Synthesis of Benzannulated Medium-ring Lactams via a Tandem Oxidative Dearomatization-Ring Expansion Reaction

Medium-ring natural products exhibit diverse biological activities but such scaffolds are underrepresented in probe and drug discovery efforts due to the limitations of classical macrocyclization reactions. We report herein a tandem oxidative dearomatization-ring-expanding rearomatization (ODRE) reaction that generates benzannulated medium-ring lactams directly from simple bicyclic substrates. The reaction accommodates diverse aryl substrates (haloarenes, aryl ethers, aryl amides, heterocycles) and strategic incorporation of a bridgehead alcohol generates a versatile ketone moiety in the products amenable to downstream modifications. Cheminformatic analysis indicates that these medium rings access regions of chemical space that overlap with related natural products and are distinct from synthetic drugs, setting the stage for their use in discovery screening against novel biological targets.

Synergistic catalysis: cis-cyclopropanation of benzoxazoles

In the present paper we report our latest efforts in pushing the boundaries of synergistic catalysis. We propose the use of 3 different catalytic cycles working in concert for the formation of cis cyclopropane derivatives of benzoxazoles with excellent stereoselectivities. This is the proof of concept that synergistic catalysis could be successfully used in cascade reactions.

Catalytic asymmetric transannulation of NH-1,2,3-triazoles with olefins

A convenient one-pot asymmetric synthesis of 2,3-dihydropyrroles from in situ generated triflated triazoles and olefins is described that further expands the utility of azavinyl carbene chemistry and provides access to an important class of cyclic enamides. Mechanistic investigations support the involvement of triflated cyclopropylaldimine intermediates in the formation of 2,3-dihydropyrrole. To the best of our knowledge, this is the first example of a chiral Brønsted acid catalyzed rearrangement of cyclopropylimines into enantioenriched 2,3-dihydropyrroles.

Reactions of ruthenium-allenylidene complexes tethering a cyclopropyl group

Two cyclopropyl allenylidene complexes [Ru]=C=C=C(R)(C(3)H(5)) ([Ru]=[RuCp(PPh(3))(2)], Cp=Cyclopentadienyl; R=thiophene (2a) and R=Ph (2b)) are prepared from the reactions of [Ru]Cl with the corresponding 1-cyclopropyl-2-propyn-1-ol in the presence of KPF(6). Thermal treatment, halide-anion addition, and palladium-catalyzed reactions of 2a and 2b all lead to a ring expansion of the cyclopropyl group, giving the vinylidene complexes 4a and 4b, respectively, each with a five-membered ring. This ring expansion proceeds by C-C bond formation between Cβ of the cumulative double bond and a methylene group of the cyclopropyl ring. In the reaction of 2a with pyrrole, consecutive formation of two C-C bonds, one between C-2 of pyrrole and Cγ of 2a and the other between C-3 of pyrrole and Cα, results in the formation of 6a. The reaction proceeds by addition of pyrrole and 1,3-proton shifts. The hydrogenation of 2a by NaBH(4) is carried out in different solvents. The cumulative double bonds are reduced regioselectively to give a mixture of 7a and 8a. Interestingly, use of different solvents leads to different ratios of 7a and 8a. Presence of a protic solvent like methanol in dichloromethane or chloroform solution increases the yield of 8a, thus revealing that both the rates of hydroboration and deboronation increase. The structures of two new complexes 4a and 6a have been firmly established by X-ray diffraction analysis.