A Pd(0)-mediated indole (macro)cyclization reaction

Herein we report a systematic study of the Larock indole annulation designed to explore the scope and define the generality of its use in macrocyclization reactions, its use in directly accessing the chloropeptin I versus II DEF ring system as well as key unnatural isomers, its utility for both peptide-derived and more conventional carbon-chain based macrocycles, and its extension to intramolecular cyclizations with formation of common ring sizes. The studies define a powerful method complementary to the Stille or Suzuki cross-coupling reactions for the synthesis of cyclic or macrocyclic ring systems containing an embedded indole, tolerating numerous functional groups and incorporating various (up to 28-membered) ring sizes. As a result of the efforts to expand the usefulness and scope of the reaction, we also disclose a catalytic variant of the reaction, along with a powerful Pd(2)(dba)(3)-derived catalyst system, and an examination of the factors impacting reactivity and catalysis.

Synthesis and stereochemical determination of complestatin A and B (neuroprotectin A and B)

Recently, we reported the first total synthesis of chloropeptin II (1, complestatin), the more strained and challenging of the two naturally occurring chloropeptins. Central to the design of the approach and by virtue of a single-step, acid-catalyzed ring expansion rearrangement of chloropeptin II to chloropeptin I, the route also provided a total synthesis of chloropeptin I. Herein, we report a complementary and divergent oxidation of chloropeptin II (1, complestatin) to either complestatin A (2, neuroprotectin A) or complestatin B (3, neuroprotectin B), providing the first synthesis of the natural products and establishing their remaining stereochemical assignments. Key to the approach to complestatin A (2, neuroprotectin A) was the development of two different single-step indole oxidations (HCl-DMSO and NBS, THF-H(2)O) that avoid the rearrangement of chloropeptin II (1) to chloropeptin I (4), providing the 2-oxindole 2 in superb yields (93% and 82%). With a mechanistic understanding of features that impact the latter oxidation and an appreciation of the intrinsic reactivity of the chloropeptin II indole, its modification (NCS, THF-H(2)O; Cs(2)CO(3), DMF-H(2)O) provided a two-step, single-pot oxidation of chloropeptin II (1) to afford directly the 3-hydroxy-2-oxindole complestatin B (3, neuroprotectin B). Extensive studies conducted on the fully functionalized synthetic DEF ring system of chloropeptin II were key to the unambiguous assignment of the stereochemistry as well as the exploration and subsequent development of the mild oxidation conditions used in the synthesis of complestatin A and B.

Total synthesis of complestatin: development of a Pd(0)-mediated indole annulation for macrocyclization

Full details of the initial development and continued examination of a powerful intramolecular palladium(0)-mediated indole annulation for macrocyclization closure of the strained 16-membered biaryl ring system found in complestatin (1, chloropeptin II) and the definition of factors impacting its intrinsic atropodiastereoselectivity are described. Its examination and use in an alternative, second-generation total synthesis of complestatin are detailed in which the order of the macrocyclization reactions was reversed from our first-generation total synthesis. In this approach and with the ABCD biaryl ether ring system in place, the key Larock cyclization was conducted with substrate 36 (containing four phenols, five secondary amides, one carbamate, and four labile aryl chlorides) and provided the product 37 (56%) exclusively as a single atropisomer (>20:1, detection limits) possessing the natural (R)-configuration. In this instance, the complexity of the substrate and the reverse macrocyclization order did not diminish the atropodiastereoselectivity; rather, it provided an improvement over the 4:1 selectivity that was observed with the analogous substrate used to provide the isolated DEF ring system in our first-generation approach. Just as significant, the atroposelectivity represents a complete reversal of the diasteroselectivity observed with analogous macrocyclizations conducted using a Suzuki biaryl coupling.

Converting gem-dimethyl groups into cyclopropanes via Pd-catalyzed sequential C-H activation and radical cyclization

A novel route to the synthesis of cyclopropane derivatives is described. 1,1-Dimethyls in 2-(1,1-dimethylalkyl)dimethyloxazolines are first converted into 1,3-diiodide derivatives via Pd-catalyzed sequential C-H activation and then radically cyclized to provide 2-(1-alkylcylclopropyl)dimethyloxazolines. The use of EtOAc as a solvent is crucial for the diiodination of the functionalized substrates. [reaction: see text]