Preparation and crystal and molecular structure of erythro(1aα,2α,4aβ)-2-hydroxy-4a-methyl-1a,2,4,4a,5,6,7, 8-octahydro-3H-naphth-[1,8a-b]-oxirene-2β-(2′-methyl)-acetic acid methyl ester, C 15 H 24 O 4