Chemoselective Ru-Catalyzed Oxidative Lactamization vs Hydroamination of Alkynylamines: Insights from Experimental and Density Functional Theory Studies

Enantioselective Total Syntheses of Vallesamidine and Schizozygane Alkaloids

A general streamlined strategy for the enantioselective total syntheses of the schizozygane family of natural products and related alkaloid vallesamidine is described. Specifically, a catalytic enantioconvergent cross-coupling sets the quaternary stereogenic center in a pluripotent intermediate adorned with an olefin and three orthogonal carboxylate groups, upon which the modularity of the synthesis relies. A late-stage catalytic oxidative lactamization of an alkyne is instrumental in the first-generation synthesis of the schizozygane alkaloids. In the second-generation approach, a novel application of the van Leusen reaction for the generation of lactams is pivotal to accessing the schizozygane alkaloids from a common intermediate. The strategies outlined here are expected to enable detailed biological investigations as well as facilitate access to and provide solutions for the late-stage modification of bioactive alkaloids.

Acetate Assistance in Regioselective Hydroamination of Allenamides: A Combined Experimental and Density Functional Theory Study

A key factor in the development of selective nucleophilic addition to allenamides is controlling the reactivity of electrophilic intermediates, which is generally achieved using an electrophilic activator via conjugated iminium intermediates. In this combined experimental and computational study, we show that a general and highly chemoselective hydroamination of allenamides can be accomplished using a combination of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and NaOAc. Experimental mechanistic studies revealed that HFIP mediates proton transfer to activate the allenamide, while the acetate additive significantly contributes to N-selective interception. This strategy enables a general hydroamination of allenamides without the use of metals. We demonstrated that various functionalized 1,3-diamines could be readily synthesized and diversified into value-added structural motifs. Detailed mechanistic investigations using the density functional theory revealed the role of NaOAc in the formation of reactive electrophilic intermediates, which ultimately governed the selective formation of 1,3-diamine products. Critically, calculations of the potential energy surface around the proton-transfer transition state revealed that two different reactive electrophilic intermediates were formed when NaOAc was added.