An intramolecular 1,3-dipolar cycloaddition has efficiently constructed the A-ring portions of the cylindrospermopsin alkaloids. A nitro-aldol addition of an elaborated nitroalkane to a pyrimidine aldehyde followed by an intramolecular reductive guanidinylation has enabled the syntheses of all three alkaloids in this family in 18-19 steps. We report the first asymmetric synthesis of cylindrospermopsin, unambiguously assigning its absolute configuration. (c) 2006 Elsevier Ltd. All rights reserved.