Endo-Selective Quenching of Hexahydropyrrolo[2,3-b]indole-Based N-Acyliminium Ions

Radical decarboxylation of L-tryptophan-derived (2S,3aR,8aS)-8-arylsulfonyl-1,2-di(methoxycarbonyl)-1,2,3,3a,8,8a-hexahydro-2H-pyrrolo[2,3-b]indoles 8 and 9 in the presence of diphenyl diselenide results in the endo-selective formation of (2R,3aR,8aS)-8-arylsulfonyl-1-methoxycarbonyl-2-phenylselenyl-1,2,3,3a,8,8a-hexahydro-2H-pyrrolo[2,3-b]indoles 10 and 11. These selenides, in conjunction with Lewis acids, serves as precursors to the corresponding N-acyl iminium ions, which undergo selective endo-face quenching by allyltributylstannane, allyltrimethylsilane, propargyltrimethylsilane, and trimethylsilylcyanide. Stereochemical assignments rest on NMR data and crystallographic studies. The endo-selective nature of these reactions is interpreted in terms of minimization of allylic strain at the transition state for nucleophilic attack on the N-acyl iminum ion.

Can an n(O) → π* Interaction Provide Thermodynamic Stability to Naturally Occurring Cephalosporolides?

The stereocontrolled synthesis of naturally occurring products containing a 5,5-spiroketal molecular structure represents a major synthetic problem. Moreover, in a previous work, the stereocontrolled synthesis of cephalosporolide E (ceph E), which presumably was obtained from its epimer congener (ceph F) through an acid-mediated equilibration process, was reported. Consequently, we performed a theoretical investigation to provide relevant information regarding the title question, and it was found that the higher thermodynamic stability of ceph E, relative to ceph F, is caused by an n → π* interaction between a lone electron pair of the oxygen atom of the spiroketal ring (n_O) and the antibonding orbital of the carbonyl group (π*_C=O). Although similar stereoelectronic interactions have been disclosed in other molecular structures, its presence in ceph E, and very likely in other related naturally occurring products, represents a novel nonanomeric stabilizing effect that should be introduced into the chemical literature.

Chiron approach toward the synthesis of the fused tricyclic core of epi-parvistemonine A

A stereoselective synthesis of fused tricyclic framework of epi-parvistemonine A from D-glucono-δ-lactone is described. The synthetic strategic is based on the stereoselective construction of the 7-membered cyclic skeleton via a cross-metathesis reaction followed by a Michael type cyclization promoted by Tf2O.

Synthetic Approach to Chromone and Flavonoid Piperidine Alkaloids

Despite the enormous importance of chromone and flavonoid piperidine alkaloids, a general method for their synthesis has not been described. Accordingly, from simple tetrahydro-3-pyridinemethanols (A) and phenol derivatives (B), a synthetic approach to chromone and flavonoid piperidine alkaloids is presented. The access to a novel chromone and flavonoid alkaloid precursors 4-(2-hydroxyphenyl)-3-methylenepiperidines (C) is achieved in only two steps: Mitsunobu reaction followed by an intramolecular C-H phenolization via an aromatic Claisen rearrangement of the respective Mitsunobu adducts (D). Consequently, the simultaneous installation of the functionalized phenol group and the exo-methylene group within the piperidine skeleton, permits, not only the easy construction of the chromone or flavonoid cores but also the simultaneous installation of the hydroxyl group with the required cis-orientation. Additionally, the synthetic utility of this novel approach is showcased in the formal synthesis of flavopiridol, rohitukine, and their N-Moc analogues.