The ethyl ester of propionic acid (1) was brominated and then converted to the Wittig reagent using triphenylphosphine. Aldehyde 6 was obtained from allyl alcohol (4) by protection as the tert-butyldiphenylsilyl ether (5) followed by ozonolysis. Wittig reagent 3 and aldehyde 6 reacted in a Wittig reaction to give unsaturated ester 7, which was deprotected to give dienophile 8 (Scheme 1, compound 1).