cyclopentenone. The synthetic key challenge was to construct the unique bicyclic core motif in an efficient way, Ester 16 was then subjected to cross metathesis conditions. 29 Initial reactions using 2-methyl-2-butene as a dimethylvinylcarbene source resulted in a mixture of products with dimethylated and monomethylated allyl groups. Therefore, the metathesis reaction was performed with gaseous 2-methylpropene in a pressure tube providing 3-epi-hypatulin B (17) in 94% yield (Scheme 4). In conclusion we completed a synthesis of 3-epi-hypatulin B (17) in 16 steps starting from 2-which was achieved by introducing all required substituents to a cyclopentane core, followed by a Mukaiyama aldol cyclization. The introduction of the last carbonyl fragment lead to the undesired epimer of the tertiary alcohol. However, synthesis of 3-epi-hypatulin B could be achieved utilizing an oxidative cleavage of an enol ether species in presence of competing alkene functionalities. This reaction could be performed in flow allowing larger scale, shorter reaction time and a slightly higher yield. Our synthesis demonstrates the value of combining an efficient approach to a highly functionalized ring system with the virtue of flow chemistry for the robustification of late-stage transformations in natural product synthesis.