A Facile One-Pot Cascade Sequence for the Enantioselective Construction of Synthetic Steroids

            Bor-Cherng Hong’s group has developed expedient organocatalytic approaches to highly substituted cyclopentanes  and, more recently, to complex 14b-steroid derivatives. The development of convenient methods for the rapid assembly of optically active steroids from readily available synthetic building blocks, suitable to compete with classical semisynthetic approaches, is a long sought goal of organic chemists, dating back to the pioneering studies of Deslongchamps, Corey and others. Quite recently, Karl Anker Jørgensen’s group at Aarhus University (Denmark) has made an important contribution in this area. Hong’s laboratory at National Chung Cheng University (Taiwan) has now disclosed a powerfully expedient strategy to access the steroidal nucleus (5, shown below) that exploits enantioselective organocatalysis in conjunction with ‘telescoping,’ one-pot process development techniques.

            To begin, a double Michael addition sequence between the nitroalkane 1 and enal 2, catalyzed by the Jørgensen-Hayashi catalyst 3, proceeds in a stereocontrolled fashion via the intermediacy of homochiral iminium (Int-I) and enamine (see TS-I) species, to ultimately furnish the tetrasubstituted cyclohexanes (3), obtained as a diastereomeric mixture at the nitro-bearing carbogenic position. Direct addition of para-toluenesulfonic acid (pTsOH) to the reaction mixture then promotes a Robinson-type dehydrative cyclization of 3, which efficiently generates the tricyclic enone 4. Chloroform is then removed from the reaction mixture in vacuo and tetrahydrofuran (THF) and 1,8-diazabicycloundec-7-ene (DBU) are sequentially added to the crude trans-decalin 4.  Curiously, upon exposure of 4 to DBU, only the b-nitro diastereomer engages in intramolecular Henry cyclization (as depicted in TS-II) to afford the synthetic steroid derivative 5. Fortunately, the subsequent addition of tetra-ⁿbutylammonium fluoride (TBAF) to the reaction medium promotes isomerization of a-nitro epimer of 4 to the more reactive b-configuration, thereby facilitating the shuttling of additional 4 to the desired product 5 through the intermediacy of TS-II. Although this fascinating one-pot enantioselective sequence has only been demonstrated on milligram scale, the highly functionalized 14b-steroid 5 should be suitable for elaboration into a range of bioactive natural products including cardenolides, bufadienolides and pregnane glycosides.

Cafestol: A Complex Kaurene Diterpenoid Found in (Mmmm…) Coffee Drinks

            If you have ever added the sweetener Stevia to a coffee beverage that was prepared using a French press, then you have (perhaps inadvertently) added one diterpenoid natural product, steviol (or stevioside), to a drink that already contained appreciable amounts of another. As discussed in a previous post, the molecules that give Stevia its sweet taste are various glycosides derived from the diterpene aglycone steviol. Another diterpene alcohol from the kaurene family of natural products (structures shown below), cafestol, is naturally present in coffee beans. The biological properties and chemical synthesis of cafestol will be the subject of this post.

            The chemical compositions of the main lipid components of the two most important coffee species, Coffea arabica and Coffea canphora var. Robusta include triacylglycerols, sterols, and tocopherols. In addition, the lipid fraction of coffee contains structurally fascinating pentacyclic diterpenoids of the ent-kaurene family in proportions of up to 20% of the total lipids. The structures of the two main coffee diterpenes, cafestol and kahweol (1,2-dehydro-cafestol), were fully elucidated by Djerassi’s group in the late 1950’s. In coffee, cafestol and kahweol exist in esterified form with various fatty acids such as palmitic acid. In order to determine the total amount of the individual diterpenes, coffee oil must be saponified and then analyzed by gas chromatography (GC) and/or HPLC. In most Arabica and Robusto coffees, cafestol can be isolated in amounts of about 50 – 200 mg/kg of dry matter. A typical bean of Coffea arabica contains about 0.6% cafestol by weight.

            Cafestol is of special interest for its widespread human consumption as a constituent of coffee. But does our cafestol dietary intake provide any health benefits? In 1982, a study from the University of Minnesota showed that the palmitate fatty esters derived from cafestol and kahweol are potent inducers of increased glutathione (GSH) S-transferase activity in mice. GSH S-transferase is a major detoxification enzyme that catalyzes the covalent binding of a variety of reactive chemicals (e.g. the electrophilic forms of carcinogens) to GSH. Green coffee beans fed in the diet of mice also enhanced GSH S-transferase activity. But contrary to its anti-carcinogenic properties, dietary cafestol is also known for its potent cholesterol-elevating pharmacology. The mechanism by which cafestol elevates serum lipid levels involves the induction of altered hepatic expression of genes involved in lipid metabolism. Cafestol-induced disruption of gene expression and its impact on cholesterol homeostasis has been attributed to agonist activity at the nuclear hormone receptors, farnesoid X receptor (FXR) and pregnane X receptor (PXR). We should emphasize that cafestol levels are highest in Scandanavian-type (Turkish/Greek) boiled and French-press unfiltered coffee brews, which generally contain 3 – 6 mg of cafestol per cup. In filtered coffee drinks, it is present in only negligible amounts.

