Stereochemistry and Global Connectivity: An Overview - ACS

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Stereochemistry and Global Connectivity: An Overview H. N. Cheng,1,* Cynthia A. Maryanoff,2,* Bradley D. Miller,3,* and Diane Grob Schmidt4,* 1USDA

Agricultural Research Service, Southern Regional Research Center, New Orleans, Louisiana 70124, United States 2P. O. Box 339, Holicong, Pennsylvania 18928, United States 3ACS International Activities, External Affairs & Communications, Office of the Secretary and General Counsel, American Chemical Society, Washington, DC 20036, United States 4Department of Chemistry, University of Cincinnati, 301 Clifton Ct., Cincinnati, Ohio 45221, United States *E-mail: [email protected]; [email protected]; [email protected]; [email protected]

Stereochemistry is a fundamental concept in chemistry with relevance to all branches of chemistry, and all practicing chemists should have a working knowledge of this topic. Likewise, global awareness and connectivity should be of interest to chemistry professionals and students alike because the chemistry enterprise is becoming increasingly internationalized. These topics are highly significant, with many ongoing activities, both at research and professional levels, as exemplified by the chapters of this book. This article provides an overview and pays a particular tribute to the late Prof. Ernest Eliel, who excelled in both topics and was an exemplar for future scientists.

Introduction Stereochemistry is a fundamental property of many chemical substances. Most chemists and chemical engineers have been exposed to stereochemistry in © 2017 American Chemical Society Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


organic chemistry classes, where the terms cis-trans isomerism, stereoisomers, enantiomers, chirality, enantiomeric excess, and conformational analysis are taught (1–3). The same concepts are also highly relevant in other areas, such as biology, biochemistry, inorganic chemistry, and polymer science. As examples, most natural materials, e.g., amino acids, carbohydrates, nucleic acids, and fatty acids have inherent stereochemistry. Cellular and enzymatic transformations involving these materials often preserve the stereochemistry of the starting materials. Bioactive compounds (e.g., pharmaceuticals and agro-chemicals) are also sometimes enantiomerically enriched in order to achieve higher efficacy. Thus, asymmetric synthesis is a major endeavor in organic chemistry (4, 5). Moreover, stereochemistry is also important in polymer chemistry. Most biopolymers, e.g., polypeptides, polysaccharides, and polynucleotides, are enantiomerically enriched. Synthetic stereoregular polymers have been found to have useful properties, and their successful synthesis and commercialization have important economic consequences (6, 7). Likewise, stereochemistry is important in inorganic chemistry (8). Many inorganic and organometallic compounds are being used for organic asymmetric catalysis or stereospecific polymerization (7–9). One of the leading scientists who had left a major impact on stereochemistry was the late Prof. Ernst L. Eliel, who taught at Notre Dame University (1948-1972) and University of North Carolina (1972-2008). A major emphasis of his work was the stereochemistry and conformational analysis of organic molecules, including derivatives of cyclohexane and saturated heterocyclic rings (10, 11). His 1962 textbook, Stereochemistry of Carbon Compounds, has influenced generations of organic chemists (12). Prof. Eliel was also a proponent of global connectivity. As a leader in the American Chemical Society (ACS), he started programs that brought chemical scientists from Latin American and Central Europe to U.S. laboratories for short sabbaticals. He was a founding member of the U.S.-Mexico Foundation for Science. He was one of the co-organizers of three U.S.-Taiwan symposia. He initiated “Global Instrument Partners” in the early 2000s to help Latin American chemists gain access to advanced analytical instrumentation. Above all, he reached out to Cuba and initiated collaborative programs. Prof. Eliel had correctly assessed the global potential of the chemistry enterprise. Today, we are observing the increasing trend of globalization in a number of areas, such as trade, business, culture, and science (13–16). For a business, global connections can potentially help innovation through increased speed of R&D, decreased cost, access to a greater talent pool, greater responsiveness to local markets and needs, and shared risks; for a scientist, global connectivity has the benefit of increased collaboration opportunities, use of international facilities, access to global talent, and cultural enrichment (17). An overview is provided in this article of the two topics wherein Prof. Eliel had made notable contributions: stereochemistry and global connectivity. Some readers may wonder about the possible relationship between these two topics. As we know, stereochemistry deals with the spatial arrangements of atoms and molecules and the effects of these arrangements on their reactions or properties. Global connectivity deals with the spatial arrangements of people and the effects 2 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

of these arrangements on their interactions or productivity. Thus, in a heuristic and strategic manner, these two topics are related.


