No abstract

No abstract

No abstract

Cyclization of resorcinol monoalkyl ethers with aliphatic aldehydes leads to the corresponding racemic mixtures of C4‐symmetric rccc‐resorc[4]arenes. Separation of these isomers was achieved by mono‐O‐functionalization of the rccc‐2,8,14,20‐tetramethylresorc[4]arene (2) with (S)‐(+)‐10‐camphorsulfonyl chloride leading to a diastereomeric mixture of (+)‐5a and (−)‐5b. After removal of the chiral auxiliary the inherently chiral pure enantiomers (+)‐2 and (−)‐2 were obtained. Further enantiomerically pure rccc‐resorc[4]arenes were obtained by cyclization of (+)‐3‐[(2S)‐2‐methylbutoxy]phenol (6) followed by chromatographic separation. The resulting diastereomeric resorc[4]arenes (+)‐7a and (−)‐7b were examined by CD spectroscopy, showing a perfect mirror image in all solvents examined. This indicates that the resorcarene cavities of (+)‐7a and (−)‐7b are essentially enantiomers of each other. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Lignans in higher plants represent an ideal class of natural products to be investigated for the origin of stereochemical diversity since chiral lignans occur in pure enantiomeric form as well as in enantiomeric mixtures. Seeds of Linum usitatissimum contain 8S, 8'S-(+)- and 8R, 8'R-(-)-secoisolariciresinol [SS-(+)- and RR-(-)-secoisolariciresinol, respectively] as diglucosides (SS- and RR-secoisolariciresinol diglucosides) whereas aerial parts of flowering L. usitatissimum accumulate only lignans derived from RR-(-)-secoisolariciresinol. Pinoresinol-lariciresinol reductase (PLR) catalyzes two early steps in lignan biosynthesis. Up to now, only a cDNA encoding a PLR ( PLR-LU1) which is enantiospecific for the conversion of 8S, 8'S-(-)-pinoresinol (SS-pinoresinol) via 8S, 8'S-(-)-lariciresinol (SS-lariciresinol) to SS-(+)-secoisolariciresinol was cloned. Here we present the cloning of a cDNA encoding a RR-pinoresinol-RR-lariciresinol reductase ( PLR-LU2) from the leaves of L. usitatissimum which converts only RR-pinoresinol to RR-secoisolariciresinol. In leaves and stems of L. usitatissimum accumulating the 8R, 8'R-enantiomers of lignans, only PLR-LU2 was transcriptionally active. Both PLR-LU1 and PLR-LU2 transcripts were observed in seeds and contribute to the synthesis of SS- and RR-secoisolariciresinol, respectively. Thus, the enantiomeric composition of lignans in the organs of L. usitatissimum appears to be determined by the relative action of two PLRs with opposite enantiospecificities rather than by a single enzyme of low enantiospecificity.