[1S,1'S,4S,5R,(+)]-3',3'a,4',5',6',6'a-Hexahydro-1'α,3'aα,4'α,6'aα-tetramethylspiro[2,6-dioxabicyclo[3.1.0]hexane-4,2'(1'H)-pentalene]-3-one