A facile and efficacious route to the benzooxabicyclo[3.2.1]octane system has been developed and applied to a synthesis of filiformin (1). The cycloaddition of ethylene to the methoxychromone 13 furnished the oxetanol 14 through a tandem cycloaddition and gamma-hydrogen abstraction sequence. Lithium aluminum hydride reduction to the diol 15 followed by acid-catalyzed rearrangement produced benzooxabicyclooctanone (16), arising from exclusive external bond migration. Similarly, ethoxychromone (17) under the same sequence of reactions afforded the homologous bridged ketone 20. For the synthesis of filiformin (1), methoxychromone 24 on ethylene cycloaddition followed by reduction of resultant oxetanol 25 with lithium aluminum hydride furnished diol 10. Acid-catalyzed rearrangement of 10 provided the bridged ketone 11 which was brominated to give 26. This bromo ketone had previously been converted to filiformin (1), and also aplysin 9, and hence, the present work represents a short, high-yield formal synthesis of these sequiterpenes from a single starting material.

Acid-catalyzed rearrangement of cyclobutachromanol (9) furnished the benzo-1,3-dioxane 13, arising from a de Mayo-type rearrangement. The structure of 13 was also proved by synthesis. Similarly, rearrangement of cyclobutachromanol 10 resulted in the benzo-1,3-dioxane compound 14. The benzo-1,3-dioxane ring system is the characteristic structural feature of averufin, an intermediate in the biosynthesis of aflatoxins.

A short and convenient route to the synthesis of A-ring aromatic trichothecene analogue 2 is described, employing a cyclobutyl carbinol rearrangement as the key step. Cycloaddition of ethylene to the methoxychromone 6 furnished the oxetanol 7 along with some cycloadduct 8, the latter arising from cleavage of 7. Lithium aluminum hydride reduction of 7 to the diol 9 followed by acid-catalyzed rearrangement afforded the benzooxabicyclo[3.2.1]octanone 10, through exclusive external bond migration. Interaction of 10 with dimethyloxosulfonium methylide furnished the desired anti-epoxide 2, whereas dimethyl sulfonium methylide yielded the syn-epoxide 12. Reduction of these epoxides provided the alcohols 13 and 14, respectively. Addition of methylmagnesium iodide to ketone 10 furnished exclusively alcohol 14, supporting the stereoassignments.

Acid-catalysed rearrangement of cyclobutachromanols 4 and 5 furnishes the benzo-lr3-dioxane compounds 6 and 7, respectively, arising from a de Mayo-type rearrangement.Averufin 1,' the pivotal intermediate in the biogenesis of

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