- A practical entry to the crambescidin family of guanidine alkaloids. Enantioselective total syntheses of ptilomycalin A, crambescidin 657 and its methyl ester (neofolitispates 2), and crambescidin 800
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Among the most structurally remarkable guanidine natural products are the crambescidin/ptilomycalin A family of marine alkaloids. The evolution of a practical strategy for preparing pharmacologically significant crambescidin/ptilomycalin A alkaloids that lack oxidation at C13 is described. The first total syntheses of crambescidin 800 (2), crambescidin 657 (6), and neofolitispate 2 (7) are reported in full detail. The central strategic step in these convergent total syntheses is tethered Biginelli condensation of β-keto ester 24 with ureido aminal 61 to combine all carbons of the guanidine nucleus and set the pivotal C10-C13 stereorelationship. The total synthesis of crambescidin 800 (2) was accomplished in 3% overall yield from commercially available 3-butyn-1-ol by way of 16 isolated and purified intermediates. Full details of our earlier total synthesis of ptilomycalin A (1) are also presented. The total syntheses described in this disclosure confirm the stereochemical assignments of 1, 2, 6, and 7 and rigorously establish that the absolute configuration of the hydroxyspermidine side chain of crambescidin 800 (2) is S.
- Coffey, D. Scott,McDonald, Andrew I.,Overman, Larry E.,Rabinowitz, Michael H.,Renhowe, Paul A.
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p. 4893 - 4903
(2007/10/03)
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