A total synthesis of allamcin. An approach to antileukaemic iridoid lactones and formal syntheses of plumericin and allamandin

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Abstract
A total synthesis of the iridoid lactone (±)-allamcin (3) found in Allamanda neriifolia is described. Starting from the readily available bicyclo[3.3.0]octenone (10), the synthesis uses a strategy based on: (i) elaboration of the key acetoxy-aldehyde intermediate (9) through epoxidative rearrangement of the enol acetate (71) derived from (10)via the unsaturated aldehyde (57), (ii) fusion of the β-oxy-γ-butyrolactone ring system on to (9) using the dianion (20) derived from 2-phenylthiobutanoic acid, (iii) chemo-selective vic-bishydroxylation of the ring B carbon-to-carbon double bond in (75), using osmium tetraoxide, leading to (76), (iv)in situ oxidation and elimination of phenylsulphenic acid from (76) producing the (E)-ethylidenefuranone (78), and (v) oxidative-cleavage and cyclisation from the 1,2-diol (78) in the presence of sodium metaperiodate. Allamcin was also produced when the allylic transposed intermediate (84) derived from compound (83)(MeOH–K2CO3) was treated with periodic acid. Since allamcin has previously been converted into plumericin (4) and allamandin (5), the synthesis of compound (3) also constitutes formal syntheses of (±)-(4) and (±)-(5).

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