Elisapterosin B (1) is a polycyclic diterpene isolated and characterized by Rodriguez and co-workers from the West Indian sea whip Pseudopterogorgia elisabethae (Bayer). [1] Among its cogeners, it showed the most potent inhibitory activity against Mycobacterium tuberculosis H37Rv. Rodriguez and co-workers proposed that the uncommon cagelike skeleton of 1 might arise by cyclization of a serrulatane diterpene such as 2, which was isolated from the same organism (Scheme 1). [2] An alternative biosynthetic cyclization of a serrulatane core leads to colombiasin A (3), another unusual structure isolated from the P. elisabethae. [3] We became interested in the possible [5þ2] and [4þ2] cyclizations of a serrulatane skeleton as biomimetic routes to elisapterosin B (1) and colombiasin A (3), respectively. Nicolaou and co-workers recently reported the synthesis of 3 by an intramolecular Diels-Alder cyclization of a serrulatane diene. [4,5] The [5þ2] cycloaddition was discovered by Joseph-Nathan many years ago [6] but has only rarely found use in synthesis. [7, 8] Herein we outline the first synthesis of elisapterosin B (1) using an intramolecular [5þ2] cycloaddition as well as an efficient route to colombiasin A (3).