An Efficient Palladium Catalyzed Stereocontrolled Synthesis of 4a-Angular Methyl Substituted Hydrofluorenes

Heteroaryl radicals. Part 1. Synthesis of linear pyridine-fused ring systems by endo-selective 2-pyridyl radical cyclizationsThis paper has been dedicated to Professor U. R. Ghatak on the occasion of his 70th birthday.

“…The assigned structure of product 8 is based upon 1 H and 13 C NMR analysis, and the stereochemistry was deduced by analogy. 16 Cyclization of 4 was carried out with different concentrations of n-Bu 3 SnH and the products analysed by GC (Table 1). This showed the formation of the exo product 8 (in minor amounts) and the debrominated olefin 9 besides the endo product 7, the ratio being dependent on the concentrations employed.…”

The Taiwaniaquinoids: A Review

“…As shown in Scheme , the cyclization precursor 82 was prepared by coupling benzyl bromide 81 (derived from vanillin) to Hagemann’s ester ( 80 ), followed by hydrolytic decarboxylation of the C-4 ester, conjugate addition of a methyl group to C-5, and olefination of the C-1 carbonyl. This sequence is also similar to Ghatak’s synthetic strategy . After cyclization of 83 to 84 , the C-12 benzyl group was replaced with an acetyl moiety, followed by a Fries rearrangement to install an acyl moiety at C-13 ( 84 → 85 ).…”

“…These were converted exclusively to 4a-methylhydrofluorenes (e.g., 58), offering a flexible and direct approach to this framework. 29 In contrast, Bailey studied the formation of 4amethylhydrofluorenes by anionic attack of aryllithiums tethered to substituents bearing an exomethylene (e.g., 59 f 60, eq 13). 30 Ishibashi and co-workers, 31 while investigating factors controlling endo-versus exo-selection in radical cyclizations, showed that exo vinyl sulfide 61 gave only exo-cyclization to 62; the thio group was removed with Raney-nickel to give 60 (eq 14).…”

“…This sequence is also similar to Ghatak's synthetic strategy. 29 After cyclization of 83 to 84, the C-12 benzyl group was replaced with an acetyl moiety, followed by a Fries rearrangement to install an acyl moiety at C-13 (84 f 85). Completion of the isopropyl unit and reprotection to form 86 required four transformations.…”

“…Ghatak’s work on hydroanthracene derivatives led him to investigate intramolecular Heck reactions of exo-olefins such as 57 (eq 12). These were converted exclusively to 4a-methylhydrofluorenes (e.g., 58 ), offering a flexible and direct approach to this framework . In contrast, Bailey studied the formation of 4a-methylhydrofluorenes by anionic attack of aryllithiums tethered to substituents bearing an exomethylene (e.g., 59 → 60 , eq 13) .…”