Propargyl aryl ethers

A particularly useful chromene synthesis involves the thermal rearrangement in an inert solvent of aryl propargyl ethers (33) (62CPB926,63CPB1042) which are conveniently prepared from a phenol and a chloroalkyne. An indication of the ease of cyclization is apparent from the presence of chromene in the ethers prepared in this manner. 

Electron-withdrawing m-substituents will decrease the electron density at the aromatic terminus of the [3,3]-rearrangement, thereby retarding the reaction. This allows polymerization of the terminal alkyne to compete successfully with the cyclization. However, the presence of a m -substituent has a much more significant effect the cyclization of m-substituted aryl propargyl ethers can lead to two isomeric products (46a and b). 

Rearrangement of aryl propargyl ethers.1 Claisen rearrangement of the naphthyl propargyl ether 1 at 215° results in the benzopyran 2 as the only isolable product (40% yield). Addition of 1 equiv. of CsF results in the benzofuran 3 as the major product, presumably formed via an a-allenyl ketone (a). Related salts such as KF, RbF, or BaF2 are completely ineffective. This modified Claisen rearrangement provides a route to o-hydroxy aldehydes such as 4 from a phenol. 

The now classical route to 2/7-1 -benzopyrans, cyclisation of aryl propargyl ethers, has been utilised in the synthesis of mollugin <06T8419> and to prepare novel photochromic naphtho[2,l-b]pyrans <06T737>. 

In the synthesis of 2//-naphtho[l,2-i]pyrans from 1-naphthols and l,l-diarylprop-2-yn-l-ols, initial protonation and loss of water from the latter generate an alkynyl carbocation, normally converted to the aryl propargyl ether. However, the concomitant formation of the highly coloured propenylidenenaphthalenones 12, a new class of merocyanine dyes, suggests attack of the allenic form of the cation at the 4-position of the naphthol followed by a 1,7-H shift <03EJ01220>. 

Indium metal reduces the terminal triple bond of aryl propargyl ethers, amines, and esters in aqueous ethanol to produce the corresponding alkenyl compounds (Tab. 8.23) [119]. Indium metal in aqueous ethanohc ammonium chloride reduces the carbon-carbon double bond of activated conjugated alkenes such as a,a-dicya-no olefins, /faryleriories, and enone esters (Scheme 8.88) [120]. 

D. Bell, M.R. Davies, G.R. Geen, I.S. Mann, Copper(I) iodide A catalyst for the improved synthesis of aryl propargyl ethers. Synthesis. (1995) 707. 

SCHEME 7.5 Conducting the Claisen rearrangement of the aryl propargyl ether in diethylanihne at elevated temperatures. Formation of the desired chroman product is accompanied by generation of furan by-product in successively increasing amounts. 

Representative procedure for the gold-catalyzed cyclization of aryl propargyl ethers. 2H-Chromen ... 

Ph3PAuNTf2 catalyses the regioselective cycloisomerization of substituted aryl propargyl ethers into 2//-chromenes (Scheme 119). ... 

The Claisen rearrangements of aryl propargyl ethers 133 are used in the synthesis of flavonoids [94]. 

SCHEME 23.2 Boron tribromide-mediated aryl propargyl ether cleavage. 

Yao, M.-L., Reddy, M. S., Zeng, W.-B., Hall, K., Walfish, 1., Kabalka G. W. 2009. Identification of a boron-containing intermediate in the boron tribromide mediated aryl propargyl ether cleavage reaction. J. Org. Chem. 74 1385-1387. 

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