Sunday, January 17, 2016
Partial Synthesis of Linckosides A and B, Unique C8-Hydroxylated Steroids from the Okinawan Blue Starfish
Linckia laevigata (also known as the Okinawan blue sea starfish) is a bright blue species of starfish that inhabits the tropical waters of the Indian and Pacific oceans. It has five cylindrical arms with a bright blue body color and yellow tube feet (See Image above). The distinctive color comes from a blue pigment called linckiacyanin as well as some accessory yellow carotenoids. The intense blue coloring of L. laevigata likely warns potential predators of toxicity, although there are no known adverse effects of the starfish on humans. Interestingly, these animals possess remarkable regenerative capabilities. For example, the blue seastar can use autotomy, or self-severance of a limb, to escape predation. Moreover, body parts lost to predators can also be regenerated by the starfish. Due to the aesthetic quality derived from their brilliant blue color, L. laevigata is popular with marine aquarium hobbyists for incorporation in personal reef aquariums, in spite of requiring slow acclimatization and being extremely sensitive to changes in temperature, oxygen level and pH.
In light of the regenerative capacities mentioned above, natural products obtained from L. laevigata were screened in the early 2000s for much sought-after neuritogenic pharmacological properties. Neuritogenic compounds or neurotrophic factors, exemplified by nerve growth factor (NGF), induce neuronal differentiation, wherein the neurons generate and extend neurites to form a functional network. For a previously reported example of a neuritogenic steroid, see. Neuritogenic activity is a vital component in the search for preventative and therapeutic agents for neurodegenerative diseases such as Alzheimer’s disease. Linckosides A and B, along with 20 minor congeners, were identified during the course of the screening campaign. Linckosides share a pentahydroxy-cholestane structural framework with variable glycosylation patterns. Polyhydroxylated steroid glycosides are commonly encountered as metabolites in starfishes. However, across bioactive and naturally occurring steroids, the hydroxylation pattern of the linckosides is relatively uncommon and hydroxyl groups located at the steroidal carbogenic position 8 (C8) are particularly rare. Very recently, Biao Yu’s group at the Shanghai Institute of Organic Chemistry completed the first semisynthetic preparation of linckosides A and B. In this post, we will outline their synthetic approach for oxidative functionalization of the steroid nucleus, paying particular attention to the strategy for introduction of the C8-hydroxyl group, an undertaking that is not well-precedented in the synthetic literature.
As depicted above, the partial synthesis of linckosides A and B begins from the well-known plant-derived sterol diosgenin, which was converted into the side-chain degradant vespertilin acetate by a high yielding four-step sequence that was conducted by Yu’s group on 33-gram scale. The steroidal C7 position was then modified under standard allylic bromination conditions to provide a functional handle that facilitated eventual access to the elusive C8 position. In brief, exposure of a C7 b-thioarylether intermediate to an oxaziridine reagent furnished the corresponding sulfoxide as an epimeric mixture. Subsequent treatment with a mild base at 80 oC promoted cis-elimination of the sulfoxide to yield a 5,7-diene. The D7,8 unit of unsaturation within the diene system is crucial to the overall strategy to ultimately hydroxylate the embedded C8 position. However, the more accessible D5,6 olefin was first selectively epoxidized using methyltrioxorhenium and urea-hydrogen peroxide and hydrolysis of the resultant oxirane generated the advanced 5a,6b-diol intermediate shown above.
The Mukaiyama hydration reaction was used to install the C8-hydroxyl group onto the steroidal skeleton of the critical polyhydroxylated D7,8 olefin intermediate. A similarly impressive cobalt-catalyzed Mukaiyama hydration reaction was previously employed by Baran’s group for the stereocontrolled functionalization of a trisubstituted D4,5 double bond within a complex steroid system (See Scheme above, top panel). The formamido-diol product thus obtained was then condensed with trimethyl orthoformate, furnishing a protected intermediate that was suitable for eventual conversion into the natural product cortistatin A. In the current case of the linckoside synthetic campaign, Mukaiyama hydration of the aforementioned key substrate provided the desired C8 b-hydroxy product in a yield that can be considered acceptable in view of the novelty and exquisite stereoselectivity of the transformation. This critical reaction likely proceeds through the intermediacy of a cobalt(III)-peroxy intermediate as the active species, which adds to the least hindered p-face of the olefin with Markovnikov selectivity. The reaction was demonstrated on 330-milligram scale.
Yu’s group appended the final monosaccharide unit to the western C3-hydroxyl using an o-alkynylbenzoate building block as the glycosyl donor. This type of donor, which was developed in Biao Yu’s laboratory, is activated in the presence of a gold(I) cationic catalyst and, in this example, the 2-deoxy-b-glycosidic linkage was constructed with a high level of stereocontrol, even in the absence of a participatory C2-neighboring group within the glycosyl donor. A final global deprotection operation (methanolysis) effectively cleaved the acyl linkages of penultimate synthetic precursor to yield about 7 milligrams of the neuritogenic natural product linckoside A, for the first time in a laboratory setting. The chemical synthesis of linckosides A and B proceeds by a longest linear sequence 32 steps (44 total) and in 0.5% overall yield. The work is particularly notable as it provides a good blueprint for oxidative functionalization of the steroid C8 position in the context of a highly complex and biologically relevant synthetic target molecule.