Studies on Difficult Intramolecular Hydroaminations in the Context of Four Syntheses of Alkaloid Natural Products

Oxidative Syntheses of N,N-Dialkylhydroxylamines

Despite the wide utility of hydroxylamines in organic synthesis, relatively few are commercially available, and there is a need for direct, efficient, and selective methods for their synthesis. Herein, we report two complementary methods to accomplish direct oxidation of secondary amines using UHP as an oxidant. The first method uses 2,2,2-trifluoroethanol (TFE) and a large excess of amine. Isolation of hydroxylamine products is enabled by selective salt formation, and recovery of excess amine is demonstrated. The second method uses hexafluoroacetone as an additive and is highlighted by the 1:1 stoichiometry between the oxidant and amine.

Enamine N-oxides: Design, Synthesis, and Function in Bioorthogonal Reactions

Enamine N-oxides act as a chemical linchpin bridging two bioorthogonal associative and dissociative reactions. This article describes the design of enamine N-oxides; their synthesis through the retro-Cope elimination reaction; the use of solvent, hyperconjugation, strain, and rehybridization effects to achieve bioorthogonal reactivity; and their rapid reductive cleavage with diboron reagents. The coordinated assembly and disassembly of the enamine N-oxide motif constitutes a powerful chemical operation that enables the attachment and detachment of small molecules from biomacromolecules in a biological setting.

Toward the Asymmetric Synthesis of Cardenolides and Related Steroidal Systems: syn-SN2' of Organometallics with C14-C17 Vinylepoxides

In a program aimed at establishing a common sequence of C-C bond-forming reactions for asymmetric construction of tetracyclic triterpenoid natural products and related synthetic systems, effort has been directed toward introducing C17β-substitution by late-stage functionalization of stereodefined "steroidal" D-ring vinylepoxides (spanning C14-C17). It has been found that cyanocuprates participate in syn-SN2' reactions that result in products bearing various C17β-substituents and containing a β-OH at C14.

A redox-enabled strategy for intramolecular hydroamination

Metal- or acid-catalyzed intramolecular hydroamination and Cope-type intramolecular hydroamination, a distinct concerted approach using hydroxylamines, typically suffer from significant synthetic limitations. Herein we report a process for intramolecular hydroamination that uses a redox-enabled strategy relying on efficient in situ generation of hydroxylamines by oxidation, followed by Cope-type hydroamination, then reduction of the resulting pyrrolidine N-oxide. The steps are performed sequentially in a single pot, no catalyst is required, the conditions are mild, the process is highly functional group tolerant, and no chromatography is generally required for isolation. A robustness screen and a gram-scale example further support the practicality of this approach.

A general strategy for the synthesis of indoloquinolizine alkaloids via a cyanide-catalyzed imino-Stetter reaction

A general synthetic strategy applicable to indoloquinolizine alkaloids has been developed using a cyanide-catalyzed imino-Stetter reaction as a key step.

Cascade reactions of nitrogen-substituted isocyanates: a new tool in heterocyclic chemistry

In contrast to normal C-substituted isocyanates, nitrogen-substituted isocyanates (N-isocyanates) are rare. Their high reactivity and amphoteric/ambident nature has prevented the scientific community from exploiting their synthetic potential. Recently, we have developed an in situ formation approach using a reversible equilibrium, which allows controlled generation and reactivity of N-isocyanates and prevents the dimerization that is typically observed with these intermediates. This blocked (masked) N-isocyanate approach enables the use of various N-isocyanate precursors to assemble heterocycles possessing the N-N-C[double bond, length as m-dash]O motif, which is often found in agrochemicals and pharmaceuticals. Cascade reactions for the rapid assembly of several valuable 5- and 6-membered heterocycles are reported, including amino-hydantoins, acyl-pyrazoles, acyl-phthalazinones and azauracils. Over 100 different compounds were synthesized using amino-, imino- and amido-substituted N-isocyanates, demonstrating their potential as powerful intermediates in heterocyclic synthesis. Their reactivity also enables access to unprecedented bicyclic derivatives and to substitution patterns of azauracils that are difficult to access using known methods, illustrating that controlled reactivity of N-isocyanates provides new disconnections, and a new tool to assemble complex N-N-C[double bond, length as m-dash]O containing motifs.

Simple indole alkaloids and those with a nonrearranged monoterpenoid unit

This review summarizes the isolation, structure determination, total syntheses and biological activities of simple indole alkaloids and those with a nonrearranged monoterpenoid unit, with literature coverage from 2012 to 2013.