Baran's Expedient Chemical Synthesis of (+)-Ingenol

          The phorbol esters are well-known tigliane natural products derived from Croton tiglium, the source of croton oil, as well as from other plants of the family Euphorbiaceae. Phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate (TPA) are useful pharmacological tool compounds in models of carcinogenesis due to their potent activity as mouse skin tumor promotors. Various esters of phorbol such as TPA, which mimic the chemical structure of diacylglycerol, bind to and activate protein kinase C. This type of modulation of one of the major signal transduction mechanisms within cells enables the phorbol esters to elicit a broad range of biological activities. TPA has been used extensively in biomedical research as a probe to identify the physiological systems in which protein kinase C is involved. Chemical syntheses of phorbol are notoriously challenging and have required between 36 and 52 total steps.

          Ingenol mebutate (trade name Picato) is the ester derived from a related ingenane diterpene, ingenol, and angelic acid. This drug was recently approved by the FDA as a first-in-class treatment for actinic keratosis, a precancerous skin condition. Unlike the tigliane/phorbol-type diterpenoids, the ingenane carbocyclic framework consists of a unique in,out-[4.4.1]bicycloundecane core. This ring skeleton is quite strained due to the "inside-outside" intrabridgehead stereochemistry of the BC ring system, wherein the C8 and C10 configurations are trans to one another. This renders ingenol a formidable synthetic target for total synthesis. The cis-triol of the AB ring fragment is also difficult to elaborate and often requires lengthy synthetic sequences. The recent chemical synthesis of ingenol by Baran's group at the Scripps Research Institute will be the subject of this post.

          Baran's route, like many previous synthetic studies targeting ingenol, starts from a readily available terpene, (+)-carene. The cyclohexanone intermediate 2 is assembled by means of a stereocontrolled aldol condensation between an elaborated carene derivative and the allene-containing aldehyde 1. Treatment of 2 with ethynyl magnesium bromide gave the diol 3  (d.r. 10:1) which was subsequently protected as the bis-silyl ether 4. Baran then beautifully applied Kay Brummond's (University of Pittsburgh) rhodium(I)-catalyzed allenic Pauson-Khand-type chemistry to the intricate substrate 4, which fashioned the carbocyclic tigliane carbon skeleton in an extremely concise manner compared to previously demonstrated synthetic strategies.

          The key step in Baran's ingenol synthesis is the vinylogous 1,2-pinacol rearrangement of the advanced tigliane/phorbol-type system 6, which sets the requisite inside-outside (trans) intrabridgehead stereochemistry of 7. This fascinating skeletal transformation, speculated to be of biosynthetic relevance, involves a 1,2-alkyl shift and is reminiscent of the Tsuchihashi-Suzuki rearrangement of 2,3-epoxy alcohols. In order to be successful, the C9-C11 bond of 6 must selectively undergo the desired migration to the developing partial positive charge at C10. In the event, exposure of intermediate 6 to Lewis acidic conditions at low temperature induced the projected vinylogous pinacol-type rearrangement which established the quaternary center at C10 in a highly stereocontrolled fashion. The advanced intermediate 7 then underwent sequential, chemoselective oxidation reactions that culminated in fully synthetic ingenol in only 14 total operations.

          It should be noted that pinacol-type rearrangements involving 1,2-alkyl migrations have been previously applied to the synthesis of ingenol. For example, in the second total synthesis of ingenol, Kuwajima and co-workers utilized this type of skeletal reorganization with a ring system (11) that was derived from a functionalized trans-decalin (10). In a related study, Cha's group assembled the Tsuchihashi-Suzuki rearrangement substrate 14 using olefin metathesis. In these examples, stereoelectronic requirements dictate that an antiperiplanar alignment of the C9-C11 bond with respect to the C10-O is required for the desired migration to occur. While the intended rearrangements in both examples were quite efficient from a yield perspective, both Kuwajima's and Cha's substrates lacked the intact cis-triol of the AB ring fragment and, thus, multistep oxidative elaboration following the skeletal reorganization was required. In comparison, Baran's substrate (6) is expertly designed. The relative stereochemistry of 6 satisfies all of the stereoelectronic requirements for bond migration and the vinylogous SN2' nature of the transformation directly installs the olefin between C1 and C2 of the A-ring. Baran clearly benefited from a discerning knowledge of the literature precedent, but also designed and executed a virtuosic refinement to the existing technology. His chemical synthesis compares favorably to isolated yields of ingenol from plant material and may facilitate the commercial manufacture of Picato.

Baran's Total Synthesis of Racemic Steviol, the Aglycone of Stevia's Sweet Glycosides

A Steviol Glycoside.

