... Chemical shifts are reported as 8 in units of parts per million relative to an internal standard of tetramethylsilane. ... Ber., 101, 41 (1968); T. A. Lessen, DM Demka and SM Weinreb, Tetrahedron Letters, 31, 2105 (1990); Jean-Pierre Vors, J. Heterocyclic Chem., 28,1043 (1991). ...