Aryl-methyl-dihydrobenzofurans

The 2-methyl-dihydrobenzofuran 12 was the most active. The aromatic benzofurans 2Q were less active. Comparable in activity were the 2,2-dimethyl, the substituted aryl and parent dihydrobenzofuran. The 6-membered pyran ring 24 was less active than the 5-membered ring 22 and the multiple substituted, in this case the 2,3-disubstituted compounds (25) were the least active. All of these sulfonylureas were herbicidal. 

The value of Eox for BH 4 is not known, but the borohydride radical, BH4 has been characterized by e.s.r and UV spectroscopy in oxidations of the anion by hydroxyl or azide radicals under pulse-radiolysis conditions (Symons et al., 1983 Horii and Taniguchi, 1986). Most borohydride reductions seem to be straightforward hydride transfers, but stepwise processes occur in the reductions of aryl bromides or iodides under photochemical or di-t-biftyl peroxide initiation. Radical intermediates are shown by the formation of 3-methyl-2,3-dihydrobenzofuran in the reduction of o-allyloxyiodobenzene (Abeywickrama and Beckwith, 1986). 

Flavanones undergo a variety of transformations by HTI or other hypervalent iodine reagents. In methanol, at room temperature, they were dehydrogenated to the corresponding flavones [40], A similar dehydrogenation occurred in chromanones [41]. Dramatic solvent effects occurred when methanol was replaced by other solvents. Thus, in trimethyl orthoformate ring-contraction was the main pathway and methyl 2-aryl-2,3-dihydrobenzofuran-3-carboxylates the major products, accompanied by substantial amounts of 3-methoxyflavones [42] ... 

Lamaty and coworkers [132] described a nice combination of three palladium-catalysed transformations first, an intermolecular nucleophilic substitution of an allylic bromide to form an aryl ether second, an intramolecular Mizoroki-Heck type transformation in which the intermediate palladium(II) species is intercepted by a phenylboronic acid. Thus, reaction of a mixture of 2-iodophenol (255), methyl 2-bromomethylacrylate (256) and phenylboronic acid in the presence of catalytic amounts of Pd(OAc)2 led to 3,3-disubstituted 2,3-dihydrobenzofuran 257. Besides phenylboronic acid, several substituted boronic acids have also been used in this process (Scheme 8.65). 

Senboku H, Michinishi J, Kara S (2011) Facile synthesis of 2,3-dihydrobenzofuran-3-ylacetic acids by novel electrochemical sequential aryl radical cycliza-tion-carboxylation of 2-allyloxybromobenzenes using methyl 4-tert-butylbenzoate as an electron-transfer mediator. Synlett 1567-1572. doi 10.1055/ s-0030-1260794... 

The Claisen rearrangement was also investigated in aqueous medium at high temperature by using a pressurized microwave reactor. The example in Scheme 5.42 is the first Claisen rearrangement of an allyl aryl ether in water. Under neat condition, the transposition of allyl phenyl ether (128) occurred at 290°C, while in water a temperature of 240°C was enough. The control of temperature was important at 245°C a mixture of 2-allylphenol (46%), 2-(2-hydroxyprop-l-yl)-phenol (31%), and 2-methyl-2,3-dihydrobenzofuran (23%) was obtained. 

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