In mammals, the adrenal glands are
responsible for releasing hormones in response to stress. The adrenal cortex,
situated along the perimeter of the adrenal gland, is responsible for the
biosynthetic production of mineralocorticoids, which control electrolyte and
water levels, glucocorticoids, which possess historic anti-inflammatory
properties, and androgen hormones.
In the fall of 1941, just prior to the entry of the United States into World War II, adrenal cortical hormones were sought as potential therapeutics for the treatment of shock and battle fatigue. This was, in part, due to a bizarre rumor, erroneously circulated by allied intelligence, which suggested that Germany was buying the adrenal glands of cattle from Argentinian slaughterhouses and administering extracts to their Luftwaffe pilots. The prevailing thought at the time was that German scientists had unraveled the secret of the adrenal glands and were providing the Nazis with an adrenal product that enabled pilots to fly at unusually high altitudes without suffering ill effects from lack of oxygen. At the behest of the US government, industrial, university and foundation laboratories were joined in a massive effort to uncover and provide allied forces with this coveted ‘miracle drug’ substance of unknown molecular composition.
By 1941, 26 cortical steroids had been isolated from the adrenal cortex by the independent laboratories of E. C. Kendall, T. Reichstein and Wintersteiner. Only a few milligrams of these compounds can be isolated from a ton of adrenals and, thus, synthesis is required to procure meaningful amounts. During the war, a 30-step partial synthesis of Kendall’s ‘Compound A’ was accomplished by Merck and Co. By 1944, nearly 100 grams of Kendall’s A, 11-dehydrocorticosterone, had been synthesized. The molecule was shown to be devoid of any significant biological activity. However, in the same year, L. H. Sarett (depicted below) of Merck prepared the 17-hydroxylated analog of Kendall’s A by a 39-step synthetic sequence starting from cholic acid (vida infra). Kendall referred to this substance as ‘Compound E,’ but to avoid confusion with ‘vitamin E,’ Kendall agreed to call it ‘cortisone.’ Cortisone failed in the treatment of adrenal insufficiency (Addison’s Disease).
Since the late-1920s, the nobel laureate-to-be, P. S. Hench of the Mayo Foundation, had astutely observed that the condition of women suffering from rheumatoid arthritis improved during pregnancy. He hypothesized that the improvement was due to the release of some hormone. In 1948, acting solely on the basis of this rather vague hypothesis, Hench injected 100 mg of Sarett’s synthetic cortisone into a patient suffering from a serious case of rheumatoid arthritis. The dramatic results are now legendary. In a few days the previously bedridden patient went downtown on a shopping spree and the event was covered extensively by the popular press of the day. Demand for the new wonder drug for the treatment of inflammatory diseases became enormous.
In the fall of 1941, just prior to the entry of the United States into World War II, adrenal cortical hormones were sought as potential therapeutics for the treatment of shock and battle fatigue. This was, in part, due to a bizarre rumor, erroneously circulated by allied intelligence, which suggested that Germany was buying the adrenal glands of cattle from Argentinian slaughterhouses and administering extracts to their Luftwaffe pilots. The prevailing thought at the time was that German scientists had unraveled the secret of the adrenal glands and were providing the Nazis with an adrenal product that enabled pilots to fly at unusually high altitudes without suffering ill effects from lack of oxygen. At the behest of the US government, industrial, university and foundation laboratories were joined in a massive effort to uncover and provide allied forces with this coveted ‘miracle drug’ substance of unknown molecular composition.
By 1941, 26 cortical steroids had been isolated from the adrenal cortex by the independent laboratories of E. C. Kendall, T. Reichstein and Wintersteiner. Only a few milligrams of these compounds can be isolated from a ton of adrenals and, thus, synthesis is required to procure meaningful amounts. During the war, a 30-step partial synthesis of Kendall’s ‘Compound A’ was accomplished by Merck and Co. By 1944, nearly 100 grams of Kendall’s A, 11-dehydrocorticosterone, had been synthesized. The molecule was shown to be devoid of any significant biological activity. However, in the same year, L. H. Sarett (depicted below) of Merck prepared the 17-hydroxylated analog of Kendall’s A by a 39-step synthetic sequence starting from cholic acid (vida infra). Kendall referred to this substance as ‘Compound E,’ but to avoid confusion with ‘vitamin E,’ Kendall agreed to call it ‘cortisone.’ Cortisone failed in the treatment of adrenal insufficiency (Addison’s Disease).
Since the late-1920s, the nobel laureate-to-be, P. S. Hench of the Mayo Foundation, had astutely observed that the condition of women suffering from rheumatoid arthritis improved during pregnancy. He hypothesized that the improvement was due to the release of some hormone. In 1948, acting solely on the basis of this rather vague hypothesis, Hench injected 100 mg of Sarett’s synthetic cortisone into a patient suffering from a serious case of rheumatoid arthritis. The dramatic results are now legendary. In a few days the previously bedridden patient went downtown on a shopping spree and the event was covered extensively by the popular press of the day. Demand for the new wonder drug for the treatment of inflammatory diseases became enormous.
Lewis Sarett (left) and Max Tishler (right) of Merck |
Merck’s process chemistry
laboratories, then led by Max Tishler (depicted above), began to develop
Sarett’s synthetic route to cortisone for industrial-scale manufacturing. By
1949, only one year after Hench’s historic discovery, one kilogram of synthetic
cortisone was obtained through optimization and execution of Sarret’s ‘bile acid process,’ starting from animal-derived deoxycholic acid (7-DCA). By 1950,
one ton of cortisone acetate had been prepared. As a direct result of this
research and development effort, between the years of 1951 and 1960, the price
of cortisone decreased from $200/gram to $1.50/gram.