          The prominent and detail-oriented 'Lydia' character from the AMC series 'Breaking Bad' was wont to partake of chamomile tea sweetened with a packet of 'Stevia' during cafe meetings. Walter White eventually took advantage of her fastidious nature to replace the single packet of Stevia on her table with his long ago-isolated ricin, a potent toxin derived from Castor beans. Steviol glycosides (a representative structure is depicted above) are responsible for the sweet taste of the leaves of the stevia plant (Stevia rebaudiana Bertoni), which range in sweetness from 40 to 300 times sweeter than sucrose. Moreover, steviol glycosides do not induce a glycemic response when ingested, rendering them attractive as natural sweeteners for diabetics. The diterpene steviol, the aglycone of Stevia's sweet glycosides, was the target of recent synthetic studies led by the recently crowned MacArthur fellow Phil Baran at the Scripps Research Institute. The Baran laboratory's campaign, which culminated in the development of a practical total synthesis of (+/-)-steviol, is the subject of this post.

          Steviol is an oxidized congener of ent-kaurene, the biosynthetic precursor to the well-known  diterpene plant hormone gibberellic acid. Baran's group constructed the key early-stage tricyclic intermediate 2 in a biomimetic fashion that drew inspiration from Nature's fascinating enzymatic conversion of geranylgeranyl pyrophosphate (GGDP) into ent-kaurene. The enone 3 was then obtained from 2 by a three-step sequence involving elimination of a secondary alcohol, hydrogenolysis and Birch reduction/isomerization. In the subsequent operation, a critical allene [2 + 2] photocycloaddition installed the hindered C8 quaternary center of the advanced cyclobutane intermediate 4. Several alternate methods to install the C8 stereocenter had failed.

          Ozonolysis of 4, when conducted in methanol, induced fragmentation of the strained cyclobutane framework to generate the intermediary methyl ester 5. Next, the [2.2.2]bicyclic system of 6 was fashioned by exposure of 5 to forcing acidic conditions and subsequent reductive cyclopropanation in the presence of acetic anhydride led to the advanced diacetate 7. Finally, controlled fragmentation of 7 with methanolic hydrochloric acid, followed by an expedient methylenation/oxidation endgame sequence produced fully synthetic steviol in only 17 total steps starting from geranyl acetate.

          Notably, an intriguing skeletal rearrangement of steviol, induced by HBr, provides access to the related beyerane diterpene natural product isosteviol, which is apparently thermodynamically favored compared to its kinetic precursor. This impressive work establishes a rapid and efficient synthetic route to access minimally oxidized members of the ent-kaurane and beyerane class of diterpenes. As noted above, members of this natural product family are commercially valuable in the flavor industry as precursors natural and/or semisynthetic sweeteners.

Functionalization of Unactivated Angular Methyl Substituents on the Steroidal Molecular Framework: Yoshii’s Classic Semisynthesis of Strophanthidin

            The Baran Laboratory’s partial synthesis of the cardiotonic steroid ouabagenin starting from adrenosterone was recently highlighted here. It is intriguing to note that a related synthetic campaign was conducted no less than 35 years ago in Japan by Eiichi Yoshi’s research group. Yoshii’s team successfully synthesized strophanthidin from the readily available tetracyclic precursor pregnenolone acetate. The cardiotonic steroid strophanthidin is structurally analogous to the renowned mammalian endogenous hormone ouabagenin by virtue of its characteristic C17 butenolide system, ring junction stereochemistry and, perhaps most importantly, oxidized angular C19 methyl substituent. The installation of oxygenated functionality at ‘unactivated’ angular groups on the steroidal nucleus has been a longstanding synthetic challenge of great medicinal relevance, dating back to the manufacture of synthetic progestogens, components of the first oral contraceptive pills for women. Yoshii’s partial synthesis of strophanthidin relied on a redox relay strategy in which the oxidation state of the steroidal C6 position in the D⁵-steroid pregnenolone acetate (1) was systematically transferred to the angular C19 position. The butenolide moiety was then introduced and elaborated in a somewhat linear fashion followed by additional stereocontrolled oxidative modification of the steroidal framework to complete the synthesis. It should be noted that, in the absence of contemporary (convergent) C-C bond forming processes such as the Stille coupling, Yoshii’s butenolide construction was highly inventive for its time. Ultimately, as a result of this pioneering synthetic study, 13 mg of strophanthidin was produced and fascinating reactivity patterns associated with cardiotonic steroids were uncovered.

