Asymmetric syntheses of 7, a substance isolated from incubation of arachidonic acid with an acetone powder of the coral Plexaura homomalla, and of constanolactones A (9) and B (10), metabolites of the red alga Constantinea simplex, are described. The key step involves stannic chloride-mediated cyclization of the oxirane derived from Sharpless epoxidation of 10-hydroxy-5,8-decadienoates, and is shown to yield a trans disubstituted cyclopropane Linked to a delta-lactone. Both (R) and (S) configurations at C5 of the lactone are produced in the cyclization, the ratio being independent of olefin geometry in the acyclic precursor, Elaboration of the cyclization products via Nozaki-Kishi coupling of the derived aldehydes 26 and 37 with the appropriate (1E,5Z)-1-iodo-1,5-undecadiene led to 7, 9, and 10, thereby establishing the relative configuration of 7 and confirming the absolute configuration of 9 and 10.