Tribute to Eliel Because of his prominence, biographies have been written about Eliel (10, 11). In his chapter (18), Seeman undertook the herculean task of summarizing Eliel’s scientific career, which covered a broad range of topics, ranging from organic synthesis to stereochemistry to natural product chemistry. In addition to his outstanding scientific and professional contributions, he was also a mentor to many other scientists and a warm and helpful friend. Seeman reminisced upon his long-term association with Eliel and provided first-hand information on Eliel as his own “hidden advisor” and friend. In another chapter (19), Montero recounted the efforts by Eliel in 1993 to communicate with Cuba and the Cuban Society of Chemistry (SCQ). Despite political difficulties, these communications resulted in mutually beneficial scientific interactions. Montero indicated that Eliel provided a good example of scientific statesmanship that placed human and scientific values above social, political and economic differences. These efforts have contributed to the promising inter-society relationship between ACS and SCQ today.

Global Connectivity As noted in the Introduction, Eliel actively promoted international activities when he was in the leadership position at ACS. He was especially partial to Cuba, because he went to school there and graduated with his chemistry degree from the University of Havana. It is therefore fitting to include two chapters in this book that report on current educational collaborations among Cuban and American scientists (20, 21) and an update on ACS international activities (22). In 2015 Presidents Barack Obama and Raúl Castro started normalizing relations between the U.S. and Cuba, thereby permitting a Cuban delegation to attend the 2015 Boston ACS meeting. In their chapter (20), Attwood and Montero described a meeting of representatives from SCQ and the ACS Division of Chemical Education International Activities Committee at Boston, that led to the attendance of four ACS educators at the 2016 Simposio Internacional de Química in Cayo Santa Maria, Cuba as well as a forthcoming workshop at the University of Utah. In a separate development, Scott et al reported (21) on a U.S.-Cuban collaborative workshop that took place at the University of Havana in October 2016. The one-week workshop focused on neglected disease drug discovery and entailed both Cuban hosts and professors and students from three U.S. institutions – Indiana University-Purdue University Indianapolis (IUPUI), Santa Clara University and Colorado College. The students learnt to use IUPUI’s Distributed Drug Discovery (D3) synthetic procedures. The workshop led to improved cross-cultural understanding and enhanced training for the students involved. 3 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

In his chapter (22), Miller (Director of ACS International Activities) indicated that his office is working closely with the ACS Committee on International Activities and others to organize a large number of systematic and impactful activities in order to advance the global chemistry enterprise. These include ACS global alliances, international chapters, science and human rights, ACS International CenterTM, ACS-Pittcon collaboration, ethics workshops, and Global Innovation Imperatives.


Stereochemistry of Organic Compounds Although the basic concepts of stereochemistry have been well known for many years, there are still many nuances and details that need to be elucidated. An example was the discovery of a strong S-C-P anomeric effect in 2-diphenylphosphinoyl-1,3-dithiane, where a suitable interpretation was pending. In their chapter (23), Juaristi and Notario confirmed the anomeric effect using computational chemistry and found fluorine to be a good lone pair electron donor towards geminal sigma bonds. Another example was given by Bailey and Lambert (24) on the conformational behavior of 5-phenyl-1,3-dioxanes that bore remote substituents. They showed how a non-classical CH…O hydrogen bond might be tuned in response to the electron-withdrawing or electron-donating ability of substituents positioned remotely on the aryl ring: electron-withdrawing substituents decreased the conformational energy of the phenyl group while electron-donating substituents increased the conformational energy of the group. In their chapter (25), Soai and Matsumoto explored asymmetric autocatalysis, where a chiral product acted as the asymmetric catalyst for its own production. Pyrimidyl alkanol was found to be a highly efficient asymmetric autocatalyst in the enantioselective addition of diisopropyl zinc to pyrimidine-5-carbaldehyde to produce more of itself. By using asymmetric autocatalysis in a clever way, spontaneous absolute asymmetric synthesis was achieved without the intervention of any added enantiomeric material. Rivera and Paixao (26) described a recent international endeavor that combined multicomponent reactions (MCRs) with highly stereoselective organocatalysis for the synthesis of enantiomerically pure compounds. The reaction sequences comprised the asymmetric aminocatalytic functionalization of α,β-unsaturated aldehydes followed by isocyanide-MCRs with such oxo-components as enantiomeric inputs. The method provided a convergent and stereoselective way of producing natural product-like compounds such as hydroquinolines, chromenes, epoxy- and depsi-peptides. Magriotis (27) reviewed the successful approaches toward the total synthesis of ecteinascidin-743 that was isolated from the Caribbean tunicate Ecteinascidia turbinate. It is an effective anti-tumor drug approved in the EU and the US. He hopes that one or a combination of the synthetic approaches that have been developed would be adopted in the commercial production of this material. Loeb et al (28) optimized the yield of the condensation reaction between phenanthroline-5,6-diones and ethylenediamine through the combined use of 4 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


theory and experimentation. They discovered the formation of a “non-aromatic” intermediate to be the cause for the lower yield. Characterization of this intermediate permitted them to facilitate its conversion to the desired product and obtain close to quantitative yield for the reaction. Electronic circular dichroism (ECD) is a valuable tool to study the unknown absolute configuration of an optically active molecule, but it is sensitive to solvent effects. Robinson et al (29) studied the solvent effects on the ECD spectrum of the compound 3,3′-dibromo-1,1′-bi-2-naphthol. They used computational chemistry (PCM model) and obtained results that were quite close to those observed experimentally.

