LDBBA9 or LAH/DMP oxid; see Supporting Information
(SI)). Optimal DYRKR conditions (Table 1, 80 °C, pH 9) led to
efficient throughput of rac-aldehyde to the (S)-2-arylpropionalde-
hyde, particularly with m- and p-substitution. Notably, (S)-profenols
corresponding to the NSAIDs naproxen (3b, scaled to 1 g @ 98%
yield and 95% ee), ibuprofen (3d, IP), flurbiprofen (3h, FlP),
fenoprofen (3j, FP), and ketoprofen (3l, KP) were obtained in
excellent yields (up to 96%) and high enantioselectivity (up to 99%).
Naproxen is FDA-approved as the active (S)-antipode. While most
individuals can invert (R)-ibuprofen to the (S)-antipode, the pathway
is inefficient for KP10a and FlP.10b Moreover, the recent observation
that the Profen-CoA thioester intermediates in this pathway inhibit
G6PDH10c argues for “chiral switching” to single (S)-antipodes.10d
Entries into (S)-profens11,12 include asymmetric hydrogenation (NP
98% ee, IP 97% ee)11g and hydroformylation (IP, 92% ee).11f DKR
processes include enantioselective crystallization (NP >99% ee),11b
DYRKR with H2 as reductant under Ru(II) catalysis (IP 92% ee),11d
and lipase/Ru(II)-mediated DKR of allylic acetates, followed by Cu-
mediated Grignard arylation (FlP 97% ee;11c Knochel arylation:11e IP
97% ee). The hydrovinylation/oxidation approach is impressive (IP,
FP, FlP, NP >96% ee),11a but access to KP requires late stage arylation.
Thus, the broad side chain tolerance of SsADH-10 makes the method
presented here among the most generally (S)-selective.
To explore how these extended hydrophobic substrates bind to
SsADH-10, docking was carried out (Figure 2) for the (S)-antipodes
of flurbiprofenal, naproxenal, ketoprofenal, and fenoprofenal. A detailed
discussion of the approach and results is provided in the SI. Briefly,
W95 is seen as enforcing (S)-selectivity, with ligands clustering into
two distinct distal ring binding modes. “Channel-gating” L272 and
L295 appear to form a hydrophobic pocket for naproxenal and
flurbiprofenal. For the more flexible ketoprofenal and fenoprofenal,
edge-to-face π-π-interactions with W117 and F49 are proposed.
From a practical viewpoint, we have also found that SsADH-10 may
be engaged in a “thermal recycling” approach that may be generalizable
to other hyperthermophilic enzymes. Namely, while 30 vol% cosolvent
is often needed to dissolve hydrophobic DH substrates,13 we use a higher
T (80 °C) @ just 5% EtOH (solvent and biorenewable reductant).
Importantly, upon completion of the reaction, cooling to rt allows the
product to precipitate and be collected by filtration (see TOC graphic and
SI). Reclaimed SsADH may be recycled (5 cycles @ 94-96% ee). Given
the growing interest in thermophilic enzymes in synthesis,1,14 and in
engineering thermostability into mesophilic enzymes,15 this “thermal
switching” approach is likely to find broad application, well beyond the
domain of geothermal dehydrogenases.
NIH (SIG-1-510-RR-06307) for NMR facilities and the NIH (RR016544)
for lab renovation.
Supporting Information Available: Details of SsADH-10 expres-
sion, synthesis, spectra, DYRKR, and modeling. This material is
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Acknowledgment. The authors thank David L. Nelson for helpful
consultation and the Nebraska Center for Energy Research Sciences, the
NSF (CHE-0911732 to DBB), and the DOE (DE-FG36-08GO88055 to
PB) for support. We thank the NSF (CHE-0091975, MRI-0079750) and
JA910778P
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