The bacterial pigment from Pseudomonas lemonnieri. Part 2. The synthesis of 3-n-octanamidopyridine-2,5,6-trione : the structure and synthesis of lemonnierin

Abstract

Oxidation of 3-n-octanamido-5-aminocitrazinic acid (8; R = NH₂) with iron(III) chloride gave a mixture of 3-n-octanamidopyridine-2,5,6-trione (1; R = H) and lemonnierin (22; R¹= R²= H) identical with the blue pigment from our strain of Ps. lemonnieri. Various derivatives of 3-n-octanamidocitrazinic acid and of (1; R = H) are described.

Condensation of (1; R = H) with the citrazinic acid (8; R = NH₂) gave 3-(6-hydroxy-5-octanamido-2-oxo-1,2-dihydro-3-pyridylimino)-6-hydroxy-5-octanamido-2-pyridone (14)(cf. Knackmussl^3,4) which differs from lemonnierin but may be converted into lemonnierin, with which it is isomeric, by copper(II) acetate-pyridine. Lemonnierin thus appears, at least in part, to be a di-free radical, of type (22; R¹= R²= H), with g= 2.004. Experiments with deuteriated intermediates together with the synthesis of the corresponding acetyl analogue (17) of the imine (14) and the acetyl analogue (25) of lemonnierin, substantiate the structure (22; R¹= R²= H) for our pigment.

Imines of type (14) are converted into the corresponding 3,3′-bipyridylidenequinone (21) by hot acetic acid, thus providing a new and improved route to these quinones.