Rivkin, A and Gonzalez-Lopez De Turiso, F and Nagashima, T and Curran, DP  (2004) Radical and palladium-catalyzed cyclizations to cyclobutenes: An entry to the BCD ring system of penitrem D.  Journal of Organic Chemistry, 69 (11).  3719 - 3725.  ISSN 0022-3263