Chemical biology of glycoproteins: From chemical synthesis to biological impact

Recent advances have demonstrated the feasibility and robustness of chemical synthesis for the production of homogeneously glycosylated protein forms (glycoforms). By taking advantage of the unmatchable flexibility and precision provided by chemical synthesis, the quantitative effects of glycosylation were obtained using chemical glycobiology approaches. These findings greatly advanced our fundamental knowledge of glycosylation. More importantly, analysis of these findings has led to the development of glycoengineering guidelines for rationally improving the properties of peptides and proteins. In this chapter, we present the key experimental steps for chemical biology studies of protein glycosylation, with the aim of facilitating and promoting research in this important but significantly underexplored area of biology.

Total Chemical Synthesis of Human Thyroid-Stimulating Hormone (hTSH) β-Subunit: Application of Arginine-tagged Acetamidomethyl (AcmR) Protecting Groups

The β-subunit of human thyroid stimulating hormone (hTSH) has been synthesized as a single glycoform bearing a chitobiose disaccharide at the native glycosylation site. Key to the successful completion of this synthesis was the introduction of an arginine-tagged acetamidomethyl group, which served to greatly facilitate handling of a glycopeptide fragment with poor aqueous solubility. This general solution to the challenge of working with intractable peptides is expected to find wide use in protein synthesis.

Staged induction of HIV-1 glycan-dependent broadly neutralizing antibodies

A preventive HIV-1 vaccine should induce HIV-1-specific broadly neutralizing antibodies (bnAbs). However, bnAbs generally require high levels of somatic hypermutation (SHM) to acquire breadth, and current vaccine strategies have not been successful in inducing bnAbs. Because bnAbs directed against a glycosylated site adjacent to the third variable loop (V3) of the HIV-1 envelope protein require limited SHM, the V3-glycan epitope is an attractive vaccine target. By studying the cooperation among multiple V3-glycan B cell lineages and their coevolution with autologous virus throughout 5 years of infection, we identify key events in the ontogeny of a V3-glycan bnAb. Two autologous neutralizing antibody lineages selected for virus escape mutations and consequently allowed initiation and affinity maturation of a V3-glycan bnAb lineage. The nucleotide substitution required to initiate the bnAb lineage occurred at a low-probability site for activation-induced cytidine deaminase activity. Cooperation of B cell lineages and an improbable mutation critical for bnAb activity defined the necessary events leading to breadth in this V3-glycan bnAb lineage. These findings may, in part, explain why initiation of V3-glycan bnAbs is rare, and suggest an immunization strategy for inducing similar V3-glycan bnAbs.

Vaccine Elicitation of High Mannose-Dependent Neutralizing Antibodies against the V3-Glycan Broadly Neutralizing Epitope in Nonhuman Primates

Induction of broadly neutralizing antibodies (bnAbs) that target HIV-1 envelope (Env) is a goal of HIV-1 vaccine development. A bnAb target is the Env third variable loop (V3)-glycan site. To determine whether immunization could induce antibodies to the V3-glycan bnAb binding site, we repetitively immunized macaques over a 4-year period with an Env expressing V3-high mannose glycans. Env immunizations elicited plasma antibodies that neutralized HIV-1 expressing only high-mannose glycans-a characteristic shared by early bnAb B cell lineage members. A rhesus recombinant monoclonal antibody from a vaccinated macaque bound to the V3-glycan site at the same amino acids as broadly neutralizing antibodies. A structure of the antibody bound to glycan revealed that the three variable heavy-chain complementarity-determining regions formed a cavity into which glycan could insert and neutralized multiple HIV-1 isolates with high-mannose glycans. Thus, HIV-1 Env vaccination induced mannose-dependent antibodies with characteristics of V3-glycan bnAb precursors.