The Merck bile acid process required 39 linear
synthetic steps to convert cholic acid into cortisone. The route proceeded in
three phases. The sole purpose of phase 1, highlighted in the reaction scheme
above, was to transpose oxygen from carbon position 12 to 11 of the
cyclopentenophenanthrene steroid C-ring. This was necessary because, at the
time, no known, abundant, natural steroid with oxygen at C11 was available. A
key synthetic feature of phase 1 of Merck’s bile acid process involved the
formation of an oxo-bridge between the seemingly distant carbon positions 3 and
9, forged by intramolecular SN2' displacement of the allylic bromide 1 by the a-oriented C3 hydroxyl group. Next, stereoselective
bromination of the resultant intermediate 2 gave predominantly the 11b, 12a-dibromide
3. Subsequent regioselective displacement of the more reactive 11b-bromide and further oxidation with sodium
dichromate then secured the ketone 4 and completed the critical task of transposition
of oxygen from carbon position 12 (of cholic acid) to C11.
The second phase of Merck’s manufacturing process transforms the cholate side chain to the requisite dihydroxyactone of cortisone. In the course of phase 2, the C17 side chain is first converted into a diene (5) and then oxidatively cleaved to furnish the pregnane dione 7. Seven subsequent conversions are then required for additional oxidative elaboration of the C17 substituent. And, finally, phase 3, which simply introduces unsaturation into the A-ring, is completed in four steps, marking the completion of Merck’s cortisone manufacturing process.
The second phase of Merck’s manufacturing process transforms the cholate side chain to the requisite dihydroxyactone of cortisone. In the course of phase 2, the C17 side chain is first converted into a diene (5) and then oxidatively cleaved to furnish the pregnane dione 7. Seven subsequent conversions are then required for additional oxidative elaboration of the C17 substituent. And, finally, phase 3, which simply introduces unsaturation into the A-ring, is completed in four steps, marking the completion of Merck’s cortisone manufacturing process.
It should be noted that Sarett of Merck
(along with R. B. Woodward of Harvard) also completed a highly stereospecific total synthesis of optically active
cortisone in 1951. The endgame of Sarett’s total synthesis is depicted in the scheme
shown above. The protocol hinges on the formation of a key cyanohydrin intermediate,
which is dehydrated upon exposure to phosphorous oxychloride in pyridine to produce
12. Dihydroxylation of the 17,20-olefin is then achieved by treatment of 12
with potassium permanganate and hydrolysis of the transiently formed oxidation product (13)
gives cortisone acetate. Total synthesis, as is often the case, ultimately proved
too costly for industrial-scale cortisone production.
Russell E. Marker |
By the late 1940s, chemists at Syntex laboratories
in Mexico City had initiated efforts to develop an alternative to Merck’s bile
acid process. Syntex (from “Synthesis” and “Mexico”) was founded to develop and
exploit a synthetic technology invented by Russell Marker, a steroid pioneer
and former chemistry professor at Penn State University. Marker had discovered
a simple method for degradation of the steroidal sapogenin side chain, which
efficiently produces a synthetic precursor to the female sex hormone progesterone
from abundant, plant-derived starting materials. In 1951, Carl Djerassi was
recruited by Syntex to lead a program focusing on the synthesis of
11-oxygenated cortical steroids starting from a plant raw material derived from
Mexican yams (cabeza de negro root),
utilizing the Marker degradation (depicted in the scheme below).
The
development of an alternate synthesis of cortisone that did not start from a
cattle-derived bile acid but, rather, an inexhaustible plant-derived starting
material was the most crucial organic chemistry challenge of the 1940s and
early 1950s. But since all of the plant sapogenins that were known at the time
lacked an oxygenated substituent in the C-ring, the critical synthetic hurdle associated with this endeavor became introduction of oxygen into a naked steroidal ring C. Many
synthetic solutions to this daunting problem were developed by Djerassi’s group
at Syntex, but none of these saw application on an industrial scale. This was
because an Upjohn group headed by O. H. Peterson reported in 1952 a culture of
the fungus Rhizopus arrhizus from
Kalamazoo air, capable of converting progesterone into 11a-hydroxyprogesterone. Culture development
eventually raised the isolated yield of the process to ~90%, thereby solving
the problem of oxygenating C11 of the steroid framework en route to cortical
steroids. However, Upjohn’s microbiological process depended on the
availability of tonnage quantities of cheap progesterone, which, in the early
1950s, were only available from Syntex using Marker’s diosgenin degradation.
The intersection of the Marker degradation (shown above) with Upjohn’s
microbial biotransformation of progesterone (see below, conversion of 19 to 20)
provided the desired alternative to Merck’s celebrated bile acid production
process, which was shut down in 1966.
The Syntex
route to cortisone, in brief, involved a novel, stereoselective catalytic
hydrogenation of the 4,5-double bond of 20, to be followed by a highly
challenging regio- and stereoselective reduction of the pregnane-trione 22, which produces the 3a-ol advanced intermediate
23 with moderate efficiency.
The conversion of 23 into cortisone
acetate (depicted in the scheme above) relied upon synthetic methodology that was
previously described by Kritchevsky and co-workers at the Sloan-Kettering
Institute. The route features selective epoxidation of the dienol acetate 24 to
introduce the C17 dihydroxyacetone moiety followed by a bromination/elimination
sequence to install the requisite A-ring unsaturation, culminating in synthetic
cortisone.
Carl Djerassi at Syntex in 1951 |
A few years later, Glaxo in England
commercialized a cortisone process that effectively transpositioned the keto
group of hecogenin, obtained from abundant and inexpensive East African sisal wastes,
from C12 to C11. The process was based on an alternate synthesis of cortisone
that was developed at Syntex and licensed to Glaxo.