methyl (3R)-4-tert-butyldiphenylsiloxy-3-(5-phenylthio-2-furoyloxy)-2-oxobutanoate

(5R,8R,9S)-8-(tert-butyldiphenylsiloxymethyl)-9-hydroxy-9-methoxycarbonyl-1,7-dioxaspiro[4.4]non-3-ene-2,6-dione

(5S,8R,9R)-8-tert-butyldiphenylsiloxymethyl-9-hydroxy-9-methoxycarbonyl-1,7-dioxaspiro[4.4]non-3-ene-2,6-dione