Antibodies Against Specific MUC16 Glycosylation Sites Inhibit Ovarian Cancer Growth

Expression of the retained C-terminal extracellular portion of the ovarian cancer glycoprotein MUC16 induces transformation and tumor growth. However, the mechanisms of MUC16 oncogenesis related to glycosylation are not clearly defined. We establish that MUC16 oncogenic effects are mediated through MGAT5-dependent N-glycosylation of two specific asparagine sites within its 58 amino acid ectodomain. Oncogenic signaling from the C-terminal portion of MUC16 requires the presence of Galectin-3 and growth factor receptors colocalized on lipid rafts. These effects are blocked upon loss of either Galectin-3 expression or activity MGAT5. Using synthetic MUC16 glycopeptides, we developed novel N-glycosylation site directed monoclonal antibodies that block Galectin-3-mediated MUC16 interactions with cell surface signaling molecules. These antibodies inhibit invasion of ovarian cancer cells, directly blocking the in vivo growth of MUC16-bearing ovarian cancer xenografts, elucidating new therapeutic modalities.

Abstract 5587: Novel monoclonal antibodies block N-glycosylation Sites of the MUC16 ectodomain in ovarian carcinoma

The MUC16 gene encodes the CA125 antigen that imparts a negative survival impact on patients with high-grade serous ovarian cancer. Short elements from the C-terminal region of MUC16 transform 3T3 cells. Introduction of the 114 amino acid C-terminal portion of MUC16 promotes invasive growth in xenograft models. This effect is dependent on N-glycosylation sites of MUC16 and interactions present on the cell surface. Mechanisms by which the ectodomain promotes MUC16 oncogenic behaviors, the function of N-glycosylation and the implicated cell surface receptors are not fully delineated.

To unravel the complex MUC16 cancer biology, in this study we investigated the glycosylation-based mechanisms and potential partners involved in MUC16 oncogenic effects, and show that site-specific glycosylation of MUC16 plays a key role in mucin-related transformation by mediating complex cell surface interactions. These effects are mediated through MGAT5-dependent N-glycosylation of two proximal N- glycosylation sites within the 58 amino acid retained MUC16 ectodomain acting in combination with Galectin-3 and growth factor receptors. Neither N- nor O-glycosylation sites in the more distally located MUC16 regions can functionally substitute for those two sites. The tumor-enhancing properties of MUC16 depend on co-localization of MUC16, Galectin-3, and growth factors receptors on lipid rafts. Loss of either Galectin-3 expression or the glycosylation enzyme MGAT5 completely abrogates MUC16 tumor promotion. Using synthetic glycopeptides, we have developed novel monoclonal antibodies (mAbs) directed at the crucial ectodomain N-glycosylation sites to inhibit the glycosylation- dependent effects of MUC16 on metastasis and invasion. These novel antibodies were characterized in vitro and in vivo studies using various ovarian cancer cell lines. All of the antibodies against the N- glycosylation sites closest to the cell membrane of MUC16 block Galectin-3-mediated polymerization to cell surface signaling molecules and inhibit the tumor-promoting effects of MUC16. The implications are broad: both for the biological understanding of cancer mucin biology and for potential therapeutic strategies.

Citation Format: Dharmarao Thapi, Alberto Fernandez-Tejada, Abraham J. Axelrod, Nestor Rosales, Xiujun Yan, Marina Stasenko, Sahityasri Thapi, Amy Wang, Samuel J. Danishefsky, David R. Spriggs. Novel monoclonal antibodies block N-glycosylation Sites of the MUC16 ectodomain in ovarian carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5587. doi:10.1158/1538-7445.AM2017-5587

Total Chemical Synthesis and Folding of All-l and All-d Variants of Oncogenic KRas(G12V)

The Ras proteins are essential GTPases involved in the regulation of cell proliferation and survival. Mutated oncogenic forms of Ras alter effector binding and innate GTPase activity, leading to deregulation of downstream signal transduction. Mutated forms of Ras are involved in approximately 30% of human cancers. Despite decades of effort to develop direct Ras inhibitors, Ras has long been considered "undruggable" due to its high affinity for GTP and its lack of hydrophobic binding pockets. Herein, we report a total chemical synthesis of all-l- and all-d-amino acid biotinylated variants of oncogenic mutant KRas(G12V). The protein is synthesized using Fmoc-based solid-phase peptide synthesis and assembled using combined native chemical ligation and isonitrile-mediated activation strategies. We demonstrate that both KRas(G12V) enantiomers can successfully fold and bind nucleotide substrates and binding partners with observable enantiodiscrimination. By demonstrating the functional competency of a mirror-image form of KRas bound to its corresponding enantiomeric nucleotide triphosphate, this study sets the stage for further biochemical studies with this material. In particular, this protein will enable mirror-image yeast surface display experiments to identify all-d peptide ligands for oncogenic KRas, providing a useful tool in the search for new therapeutics against this challenging disease target.

