The total synthesis of the naturally occurring (Z)-2-methoxy-5-hexadecenoic acid and (Z)-2-methoxy-6-hexadecenoic acid was accomplished using as a key step Mukaiyama's trimethylsilyl cyanide addition to4- and 5-pentadecenal, respectively. These syntheses further confirm the structures of the natural marinefatty acids and corroborate their cis double-bond stereochemistry. The title compounds were antimicrobialagainst the Gram-positive bacteria Staphylococcus aureus (MIC 0.35 μmol/mL) and Streptococcus faecalis(MIC 0.35 μmol/mL).