Increase in intricacy—a tool for evaluating organic syntheses

Aryl Annulation: A Powerful Simplifying Retrosynthetic Disconnection

Retrosynthetic deconstruction of a core aromatic ring is an especially simplifying retrosynthetic step, reducing the complexity of the precursor synthetic target. Moreover, when implemented to provide a penultimate intermediate, it enables late-stage divergent aryl introductions, permitting deep-seated core aryl modifications ordinarily accessible only by independent synthesis. Herein, we highlight the use of a ketone carbonyl group as the functionality to direct such late-stage divergent aryl introductions onto a penultimate intermediate with a projected application in the total synthesis of vinblastine and its presently inaccessible analogs containing indole replacements. Although the studies highlight this presently unconventional strategy with an especially challenging target in mind, the increase in molecular complexity (intricacy) established by the synthetic implementation of the powerful retrosynthetic disconnection, the use of a ketone as the precursor enabling functionality, and with adoption of either conventional or new wave (hetero)aromatic annulations combine to define a general and powerful strategy suited for wide-spread implementation with near limitless scope in target diversification.

A Case for Bond-Network Analysis in the Synthesis of Bridged Polycyclic Complex Molecules: Hetidine and Hetisine Diterpenoid Alkaloids

A key challenge in the synthesis of diterpenoid alkaloids lies in identifying strategies that rapidly construct their multiply bridged polycyclic skeletons. Existing approaches to these structurally intricate secondary metabolites are discussed in the context of a "bond-network analysis" of molecular frameworks, which was originally devised by Corey some 40 years ago. The retrosynthesis plans that emerge from a topological analysis of the highly bridged frameworks of the diterpenoid alkaloids are discussed in the context of eight recent syntheses of hetidine and hetisine natural products and their derivatives. This Minireview highlights the extent to which network analyses of the type described here sufficed for designing synthesis plans, as well as areas where they had to be amalgamated with functional group oriented synthetic planning considerations.

Enabling strategies for step efficient syntheses

The field of natural product total synthesis has reached the point where synthetic efficiency has become more important than merely defining a viable (yet less ideal) route to the target molecule. Synthetic efficiency is best represented by the number of steps it takes to finish the target molecule from readily available starting materials, as by reducing the number of steps, all other factors of synthetic efficiency are influenced positively. By comparing several total syntheses from the recent years, the most successful strategies for step efficient syntheses will be highlighted. Each synthesis will be presented using a color-coded synthetic flowchart, in which each step is categorized by a colored box. Five categories of transformations are defined and rated according to their synthetic value. Each class will be signified by different colors so that the reader can quickly see which parts of the synthesis are productive and those that are not.

A simple recipe for sophisticated cocktails: organocatalytic one-pot reactions--concept, nomenclature, and future perspectives

Asymmetric organocatalysis has been successfully incorporated in many multistep one-pot sequences to provide simple access to structurally complex target molecules in a highly stereoselective fashion. The key feature behind this success is the ability of organocatalyzed reactions to proceed efficiently in the presence of large amounts of spectator reagents. Additionally, owing to their organic nature and substoichiometric presence, organocatalysts are also expected to become innocent bystanders in subsequent transformations. In this Minireview, an easy-to-use classification and nomenclatural system that is capable of systematically and informatively describing each one-pot reaction is introduced, and selected important contributions within the field of organocatalytic one-pot reactions are reviewed according to this new system. Finally, future developments and perspectives in the field are discussed.

Chemical process research and development in the 21st century: challenges, strategies, and solutions from a pharmaceutical industry perspective

In process research and development (PR&D), the generation and manipulation of small-molecule drugs ranges from bench-scale (laboratory) chemistry to pilot plant manufacture to commercial production. A broad range of disciplines, including process chemistry (organic synthesis), analytical chemistry, process engineering (mass and heat transfer, unit operations), process safety (chemical risk assessment), regulatory compliance, and plant operation, must be effectively applied. In the critical handover between medicinal chemistry and PR&D, compound production is typically scaled up from a few hundred grams to several kilograms. Can the methodologies applied to the former also satisfy the technical, safety, and scalability aspects that come into play in the latter? Occasionally, the transition might occur smoothly, but more often the situation is the opposite: much work and resources must be invested to design a process that is feasible for manufacturing on pilot scale and, eventually, for commercial production. Authentic examples provide enlightening illustrations of dos and don'ts for developing syntheses designed for round-flask operation into production-scale processes. Factors that are easily underestimated or even neglected in the laboratory, such as method robustness, chemical hazards, safety concerns, environmental impact, availability of starting materials and building blocks in bulk quantities, intellectual property (IP) issues, and the final cost of the product, will come into play and need to be addressed appropriately. The decision on which route will be the best for further development is a crucial event and should come into focus early on the R&D timeline. In addition to scientific and technical concerns, the parameter of speed has come to the forefront in the pharmaceutical arena. Although historically the drug industry has tolerated a total time investment of far more than 10 years from idea to market, the current worldwide paradigm requires a reduction to under 10 years for the specific segment covering preclinical development through launch. This change puts enormous pressure on the entire organization, and the implication for PR&D is that the time allowed for conducting route design and scale-up has shrunk accordingly. Furthermore, molecular complexity has become extremely challenging in many instances, and demand steadily grows for process understanding and knowledge generation about low-level byproduct, which often must be controlled even at trace concentrations to meet regulatory specifications (especially in the case of potentially genotoxic impurities). In this Account, we paint a broad picture of the technical challenges the PR&D community is grappling with today, focusing on what measures have been taken over the years to create more efficiency and effectiveness.