Fully Synthetic Granulocyte Colony-Stimulating Factor Enabled by Isonitrile-Mediated Coupling of Large, Side-Chain-Unprotected Peptides

Human granulocyte colony-stimulating factor (G-CSF) is an endogenous glycoprotein involved in hematopoiesis. Natively glycosylated and nonglycosylated recombinant forms, lenograstim and filgrastim, respectively, are used clinically to manage neutropenia in patients undergoing chemotherapeutic treatment. Despite their comparable therapeutic potential, the purpose of O-linked glycosylation at Thr133 remains a subject of controversy. In light of this, we have developed a synthetic platform to prepare G-CSF aglycone with the goal of enabling access to native and designed glycoforms with site-selectivity and glycan homogeneity. To address the synthesis of a relatively large, aggregation-prone sequence, we advanced an isonitrile-mediated ligation method. The chemoselective activation and coupling of C-terminal peptidyl Gly thioacids with the N-terminus of an unprotected peptide provide ligated peptides directly in a manner complementary to that with conventional native chemical ligation-desulfurization strategies. Herein, we describe the details and application of this method as it enabled the convergent total synthesis of G-CSF aglycone.

Human Cancer Antigen Globo H Is a Cell-Surface Ligand for Human Ribonuclease 1

Pancreatic-type ribonucleases are secretory enzymes that catalyze the cleavage of RNA. Recent efforts have endowed the homologues from cow (RNase A) and human (RNase 1) with toxicity for cancer cells, leading to a clinical trial. The basis for the selective toxicity of ribonuclease variants for cancerous versus noncancerous cells has, however, been unclear. A screen for RNase A ligands in an array of mammalian cell-surface glycans revealed strong affinity for a hexasaccharide, Globo H, that is a tumor-associated antigen and the basis for a vaccine in clinical trials. The affinity of RNase A and RNase 1 for immobilized Globo H is in the low micromolar-high nanomolar range. Moreover, reducing the display of Globo H on the surface of human breast adenocarcinoma cells with a small-molecule inhibitor of biosynthesis or a monoclonal antibody antagonist decreases the toxicity of an RNase 1 variant. Finally, heteronuclear single quantum coherence (HSQC) NMR spectroscopy showed that RNase 1 interacts with Globo H by using residues that are distal from the enzymic active site. The discovery that a systemic human ribonuclease binds to a moiety displayed on human cancer cells links two clinical paradigms and suggests a mechanism for innate resistance to cancer.

Designing synthetic vaccines for HIV

Despite three decades of intensive research efforts, the development of an effective prophylactic vaccine against HIV remains an unrealized goal in the global campaign to contain the HIV/AIDS pandemic. Recent characterization of novel epitopes for inducing broadly neutralizing antibodies has fueled research in the design and synthesis of new, well-defined antigenic constructs for the development of HIV envelope-directed vaccines. The present review will cover previous and recent efforts toward the design of synthetic vaccines based on the HIV viral envelope glycoproteins, with special emphasis on examples from our own laboratories. The biological evaluation of some of the most representative vaccine candidates, in terms of their antigenicity and immunogenicity, will also be discussed to illustrate the current state-of-the-art toward the development of fully synthetic HIV vaccines.

