Furan ring rearrangement

Hydrogen chloride 5-Subst. /5-(2-furyl)propionic acids from a,/ -ethylene-/ -(2-furyl)ketones Furan ring rearrangement... 

In the synthesis in Scheme 13.46, a stereoselective aldol addition was used to establish the configuration at C(2) and C(3) in Step A. The furan ring was then subjected to an electrophilic addition and solvolytic rearrangement in Step B. 

A review of Claisen rearrangements in aqueous solution has appeared. The synthesis of natural products utilizing tandem Diels-Alder additions with sigmatropic rearrangement processes has been reviewed, and a brief review of the regioselective synthesis of coumarins, quinolones and thiocoumarins with 3,4-fused pyran or furan ring systems by the Claisen rearrangement has been presented. ... 

The cascade rearrangement of 4-acetoxy-9-furylnaphtho[2,3-, ]furans in ethanol saturated with hydrogen chloride gas gives the tetracyclic naphthodifuran derivatives in moderate yields (Equation 93) <2006TL4117>. The reaction is thought to proceed via double recyclization of both furan rings of the initial molecule (Table 6). 

In conclusion, the pulmonary toxicity of 4-ipomeanol seems to be due to metabolic activation probably by formation of an unstable epoxide on the furan ring catalyzed by CYP4B1 in the Clara cell. The epoxide can rearrange to an a, 3 unsaturated dialdehyde (Fig. 7.37), which can interact with macromolecules and covalently bind, leading to cellular necrosis and edema. 

The reactions of the methyl furo[2,3-6]pyrrole-5-carboxylate (153) or its 6-methyl derivative (154) with DMAD proceed via [4 -I- 2] cycloaddition at the a,a -positions of the furan ring, giving cycloadducts which by subsequent 1,5-sigmatropic rearrangement give trimethyl 5-hydroxyindole-2,6,7-tricarboxylate (155) or its 1-methyl derivative (156) <94UP 70i-0i>. 

The rearrangement can be explained on the basis of the tendency with which the relatively unstable furan ring is converted into a more stable aromatic system. The authors propose the following mechanism ... 

Thomas and Ozainne have explored the tandem Qaisen-Cope rearrangement on aromatic substrates. Treatment of 2-furfuryl alcohol with diene (25 equation 9) provides a 9 1 mixture of Claisen-Cope product (39) and furan (37), respectively. The minor component arises via the forbidden [1,3] prototropic shift of the initial Claisen intermediate (38), thereby reestablishing the aromaticity of the furan ring. 

---