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The aristolochic acids are among a group of substituted 10-nitro-1-phenantropic acids, biogenetically derived from benzylisoquinoline precursors, which in turn originate from tyrosine. The biosynthesis of aristolochic acid (1) therefore begins at tyrosine (2), which is converted to dopa (3). Dopa is subsequently converted into dopamine (4) and further converstion of dopamine into 3,4-dihydroxyphenylacetaldehyde (5) results in the formation of norlaudanosoline (6) via a spontaneous Pictet-Spengler condensation of the two molecules. Evidence of these precursors was confirmed by 14C experiments where labeled compounds were administered to Aristolochia sipho and aristolochic acid was successfully extracted. Modification of norlaudanosoline is continued with a methalation of two alcohol groups to give orientaline (7). Orientaline undergoes a cyclization event to give the addition of the 5-membered ring into orientalinol (8), subsequently undergoing a methyl shift and aromatization event (9) to yield prestephanine (10). The final steps are the formation of stephanine (11) and the addition of a carboxylate to yield aristolochic acid. Comer et. al. originally proposed a postulated pathway from norlaudanosoline to aristolochic in 1968 very similar to the described pathway but Sharma et. al. confirmed the presence of the intermediate compounds via DL-[3-14C] tyrosine trapping experiments in Aristolochia bracteata in 1982.