The crystal structure of (2a′S,2a1′S,3R,5a′S,7′R)-5-(furan-3-yl)-2a′,2a1′-dihydroxy-7′-methyldecahydro-2H-spiro[furan-3,6′-naphtho[1,8-bc]furan]-2,2′(2a′H)-dione, C19H22O7