Development of Globo-H cancer vaccine

The development of anticancer vaccines requires the identification of unique epitope markers, preferably expressed exclusively on the surface of cancer cells. This Account describes the path of development of a carbohydrate-based vaccine for metastatic breast cancer, including the selection and synthesis of Globo-H as the target, the development of the vaccine conjugate and adjuvant design, the study of the immune response and consideration of class switch, and the analysis of Globo-H distribution on the surface of various cancer cells, cancer stem cells, and normal cells. The first synthesis of Globo-H was accomplished through the use of glycal chemistry; this approach delivered sufficient material for evaluation in phase I human trials. The development of a programmable one-pot synthesis method rendered the synthesis more practical and enabled the midstage proof-of-concept phase II trial and late-stage phase III trial. Finally, enzymatic synthesis of Globo-H coupled with cofactor regeneration was used for the late-stage multicenter trials and manufacture of the product. Along this path of development, it was discovered that the vaccine induced antibodies to target not only Globo-H, but also SSEA3 and SSEA4. Moreover, these three glycolipids were found to be uniquely expressed not only on the cell surface of breast cancer but on 15 additional cancer types, suggesting the broad application of this vaccine in cancer treatment and perhaps cancer prevention. In addition, a new glycolipid adjuvant was designed to target the CD1d receptor on dendritic cells and B cells for presentation to and activation of T cells to modulate the immune response and induce a class switch from IgM to IgG, thereby overcoming the common problem of carbohydrate-based vaccines that often induce mainly IgM antibodies. As demonstrated in this vaccine development, the chemical approach to the synthesis and conjugation of carbohydrate-based immunogens provides the flexibility for access to various structures and linkers to identify optimal compositions for development. The enzymatic method was then introduced to enable the practical synthesis of the vaccine candidate for clinical development and commercialization. Overall, this Account illustrates the path of development of a cancer vaccine, from selection of a unique glycan marker on breast cancer cells and the cancer stem cells as target to the use of chemistry in combination with immunology and cancer biology to enable the design and development of the Globo-H vaccine to target three specific glycan markers exclusively expressed on the cell surface of a number of different types of cancer.

Total synthesis of glycosylated proteins

Glycoproteins are an important class of naturally occurring biomolecules which play a pivotal role in many biological processes. They are biosynthesized as complex mixtures of glycoforms through post-translational protein glycosylation. This fact, together with the challenges associated with producing them in homogeneous form, has hampered detailed structure-function studies of glycoproteins as well as their full exploitation as potential therapeutic agents. By contrast, chemical synthesis offers the unique opportunity to gain access to homogeneous glycoprotein samples for rigorous biological evaluation. Herein, we review recent methods for the assembly of complex glycopeptides and glycoproteins and present several examples from our laboratory towards the total chemical synthesis of clinically relevant glycosylated proteins that have enabled synthetic access to full-length homogeneous glycoproteins.

Studies toward the total synthesis of pluraflavin A

A synthetic strategy towards the potent cytostatic agent pluraflavin A has been developed. Formation of the enantioenriched anthrapyran core bearing a halogen atom enabled the introduction of the α C-aryl glycoside by Stille cross-coupling and subsequent hydrogenation of the aryl glycal. Chemo- and stereoselective O-glycosylations of α oliose and β 3-epi vancosamine residues afforded a fully glycosylated aromatic core. Attempts to install the dimethylamino group of the C-disaccharide suggest that introduction of an azide group by displacement and subsequent reduction may pave the way to the total synthesis of pluraflavin A.

Chemical synthesis of the β-subunit of human luteinizing (hLH) and chorionic gonadotropin (hCG) glycoprotein hormones

Human luteinizing hormone (hLH) and human chorionic gonadotropin (hCG) are human glycoprotein hormones each consisting of two subunits, an identical α-subunit and a unique β-subunit, that form noncovalent heterodimers. Structurally, β-hCG shares a high degree of sequence similarity with β-hLH, including a common N-glycosylation site at the N-terminus but differs mainly in the presence of an extended C-terminal portion incorporating four closely spaced O-linked glycans. These glycoproteins play important roles in reproduction and are used clinically in the treatment of infertility. In addition, the role of hCG as a tumor marker in a variety of cancers has also attracted significant interest for the development of cancer vaccines. In clinical applications, these hormones are administered as mixtures of glycoforms due to limitations of biological methods in producing homogeneous samples of these glycoproteins. Using the powerful tools of chemical synthesis, the work presented herein focuses on the highly convergent syntheses of homogeneous β-hLH and β-hCG bearing model glycans at all native glycosylation sites. Key steps in these syntheses include a successful double Lansbury glycosylation en route to the N-terminal fragment of β-hCG and the sequential installation of four O-linked glycosyl-amino acid cassettes into closely spaced O-glycosylation sites in a single, high-yielding solid-supported synthesis to access the C-terminal portion of the molecule. The final assembly of the individual glycopeptide fragments involved a stepwise native chemical ligation strategy to provide the longest and most complex human glycoprotein hormone (β-hCG) as well as its closely related homologue (β-hLH) as discrete glycoforms.

