N1/N4 1,4-Cycloaddition of 1,2,4,5-Tetrazines with Enamines Promoted by the Lewis Acid ZnCl2

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.

Acyclic and Heterocyclic Azadiene Diels-Alder Reactions Promoted by Perfluoroalcohol Solvent Hydrogen Bonding: Comprehensive Examination of Scope

Herein, the first use of perfluoroalcohol H-bonding in accelerating acyclic azadiene inverse electron demand cycloaddition reactions is described, and its use in the promotion of heterocyclic azadiene cycloaddition reactions is generalized through examination of a complete range of azadienes. The scope of dienophiles was comprehensively explored; relative reactivity trends and solvent compatibilities were established with respect to the dienophile as well as azadiene; H-bonding solvent effects that lead to rate enhancements, yield improvements, and their impact on regioselectivity and mode of cycloaddition are defined; new viable diene/dienophile reaction partners in the cycloaddition reactions are disclosed; and key comparison rate constants are reported. The perfluoroalcohol effectiveness at accelerating an inverse electron demand Diels-Alder cycloaddition is directly correlated with its H-bond potential (pKa). Not only are the reactions of electron-rich dienophiles accelerated but those of strained and even unactivated alkenes and alkynes are improved, including representative bioorthogonal click reactions.