“…They are often present as structural sub-units in natural and non-natural products (Rappoport, 1987;Burke and Grieco, 1979;Hudlicky et al, 1985;Ho, 1988;Burgess and Ho, 1994), are frequently used as mechanistic probes to elucidate reaction pathways (Suckling, 1988;Silverman et al, 1993;Newcomb and Chestney, 1994;Caldwell and Zhou, 1994;Husbands et al, 1994), and are increasingly valuable as synthetic intermediates (Wong et al, 1989;Davies, 1991;Reissig, 1995).…”