Synthesis of Isobavachalcone and Some Organometallic Derivatives

Isobavachalcone [2',4,4'-trihydroxy-3'-(3 ''-methyl-2 ''-butenyl)chalcone, 1] is a prenylated chalcone that has broad biological activity, in particular against neuroblastomas, the most common cancer in infancy. It is currently commercially available at a cost of $190/mg by extraction from Psoralea corylifolia and a number of other African and Asian plants. Several synthetic routes have been explored, and the most efficient procedure involves the palladium-catalysed Stille coupling of 3-iodo-2,4-bis(methoxymethoxy)acetophenone (25) with prenyltributyltin, Claisen-Schmidt condensation with 4-(methoxymethoxy)benzaldehyde to form the triply MOM-protected prenylchalcone 27 and finally deprotection with 2 M HCl in methanol to form isobavachalcone in an overall yield of 15% over five steps. The X-ray crystal structures of 2,4-dihydroxy-3-iodoacetophenone (21) and of several prenylated chalcones are reported, including the elucidation of their hydrogen-bonding networks in the solid state. The synthetic route has been extended to include organometallic derivatives in which the 4-(methoxymethoxy) benzaldehyde used in the Claisen-Schmidt condensation has been replaced by formylferrocene, formylruthenocene or (eta(5)-formylcyclopentadienyl)(eta(4)-tetraphenylcyclobutadiene)cobalt to form the corresponding analogues of isobavachalcone containing organometallic sandwich moieties.