Glycosylation Reactions Involving a High Degree of Chemical Complexity: Nicolaou’s Total Synthesis of Shishijimicin A

            Naturally occurring glycosylated steroids and triterpenoids often exhibit pharmacological bioactivity superior to that of their corresponding aglycones. Prominent examples include the immunostimulatory adjuvant QS-21-apiose and the cardiotonic glycoside drug digoxin. Limonin glucoside, unlike its parent aglycone, limonin, is freely water-soluble and has been shown to lower levels of circulating biomarkers of chronic inflammatory diseases when administered orally in beverages. Moreover, while limonin glucoside is almost tasteless, low concentrations of its aglycone impart an extreme bitterness that is unacceptable to consumers.

            A number of fully carbohydrate-based polysaccharide drugs have been registered to date, but most of these active pharmaceutical ingredients (APIs) originate from natural sources. Synthetic oligosaccharides are rare, due to a reliance of carbohydrate synthesis on linear strategies involving elaborate, orthogonal protecting group schemes and cumbersome purifications. Fondaparinux (molecular structure depicted below) is a unique synthetic pentasaccharide API that was approved in 2001 for the prevention of venous thromboembolism. It has been obtained from total synthesis by a route that requires approximately 50 chemical steps. In view of the examples noted above, it is not surprising that the development of efficient and scalable chemical glycosylation protocols is of great interest to the pharmaceutical industry.

            Total synthesis studies targeting bioactive natural products often stimulate the invention of novel bond-forming methodologies that enable the eventual chemical synthesis of increasingly complex classes of drug candidates. This important area of research also provides an arena to evaluate the effectiveness of existing synthetic technologies, when applied in the context of molecular architectures at the height of chemical complexity. The endiyne antitumor antibiotics fall into this latter category. Natural products from the endiyne class such as calicheamicin g₁ (structure shown below) exhibit exquisitely potent cytotoxicity against cancer cell lines due to a fascinating mechanism of action involving Bergman cycloaromatization. This phenomenon gives rise to a 1,4-benzenoid diradical that can abstract two hydrogens from DNA, causing strand cleavage. Calicheamicin g₁ serves as the ‘warhead’ or ‘payload’ that is conjugated to an antibody in the approved antibody-drug conjugate (ADC), Mylotarg. The advent of ADCs has prompted renewed interest in high-potency cytotoxic agents that were previously unsuitable for clinical applications due to severe side effects. Recently, the total synthesis of a related endiyne natural product, shishijimicin A, was reported by K. C. Nicolaou’s laboratory at Rice University in Houston, Texas. The inarguably impressive synthesis reported by Nicolaou’s group employs a challenging glycosylative coupling reaction between two highly elaborated fragments that calls attention to a limitation of the classical Schmidt trichloroacetimidate protocol. The daunting glycosylation chemistry outlined below highlights the synthetic difficulties associated with endiyne production in the laboratory and underscores the need for new and robust oligosaccharide coupling technologies.

            In the course of Nicolaou and co-workers’ total synthesis of shishijimicin A, the two advanced intermediates, a trichloroacetimidate glycosyl donor and a hydroxy-endiyne glycosyl acceptor (Scheme below, lower panel), were prepared by a lengthy but reasonably efficient synthetic sequence of reactions. The two fragments were subjected to a standard variant of the Schmidt glycosylation reaction, wherein exposure to the Lewis acid BF₃-OEt₂ promotes formation of a transient oxonium species derived from the glycosyl donor, ultimately furnishing the b-glycoside adduct in a meager 26% yield. The reaction was exemplified on 40-milligram scale. The authors attributed the low yield to steric hindrance, although both coupling partners contain a diverse array of functionality that could potentially engage in unproductive pathways under Lewis acidic conditions. The condensation product was shown to be suitable for subsequent conversion to the natural product. Shishijimicin A is expected to serve as a uniquely powerful payload for ADC biomedical applications due to its extremely potent antitumor properties (IC₅₀ = 0.48 pM against leukemia cells). A similarly complex endiyne family member, maduropeptin, was synthesized by Masahiro Hirama’s group at Tohoku University in Japan in 2009. In the case of maduropeptin, an embedded tertiary alcohol serves as the glycosyl acceptor in the critical glycosylative fragment-coupling step. A Schmidt glycosylation using 6.4 milligrams of the endiyne glycosyl acceptor afforded 4.9 milligrams (40% yield) of the advanced maduropeptin synthetic intermediate, as a mixture of two atropisomers. Hirama et al achieved the first total synthesis and structure revision of the maduropeptin chromophore. The chemical synthesis of endiyne targets residing at the frontier of chemical complexity is instructive to organic chemists and generates precious material for biochemical investigations and ADC-related conjugation studies. However, the yields achieved in the requisite late-stage glycosylative coupling steps clearly illustrate a need for further methodological refinement within this important and medicinally relevant area of synthetic chemistry.