2.16 Wacker-Type Cyclization
A novel Wacker-type approach toward bis(benzannulated) spiroacetals has been
reported recently by Xue and coworkers [194] (Scheme 100).
In their study, a small range of simply substituted olefins 416 were subjected to
Wacker conditions (cat. PdCl 2 /CuCl 2 , O 2 , MeOH, 60
C), giving mixtures of the
desired bis(benzannulated) spiroacetals 417 in moderate yield, together with partially cyclized benzofuran 418 (Scheme 101).
Electron-withdrawing substituents at R
1 (i.e., Ph, Cl, CO 2 Et) required longer
reaction times and resulted in a higher proportion of the unwanted benzofuran
byproduct. Electron-donating substituents (Me, t-Bu) at the same position had a
beneficial effect on the spirocyclization, presumed to be due to increased nucleophilicity of the Pd–benzofuran complex.
2.17 Cyclopropane Ring-Opening Approaches Toward
Spiroacetals
A recent addition to the range of ring-expansion strategies for the synthesis of
spiroacetals includes ring opening of spirocyclopropanes (Scheme 102).
Scheme 100 Wacker-type cyclization for the synthesis of spiroacetals
Scheme 101 Wacker-type cyclization for the synthesis of bis(benzannulated)spiroacetals [194]
Synthesis of 5,6- and 6,6-Spirocyclic Compounds
257
A novel Wacker-type approach toward bis(benzannulated) spiroacetals has been
reported recently by Xue and coworkers [194] (Scheme 100).
In their study, a small range of simply substituted olefins 416 were subjected to
Wacker conditions (cat. PdCl 2 /CuCl 2 , O 2 , MeOH, 60
C), giving mixtures of the
desired bis(benzannulated) spiroacetals 417 in moderate yield, together with partially cyclized benzofuran 418 (Scheme 101).
Electron-withdrawing substituents at R
1 (i.e., Ph, Cl, CO 2 Et) required longer
reaction times and resulted in a higher proportion of the unwanted benzofuran
byproduct. Electron-donating substituents (Me, t-Bu) at the same position had a
beneficial effect on the spirocyclization, presumed to be due to increased nucleophilicity of the Pd–benzofuran complex.
2.17 Cyclopropane Ring-Opening Approaches Toward
Spiroacetals
A recent addition to the range of ring-expansion strategies for the synthesis of
spiroacetals includes ring opening of spirocyclopropanes (Scheme 102).
Scheme 100 Wacker-type cyclization for the synthesis of spiroacetals
Scheme 101 Wacker-type cyclization for the synthesis of bis(benzannulated)spiroacetals [194]
Synthesis of 5,6- and 6,6-Spirocyclic Compounds
257