            Elegant technologies for the regioselective oxidative functionalization of the angular steroidal C19 position had been previously developed by chemists at Syntex Research Laboratories and by D. H. R. Barton in the early 1960s (see below). Yoshii’s group exploited those methods to gain rapid access to the pentacyclic intermediate 2, which was then advanced to the D¹⁴-butenolide derivative 3 by an 11-step synthetic sequence.

            The synthesis of oxidatively modified steroidal congeners related to 2, bearing heteroatom functionality at the C19 methyl group, facilitated the commercialization of various synthetic 19-norsteroids that were constituents of the first combined orally active hormonal contraceptives. The combination birth control pill usually contains a synthetic estrogen and progestogen (progestin) component. These hormones induce a physiological state resembling pregnancy wherein no additional ova can mature in the ovary for eventual fertilization. Most of the commercial estrogenic and progestogenic active pharmaceutical ingredients (APIs) belong to the 19-norsteroid series in which the unactivated angular 19-methyl group is eliminated. We will briefly examine a few classical examples of C-H functionalization of the steroidal C19 methyl group before returning to an overview of Yoshii’s partial synthesis of strophanthidin.

            The 1962 Syntex route to 19-norprogesterone (8) is shown in the scheme below. The addition of hypobromous acid to the D⁵-steroid pregnenolone acetate (1) generates a 6b-hydroxy bromohydrin intermediate which, on exposure to lead tetraacetate, undergoes intramolecular functionalization of the axial 19-methyl group to afford the 6b,19-oxido-steroid (2) in high yield (Intermediate 2 is also used in Yoshii’s synthesis of strophanthidin). Subsequent zinc reduction of the bromoketone 5 then yields a 19-hydroxy enone (6) that can be readily oxidized and decarboxylated to furnish 19-norprogesterone. In this groundbreaking work, the intramolecular functionalization of the 19-methyl group is the enabling technology that provides access to a molecular species (7) from which C19 can be eliminated under mild conditions.

            In the early 1960s, D. H. R. Barton showcased his nitrite photolysis methodology in an expedient partial synthesis of 19-noraldosterone acetate, a highly active mineralocorticoid salt-retaining hormone of the adrenal cortex. In brief, photolysis of the nitrite ester 10 results in formation of a carbon-centered C19 radical and nitric oxide (see Int-II), which then recombine to give an oxime (11) upon rearrangement of Int-III. The oxime 11 is then hydrolyzed and reduced to 19-hydroxycorticosterone (13) via the intermediacy of the lactol 12. Treatment of 13 with methoxide under thermodynamic conditions effectively excises the C19 substituent by means of a retro-vinylogous aldol process in which the intermediate enolate is protonated exclusively on the b-face. A second redox relay from C11 to the angular C18 methyl group then fashions the requisite lactol and completes the semisynthesis of 19-noraldosterone acetate (15). The nitrite photolysis methodology described herein was also famously executed by Barton in his classic synthesis of b-amyrin as well as by Corey in the first total synthesis of the limonoid natural product azadiradione. Analogous oxidative remote functionalization of angular methyl groups on the steroidal skeletal framework has been accomplished by implementation of Meystre’s photochemical hypoiodite method developed at Ciba Pharmaceuticals (discussed here and here).

            Thus, synthetic access to the 6b,19-oxido-steroid 2 by utilization of the C-H functionalization technologies summarized above not only facilitated the commercial development of the first oral contraceptives for women, but also enabled pioneering synthetic work on complex cardiotonic steroids such as strophanthidin. As depicted in the scheme below, the D¹⁴-butenolide derivative 3 (obtained from 2) was successfully converted into strophanthidin by a 10-step sequence (key transformations highlighted in red). This late-stage portion of Yoshii’s route entailed installation of the 14b-hydroxy group via the intermediacy of the bromohydrin derivative 17. Subsequent nucleophilic epoxidation directed by the 19b-hydroxy functionality then led to the advanced intermediate 19, which underwent reductive oxirane cleavage upon exposure to chromous acetate. Stereoselective reduction of the C3 ketone resident in 20 was promoted by treatment with Urushibara nickel, a nonpyrophoric version of Raney nickel that is known for catalysis of highly regio- and stereoselective carbonyl reductions. Finally, exposure of the polyol 21 to chromic trioxide in hexamethylphosphoric triamide (HMPA) induced selective oxidation of the 19-hydroxy group to the requisite angular formyl moiety found in strophanthidin. The magnitude of the impact of Yoshii’s 1978 synthesis of a highly complex cardiotonic steroid cannot be overstated. Indeed, the development and optimization of semisynthetic methods analogous to the conversion of 1 into strophanthidin are currently underway more than 35 years after the disclosure of Yoshii’s JOC manuscript.