Synthesis of cyclobutane-fused oxygen-containing tricyclic framework via thermally promoted intramolecular cycloaddition of cyclohexadienone-tethered allenes

Synthesis of Tricyclic Fused 6-5-4 Carbocycles via Acid-Promoted Cascade Intramolecular Cyclization of Allenylsilane-Tethered Cyclohexadienones

The reaction of 2,5-cyclohexadienones with methylene-tethered allenylsilane in the presence of Lewis or Brønsted acids leads to a cascade of intramolecular cyclization, yielding stereoselective tricyclic fused 6-5-4 carbocycles featuring a silyl-methylenecyclobutane ring. This transformation is notable for the diastereoselective asymmetric desymmetrization of prochiral dienones, attributed to the axial chirality of allene.

Reagent-Controlled Regioselective Intramolecular [2+2] Cycloaddition between Vinylsilanes and Allenes

This paper provides the first report of the intramolecular [2+2] cycloaddition of vinylsilane. The [2+2] cycloaddition of allenes is a useful reaction that can synthesize cyclobutanes. However, no previous works have attempted to control the regioselectivity between the two double bonds of allenes only by changing the reaction conditions, although there have been some reports of the regioselective [2+2] cycloaddition of allenes by changing the substrate. In this study, we have succeeded in controlling intramolecular [2+2] cycloaddition reactions at the proximal and distal positions of allenes simply by changing the reaction conditions. We found that the atomic radius of silicon is very important for perfect control of the reaction's sites. The structures of all key compounds were determined by the crystal sponge method which could identify the structures of liquid compounds.

Steric- and Electronic-Controlled Intramolecular [2 + 2]-Cycloaddition of Cyclohexadienone-Containing 1,7-Enynes

Herein we have developed the silver-catalyzed electronic- and steric-controlled intramolecular formal [2 + 2]-cycloaddition of alkyne-tethered cyclohexadienones. Substrates with electron-rich alkynes and a less hindered quaternary carbon center afford tricyclic fused cyclobutenes through 1,7-enyne cyclization. In contrast, the formation of dihydrofurans was observed from electron-deficient alkynes via proton abstraction/C-O bond cleavage. The synthetic potential of this method was also broadened with a gram-scale reaction and various transformations on cyclobutene.

Tandem Ring Opening/Intramolecular [2 + 2] Cycloaddition Reaction for the Synthesis of Cyclobutane Fused Thiazolino-2-Pyridones

Reaction of thiazoline fused 2-pyridones with alkyl halides in the presence of cesium carbonate opens the thiazoline ring via S-alkylation and generates N-alkenyl functionalized 2-pyridones. In the reaction with propargyl bromide, the thiazoline ring opens and subsequently closes via a [2 + 2] cycloaddition between an in situ generated allene and the α,β-unsaturated methyl ester. This method enabled the synthesis of a variety of cyclobutane fused thiazolino-2-pyridones, of which a few analogues inhibit amyloid β_1–40 fibril formation. Furthermore, other analogues were able to bind mature α-synuclein and amyloid β₁₋₄₀ fibrils. Several thiazoline fused 2-pyridones with biological activity tolerate this transformation, which in addition provides an exocyclic alkene as a potential handle for tuning bioactivity.

Copper-Catalyzed Tandem Cross-Coupling/Thermally Promoted [2 + 2] Cycloaddition of 1,6-Enynes and Diazo Compounds To Assemble Methylenecyclobutane-Fused Ring System

An unprecedented copper-catalyzed tandem reaction of 1,6-enynes with diazo compounds via a cross-coupling/[2 + 2] cycloaddition sequence was reported. A library of methylenecyclobutane-fused ring systems including cyclobuta[b]indolines, cyclobuta[b]benzofuran, benzo[b]cyclobuta[d]thiophene, and bicyclo[3.2.0] structures were obtained in moderate to excellent yields under very mild reaction conditions. The reaction exhibited high proximal-regioselectivity and diastereoselectivity. Moreover, 1,6-allenene has proven to be the key intermediate and proceeds via a thermally promoted [2 + 2] cycloaddition in the absence of copper catalyst.

Crystal structure of (1aS,1a 1 S,2S)-4a-butoxy-1a,1a 1,2,4a,5,6-hexahydro-1H-cyclobuta[de]naphthalen-2-yl-4-nitrobenzoate, C22H25NO5

C22H25NO5, monoclinic, P21/c (no. 14), a = 15.7241(13) Å, b = 7.0223(5) Å, c = 18.4613(14) Å, β = 100.329(4)°, V = 2005.4(3) Å3, Z = 4, R gt (F) = 0.0495, wR ref (F 2) = 0.1564, T = 296(2) K.

Crystal structure of (6aR,6a1S,10aS)-2,4a,6a,6a 1,9,10-hexahydro-7H-4,5-methanocyclobuta[4,5]naphtho[8a,1-b]pyran, C15H16O

C15H16O, monoclinic, P21/c (no. 14), a = 5.4371(7) Å, b = 17.567(2) Å, c = 11.8840(18) Å, β = 101.043(9)°, V = 1114.1(3) Å3, Z = 4, R gt(F) = 0.0422, wR ref(F 2) = 0.1155, T = 296(2) K.

Crystal structure of (4aS,5S,6aS,6a1S, 10aS)-4a,5,6a,6a 1,9,10-hexahydro-7H-4,5-methanocyclobuta[4,5]naphtho[8a,1-b]pyran-6(2H)-one, C15H16O2

C15H16O2, triclinic, P1̄ (no. 2), a = 5.6034(7) Å, b = 8.7105(11) Å, c = 12.5661(16) Å, α = 92.790(7)°, β = 100.516(7)°, γ = 106.105(6)°, V = 576.15(13) Å3, Z = 2, R gt(F) = 0.0476, wR ref(F 2) = 0.1348, T = 296.15 K.