Sunday, December 28, 2014

Evolution of a Fully Synthetic Approach to Access the [Iso]Cyclocitrinol Family of Rearranged C25 Steroids

            Isocyclocitrinol A (structure and retrosynthetic analysis shown below) is a skeletally unique rearranged steroid that has been the subject of recent synthetic investigations. The citrinane family of C25 steroids share a nearly unprecedented carbocyclic framework comprised of a bridged bicyclo[4.4.1]undecene A/B ring system that includes an uncommon anti-Bredt bridgehead olefin. Cyclocitrinols are produced by the ubiquitous fungus Penicillium citrinum and have been isolated from terrestrial as well as marine sources. The originating member of this steroidal subclass, cyclocitrinol, was isolated from a terrestrial Penicillium citrinum and reported as a new sesterpenoid in 2000 by Kozlovsky and co-workers. Three years later, Clardy and Crews isolated isocyclocitrinol A from a marine strain of fungus obtained from an Axinella sponge collected in Papa New Guinea. They also revised the original cyclocitrinol structure reported by Kozlovsky et al based on single crystal X-ray crystallography data. Recently, a group from Ocean University of China isolated eleven new citrinane C25 steroids from cultures of fungus derived from the volcanic crater ash of an extinct volcano in Guangdong, China. The biogenetic origin of these fascinating steroids traces back to ergosterol and has been discussed here and here.
            In 2007, the laboratory of Hans-Günther Schmalz, based in Germany, reported a clever semisynthetic approach to access the isocyclocitrinol core framework. At the time of their SYNLETT publication, James Leighton’s group at Columbia University had already embarked upon an amibitious campaign to prepare totally synthetic isocyclocitrinol A. Steroid total synthesis has been a historically challenging endeavor due to the stereochemical complexity associated with a variously oxygenated tetracyclic ring skeleton, often bearing several all-carbon quaternary stereogenic centers at angular ring junction positions. A de novo approach to steroid synthesis rarely approaches the relative efficiency and practicality of semisynthetic alternatives. Moreover, abundant and enantiopure plant-derived sterols and related synthetic precursors such as prednisone ($1.20/gram) are cost-effective chiral pool substances that are readily elaborated and processed. The cyclocitrinols are particularly daunting steroidal targets for total synthesis owing to a lack of viable synthetic methods to construct the A/B bicyclo[4.4.1]undecene, a carbocyclic motif that is rarely encountered in Nature. In spite of these significant scientific hurdles, Leighton’s group has successfully developed an impressive sequence of tandem reactions that provides reasonably expedient access to the complex citrinane core structure, sans D-ring. The Columbia University total synthesis of advanced intermediates en route to isocyclocitrinol A will be the subject of this post.
            The genius of the Leighton approach to isocyclocitrinol is derived from the recognition that its silyl enol ether retrosynthetic precursor (a Saegusa oxidation substrate, in the forward sense) is the product of a Cope rearrangement of a bicyclo[3.2.1]octane bearing diene functionality. A ten-membered ring tether that is, ideally, fused to the steroidal cyclopentane D-ring would bring the termini of the reacting alkenes into the necessary proximal orientation required for p overlap. The Cope rearrangement was projected to proceed under mild reaction conditions as the electrocyclic reaction would be accelerated by the release of significant strain in the ten-membered ring of the substrate. Another key insight by Leighton’s group was that the requisite bicyclo[3.2.1] system could be obtained from a semipinacol-type rearrangement of a simplified [3.3.0] bicyclic alkene. The latter is accessible by way of a Pauson-Khand reaction with an optically active enyne. Based on the publication dates of two PhD dissertations describing progress toward the execution of this intriguing synthetic hypothesis, it appears that the Columbia team initiated their efforts shortly after the structural revision of cyclocitrinol was reported by Clardy and Crews, about eleven years ago.
            The total synthesis begins with stereospecific elaboration of enantiopure (R)-epichlorohydrin, giving rise to the enyne 4 after a brief four-step reaction sequence. The enyne 4 is then subjected to the Pauson-Khand reaction conditions of Yang (catalytic Co2(CO)8 and the ligand, tetramethylthiourea or TMTU), methodology that has been famously applied to a monumental total synthesis of schindilactone A (discussed here). Regrettably, the annulation reaction affords the bicyclo[3.3.0]octanone 5 in an unselective fashion, delivering a separable mixture of stereoisomers. The desired diastereomeric bicyclic enone was obtained in moderate (43%) yield, reproducibly, on multigram scale. Nucleophilic epoxidation of 5 proceeds from the anticipated convex face to provide the epoxyketone 6 with good efficiency. Next, a methylene Wittig reaction was used to install an exocyclic olefin handle for subsequent cross-metathesis (CM), furnishing the alkene 7 in 85% yield.
            Then, upon exposure to a catalytic amount (5 mol%) of the Hoveyda-Grubbs Generation II (HG-II) catalyst in refluxing chloroform, the bicyclic epoxyalkene 7 undergoes a remarkable one-pot tandem cross-metathesis/semipinacol rearrangement to fashion the requisite (3S,E)-6-alkylidene-3-hydroxybicyclo[3.2.1]octan-8-one (9) in a single operation, with full control over the stereochemistry of the challenging trisubstituted alkene. Based on the results of several mechanistic control experiments, the authors conclude that a mildly Lewis acidic ruthenium methylidene species [LnRu=CH2], generated upon completion of the intial E-selective cross-metathesis reaction, serves as the catalyst for the subsequent semipinacol-type 1,2-bond migration that ultimately leads to 9. The extraordinary conciseness and stereospecificity of the Columbia route to the complex hydroxybicyclo[3.2.1]octanone intermediate 9, beginning from an inexpensive three-carbon building block, is diminished only slightly by the lack of stereocontrol in the Pauson-Khand annulation step (4 à 5).
