Pyrrolizidine alkaloids. A concise entry to (−)-pyrrolam A

Total Synthesis of (-)-Kopsifoline A and (+)-Kopsifoline E

We report the first total synthesis of (-)-kopsifoline A and (+)-kopsifoline E. Our synthetic strategy features a biogenetically inspired regioselective C17-functionalization of a versatile intermediate containing the pentacyclic core of aspidosperma alkaloids. The vinylogous urethane substructure of this intermediate affords (-)-kopsifoline D via C3-C21 bond formation under the Mitsunobu reaction conditions, while it enables selective C17-functionalization en route to (-)-kopsifoline A and (+)-kopsifoline E.

Synthesis of (−)-(R)-Pyrrolam A and Studies on Its Stability: A Caveat on Computational Methods

The asymmetric synthesis of (-)-(R)-pyrrolam A was achieved in three operations from N-Boc pyrrolidine via an alpha-(N-carbamoyl)alkylcuprate vinylation reaction followed by N-Boc deprotection and cyclization. One-pot deprotection-cyclization procedures led to mixtures of pyrrolam A and its double bond isomers. These isomerization events could be circumvented by use of a two-step procedure. To guide aspects of the experiments, a series of computational energy evaluations and chemical shift predictions were performed with molecular mechanics, semiempirical, ab initio, and density functional methods. The relative stabilities of the double bond isomers, as estimated by experiment, challenged a number of computational methods, and only the MP2 model with its moderate degree of electron correlation came close to matching the experimental data. The MP2 method was further applied to an unusual aspect of the double bond migration between pyrrolam A and its isomer 9. The reaction (1 to 9) on neat samples is irreversible without racemization, and the alumina-mediated equilibration is accompanied by complete loss of enantiomeric excess. The source of the irreversibility was traced to asymmetric charge distribution in the intermediate dienolate anion. The analysis ultimately led to a semiquantitative sketch of the pyrrolam energy surface.

Stereoselective Carbon−Carbon Bond Formation via the Mitsunobu Displacement of Chiral Secondary Benzylic Alcohols

[reaction: see text] The stereoselective displacement of a variety of chiral benzylic alcohols with triethylmethanetricarboxylate (TEMT) under Mitsunobu conditions (DEAD, PMe(3)) has been demonstrated to proceed in good yield (70-94%) and with a high degree of inversion. Subsequent saponification and decarboxylation of the products thus obtained provide chiral 3-aryl-3-substituted propanoic acids without racemization.