attenol A, the authors constructed the requisite endocyclic enol ether 100 via a
two-step Suzuki–Miyaura coupling/RCM sequence. Deprotection of the silyl ethers
with TBAF and subsequent acidic treatment afforded the 5,6-spiroacetal 101 in
good yield.
Synthesis of the C9–C28 spiroacetal fragment of didemnaketal B was achieved
using a similar strategy. Suzuki–Miyaura coupling of an alkyl borate (derived from
iodide 102 and B-methoxy-BBN) to the phosphate 103 afforded endocyclic enol
ether 104. Spirocyclization took place after cleavage of the silyl ethers with
subsequent treatment with mild acid (PPTS), affording the doubly anomerically
stabilized spiroacetal 105 in high yield.
A related Suzuki–Miyaura coupling/acidic spirocyclization sequence has
recently been extended to the synthesis of benzannulated spiroacetals by Cossy
et al. [74] (Scheme 28). The boronate 107 in this case was derived from the desired
Scheme 28 Cossy et al.’s Suzuki–Miyaura coupling approach toward benzannulated spiroacetals
[74]
210
M.A. Brimble and L.A. Stubbing