Expedient synthesis of unsaturated amide alkaloids from Piper spp: Exploring the scope of recent methodology

The Sakai aryl aldehyde – cyclic ketone aldol – Grob fragmentation sequence was extended to cinnamaldehyde and cyclohexanone, and the product was elaborated to analogues of the alkaloid piperstachine. The effects of substituents on the reaction involving cinnamaldehyde were studied. The aldol-fragmentation sequence failed with benzaldehyde when cyclooctanone or cyclobutanone was substituted for cyclohexanone or cyclopentanone, and the reasons for this failure were examined. Four-carbon Wittig homologation of the piperonal–cyclobutanone aldol-fragmentation product, a hypothetical route to alkaloids such as retrofractamide A, was thus not viable. Instead, three-carbon homologation of the readily available piperonal–cyclopentanone product, using alkyne chemistry recently disclosed by Lu and Trost, afforded these alkaloids in excellent overall yield. The alkyne isomerization was also used to effect efficient syntheses of pellitorine and several other non-aromatic 2E,4E-dienoic Piper amide alkaloids. Key words: Piper, amides, alkaloids, insecticides, aldol, fragmentation, cinnamaldehydes, alkyne, redox, isomerization.

Ion-molecule reactions of formic acid. II. Rearrangement reactions

Rate coefficients for a number of rearrangement reactions of positive ions with HCOOH are reported. These reactions may be represented by the general equation �������������������������� Y+ + HCOOH → products Most reactant ions resulted in the fragmentation product HCO+ and five reactant ions, N+, O+, CN+, N2+ and CO+, were observed to have rate coefficients much greater than predicted by the ADO model. An explanation for the high rates observed for these reactant ions is proposed and involves dissociative charge transfer by a non-orbiting electron-jump mechanism.

REARRANGEMENT STUDIES WITH 14C: XXI. SIDE CHAIN REARRANGEMENT AND FRAGMENTATION FOLLOWING FRIEDEL–CRAFTS ALKYLATION OF BENZENE WITH β-14C-ETHYL IODIDE

The aluminum chloride catalyzed reaction between β-14C-ethyl iodide and benzene in 1,2,4-trichlorobenzene as solvent was studied under different conditions of temperature and reaction time. Yields of ethylbenzene were estimated by isotopic dilution. Up to 20% rearranged product, α-14C-ethylbenzene, was found. The trend indicated that yield of ethylbenzene was lower and side chain rearrangement greater when the reaction mixture was heated at higher temperature for longer time. The rearrangement apparently occurred after the initial formation of β-14C-ethylbenzene. After a reaction time of 5 min at 154 °C, only 0.1–0.3% unreacted and unrearranged β-14C-ethyl iodide was recovered while the ethylbenzene obtained from the reaction showed about 2% rearrangement. When β-14C-ethylbenzene was heated with aluminum chloride in 1,2,4-trichlorobenzene for 2 or 24 h, rearrangement to α-14C-ethylbenzene was observed. In some experiments, a fragmentation product, toluene, was isolated. Its specific activity suggested that 1 mole of active ethylbenzene gave 2 moles of toluene only one of which was active. The mechanistic implications of these findings are discussed.