Explorations on the Total Synthesis of the Unusual Marine Alkaloid Chartelline A
- 7 March 2006
- journal article
- research article
- Published by American Chemical Society (ACS) in The Journal of Organic Chemistry
- Vol. 71 (8), 3159-3166
- https://doi.org/10.1021/jo060084f
Abstract
In work directed toward a total synthesis of chartelline A (1a), a strategy was investigated to construct the 10-membered ring of this marine alkaloid via an intramolecular aldehyde/β-lactam cyclocondensation to form the macrocyclic enamide functionality. Therefore, spiro-β-lactam and imidazole fragments were first prepared. Tribromooxindole β-lactam 24 was synthesized from commercially available 5-nitroisatin (18) in seven steps and 30% overall yield via a Staudinger ketene−imine [2 + 2]-cycloaddition strategy. The requisite 2-bromoimidazole subunit 40 bearing a terminal alkyne and a masked aldehyde was efficiently prepared from the readily available imidazole ester 25 in 10 steps. With both advanced intermediates available, the addition of the lithium acetylide generated from 2-bromoimidazole subunit 40 to the γ-lactam carbonyl group of N-Boc-tribromooxindole 24 was investigated, affording the desired N-Boc-aminal 41. Hydrolysis of the acetonide moiety of 41, followed by oxidative cleavage of the resulting diol, gave the aldehyde 42. Unfortunately, treatment of aldehyde 42 with p-toluenesulfonic acid did not give the desired 10-membered macrocyclic (Z)-enamide 46, but rather the highly unsaturated seven-membered ring compound 44.Keywords
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