Rhodium-Catalyzed C-H Alkenylation/Electrocyclization Cascade Provides Dihydropyridines That Serve as Versatile Intermediates to Diverse Nitrogen Heterocycles

Abstract

Nitrogen heterocycles are present in approximately 60% of drugs, with nonplanar heterocycles incorporating stereogenic centers being of considerable interest to the fields of medicinal chemistry, chemical biology, and synthetic methods development. Over the past several years, our laboratory has developed synthetic strategies to access highly functionalized nitrogen heterocycles with multiple stereogenic centers. This approach centers on the efficient preparation of diverse 1,2-dihydropyridines by a Rh-catalyzed C-H bond alkenylation/electrocyclization cascade from readily available α,β-unsaturated imines and alkynes. The often densely substituted 1,2-dihydropyridine products have proven to be extremely versatile intermediates that can be elaborated with high regioselectivity and stereoselectivity, often without purification or even isolation. Protonation or alkylation followed by addition of hydride or carbon nucleophiles affords tetrahydropyridines with divergent regioselectivity and stereoselectivity depending on the reaction conditions. Mechanistic experiments in combination with density functional theory (DFT) calculations provide a rationale for the high level of regiocontrol and stereocontrol that is observed. Further elaboration of the tetrahydropyridines by diastereoselective epoxidation and regioselective ring opening furnishes hydroxy-substituted piperidines. Alternatively, piperidines can be obtained directly from dihydropyridines by catalytic hydrogenation in good yields with high face selectivity.When trimethylsilyl alkynes or N-trimethylsilylmethyl imines are employed as starting inputs, the Rh-catalyzed C-H bond alkenylation/electrocyclization cascade provides silyl-substituted dihydropyridines that enable a host of new and useful transformations to different heterocycle classes. Protonation of these products under acidic conditions triggers the loss of the silyl group and the formation of unstabilized azomethine ylides that would be difficult to access by other means. Depending on the location of the silyl group, [3 + 2] cycloaddition of the azomethine ylides with dipolarophiles provides tropane or indolizidine privileged frameworks, which for intramolecular cycloadditions yield complex polycyclic products with up to five contiguous stereogenic centers. When different types of conditions are employed, loss of the silyl group can result in either rearrangement to cyclopropyl-fused pyrrolidines or to aminocyclopentadienes. Mechanistic experiments supported by DFT calculations provide reaction pathways for these unusual rearrangements.The transformations described in this Account are amenable to natural product synthesis and drug discovery applications because of the biological relevance of the structural motifs that are prepared, short reaction sequences that rely on readily available starting inputs, high regiocontrol and stereocontrol, and excellent functional group compatibility. For example, the methods have been applied to efficient asymmetric syntheses of morphinan drugs, including the opioid antagonist (-)-naltrexone, which is extensively used for the treatment of drug abuse.

Scheme 1.

One-Pot Rh(I)-Catalyzed C–H Activation/Alkenylation and 6π Electrocyclization Cascade.

Scheme 2.

Regio- and Diastereoselective Protonation and Reduction of 4.

Scheme 3.

Four Possible Iminium Ions Resulting from Protonation of 4.

Scheme 4.

Proposed Mechanistic Pathway.

Scheme 5.

Proposed Mechanisms for the Divergent Rearrangements of 16.

Scheme 6.

Synthesis of (+)-Ketorphanol from the Rh(I) Cascade Starting from Imine and Tethered Alkyne. (a) [RhCl(coe)₂]₂ (5 mol %), 4-(diethylphosphino)-N,N-dimethylaniline (10 mol %), toluene, 55 °C; (b) NaHB(OAc)₃, AcOH, EtOH, 0 to 23 °C; (c) 85% H₃PO₄, 125 °C; (d) H₂, Pd/C, NaHCO₃, EtOH, 23 °C.

Scheme 7.

Synthesis of (−)-Naltrexone. (a) [RhCl(coe)₂]₂ (5 mol %), (pNMe₂)PhPEt₂ (10 mol %), PhMe, 85 °C; (b) NaBH(OAc)₃ (5.0 equiv), AcOH, EtOH, 0 °C; (c) 55% H₃PO₄, 125 °C; (d) Br₂, AcOH, 23 °C; NaOH (aq), 23 °C; (e) Tf₂O, pyridine, 0 °C; (f) Pd(TFA)₂ (1.4 equiv), TFA, DMSO, 80 °C; (g) CuSO₄ (2 mol %), ketoglutaric acid, pyridine, 23 °C, O₂; (h) Et₃N, Pd(OH)₂ (20 wt%), 1:3 EtOAc/MeOH, H₂, 23 °C; (i) BBr₃, CH₂Cl₂, −40 to 0 °C.