Synthetic Applications of Cationic Rearrangements and Cyclizations

摘要

A collection of nonplanar polycyclic aromatic hydrocarbons was synthesized using cationic rearrangements and cyclizations. Dibenzo[g,p]chrysene (DBC) was synthesized by the pinacol pinacolone Stone-Wales rearrangement route (PPSW) and its central double bond reactivity was investigated. Symmetrical extension of DBC along the aromatic ring leads to 3 structural isomers of C42H24:bis[5]helicene, bispentaphene, and bispicene. Bis[5]helicene was successfully synthesized by photocyclization and acid-catalyzed oxidative cyclization from ?-tetranaphthylethylene. Multiple synthetic routes were designed for the synthesis of bispicene including the PPSW route, McMurry coupling of 1,1'-dinaphthylmethanone, nucleophilic coupling of 1,1-dinaphthylmethane carbanion and 1,1'dinaphthylmethanone, and cross coupling reaction; however, all attempts were unsuccessful. The PPSW multistep sequence was also used to synthesize 5 novel pinacol derivatives and 3,6,11,14-tetrabromodibenzo[g,p]chrysene. In addition to the synthesis of nonplanar aromatic hydrocarbons, a broadly applicable synthetic methodology was developed in which fluorene derivatives were synthesized from diarylmethanols by Nazarov-type cyclization in polyphosphoric acid using a microwave reactor. Density functional theory (DFT) was used to calculate the free energy barriers and to reliably predict the regioselectivity of these cyclizations. 1,2-Acyl group migrations in aromatic ketones was studied. In triflic acid, 1-acetonaphthone rearranges to 2-acetonaphthone. Similar results were observed with 1-benzoylnaphthalene, 1,1'-dinaphthylmethanone, and 1,2'-dinaphthylmethanone. Theory was used to understand the mechanism of the rearrangement and the relative propensity for deacylation. It is proposed that the rearrangement proceeds through ipso arenium ions and a pi-complex. Computational results indicate both the endo and exo mechanisms to have reasonable energy barriers to be operative. Theory was also used to understand the reaction mechanism and stereoselectivity observed in the biomimetic synthesis of griffipavixanthone, a natural product with a dimeric xanthone core. Computational results support a cationic cascade mechanism which involves an ionic Diels-Alder reaction followed by arylation. Product stereochemistry is determined in the arylation step. Using methanol as a model for serine, the ring opening of a series of carbapenem derivatives was modeled by DFT methods. This is a multistep process in which predicted energetics is highly dependent on the placement of heteroatoms.