Biosynthesis of a syringyl 8-O-4′ neolignan in Eucommia ulmoides:: formation of syringylglycerol-8-O-4′-(sinapyl alcohol) ether from sinapyl alcohol

To investigate the biosynthesis and stereochemistry of syringylglycerol-8-O-4'-(sinapyl alcohol) ether (SGSE), a syringyl 8-O-4' neolignan, feeding experiments and enzyme assays using Eucommia ulmoides were carried out. Diastereoselective formation of erythro-SGSE was found. When [8-C-14] sinapyl alcohol was administered to excised shoots of E. ulmoides, C-14 was incorporated into free SGSE and SGSE glucosides. In stems, incorporation into (+)-erythro-[C-14] SGSE (0.037%) with 9.1% enantiomeric excess (% e.e.) was found; incorporation into the threo isomer was not detectable. Erythro-[C-14] SGSE glucosides (0.047%) dominated over threo forms (0.007%) with 74.0% diastereomeric excess (% d.e.); both diastereomers were levorotatory with 32.0% e.e. and 18.3% e.e., respectively. In leaves, higher incorporation into H-erythro-[C-14] SGSE (0.500%, 15.9% e.e.) than into the threo isomer (0.206%, 7.4% e.e.) was observed (41.6% d.e.). (-)Erythro-[C-14]SGSE glucosides (1.692%, 25.0% e.e.) were produced at higher rates than threo isomers (0.177%, 16.4% e.e.) with 81.0% d.e. In incubations of a mixture of [8-C-14]sinapyl and [8-C-14]coniferyl alcohols with an insoluble enzyme preparation from stems of E. ulmoides, erphro-SGSE was preferentially produced. The highest % d.e. (82.8) was observed at 60min with the (+)-erythro isomer (21.4% e.e.) and the (-)-threo form (4.3% e.e.).