Studies on Acetylenic Compounds. XXXII. Ring Closure of Propargyl Ethers. (2).

The Claisen rearrangement of 1,4-bis(m-methoxyphenoxy-)2-butyne

“…Unfortunately we could not identify a single set of conditions producing a specific Ila-methylpterocarpan isomer (3b, 3c, 3d, or 3e) in high yield and purity such as lo obviate elaborate chromatographic separation procedures. The Claisen rearrangement of l b is clearly not regiospecific, in agreement with the widely fluctuating isomer ratios reported in the literature for the chromanes and chromenes formed in the corresponding reactions of other meta-substituted ally1 and propargyl aryl ethers (13,14,17). However, it can be stated that the two [3,3] sigmatropic shifts occur preferentially to the aromatic carbon para to the methoxy group, i.e., the sterically least crowded isomers 3b and 4b constitute the major portion of the product mixture in all experiments.…”

Furo-fused 2H-chromenes: synthesis and photochromic properties

“…These compounds can be obtained through a multistep sequence from chromanones, involving the Kabbe synthesis (14,15). This method is not readily applicable to 2,2-diary1 analogues (10) and, alternatively, a thermal cyclization of propargylaryl ethers can be considered (16)(17)(18). This approach (method A), which appears particularly attractive because of availability of the starting phenols and chloro-or hydroxyalkynes, has been useful in synthesizing diaryl-naphthopyrans (9).…”

A simple two steps ytterbium triflate‐catalysed preparation of 2,2‐dimethyl‐2h‐chromenes from salicylaldehydes and 2‐methylpropene

“…One of the most used methods to prepare chromenes [2] is to build the pyran ring via an alkylation of the corresponding phenol with 3-chloro-3-methylbut-1-yne followed by a Claisen rearrangement of the resulting propargyl ether into the target compound [19][20][21]. If the introduction of a copper-catalysed alkylation method did wonder in yield improvement of this first step [22,23], the following thermal Claisen rearrangement/cyclization is, in some cases, problematic [23,24].…”