The interaction of OsO4 and protected derivatives of
3,4-dehydro-DL-proline gives high yields of 2,3-trans-3,4-cis-3,4-dihydroxy-DL-proline derivatives, with only traces of the
2,3-cis-3,4-cis diastereoisomers. Corresponding α-glycolation with KMnO4
gives mixtures of the 2,3-trans-3,4-cis and 2,3-cis-3,4-cis isomers in the ratio 1 : 1. Separation of the components of
such mixtures is extremely difficult. The free amino acids in
these two families of the 3,4-dihydroxy-DL-prolines
have been characterized. Epimerization at C2 of 2,3-cis derivatives occurs readily. O-Tosylation
of the 2,3-cis series occurs more
readily than for other hydroxyprolines. Esters of N,O,O-tritosyl-2,3-trans-3,4-cis-3,4-dihydroxyproline are converted very rapidly by mild
alkali into the equivalent esters of X-tosylpyrrole-2-carboxylic
acid. For the corresponding N,O,O-tritosyl carboxylic
acid, decarboxylation also takes place during aromatization, to yield N-tosylpyrrole.
3,4-Dehydro-DL-proline reacts with a number of a-dicarbonyl compounds to form products in which the pyrroline ring is converted into an N-substituted pyrrole. Depending on the reactivity of the a-dioarbonyl compound, a second molecule may (e.g. ninhydrin) or may not (e.g, isatin) react further by a-substitution of the pyrrole. The structures of the products account for the different colour reactions of dehydroproline compared with proline. cis-and trans-4-Chloro-L-proline yield the same products as dehydroproline with ninhydrin and isatin.
Indole-2-carboxamide is
reduced by phosphonium iodide and fuming hydriodic acid to
DL-indoline-2-carboxamide, hydrolysis of which readily gives indoline-2-carboxylic
acid. The chemistry of this new amino acid and some of its derivatives has been
explored. Benzimidazole-2-carboxamide is inert to phosphonium iodide/hydriodic
acid.
Reduction by lithium
aluminium hydride of the adducts from the interaction of 3,4-dehydroproline and
various isatins is described. The parent mono-adduct, 3-(1-pyrrolyl)oxindole,
is normally inert but yields indole under special conditions. The 1-methyl and
1-ethyl analogues give mixtures of the corresponding 3-(1-pyrrolyl)-indoles and
3-(1-pyrrolyl)indolines. These indolines are extremely weak bases. The
di-adduct from 1-methylisatin gives products which confirm its hemiacetal
structure.
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