Structural and conformational studies of derivatives of 2,4,8-trihydroxy-5-oxo-10-methylbenzo[1,2-c]chromene (alternariol). II: Crystal and molecular structure of (±)-1-methoxycarbonyl-1-hydroxy-2-oxa-3,4-dioxo-7-methoxy-9-methylcyclopenteno [3,4-c]chromene