<span class="citations"><p>Hector F. DeLuca, Praveen K. Tadikonda, "Method of synthesis of retinoic acid." U.S. Patent US5808120, issued December, 1975.</p>
</span><a href="https://www.google.com/?tbm=pts#q=5808120&amp;tbm=pts" rel="noopener" target="_blank">US5808120</a>