2.3- dihydro-l-benzoxepines

When the analogous starting material without the hydroxy group was used, 2,3-dihydro-l-benzoxepin was obtained in low yield (8%). The main product (87%) was 2-methyl-2//-chromene.97... 

The dehydrohalogenation reaction has been extended to benzannulated oxepins. Elimination of hydrogen bromide from 3-bromo-4-phenyl-2,3-dihydro-l-benzoxepin with 1,5-diazabicyclo-[4.3.0]non-5-ene gives 4-phenyl-1-benzoxepins 15a15 and 15b16 in low yield. 

A further development of this route (71JOC4028), involving the cyclization of 3-(o-formylphenoxy)propyltriphenylphosphonium salts in methanol, has led to high yields of 2-methylchromene. In the detailed mechanism which has been proposed, an o-quinoneallide is considered to arise by the ring opening of an initially formed benzoxepin derivative. Indeed, 2,3-dihydro-l-benzoxepin is the major product of the reaction in aprotic solvents. 

Expansion of the pyrone ring of chromanones has been achieved by reaction with ethyl diazoacetate and BuLi at —78°C [to produce the a-diazo-ester (154)] followed by acid-catalysed conversion, at room temperature, into the benzoxepin (155). De-ethoxycarbonylation led to the parent 2,3-dihydro-l-benzoxepin-4-(5jF/)-one (156). By a similar sequence, isochroman-4-one (157) was converted into the hitherto unknown l,5-dihydro-2-benzoxepin-4(3H)-one (158). ... 

One Oxygen Atom.—2,5-Dihydro-l-benzoxepins (89) have been reported as products of thermolysis of the vinylallylphenols (88), formed possibly via an intermediate such as (90). The products are thermally unstable and isomerize to cis- and /ra j -3-methyl-2-vinylcoumarans (91). Condensation of phthalic... 

Also obtained by catalytic hydrogenation of 9-acetyl-2,5-dihydro-3-methyl-l-benzoxepin-6,8-diol in ethyl acetate in the presence of Adams catalyst (25%) [3499]. 

Dichloro- and l,2-dibromo-l,2-dihydro-3-benzoxepins are dehalogenated with zinc in methanol to give 3-benzoxepin (6a) in excellent yield.93 A methoxycarbonyl group in the 2-position, however, diminishes the yield of 6b considerably.91... 

Irradiation of anft -9,10-epoxy-1.4-dipropyl-l,4-dihydro-l,4-ethanonaphthalene-2,3-dione results in a stepwise loss of two carbonyl groups and the formation of l,5-dipropyl-3-benzoxepin as the major product (25%).171... 

This reaction can also be used for the synthesis of substituted 1-benzoxepins with one modification instead of the 4/T-benzopyran the 2/7-isomer must be used. 2-[Diazo(phosphoryl)meth-yl]-2//-benzopyrans decompose in the presence of ))3-allylpalladium chloride dimer with elimination of nitrogen to give 1-benzoxepins 2.192 In some cases, the reaction takes a different course and gives 2-methylene-2//-benzopyrans 3.192 In this respect, the bicyclic system behaves differently to the monocyclic diazo(pyranyl)methane. The 2-isomers of the latter structure could not be isolated and gave l//-l,2-diazepines.190 The 4//-benzopyrans do not form benzoxepins but undergo an intramolecular [2+1] cycloaddition to 3,4-dihydro-2,3,4-metheno-2//-ben-... 

When dimethyl 3-benzoxepin-2,4-dicarboxylate is reacted under controlled conditions (15 min) with hydrogen it is possible to isolate dimethyl l,2-dihydro-3-benzoxepin-2,4-dicarboxylate.17 Hydrogenation of 2-nitrodibenz[, /]oxepin-10-carboxylic acid in the presence of palladium on potassium carbonate results in the reduction of the nonaromatic C C double bond and the nitro group to give 2-amino-10,l l-dihydrodibenz[i,/]oxepin-10-carboxylic acid.107... 

The oxidation of 1,4-dihydronaphthalene with oxygen with irradiation gave l,2-dihydro-2-naphthyl hydroperoxide that decomposed thermally to 3-benzoxepin (3).189... 

Benzoxepin undergoes addition of bromine and chlorine to give l,2-dibromo-l,2-dihydro-3-benzoxepin (2 a) and 1,2-dichloro-l, 2-dihydro-3-benzoxepin (2b), respectively.93 The bromination reaction of dimethyl 3-benzoxepin-2,4-dicarboxylate takes the same course to give 2c.91... 

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