Pigments of Fungi. Part 70.* Total Synthesis of ( R )-Semixanthomegnin and the X-Ray Crystal Structure of (±)-7-Chloro-10-methoxy-3-methyl-3, 4-dihydro-1 H -naphtho[2,3- c ]pyran-1,6,9-trione

( R )-Semixanthomegnin (8), which has been converted by others to the mould metabolite (3 R ,3' R )-xanthomegnin (9), is prepared in seven steps from ( R )-propylene oxide (16). A single-crystal X-ray structure analysis of (±)-7-chloro-10-methoxy-3-methyl-3,4-dihydro-1 H -naphtho[2,3- c ]pyran-1,6,9-trione (25) confirms the regiochemistry of the cycloaddition reaction between 2,5-dichloro-1,4-benzoquinone and the new, highly oxygenated, chiral butadiene derivative (2).