Intramolecular Diels-Alder reactions of cycloalkenones: stereoselectivity, Lewis acid acceleration, and halogen substituent effects

The intramolecular Diels-Alder reactions of cycloalkenones and terminal dienes occur with high endo stereoselectivity, both thermally and under Lewis-acidic conditions. Through computations, we show that steric repulsion and tether conformation govern the selectivity of the reaction, and incorporation of either BF3 or α-halogenation increases the rate of cycloaddition. With a longer tether, isomerization from a terminal diene to the more stable internal diene results in a more facile cycloaddition.

Erythropoietin derived by chemical synthesis

Erythropoietin is a signaling glycoprotein that controls the fundamental process of erythropoiesis, orchestrating the production and maintenance of red blood cells. As administrated clinically, erythropoietin has a polypeptide backbone with complex dishomogeneity in its carbohydrate domains. Here we describe the total synthesis of homogeneous erythropoietin with consensus carbohydrate domains incorporated at all of the native glycosylation sites. The oligosaccharide sectors were built by total synthesis and attached stereospecifically to peptidyl fragments of the wild-type primary sequence, themselves obtained by solid-phase peptide synthesis. The glycopeptidyl constructs were joined by chemical ligation, followed by metal-free dethiylation, and subsequently folded. This homogeneous erythropoietin glycosylated at the three wild-type aspartates with N-linked high-mannose sialic acid-containing oligosaccharides and O-linked glycophorin exhibits Procrit-level in vivo activity in mice.

A vision for vaccines built from fully synthetic tumor-associated antigens: from the laboratory to the clinic

Cancer cells may be distinguished from normal cells by cell surface displays of aberrant levels and types of carbohydrate domains. Accordingly, these tumor-associated carbohydrate antigens (TACAs) represent promising target structures for the design of anticancer vaccines. Over the past 20 years, our laboratory has sought to use the tools of chemical synthesis to develop TACA-based anticancer vaccine candidates. We provide herein a personal accounting of our laboratory's progress toward the long-standing goal of developing clinically viable fully synthetic carbohydrate-based anticancer vaccines.

Chemical synthesis of highly congested gp120 V1V2 N-glycopeptide antigens for potential HIV-1-directed vaccines

Critical to the search for an effective HIV-1 vaccine is the development of immunogens capable of inducing broadly neutralizing antibodies (BnAbs). A key first step in this process is to design immunogens that can be recognized by known BnAbs. The monoclonal antibody PG9 is a BnAb that neutralizes diverse strains of HIV-1 by targeting a conserved carbohydrate-protein epitope in the variable 1 and 2 (V1V2) region of the viral envelope. Important for recognition are two closely spaced N-glycans at Asn(160) and Asn(156). Glycopeptides containing this synthetically challenging bis-N-glycosylated motif were prepared by convergent assembly, and were shown to be antigenic for PG9. Synthetic glycopeptides such as these may be useful for the development of HIV-1 vaccines based on the envelope V1V2 BnAb epitope.

The winding pathway to erythropoietin along the chemistry-biology frontier: a success at last

The total synthesis of a homogeneous erythropoietin (EPO), possessing the native amino acid sequence and chitobiose glycans at each of the three wild-type sites of N glycosylation, has been accomplished in our laboratory. We provide herein an account of our decade-long research effort en route to this formidable target compound. The optimization of the synergy of the two bedrock sciences we now call biology and chemistry was central to the success of the synthesis of EPO.

