Silacyclohex-4-enes: novel silene-cycloadducts for organic synthesis

Silenes, compounds containing a Si=c double bond, are highly reactive and are normally observed as transient intermediates which readily dimerise. Evidence for their existence was first reported in 1967 by Gusel'nikov and Flowers.(^1) However, since then only minimal effort has been made to exploit their unique reactivity in organic synthesis. This thesis describes research concerning the chemistry of silenes and more specifically their Diels-Alder adducts, silacyclohex-4-enes. These cycloadducts were utilised as building blocks for organic synthesis, enabling the total synthesis of prelactone B (R = 'Pr) and an analogue (R = Ph) to be achieved in high yields over 5 steps (Figure I) Figure 1 In addition, a unique application of the Hosomi-Sakurai reaction to the cycloadducts provides access to a unique 1,4-monoprotected diol and tetrahydronaphthalene, both possessing four contiguous chiral centres. This methodology was then applied to the total synthesis of the podophyllotoxin analogue, epipicropodophylin (Figure 2)