Catalytic Ynone–Amidine Formal [4 + 2]-Cycloaddition for the Regioselective Synthesis of Tricyclic Azepines

Enantioselective Synthesis of Nonfused Eight-Membered O-Heterocycles by Sequential Catalysis

This work describes a chiral bifunctional squaramide/DBU sequential catalytic strategy for the enantioselective synthesis of nonfused chiral eight-membered O-heterocycles through the asymmetric addition of ynones to β,γ-unsaturated α-ketoesters followed by the regio- and diastereoselective cyclization of the adduct intermediates. Mechanistic experiments revealed that an isomerization process should be involved in the ring formation step, and the origin of the high regioselectivity and diastereoselectivity has also been elucidated by the DFT calculations.

Macrocyclic Bridgehead Fluorophores, Pyrrolyl-diazabicyclo[8.3.1]tetradecadienones, with Giant Stokes Shifts

A previously unknown class of fluorophores was discovered, which represents 14-membered bridgehead heterocycles, pyrrolyl-diazabicyclo[8.3.1]tetradecadienones, herein referred to as PY-14-ONEs. The new fluorophores are characterized by giant Stokes shifts of ∼8000-10,250 cm-1 and virtually zero overlap of the absorption and emission bands. They exhibit fluorescence maxima in the blue-green region (454 ≤ λem ≤ 513 nm, MeCN), which shift to the red side when converted to their water-soluble salts by alkylation with MeI (478 ≤ λem ≤ 516 nm, water). PY-14-ONEs were obtained by an original synthesis from DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene, which reacts with acylethynylpyrroles without catalysts under mild conditions to afford PY-14-ONEs in a 34-58% yield. The reaction represents a ring expansion of DBU. Since acylethynylpyrroles are readily available, the discovered reaction opens promising possibilities for the development of new fluorophores. The results of our time-dependent DFT calculations indicate that the pyrrole ring in PY-14-ONEs plays an important role in the formation of the Stokes shifts, which can be further enhanced by attaching appropriate substituents to it, capable of creating in S1 an extended conjugated system and causing a substantial alternation of the molecular structure via its planarization.

Diastereoselective Synthesis of Fused Tricyclic Pyridopyrimidines via Tandem Cyclization of Allenoates and Cyclic Amidines

The tandem cyclization of easily accessible allenoates and cyclic amidines for the synthesis of functionalized tricyclic pyridopyrimidines is reported herein. The annulation featured a broad substrate scope with good functional group tolerance under very mild conditions (35 examples, 32-85% yields). The pyridopyrimidines were obtained in a very short reaction time (1 min), at room temperature, under neat conditions, which offers an alternative way to the sustainable synthesis of functionalized pyridopyrimidines. The scalability of the developed protocol is further demonstrated by a gram-scale synthesis.

Decomposition of Alkynyl Hydrazones: Synthesis of Allenoates, Dihaloallenoates, and Angularly Fused Tricyclic Azepines

An unprecedented decomposition of unprotected alkynyl hydrazones is attempted that has provided allenoates, tetrasubstituted α,γ-dihaloallenoates, and functionalized tricyclic azepines. A reaction of alkynyl hydrazones with N-halosuccinimides captures the electrophile in 2-fold that delivers fully substituted dibromo- and diiodoallenoates in good yields. In addition, a DABCO-promoted Wolff-Kishner reduction of hydrazones, followed by isomerization, provides versatile allenoates under mild conditions. In contrast, a similar decomposition with ambiphilic DBU furnishes a completely different tricyclic azepine scaffold in excellent yield and diastereoselectivity.

Highly Diastereoselective One-Pot Synthesis of 4,5-Dihydrofuro[2,3-b]azocin-6-one Derivatives through Cyclization/[4+4] Annulation Reactions

A variety of 4,5-dihydrofuro[2,3-b]azocin-6-one derivatives were expediently assembled through Au(I)-catalyzed cyclization and 2-(tert-butyl)-1,1,3,3-tetramethylguanidine (BTMG)-mediated [4+4] annulation reactions of enyne-amides and ynones. The reactions exhibit high efficiency with excellent regio- and diastereoselectivity. A broad spectrum of substrates was utilized. The products with an eight-membered ring might be useful in biological chemistry and medicinal science. Furthermore, the products could be facilely converted into various derivatives.

Phosphine-catalysed denitrative rearomatising (3 + 2) annulation of α,β-ynones and 3-nitroindoles

We describe a metal-free strategy to access various α-arylidene cyclopenta[b]indoles via phosphine-catalysed (3 + 2) annulation of α,β-ynones and 3-nitroindoles. For the first time, the rearomatisation of the indole nucleus was observed in such an annulative transformation. The method was extended to the synthesis of an antimalarial natural product, bruceolline E.

Catalyst-Free [3 + 3] Annulation/Oxidation of Cyclic Amidines with Activated Olefins: When the Substrate Olefin Is Also an Oxidant

Herein we describe a catalyst-free regioselective [3 + 3] annulation/oxidation reaction of cyclic amidines such as DBU (1,8-diazabicyclo(5.4.0)undec-7-ene) and DBN (1,5-diazabicyclo(4.3.0)non-5-ene) with activated olefins, i.e., 2-arylidenemalononitriles and 2-cyano-3-aryl acrylates, to afford tricyclic 2-pyridones and pyridin-2(1H)-imines, respectively. The mechanism has been proposed based on DFT calculations. In the reaction, the cyclic amidines serve as C,N-bisnucleophiles for the cyclization, while the olefins play a dual role by acting as both reactants and oxidants.