A Comparison of Signaling Activities Induced by Taxol and Desoxyepothilone B

Desoxyepothilone B (dEpoB), currently in clinical trials, is a novel microtubule inhibitor with similar mode-of-action to paclitaxel (Taxol). Intriguingly, it is effective in some cell lines and tumor xenografts refractory to Taxol. The purpose of this study is to compare signaling induced by the two drugs and identify a molecular basis for increased efficacy of dEpoB in resistant lines. The importance of ERK signaling, already established for Taxol, was shown for dEpoB and other G2-blocking agents. However, a role in differential sensitivity was not observed. Affymetrix analysis shows similar gene modulation by either agent, alone or in combination with MEK inhibitor. Differential sensitivity in a set of Taxol-resistant lines correlated to the expression of P-glycoprotein (P-gp), and its importance was demonstrated directly. These results suggest that Taxol and dEpoB elicit similar cell death pathways, and the increased efficacy of dEpoB in resistant tumor lines lies in differential susceptibility to P-gp.

Building biologics by chemical synthesis: practical preparation of di- and triantennary N-linked glycoconjugates

A unified strategy for the syntheses of bi- and triantennary fully sialylated N-glycans is described. The synthesis capitalizes on a global glycosylation strategy that delivers the desired undeca- and tetradecasaccharide in excellent yields. Finally, conjugation of the glycan to PSMA oligopeptide is described.

Halocycloalkenones as Diels-Alder dienophiles. Applications to generating useful structural patterns

Halocycloalkenones are demonstrated to function as potent dienophiles in inter- and intramolecular Diels-Alder cycloadditions. We have found 2-brominated cycloalkenone dienophiles to be both highly endo selective and significantly more reactive than their nonhalogenated parent compounds. A method for the facile conversion of brominated cyclobutanone DA adducts to synthetically useful cyclopropyl functional handles is described.

An advance in the chemical synthesis of homogeneous N-linked glycopolypeptides by convergent aspartylation

We describe a useful advance in glycopeptide synthesis. We have developed a one-flask aspartylation/deprotection method, wherein long peptide fragments, bearing proximal pseudoproline functionality are merged with complex glycan domains. Following aspartylation, acidmediated global deprotection reveals the elaborated glycopeptide. The temporary pseudoproline functionality serves to suppress formation of aspartimide side products during solid phase peptide synthesis and aspartylation.

Total synthesis of (±)-maoecrystal V

The total synthesis of racemic maoecrystal V has been accomplished. Key steps include an intramolecular Diels-Alder cyclization to rapidly construct the core system from simple starting materials and the creation of the A-C ring trans-fusion through intramolecular delivery of a hydrogen to the hindered β-face of the ring system.

Solving the convergence problem in the synthesis of triantennary N-glycan relevant to prostate-specific membrane antigen (PSMA)

The first total synthesis of triantennary, fully sialylated N-glycan of complex type is described. Two strategies for installation of sialylated antennae are explored, and both approaches converge on a global glycosylation step that delivers the desired tetradecasaccharide in good yields.

Intramolecular Diels-Alder reactions of cycloalkenones: translation of high endo selectivity to trans junctions

Intramolecular Diels-Alder reactions of cyclobutenone and larger cycloalkenones are described. High levels of endo addition attained from Lewis acid catalysis translate to trans hydrindene junctions upon fragmentation of the tricyclic adducts.

Engineering of therapeutic polypeptides through chemical synthesis: early lessons from human parathyroid hormone and analogues

Application of chemical synthesis to gain access to high purity hPTH as well as more stable analogues was accomplished through a menu of extended NCL followed by metal free dethiylation.

Revisiting oxytocin through the medium of isonitriles

The reaction of thioamino acids and N-terminal peptides, mediated by hindered isonitriles and hydroxybenzotriazole, gives rise to peptide bonds. In one pathway, oxytocin was synthesized by eight such reiterative amidations. In another stereospecific track, oxytocin was constructed by native chemical ligation, wherein the two building blocks were assembled by thioacid amine amidation. The NMR spectra of oxytocin and dihydrooxytocin suggest a high level of preorganization in the latter, perhaps favoring oxidative folding.

Enhancing the scope of the Diels-Alder reaction through isonitrile chemistry: emergence of a new class of acyl-activated dienophiles

α,β-Unsaturated imides, formylated at the nitrogen atom, comprise a new and valuable family of dienophiles for servicing Diels-Alder reactions. These systems are assembled through extension of recently discovered isonitrile chemistry to the domain of α,β-unsaturated acids. Cycloadditions are facilitated by Et(2)AlCl, presumably via chelation between the two carbonyl groups of the N-formyl amide. Applications of the isonitrile/Diels-Alder logic to the IMDA reaction, as well as methodologies to modify the N-formyl amide of the resultant cycloaddition product, are described. It is expected that this easily executed chemistry will provide a significant enhancement for application of Diels-Alder reactions to many synthetic targets.