Carbohydrates The carbohydrate molecules contain chiral centers and they are rich in stereochemistry. In their chapter (30), Serianni et al showed a large number of high-impact problems solved through the combination of NMR, isotopic enrichment, and computational methods. Examples included the use of stable isotopes to detect and quantify the cyclic and acyclic forms of reducing sugars in solution and to investigate relationships between saccharide structure, conformation and the kinetics of anomerization. Other examples included cis-trans isomerization of the N-acetyl side-chains, conformational analysis, and mechanistic studies. Giuliano et al (31) synthesized 2,3-dideoxy glycosyl cyanides using the Ferrier reaction of glycals with trimethylsilyl cyanide and Lewis acid catalysts, chemoselective reduction of the double bond in the products, and deacylation. X-ray crystallographic analysis of one of the products revealed features consistent with the anomeric effect of the cyano group, similar to other glycosyl cyanides. The results were confirmed with computational studies. Kisiliak and Livney (32) studied the role of sugars in enhancing the thermal stability of globular proteins. They chose stereochemically different mono- and di-saccharides and found rising protein denaturation temperature with increasing sugar concentration. No binding was found between the sugars and the protein. Sugars affected protein mainly indirectly via the water. The extent of thermal protection conferred to the protein correlated with the hydration number of the sugar within each group of isomers. Rivas and Sanchez (33) synthesized the water-soluble polymer, poly(glycidyl methacrylate-N-methyl-D-glucamine) and used it to remove arsenate, chromate and borate in water as a function of pH, polymer concentration, and presence of interfering ions. Thus, this polymer in combination with ultrafiltration might be regarded as a new separation system.

NMR Applications NMR is a versatile method that can be used to study stereochemistry. One of the techniques developed was rapid-injection NMR (RI-NMR), devised by McGarrity (34) and used extensively by Eliel et al (35–37). In their chapter, 5 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Thomas and Denmark (38) provided an authoritative review of RI-NMR, including the background, the design of the setup, and many applications of RI-NMR in organic chemistry. Dybowski (39) provided an excellent review of solid state NMR and many examples of its applications, including studies of orientation in polymer materials, identification of species at catalyst surfaces, estimates of porosity in porous materials, and chemical changes in art masterworks. In many artworks slow reactions between free fatty acids and pigment-derived ions produce soaps that cause the appearance of protrusions (soap aggregates), clear spots in paintings, and crazes. Through a combination of 13C, 207Pb and 119Sn NMR, he was able to gain a better understanding of the chemistry involved. For many years Tonelli has focused on polymer conformation to explain structure/property correlations, particularly 13C NMR and polymer microstructures. In his chapter (40), he reviewed the use of the conformationally sensitive γ-effects to assign NMR spectra of polymers in solutions and melts. He also discussed the studies of conformations in solid polymers. Silk is a natural protein fiber that exhibits outstanding end-use properties. Asakura and his group have used NMR in ingenious ways to study the structure and conformation of silk fibroins. In their chapter (41), they reviewed their work relating to the silk fibroins stored in Bombyx mori and Samia cynthia ricini silkworms. Detailed information on the helix or coil conformation of different structural segments was obtained, and a good understanding of the strength of the silk fiber was achieved. In the last chapter of the book, Biswas, et al (42) used biodiesel as a renewable feedstock for new polymers. They introduced the epoxide functionality into biodiesel and converted it into homopolymers (and copolymers) through cationic polymerization with fluorosulfonic acid. Because of the stereochemistry involved, both linear and cyclic products were found. NMR was used to determined polymer structures and reaction mechanisms.

Conclusions This article provides a synopsis of the papers given in both volumes of this book, providing a glimpse of the advances and recent developments in stereochemistry and global connectivity. The authors hope that this article may give the readers a better appreciation and more in-depth knowledge of these two topics. Since Prof. Eliel had made important contributions to these topics in his long and illustrious career, these two topics serve as part of his legacy to us. It is fitting therefore to report on the progress in these areas as we honor his memory.

Acknowledgments The authors thank the authors of the book chapters for their contributions to this book and Dr. K. T. Klasson for helpful comments. Mention of trade names 6 Cheng et al.; Stereochemistry and Global Connectivity: The Legacy of Ernest L. Eliel Volume 1 ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.

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Stereochemistry and Global Connectivity: An Overview - ACS

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