Strategic efficiency — The new thrust for synthetic organic chemists

A standard for gauging the efficiency of synthetic strategies has been proposed. Synthetic efficiency is correlated to the molecular-complexity index, which is the cumulative total number of chiral centers, independent functional groups, and rings contained in a molecule. The number of reaction steps for an efficient synthetic design must be smaller than that of molecular-complexity index. This standard will motivate organic chemists to work harder on the synthetic design prior to execution.Key words: strategic efficiency, total synthesis, synthetic design, complexity index.

The Domino Way to Heterocycles

Sequential transformations enable the facile synthesis of complex target molecules from simple building blocks in a single preparative step. Their value is amplified if they also create multiple stereogenic centers. In the ongoing search for new domino processes, emphasis is usually placed on sequential reactions which occur cleanly and without forming by-products. As a prerequisite for an ideally proceeding one-pot sequential transformation, the reactivity pattern of all participating components has to be such that each building block gets involved in a reaction only when it is supposed to do so. The development of sequences that combine transformations of fundamentally different mechanisms broadens the scope of such procedures in synthetic chemistry. This mini review contains a representative sampling from the last 15 years on the kinds of reactions that have been sequenced into cascades to produce heterocyclic molecules.

Asymmetric Synthesis of All Eight Seven-Carbon Dipropionate Stereotetrads

Enantiopure cycloheptadienyl sulfones 6 and 7 are diastereoselectively epoxidized to yield epoxyvinyl sulfones 8, 9, 14, and 16 in high yields and diastereomeric ratios. Syn and anti methylation of epoxides 8, 9, 14, and 16 enables access to all eight possible diastereomeric stereotetrads, seven of which are commonly found in polypropionate natural products. Anti methylations of the above epoxides are possible by either the reaction of methyl organometallics promoted by copper(I), or via reaction with trimethylaluminum to yield stereotetrads 11, 12, 22, and 24. Syn methylations are achieved via Lawton SN2' reaction in the case of stereotetrads 10, 15, and 38, while stereotetrad 13 is accessed by an oxidation/reduction alcohol inversion sequence from stereotetrad 11. All stereotetrads were obtained in high diastereomeric ratios and yields, and their relative stereochemistry was confirmed by X-ray crystallography. Oxidative cleavage of the cyclic stereotetrads yields termini-differentiated acyclic heptanyl stereotetrads ready for use in building larger fragments in the course of target syntheses.

Computer-aided organic synthesis

It is tempting for those in the field of organic synthesis to liken the process of retrosynthesis to a game of chess. That the world chess champion was recently defeated by a computer leads us to think that perhaps new and powerful computing methods could be applied to synthetic problems. Here the analogy between synthesis and chess is outlined. Achievements in the 35-year history of computer-aided synthetic design are described, followed by some more recent developments.

Polycyclic compounds from aminopolyols and α-dicarbonyls: structure and application in the synthesis of exoditopic ligands

The structure and stereochemistry of the crystalline 2 : 2 condensation product ("glytham") of glyoxal and tris(hydroxymethyl)aminomethane was conclusively determined by X-ray diffractometric analysis. The singular disposition of the heteroatoms suggests the employment of glytham as starting material for the synthesis of ditopic ligands for metal ions. Some derivatives of glytham were prepared and their binding properties towards alkaline metal ions were preliminarily investigated by ESI-MS and NMR.

Synthesis of termini-differentiated 6-carbon stereotetrads: an alkylative oxidation strategy for preparation of the c21-c26 segment of apoptolidin(1)

[reaction: see text] Two methods have been developed for the synthesis of epoxide 36. The first uses (+)-pulegone 25 as an enantiopure starting material and introduces the requisite intricacy of target 22 in 12 operations. The second method employs an enantiospecific catalytic Jacobsen epoxidation of 1a and is five operations shorter. The second sequence features an oxygen-directed alkylative oxidation reaction that re-establishes the dienyl sulfone functionality with concomitant 1,3-transposition of the sulfone moiety.