Stereocontrolled and versatile total synthesis of bispyrrolidinoindoline diketopiperazine alkaloids: structural revision of the fungal isolate (+)-asperdimin.

Homo- and heterodimeric bispyrrolidinoindoline diketopiperazine alkaloids have been synthesized following a concise, versatile, and stereoselective route. Highlights of the sequence are a diastereoselective construction of the C3a-bromo-hexahydropyrrolo[2,3-b]indole nucleus, its Co(I)-induced C3a-C3a' dimerization, and the twofold or sequential amide-bond formation before cyclization to the diketopiperazine of the homo- or heterodimeric alkaloids, respectively. Stereochemical diversity is achieved through the choice of the appropriate amino acids combined with the base-induced epimerization of the C2-acyl-hexahydropyrrolo[2,3-b]indole at C2. According to this strategy, the natural products (+)-WIN 64821 1, (+)-WIN 64745 2 and (+)-asperdimin 6 as well as analogues (5, 22, 32, 44) with different relative and absolute configuration have been efficiently synthesized. The flexibility of this synthetic methodology has facilitated the structural revision of the natural product (+)-asperdimin, whose structure has been corrected to diastereomer 6.

Mechanistic insights into the stereocontrolled synthesis of hexahydropyrrolo[2,3-b]indoles by electrophilic activation of tryptophan derivatives.

A three-step mechanism involving the formation and rearrangement of an intermediate with indoline-azetidine spirocyclic core structure was shown by DFT computations to account for the electrophilic cyclization of tryptophan derivatives to hexahydropyrrolo[2,3-b]indoles. The corresponding 3a-bromo derivatives have been obtained in high yields and synthetically useful exo/endo ratios.

Bispyridinium dienes: histone deacetylase inhibitors with selective activities.

A novel synthetic route to the cyclostellettamines 1 using as the key step a microwave-mediated macrocyclic ring-closing metathesis of precursors bispyridinium dienes 10 followed by catalytic hydrogenation has been developed. The open-chain bispyridinium dienes 10 showed uniformly higher histone deacetylase (HDAC) inhibitory potency than the natural products. Diene 10b inhibited HDAC1 and was inactive on HDAC4, whereas 10a showed a weak inhibition of HDAC1 and a higher activity on HDAC4. Neither 10b nor 10a inhibited isoforms HDAC2 and HDAC3. Cell cycle analysis, cell differentiation, and apoptosis as well as evaluation of the acetylation status of H3 lysine tails, up-regulation of p21WAF1/CIP1, and alpha-tubulin acetylation induced by the dienes 10 and cyclostellettamines 1 were also carried out on the human leukemia U937 cell line. These enzymatic and functional assays suggest that 10b is a HDAC1-selective inhibitor and 10a is a HDAC subclass IIa-selective inhibitor.