            For synthetic chemists, the intriguing structural features of cafestol alone, which include a somewhat sensitive furan and oxygenated bicyclo[3.2.1]octane system, render it a compelling and irresistibly confounding target for organic chemistry inquiries. Indeed, since its structure elucidation over fifty years ago, only two total syntheses of the pentacyclic diterpenoid have been reported, both in racemic form. The landmark achievement of E. J. Corey’s group in 1987 involved controlled fragmentation of a strained cyclopropane embedded within a complex bicyclo[2.2.2]octane ring system to fashion the kaurene skeleton. Corey’s cleverly designed carbocyclization strategy (shown below) exploited the electron-rich nature of cafestol’s western furanyl substructure and simultaneously installed the exocyclic C16 olefin that is a characteristic structural feature of the ent-kaurene diterpenoids. A similar advanced intermediate was accessed in the course of Baran’s recent total synthesis of steviol.

            The key polyolefin carbocyclization step in Corey’s total synthesis was promoted by activation of the primary alcohol 5 as its corresponding triflate derivative. At low temperature, the transiently generated triflate is intercepted by the proximally tethered vinyl furan, which fragments the labile cyclopropane ring in the course of a skeletal reorganization that culminates with expulsion of the triflate leaving group (mechanistic arrows shown above). Rearomatization of the furan occurs by deprotonation of the C6 position to ultimately afford the kaurene derivative 6. The reaction is stereospecific and proceeds with outstanding efficiency, given the complexity of the overall transformation. Additionally, the expedient stereocontrolled assembly of the complex carbocyclization substrate (5) also deserves comment. The cyclopropane of 5 is formed by a selective internal [2 + 1]-cycloaddition to one of the C-C double bonds of the diazo intermediate 2, promoted by a bis-(N-tert-butylsalicyclaldiminato)copper(II) catalyst (3). The generality of this reaction was described in 2013 by Corey and Newhouse. The remainder of Corey’s cafestol synthesis involves regio- and/or stereoselective olefin reductions and implementation of a triisopropylsilyl protecting group, to impede furan over-reduction in a late-stage hydrogenation. The first total synthesis of cafestol proceeds in a longest linear sequence of 26 operations.

            In April of this year, Ran Hong’s group at the Shanghai Institute of Organic Chemistry reported the second total synthesis of cafestol, 27 years after Corey’s original disclosure. Hong’s route, involving late-stage construction of the western furan ring, is putatively biomimetic. To begin, a tandem aldehyde-ene/Friedel-Crafts carbocyclization sequence forges the tricyclic decalin derivative 13 in a stereocontrolled fashion on multigram scale. The 1,2-cis selectivity observed in the ene reaction (11 à 12) is thought to be governed by the transition state model, Int-I. Next, the hindered all-carbon quaternary stereocenter at C8 of cafestol is installed with an Eschenmoser-Claisen rearrangement, conducted under neutral conditions, to secure 15 through the intermediacy of Int-II. The N,N-dimethylamide 15 is subsequently elaborated to the lactol 16 by a four-step sequence involving iodolactonization followed by base-promoted elimination of HI to migrate the C-ring double bond to C12-C13. A critical radical cyclization mediated by samarium iodide then fashions the requisite bicyclo[3.2.1]octane motif of the advanced intermediate 17 with remarkable synthetic efficiency (98% yield). The complex diol 17 can be advanced to the intact ent-kaurene framework (18) in four additional operations.

            Akai’s cationic gold(I)-mediated intramolecular cyclization of 3-alkyne-1,2-diols was projected to accomplish fusion of a heterocyclic furan ring to the advanced A-ring ketone derivative 18. Toward that end, the corresponding lithium enolate derived from 18 was hydroxylated at the a-position with Vedejs’ reagent and addition of ethynylmagnesium bromide to the resultant ketol delivered the furan cyclization substrate 19 as a single diastereomer. In the event, a procedural variation of Akai’s protocol for Au(I)-promoted cyclization (involving removal of a silver salt by filtration) provided the desired furan 20 in excellent yield. Finally, dihydroxylation of the C16 exocyclic olefin of penultimate 20 with stoichiometric osmium tetroxide followed by cleavage of the intermediate bisosmate with sodium sulfite completed the second total synthesis of racemic cafestol. It will be interesting to see if Hong’s synthetic route to cafestol can be rendered enantioselective in the future, perhaps by application of chiral Brønsted or Lewis acid catalysis to the tandem aldehyde-ene/Friedel-Crafts carbocyclization sequence (11 à 13).