127154-81-8Relevant articles and documents
Formation of Δ4-oxocenes from Lewis acid promoted cyclizations of 5-hexenyl acetals. Evidence for a concerted ene cyclization mechanism
Blumenkopf, Todd A.,Look, Gary C.,Overman, Larry E.
, p. 4399 - 4403 (2007/10/02)
Both the intermolecular (kinetic) and intramolecular (product) hydrogen-deuterium isotope effects were determined to be 1.65 for the formation of 2-methyl-4-(trimethylsilyl)-Δ4-oxocene (20) from the SnCl4-promoted cyclization of acetals 19, 30, and 31 (eq 6). In other experiments silyl acetal 32 was found to cyclize in the presence of SnCl4 to form the silyl-Δ4-oxocene 34 and the alkylideneoxepane 35 in 2:1 ratio (eq 7). Both results provide strong evidence that the formation of 4-(trimethylsilyl)-Δ4-oxocenes from SnCl4-promoted cyclizations of 5-(trimethylsilyl)-5-hexenyl acetals takes place by a concerted intramolecular ene mechanism. Also reported are SnCl4-promoted exchange reactions of formaldehyde- and aldehyde-derived acetals, which occur readily at -10 to 0 °C and -70 °C, respectively (eqs 2 and 3).
