Syntheses and Applications of Chiral a-Hydroxyphosphonates

Chiral 1-hydroxyphosphonates were prepared by enzymatic hydrolysis and the phosphate-phosphonate rearrangement and transformed into 1-aminophosphonic acids. Some of them were tested as inhibitors of the phenyalanine ammonia lyase. Phosphonic acids and their derivatives are important classes of compounds in organophosphorus chemistry. These compounds contain a carbon-phosphorus bond. Some of them, especially the 1-aminophosphonic acids, have interesting biological properties. They block enzymes und thus disrupt the metabolism of living cells. A small group of natural products are also phosphonic acids, of which the majority are antibiotics. The best known compound is fosfomycin, which is used clinically. 1-Hydroxyphosphonates are of central importance for the project. Many other phosphonates and phosphonic acids can be prepared thereof. Racemic hydroxyphosphonates can be prepared easily, but not chiral, nonracemic ones. We have used two methods to prepare them, the enzymatic resolution and the phosphate-phosphonate rearrangement. Enzymes are used in organic chemistry more and more to resolve racemates, because only antipodes are of biological relevance. We started with racemic hydroxyphosphonates, esterified them with acetic or chloroacetic anhydride and tested whether they are hydrolysed by hydrolases such as lipases and proteases in a biphasic system, consisting of phosphate buffer and an organic solvent. The released acid was neutralised with standard sodium hydroxide. The second method we used for the preparation of 1-hydroxyphosphonates was the phosphate-phosphonate rearrangement, primarily for the preparation of racemic 1-hydroxyphosphonate. Later on one enantiomer should be synthesised preferentially. The main aspect of this rearrangement is the treatment of a phosphoric acid ester with a strong base, which abstracts a proton. The species formed is very reactive and isomerises to the hydroxyphosphonates. Interestingly, this reaction can also be performed with phosporamidates which are derived from amines. Thus aminophosphonic acids are accessible directly. Some of the chiral, nonracemic 1-hydroxyphosphonates were transformed into phosphonic acid analogues of proteinogenic and nonproteinogenic amino acids. The Mitsunobu reaction was used for the introduction of the nitrogen. The protecting groups were removed and the aminophosphonic acids were purified and crystallised. The optical purity of the final products was determined by high pressure liquid chromatography on chiral stationary phases (in cooperation with Prof. W. Lindner from the Department of Analytical Chemistry of the University of Vienna). Additionally, phosphonic acids analogue of compounds structurally related to phenylalanine were prepared, which are potential inhibitors of the phenylalanine ammonia lyase. Blockers of this key plant enzyme are interesting as potential herbicides. The compounds were tested for their biological activity by two foreign cooperation partners (in Switzerland and Germany).