General route to 4a-methylhydrofluorene diterpenoids: Total syntheses of (plusmn)-taiwaniaquinones D and H, (plusmn)-taiwaniaquinol B, (plusmn)-dichroanal B, and (plusmn)-dichroanone

A convenient route for the total synthesis of a number of hydrofluorene natural products and their analogues is developed. The protocol is followed to realize the total synthesis of (plusmn)-dichroanal B, (plusmn)-dichroanone, (plusmn)-taiwaniaqunol B and (plismn)-taiwaniaquinones D and H.