Isousnic acid

Fig. 48. A Didymic acid, GE. Bar lOjim. B Isousnic acid, GE. Bar lOOiLim. C Isousnic acid. An. Bar 100 tm. D Placodiolic acid, GE. Bar 100 jim...

StL (+)-Isousnic aid Cladonia mitis Sandst., (-)-isousnic acid Cladonia pleurota (Florke) Schaer. 

Huneck S, Akinniyi JA, Cameron AF, Connolly JD, MulhoUand AG (1981) The absolute configurations of (+)-usnic and (+)-isousnic acids. X-Ray analyses of the (-)-a-phenyl-ethylamine derivative of (+)-usnic acid and of (-)-pseudoplacodiolic acid, a new dibenzofuran from the lichen Rhizoplaca chrysoleuca. Tetrahedron Lett 22 351-352... 

Taguchi H, Shibata S (1970) The structure of isousnic acid with reference to isodihydrousnic acid derived from dihydrousnic acid. Chem Pharm BuU 18 374-378... 

The isousnic acids are known only in Cladonia, but extensive surveys for the distribution of these compounds have not yet been reported. In Cladonia, both (+)-and (—)-usnic acids occur, but in this genus as in others, both optical isomers are never found in a single plant. Some genera produce only one optical isomer. It is probable that the methylphloroacetophenone unit of usnic acid is a common aromatic lichen product for which mechanisms of coupling and cyclodehydration have arisen evolutionarily many times. 

Scheme 61. Conversion of (— )-placodiolic acid to ( —)-isousnic acid...

Three-dimensional crystal structures of usnic acid and 2-desacylusnic acid were determined by X-ray diffraction 236, 259), but it was not until 1981 that Huneck etal. 178) established the absolute configurations of the usnic acids. X-Ray diffraction of the (— )-a-phenylethylimine of (H-)-usnic acid established the configuration 4a(R) for this isomer. By utilising the relationship of (-f-)-usnic acid and (+ )-isousnic acid to (— )-dihydrousnic acid (Scheme 63), (H-)-isousnic acid must also possess a 4a(R) configuration. 

A synthesis of usnic acid derivatives, incorporating a Diels-Alder reaction similar to that of the synthesis of di-O-methylstrepsilin (Scheme 58), was reported by Elix and Tronson 133). Extensive studies on the chemistry of usnic acid and derivatives have been conducted by Kutney etal. 217—225) and Takahashi etal. 315—321, 324, 326). Included in the work of Kutney etal. was a base-catalysed usnic acid-isousnic acid rearrangement 224, 225), which constituted the first synthesis of isousnic acid (476) (Scheme 64). A mechanism for this rearrangement was proposed (221). 

Scheme 64. Base catalysed usnic acid-isousnic acid rearrangement...

Chemistry of some Yellow Acarospora Species. Lichenologist 12, 239 (1980). Huneck, S., J. A. Akinniyi, A. F. Cameron, J. D. Connolly, and A. G. Mulholland The Absolute Configurations of ( + )-Usnic and ( + )-Isousnic Acid. X-Ray Analyses of the (— )-a-Phenylethylamine Derivative of ( + )-Usnic Acid and of ( —)-Pseudo-placodiolic Acid, A New Dibenzofuran, from the Lichen Rhizoplaca chrysoleuca. Tetrahedron Letters 22, 351 (1981). 

StudiesintheUsnicAcidSeries. V. The Base Catalysed Usnic Acid-Isousnic Acid Rearrangement. Part III. (— )-Usnic Acid Isomethoxide Monoacetate. Can. J. Chem. 55, 1079 (1977). 

The structure of (—)-placodiolic acid from Lecanora mbina (Vill.) Ach. has been established as (—)-isousnic acid isomethoxide (51a). 

Kinoshita Y, Yamamoto Y, Yoshimura 1, Kurokawa T, Huneck S (1997) Distribution of Optical Isomers of Usnic and Isousnic Acids Analyzed by High Performance Liquid Chromatography. J Hattori Bot Lab 83 173... 

Kutney JP, Sanchez IH, Yee T (1977) Studies in the Usnic Acid Series. IV. The Base Catalyzed Usnic Acid - Isousnic Acid Rearrangement. Part II. An Improved Synthesis of (+)-Isousnic Acid. Can J Chem 55 1073... 

---