Oxetanone 1233A 1 has been synthesized in 22 steps and 3.43% overall yield from 3-methylglutaricanhydride. Asymmetric carbon at C7 was introduced by a diastereoselective esterification of3-methylglutaric anhydride, whereas the two asymmetric carbons at C2‘ and C3‘ were controlled byan asymmetric [2 + 3] cycloaddition using camphor-derived oxazoline N-oxide as dipole.