substitution of the sulfoximines 141 generates the exocyclic enol ethers 143.
α-Lithiation and subsequent addition of an epoxide incorporates the alkyl chain,
and finally, an intramolecular oxa-Michael addition affords the 5,5-spiroacetals 147.
The sulfoximine-substituted spiroacetals 147 were obtained in good yield using
this approach. Reductive removal of the sulfoximine over Al/Hg amalgam afforded
the simple 5,5-spiroacetals 149 in moderate to high yield, as approximately 1:1
mixtures of diastereomers.
Alternatively, Mootoo and coworkers [82, 83] have developed a two-step iodoetherification/Ag-catalyzed cyclization method for the synthesis of spiroacetals from
homoallylic alcohols 150 (Scheme 37). Treatment of the diol with iodonium dicollidine
perchlorate (IDCP), followed by exposure to silver triflate in the presence of collidine
affords the spiroacetals 154 in moderate to good yield as diastereomeric mixtures. This
method was most recently applied in their synthesis of a simplified monensin analogue,
polyether spiroacetal 156 [84]. Thus, two-step treatment of diol 155 under the
established conditions provided spiroacetal 156 as a 2:1 mixture of epimers.
2.6 Transition Metal-Catalyzed Hydroalkoxylation
of Internal Alkynes
The transition metal-catalyzed intramolecular hydroalkoxylation approach for the
synthesis of spiroacetals has increased in popularity in the last decade. A variety of
Scheme 34 Spirocyclization of an exocyclic enol ether
Scheme 35 Goekjian et al.’s approach toward the spiroacetal of bistramide A [79]
Synthesis of 5,6- and 6,6-Spirocyclic Compounds
215
α-Lithiation and subsequent addition of an epoxide incorporates the alkyl chain,
and finally, an intramolecular oxa-Michael addition affords the 5,5-spiroacetals 147.
The sulfoximine-substituted spiroacetals 147 were obtained in good yield using
this approach. Reductive removal of the sulfoximine over Al/Hg amalgam afforded
the simple 5,5-spiroacetals 149 in moderate to high yield, as approximately 1:1
mixtures of diastereomers.
Alternatively, Mootoo and coworkers [82, 83] have developed a two-step iodoetherification/Ag-catalyzed cyclization method for the synthesis of spiroacetals from
homoallylic alcohols 150 (Scheme 37). Treatment of the diol with iodonium dicollidine
perchlorate (IDCP), followed by exposure to silver triflate in the presence of collidine
affords the spiroacetals 154 in moderate to good yield as diastereomeric mixtures. This
method was most recently applied in their synthesis of a simplified monensin analogue,
polyether spiroacetal 156 [84]. Thus, two-step treatment of diol 155 under the
established conditions provided spiroacetal 156 as a 2:1 mixture of epimers.
2.6 Transition Metal-Catalyzed Hydroalkoxylation
of Internal Alkynes
The transition metal-catalyzed intramolecular hydroalkoxylation approach for the
synthesis of spiroacetals has increased in popularity in the last decade. A variety of
Scheme 34 Spirocyclization of an exocyclic enol ether
Scheme 35 Goekjian et al.’s approach toward the spiroacetal of bistramide A [79]
Synthesis of 5,6- and 6,6-Spirocyclic Compounds
215