Advances in proline ligation

Application of native chemical ligation logic to the case of an N-terminal proline is described. Two approaches were studied. One involved incorporation of a 3R-substituted thiyl-proline derivative. Improved results were obtained from a 3R-substituted selenol function, incorporated in the context of an oxidized dimer.

A fascinating journey into history: exploration of the world of isonitriles en route to complex amides

We describe herein our recent explorations in the field of isonitrile chemistry. An array of broadly useful coupling methodologies has been developed for the formation of peptidyl and glycopeptidyl amide bonds. We further describe the application of these methods to the syntheses of complex systems, including the cyclic peptide cyclosporine A, constrained peptide systems, and heterocycles.

Total synthesis of the α-subunit of human glycoprotein hormones: toward fully synthetic homogeneous human follicle-stimulating hormone

Described herein is the first total chemical synthesis of the unique α-subunit of the human glycoprotein hormone (α-hGPH). Unlike the biologically derived glycoprotein hormones, which are isolated as highly complex mixtures of glycoforms, α-hGPH obtained by chemical synthesis contains discrete homogeneous glycoforms. Two such systems have been prepared. One contains the disaccharide chitobiose at the natural N-glycosylation sites. The other contains dodecamer oligosaccharides at these same sites. The dodecamer sugar is a consensus sequence incorporating the key features associated with human glycoproteins.

Expanding the limits of isonitrile-mediated amidations: on the remarkable stereosubtleties of macrolactam formation from synthetic seco-cyclosporins

The scope of isonitrile-mediated amide bond-forming reactions is further explored in this second-generation synthetic approach to cyclosporine (cyclosporin A). Both type I and type II amidations are utilized in this effort, allowing access to epimeric cyclosporins A and H from a single precursor by variation of the coupling reagents. This work lends deeper insight into the relative acylating ability of the formimidate carboxylate mixed anhydride (FCMA) intermediate, while shedding light on the far-reaching impact of remote stereochemical changes on the effective preorganization of seco-cyclosporins.

Permuting Diels-Alder and Robinson Annulation Stereopatterns

Controlled isomerization of the double bond of certain Diels-Alder reactions provides substrates that, upon oxidation, give rise to products whose gross structure corresponds to that of a Robinson annulation. In these cases, the stereochemistry of the Robinson annulation product reflects the fact that the initial combination occurred in a Diels-Alder mode. Using these principles, we have synthesized carissone and cosmosoic acid. In the latter case, our total synthesis raised serious questions as to the accuracy of the assigned structure of the natural product.

Synthetic studies toward (+)-cortistatin A

We describe herein the synthesis of a late-stage intermediate en route to cortistatin A. Key transformations included a Snieckus-like cascade sequence culminating in a 6π-electrocyclization, an alkylative dearomatization, and the stereoselective functionalization of the cortistatin A-ring. While the total synthesis we sought was not accomplished, the work sets the stage for several approaches to the preparation of novel analogs via diverted total synthesis.

Toward Homogeneous Erythropoietin: Application of Metal Free Dethiylation in the Chemical Synthesis of the Ala79-Arg166 Glycopeptide Domain

We describe herein the assembly of hEPO(79-166), a key glycopeptide segment en route to erythropoietin, in minimally protected form. Key to the success of this synthetic endeavor was the application of our two-step cysteine-free native chemical ligation strategy, by which we achieved formal ligation at alanine and proline residues through the use of an N-terminal amino acid surrogate presenting a readily removable thiol functionality.

An advance in proline ligation

Native chemical ligation (NCL) is widely applicable for building proteins in the laboratory. Since the discovery of this method, many strategies have been developed to enhance its capability and efficiency. Because of the poor reactivity of proline thioesters, ligation at a C-terminal proline site is not readily accomplished. Here, we demonstrate that ligation at an N-terminal protein is feasible using the combined logic of NCL and metal-free dethiylation (MFD).