1. a) B.A. Omar, S.C. Flores and J.M. McCord, Adv. Pharmacol., (San Diego) 1992, 23, 109;
2. D.T. Sawyer and J.S. Valentine, Acc. Chem. Res., 1981, 14, 393.
3. a) E.J. Corey, K.C. Nicolaon and M. Shibasaki, J. Chem. Soc., Chem. Comm., 1975, 658.
4. M. Singh and R.A. Misra, Synthesis, 1989, 403;
5. M. Singh, K.N. Singh, S. Dwivedi and R.A. Misra, Synthesis, 1991, 291;