Lewis acid catalyzed [4+2] annulation of bicyclobutanes with dienol ethers for the synthesis of bicyclo[4.1.1]octanes

Bicyclic carbocycles containing a high fraction of Csp3 have become highly attractive synthetic targets because of the multiple applications they have found in medicinal chemistry. The formal cycloaddition of bicyclobutanes (BCBs) with two- or three-atom partners has recently been extensively explored for the construction of bicyclohexanes and bicycloheptanes, but applications to the synthesis of medium-sized bridged carbocycles remained more limited. We report herein the formal [4+2] cycloaddition of BCB ketones with silyl dienol ethers. The reaction occurred in the presence of 5 mol% aluminium triflate as a Lewis acid catalyst. Upon acidic hydrolysis of the enol ether intermediates, rigid bicyclo[4.1.1]octane (BCO) diketones could be accessed in up to quantitative yields. This procedure tolerated a range of both aromatic and aliphatic substituents on both the BCB substrates and the dienes. The obtained BCO products could be functionalized through reduction and cross-coupling reactions.

Enantioselective Total Synthesis of 10-Desoxy Analogue of a Previously Reported Natural Peroxyguaidiol

Described herein is an enantioselective synthesis of an analogue of a previously reported guaiane endoperoxide isolated from aerial parts of Croton arboreous. The polycyclic framework of the target structure was constructed with the C-7 stereogenic center derived from L-(-)-carvone and other stereogenic centers installed via substrate chirality-induced asymmetric reactions, starting with the synthesis of the seven-membered ring through regioselective enolization of carvone, ring-expansion, and installation of a conjugated C═C bond. Further functionalization was then achieved through regioselective enolization, triflation, and installation of an isopropenyl group. During the synthesis, some exceptions to the well-known rules of "thermodynamic control" and "kinetic control" in enolization of asymmetric cyclic ketones were observed. In construction of the bridged five-membered and endoperoxy rings, a peroxycarbenium [3 + 2] cycloaddition reaction with alkenes was carried out with several alkenes-silyl-gem-dihydroperoxides of different relative configurations. However, no expected [3 + 2] products were observed. Finally, the five-membered ring was smoothly installed through an intramolecular Darzens reaction, and the peroxy functionality was introduced via a carbon-centered radical-mediated reaction with triplet oxygen, followed by an intramolecular etherification under acidic conditions. Comparison of the ¹H and ¹³C NMR spectra of the synthetic analogue and the natural product revealed that the latter was definitely not an endoperoxide.

NHC-BIAN-Cu(I)-Catalyzed Friedländer-Type Annulation of 2-Amino-3-(per)fluoroacetylpyridines with Alkynes on Water

The direct catalytic alkynylation/dehydrative cyclization of 2-amino-3-trifluoroacetyl-pyridines on water was developed for the efficient synthesis of a broad range of fluorinated 1,8-naphthyridines from terminal alkynes. A novel N-heterocyclic carbene (NHC) ligand system that combines a π-extended acenaphthylene backbone with sterically bulky pentiptycene pendant groups was successfully utilized in a copper- or silver-mediated cyclization. Computational analysis of the reaction pathway supports our explanation of the different experimental conversions and yields for the set of copper and silver catalysts. The impact of steric hindrance at the metal center and the flexibility of substituents on the imidazole ring of the NHC on catalytic performance are also discussed.

One-Pot Synthesis of Symmetrical and Asymmetrical 3-Amino Diynes via Cu(I)-Catalyzed Reaction of Enaminones with Terminal Alkynes

An economical and efficient protocol for the direct construction of amino skipped diynes through the Cu(I)-catalyzed reaction of enaminones and terminal alkynes has been described. Different kinds of symmetrical and asymmetrical 3-amino diynes could be obtained in up to 83% yield through a one-pot reaction under mild conditions.

Nickel-Catalyzed Conversion of Enol Triflates into Alkenyl Halides

A Ni-catalyzed halogenation of enol triflates was developed and it enables the synthesis of a broad range of alkenyl iodides, bromides, and chlorides under mild reaction conditions. The reaction utilizes inexpensive, bench-stable Ni(OAc)2 ⋅4 H2 O as a precatalyst and proceeds at room temperature in the presence of sub-stoichiometric Zn and either 1,5-cyclooctadiene or 4-(N,N-dimethylamino)pyridine.

The Direct Conversion of α-Hydroxyketones to Alkynes

Alkynes are highly important functional groups in organic chemistry, both as part of target structures and as versatile synthetic intermediates. In this study, a protocol for the direct conversion of α-hydroxyketones to alkynes is reported. In combination with the variety of synthetic methods that generate the required starting materials by forming the central C-C bond, it enables a highly versatile fragment coupling approach toward alkynes. A broad scope for this novel transformation is shown alongside mechanistic insights. Furthermore, the utility of our protocol is demonstrated through its application in concert with varied α-hydroxyketone syntheses, giving access to a broad spectrum of alkynes.

Total Synthesis of Aplysiasecosterol A

Aplysiasecosterol A (1) is a structurally unusual 9,11-secosteroid isolated from the sea hare Aplysia kurodai. We have accomplished the first and asymmetric total synthesis of 1 in a convergent fashion. The left-hand segment bearing three adjacent stereocenters was constructed through desymmetrizing reduction, ketalization, and radical cyclization. A strategy of asymmetric 2-bromoallylation followed by spontaneous desymmetrizing lactolization enabled a more expeditious access to this segment. The right-hand segment was prepared through two different approaches: one featuring Myers alkylation and Suzuki-Miyaura coupling and the other relying upon Aggarwal lithiation-borylation and Zweifel-Evans olefination. The two fragments were coupled by a Reformatsky type reaction. The three consecutive stereocenters embedded in the central domain of 1 were generated by an iron-mediated, hydrogen atom transfer based radical cyclization reaction.