            As with any conceptually interesting total synthesis endeavor, a major roadblock was encountered. The hypothetical ten-membered ring substrate for the projected Cope rearrangement (discussed above) to form the bridged bicyclic A/B core of isocyclocitrinol A is expected to be considerably strained. As such, the problem of constructing the elusive anti-Bredt bicyclo[4.4.1]undecene of the citrinanes has been exchanged for that of building a highly strained ten-membered ring. But, as noted above, if such an intermediate could be accessed, its rearrangement should be strain-accelerated and facile. Indeed, significant obstacles were encountered in the course of the first series of attempts to synthesize the ten-membered Cope substrate, containing a pre-installed [3.2.1] bicycle and cyclopentane D-ring. Two strategies based on ring-closing metathesis (RCM) as a means to establish the medium-sized ring (attempted transformations shown above) were met with failure. Testing the team’s persistence further, at least three alternate approaches to the challenging cyclization were similarly unsuccessful, presumably due to the inherent strain of the ten-membered system.
            The ring formation strategy that ultimately proved successful was inspired by the work of Funk and co-workers who, in the course of studies directed toward the diterpenoid ingenol, synthesized a superfluously large (and correspondingly less-strained) macrolactone, and then performed an Ireland-Claisen ring contraction to forge the natural product’s strained carbocyclic core structure. Leighton and co-workers hoped that the unanticipated contraction of the larger and more synthetically accessible ring could be combined, again, in a tandem one-pot process, with the designed strain-accelerated Cope rearrangement. A 14-membered macrolactone (11) could, indeed, be efficiently prepared in three steps from intermediate 9. Then, under carefully optimized reaction conditions, the macrolactone 11 underwent the desired tandem Ireland-Claisen ring contraction/Cope rearrangement to fashion the intact A/B/C tricyclic core of isocyclocitrinol A. The authors invoke an intermediary silyl ketene acetal, which undergoes the Ireland-Claisen rearrangement through a boat-like conformation (see TS-I above) to transiently generate the strained medium-sized ring. The in situ-formed ten-membered ring species then immediately succumbs to the projected electrocyclic Cope rearrangement as depicted in TS-II, furnishing the advanced intermediate 12, following desilylation and esterification with diazomethane. It should be noted that the C14 configuration of the advanced tricycle 12 is epimeric to that of natural isocyclocitrinol A.
            The tandem Ireland-Claisen/Cope rearrangement protocol delivers a product that lacks the requisite C6 enone as well as the D-ring cyclopentane and C17 side chain. Moreover, the immediate product of the tandem sigmatropic sequence, prior to acid treatment, is a silyl enol ether (e.g. 13). Interestingly, the authors did attempt to isolate 13 from the Cope rearrangement and directly subject it to Saegusa oxidation conditions (stoichiometric palladium acetate) to accomplish the requisite dehydrogenation at C6. Unfortunately, the vinyl substituent projecting from the C-ring was activated more readily than the silyl enol ether, giving rise to a cyclic enol acetate moiety in combination with a low yield of the desired enone (e.g. 14). It was later demonstrated that selenoxide elimination was a preferable method for enone installation in terms of overall efficiency.
            The Leighton group exploited the propensity for the vinyl subsituent to undergo Wacker oxidation for D-ring formation. First, the keto group of advanced intermediate 15 was protected as a cyclic ketal using the method of Noyori. Standard Wacker oxidation conditions then afforded the expected ketoester 16 in good yield. Finally, treatment of 16 with a strong base followed by diazomethane furnished the annulated vinylogous ester tetracycle 17, bearing the full citrinane carbocyclic framework, albeit with an inverted configuration at C14.
            Recently, a new method for formation of the bridged seven-membered ring system of the citrinanes (i.e. the bicyclo[4.4.1]undecane) involving intramolecular oxidopyrilium-mediated [5+2] cycloaddition has emerged. A team from Shenzhen, China, led by Chuang-chuang Li, used their direct and efficienct thermal cycloaddition methodology to prepare the strained polycyclic cores of ingenol and cyclocitrinol (19). Thus, exposure of the acetoxypyranone 18 to 1.5 molar equivalents of 2,2,6,6-tetramethylpiperidine transiently generates an oxidopyrilium species containing a tethered dienophile (TS-I). A type II intramolecular [5+2] cycloaddition reaction then ensues, producing the strained carbocyclic core of cyclocitrinol (19). Importantly, the cycloadduct 19 is oxygenated at C3 in the correct stereochemical configuration and contains the notoriously challenging bridgehead double bond. The cycloaddition method of Li constitutes one of only three synthetic strategies that are uniquely suitable for the construction of the unusual bridged, anti-Bredt A/B ring system of the citrinane steroids.
            The cyclocitrinol family of C25 steroids has not been shown to exhibit overwhelmingly enticing bioactivity. Weak antibacterial activity (MICs in the ~100 ug/mL range) and effects on activation of potentially neuritogenic GPR-12 have been reported. It is largely the unmet synthetic needs that are illuminated by the confounding molecular architecture of the citrinanes that have stimulated interest and effort from the synthetic community. The chemical structures of the citrinanes have inspired the invention of innovative and virtuosic synthetic technologies from laboratories based in Germany (Schmalz), China (Li) and the United States (Leighton). Steroids, particularly those of fundamentally new structural types, continue to play an important role in the development of the field of synthetic organic chemistry.