Propargylic aryl ethers

With a low concentration of thiol, propargyl aryl ethers or amines show vicinal bis(stannation), and it is suggested that this results from stabilization of the initial radical by cyclization onto the aromatic ring (Equation (17)). 

Propargylic aryl ethers (also esters) are cleaved by benzyltriethylammonium tetrathiomolyb-date in acetonitrile at room temperature. Allyl esters are not cleaved under these conditions. Electroreduction in the presence of Ni-bipyridine complex as catalyst is another method to affect the deprotection of propargyl ethers (Scheme 1.22). 

Arylbismuth 671, 673 Arylboronic acid 671, 673 Arylbutenes, formation of 613 Aryl-2-cyclohexenones 653 Aryl ethers—see also AUyl aryl ethers. Diaryl ethers. Phenyl ethers, Propargyl aryl ethers formation from calixarenes 1387 Aryl haUdes, as phenol precursors 396, 397 Ai-Arylhydroxylamines, isomerization of 801-805 oxidation of 419 3-Arylindoles, synthesis of 1236 Aryl ketones, oxidation of 424, 425 Aryloxylium cations 179 Asatone, synthesis of 1178, 1179 Ash, from incineration of municipal waste, phenoUc compounds in 938 Aspersitin, synthesis of 1327, 1328 Aspirin 10, 11... 

Prooxidant effect 845, 849, 891, 892, 895, 899 Propargyl aryl ethers, rearrangement of 770, 773, 779-782 o-Propenylphenols 301 5-Propylguaiacol, mass spectrum of 303, 304 Protocatechuic acid, irrachation of 1079... 

Propargyl aryl ethers undergo a Claisen rearrangement and then ring closure to produce 2-methyl-benzofurans directly. ... 

Propargyl aryl ether is also an attractive substrate of the Claisen rearrangement for the construction of heterocycles such as chromene derivatives. 

The substrate can be prepared by standard etherification reaction of a phenol derivative with a propargylic halide [lb]. Further carbon-carbon bond forming reactions of the terminal acetylenic carbon via metal acetylide provides a variety of substituted propargyl aryl ethers. Direct preparation using substituted propargyl ether is also possible. 

Rearrangement of propargyl aryl ether 19 smoothly proceeds under thermal conditions to afford chromene derivatives 22 [23, 24]. The mechanism involves the formation of ortho allenyl phenol 20 followed by a 1,5-hydrogen shift and elec-trocyclic ring closure sequence via 21. For example, the Claisen rearrangement of propargyl aryl ether 23 prepared by the Mitsunobu reaction smoothly took place at 180 °C to give a flav-3-ene derivative 24 in excellent yield [25]. 

Keywords Propargylic aryl ethers, iodine, sodium bicarbonate, nitromethane, room temperature, electrophilic iodocyclization, 3,4-disubstituted 2//-benzopyrans... 

A new versatile synthesis of coumarins has been developed, based upon the rearrangements of allylic or propargylic aryl ethers, These processes (a) avoid the drastic acid conditions of conventional methods, (Jb) avoid orientation limitations, and (c) give good yields. The intermediate (49) is, however, very unstable and requires careful handling. 

The high-valent metal species required for activation of an alkyne has also been generated by the oxidative addition to an allylic or propargylic system. For example, with an allyl aryl ether as the substrate, this type of reaction achieves a cycloisomerization that occurs through an 0- to C-allyl migration (Equation (92)) 323,324 similarly, (9-propargyl derivatives lead to a mixture of allenyl and propargyl products (Equation (93)).325,326... 

In general, allylic and propargyllic electrophiles are much more reactive than alkenyl and alkynyl electrophiles. Thus, a wide variety of electrophiles containing halogens, e.g. I, Br, and Cl, and oxygen groups, e.g. sulfonates, phosphates, carboxylates, carbonates, alkyl and aryl ethers, and even silyl... 

Kirsch has reported the conversion of propargyl vinyl ethers to form pyrroles via a multi-step transformation involving the silver-catalyzed propargyl Claisen rearrangement to form the a-allenyl P-keto ester, condensation with a primary aryl amine. 

The Ugi adduct 45 was next subjected to TFA in an attempt to deprotect the Boc and PMB ester groups, prior to macrocyclization (Scheme 10). However, the acidic conditions cleaved the tertiary alkyl-aryl ether that we had just constructed in the AAA reaction Joullie s route invokes a propargyl ether 33, which survives treatment with 30% TFA in CH2CI2, in place of our allylic... 

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