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The Cope rearrangement is an extensively studied organic reaction involving the [3,3] sigmatropic rearrangement of 1,5-dienes. [ 14 ] [ 15 ] [ 16 ] It was developed by Arthur C. Cope . For example, 3,4-dimethyl-1,5-hexadiene heated to 300 °C yields 2,6-octadiene.
Rearrangement of Mariano's diene. One example was the photolysis of Mariano's compound, 3,3‑dimethyl-1,1,5,5‑tetraphenyl-1,4‑pentadiene. In this symmetric diene, the active π bonds are conjugated to arenes, which does not inhibit the reaction. [4] [5] [6] Pratt's diene has two possibilities for rearrangement: a and b.
The rearrangement is widely used in organic synthesis. It is symmetry-allowed when it is suprafacial on all components. The transition state of the molecule passes through a boat or chair like transition state. An example of the Cope rearrangement is the expansion of a cyclobutane ring to a cycloocta-1,5-diene ring:
The benzonorcaradiene diterpenoid (below left) was rearranged into the benzocycloheptatriene diterpenoid isosalvipuberlin (right) by boiling a methylene chloride solution. This transformation can be envisaged as a disrotatory electrocyclic reaction, followed by two suprafacial 1,5-sigmatropic hydrogen shifts, as shown below.
Two further early examples were the rearrangement of 1,1,5,5-tetraphenyl-3,3-dimethyl-1,4-pentadiene (the "Mariano" molecule) [22] and the rearrangement of barrelene to semibullvalene. [23] We note that, in contrast to the cyclohexadienone reactions which used n- π * excited states, the di- π -methane rearrangements utilize π - π * excited ...
Bergman reaction of cyclodeca-3-ene-1,5-diyne Naturally occurring compounds such as calicheamicin contain the same 10-membered ring and are found to be cytotoxic . These compounds generate the diradical intermediate described above which can cause single and double stranded DNA cuts. [ 4 ]
The effect of the solvent of the cheletropic reaction of 3,4-dimethyl-2,5-dihydrothiophen-1,1-dioxide (shown at right) was kinetically investigated in 14 solvents. The reaction rate constants of the forward and reverse reaction in addition to the equilibrium constants were found to be linearly correlated with the E T (30) solvent polarity scale.
Depending on the substrate chosen, the HDDA reaction can be initiated thermally or by the addition of a suitable catalyst, often a transition metal. [1] [2] [4] [5] The prevailing mechanism for the thermally-initiated HDDA reaction is a [4+2] cycloaddition between a conjugated diyne (1,3-dialkyne) and an alkyne (often referred to as a diynophile in analogy to the Diels–Alder dienophile) to ...