We speculated that the easier accessible hypatulin B (3) could be transformed into hypatulin A (2) via cyclization. Simplification of hypatulin B (3) by The synthesis commenced with cyclopentanone 5, which was prepared in 97% enantiomeric excess in analogy to a procedure by Taber and co-workers in four steps starting from 2-cyclopentenone. Enolization under thermodynamic control afforded silyl enol ether 6 in 94% yield and in a 93:7 ratio of constitutional isomers (Scheme 2). 8 Subsequent Mukaiyama aldol addition with aldehyde 7 proceeded in 91% yield and generated the first of three quarternary stereocenters of the cyclopentane core. A 3:2-mixture of two diastereomers was obtained, providing, after separation, one diastereomer as a crystalline solid. X-ray crystal structure analysis revealed (10R)-8 to possess the desired cis-relationship between methyl and allyl substituent. Both diastereomers were subjected to a tandem deacetalization/elimination affording enone 9. As both diastereomers 8 converge into enone 9, it can be deduced that they are diastereomers at C-10, thus rendering the low diastereoselectivity inconsequential for the further synthesis (Scheme 2). With enone 9 in hand, construction of the second quarternary center bearing a methyl ester and an allyl group was pursued. Initial attempts to install the ester first, followed by an allylation resulted in the exclusive formation of the undesired diastereomer. Thus, the order of the electrophile additions was reversed. Interestingly, allylation of the corresponding enolate with allyl iodide in the presence of N,N'-dimethylpropyleneurea (DMPU) at -78 °C gave a mixture of unreacted starting material, mono-, and bisallylated products. However, by decreasing the reaction temperature to -100 °C monoallylation was achieved exclusively. Subsequent methoxycarbonylation was performed by deprotonation with NaHMDS and treatment with methyl cyanoformate providing the desired isomer 10 in 59% over two steps (Scheme 3).