V. Gevorgyan et al., Effective synthesis of aryl ethers and coumaranones employing the palladium-catalyzed enyne-diyne [4+2] cycloaddition protocol, J ORG CHEM, 65(2), 2000, pp. 568-572
Alkyl-, alkoxy-, and aryloxy-substituted conjugated enynes 1 in the presenc
e of Pd(PPh3)(4) catalyst smoothly underwent the regiospecific [4 + 2] cycl
oaddition reaction with conjugated alkyl- and alkoxy-substituted symmetric
diynes 2 to give multisubstituted aryl ethers 3 in good to high yields. Ben
zannulation of enynes 1d-g with unsymmetric diyne 6, possessing alkyl and a
lkoxy groups at acetylenic terminii, in most cases produced an aromatic pro
duct 8 with an alkoxy group of diyne attached to the ethynyl moiety of the
aromatic product. Remarkably, alkoxy-substituted diynes 2c and 6 underwent
the benzannulation reaction with 1 at an unusually low temperature of 0 deg
rees C! One-pot consecutive benzannulation of alkyl-substituted enynes 1d,e
and alkoxy-substituted enynes 1f,g with alkoxy-substituted diynes 2c and 6
followed by protonolysis with TsOH afforded coumaranones 9a-c and 10 in re
asonable to high overall yields.