Na+,K+-ATPase, often referred to as the sodium-potassium pump, is an integral membrane protein that catalyzes the
enzymatic ATP-dependent transport of three sodium ions out of the cell and two
potassium ions in, generating a Na+ gradient across the cell. The
resulting gradient drives a number of vital processes including the transport
of glucose and other nutrients across the cell membrane. Apart from its
transport function, Na+,K+-ATPase can also act as a
functional receptor for the initiation of a variety of signaling events that
can alter cellular functions and cell growth in a cell-specific manner. Some of
these Na+,K+-ATPase-mediated signaling pathways can be
activated in certain mammals by circulating endogenous cardiotonic steroids, which
are inhibitory ligands of the sodium-potassium pump. Altered plasma levels of
endogenous cardiotonic (cardenolide) steroids have been associated with the
development of pathological conditions such as chronic renal failure,
hypertension and congestive heart failure. Moreover, certain cardenolide
steroids such as ouabain and digoxin are widely prescribed therapeutic
treatments for heart failure and other cardiac conditions. The practical chemical
synthesis of structurally complex cardiotonic steroids and analogues thereof
remains a significant obstacle to the task of improving the narrow therapeutic
index of this important class of cardiac drugs.
Recently, Phil Baran’s group at the Scripps Research Institute
disclosed a truly groundbreaking synthesis of the polyhydroxylated steroid
ouabagenin (the aglycone of ouabain, structure depicted above) starting from a
readily available steroidal precursor. The route was demonstrated on a moderate
laboratory scale: 500 mg of a late-stage intermediate was obtained from a
single batch run. Baran refers to his synthetic approach to ouabagenin as ‘quasibiomimetic,’
utilizing the concept of ‘redox relay,’ broadly defined as the transfer of
redox information from one carbon position to another site within a given
molecular framework. In this case, the 11-oxo functionality resident in the
starting material cortisone is sequentially relayed, first, to the angular C19
position and then subsequently to C1 and C5 on the A-ring through the
intermediacy of structures 3 and 4. Intermediate 3 is the product of Norrish
type II photochemical functionalization the C19 methyl group by the proximal C11
carbonyl to furnish the cyclobutanol system (of 3) after stereocontrolled recombination
of a transiently generated 1,4-biradical. A related photochemically-mediated
C-C bond formation (shown below) to selectively functionalize the C18 angular
methyl group of the ouabain derivative 5 was previously described in 1999 by
Corey and Stoltz.
As a temporary aside, it must be noted tangentially that the
‘endogenous ouabain’ structure elucidation story is a complicated and
fascinating one. For the interested reader, a leading reference can be found
here. In brief, during the isolation, structural analysis and physiological
characterization of ‘endogenous ouabain,’ Nakanishi and co-workers uncovered a
facile ouabain-borate coordination phenomenon (depicted below) that occurs
readily on exposure to borosilicate glassware. The polyhydroxylated steroid
framework was shown to serve as a polydentate ligand capable of complexation to
soluble inorganic borate derived from either borosilicate glass or the chemical
reagent sodium tetraborate. Baran and co-workers insightfully exploited
ouabain’s unique capacity for borate complexation in their A-ring protecting
group strategy, highlighted below.
Returning to the Baran synthetic route to
ouabagenin, stereoselective Super-Hydride® reduction of the less hindered C3 ketone
of the advanced dione 1 was accompanied by formation of a cyclic ethyl boronic
ester (8) which was elegantly utilized as a robust protecting group for the
A-ring cis-1,3-diol. Next, reduction of the internal C11 carbonyl with
lithium/ammonia under thermodynamically favorable conditions provided the
desired 11-OH a-stereoisomer 9 in good yield (70%). The advanced
intermediates 8 and 9 are reminiscent of the ouabain-borate complexes (see
above) that were previously discovered and characterized by Nakanishi and
co-workers. The bridging ethyl boronic ester functionality resident in 8 and 9
constitutes an unconventional yet effective A-ring diol blockade that was shown
to be sufficiently stable throughout the entirety of the endgame reaction
sequence leading to the target structure. A final redox relay from C17 (of 9) to
C14 with inversion of the C/D ring junction configuration was now necessitated
in order to obtain a 14b-hydroxy-androstane derivative suitable for
completion of the synthesis of ouabagenin.
Two other research groups have previously disclosed potentially
applicable methodologies for the transformation of readily available sapogenin
or androstane systems into C/D cis-fused cardenolide-type steroids. Baran’s laboratory
chose to develop a new solution to this longstanding synthetic problem. Their
innovative method for elaboration of the eastern cardenolide substructural
framework must be assessed as superior to that of Heathcock and Wiesner (both of
the previous technologies are discussed in detail here) with regard to step
economy and modularity. Wiesner’s method, previously regarded as the state of
the art in semi-synthetic approaches to cardiotonic steroids, requires about 8
steps to obtain a C/D cis-fused 14b-hydroxy-cardenolide
derivative from a synthetic intermediate derived from testosterone. Baran’s methodology
requires only three synthetic operations to accomplish the same objective: (1)
Saegusa oxidation for D-ring (D14,15)
dehydrogenation; (2) a novel olefin isomerization (conversion of 10 into 11) and (3) Mukaiyama olefin hydration
to secure the advanced intermediate 12. Moreover, the corresponding vinyl
iodide (13) obtained from 12 by implementation of Barton’s protocol constitutes
a valuable medicinal chemistry branch point for the installation of a variety
of heterocyclic appendages to the C17 position in the course of forthcoming structure-activity
relationship (SAR) studies. Wiesner’s analogous ‘furan methodology,’ developed
in the early 1980’s, is far less flexible with regard to synthetic accessibility
of C17 ouabain analogues. The advanced vinyl iodide 13 can be converted to
ouabagenin in just four additional steps. The overall route proceeds in 20 steps
from andrenosterone (2) in 0.56% yield. It is a testament to the practicality and
potential medicinal relevance of Baran’s synthetic cardenolide technology that
this work, a steroid semi-synthesis (not exactly a new concept…), was published
in the journal Science in the year 2013.
