spirocyclization of precursor pyrrole 70 was not selective, affording spiroacetals 71
and epi-71 as a separable 1:1 mixture. Spiroacetal 71, containing the desired stereochemistry, was then elaborated to acortatarin A 37 in 5 steps in moderate yield.
Alternatively, Cossy et al. [65] have recently disclosed a Fe(III)-catalyzed
spirocyclization of a ζ-hydroxy allylic acetate, 73 (Scheme 20). Formation of
hemiacetal 74, followed by Fe(III)-catalyzed addition of the alcohol and elimination gives the doubly anomeric spiroacetal 75 in good yield and selectivity. This
method has subsequently been used in synthetic studies toward the cytotoxic
marine metabolites, the bistramides [66]. Thus, treatment of the hemiacetal 76
under the same conditions afforded the bistramide-like spiroacetal 77 in moderate
yield.
Scheme 18 Hirai et al.’s Pd-catalyzed synthesis of 5,5-spiroacetals [63]
204
M.A. Brimble and L.A. Stubbing
and epi-71 as a separable 1:1 mixture. Spiroacetal 71, containing the desired stereochemistry, was then elaborated to acortatarin A 37 in 5 steps in moderate yield.
Alternatively, Cossy et al. [65] have recently disclosed a Fe(III)-catalyzed
spirocyclization of a ζ-hydroxy allylic acetate, 73 (Scheme 20). Formation of
hemiacetal 74, followed by Fe(III)-catalyzed addition of the alcohol and elimination gives the doubly anomeric spiroacetal 75 in good yield and selectivity. This
method has subsequently been used in synthetic studies toward the cytotoxic
marine metabolites, the bistramides [66]. Thus, treatment of the hemiacetal 76
under the same conditions afforded the bistramide-like spiroacetal 77 in moderate
yield.
Scheme 18 Hirai et al.’s Pd-catalyzed synthesis of 5,5-spiroacetals [63]
204
M.A. Brimble and L.A. Stubbing