Permethylated pericyclynes

In cases where stable carbocations can be generated, e.g., tertiary propargylic cations, the rather low nucleophilicity of silylated terminal alkynes is still sufHcient for a coupling reaction. Ring closures have been carried out in this fashion to give the permethylated pericyclynes, which are discussed by L. T. Scott and M. J. Cooney in Chapter 9 of this monograph [8]. 

Figure 9-3 Homologation sequence for the construction of synthetic precursors to permethylated pericyclynes (a) 2 equiv n-BuLi, then 2.5 equiv TMSCl, then cone. HCl, Cu powder, CaClj, 71% overall (b) NaH, Me2S04, 80%, (c) EtMgBr, then CuCl, then 8, 83% for 9 - 10, 62-83% for each coupling in 10 - 11 (d) KOH, MeOH, 76-80% for each desilylation in 10 11.

An X-ray crystal structure of the 18-membered ring compound, 63 [24], shows that it adopts a giant chair conformation not unlike that of cyclohexane, albeit slightly less puckered, with essentially linear diynes and internal angles of 115.5° at the comers. Unlike the permethylated derivatives of exploded pericyclynes, crystals of the perspirocyclopropanated derivative, 63, explode when struck [33]. 

---