CHIRAL ETA(6)-ARENE-CR(CO)(3) COMPLEXES IN ORGANIC-SYNTHESIS - A SHORT AND HIGHLY SELECTIVE SYNTHESIS OF THE 18-NOR-SECO-PSEUDOPTEROSIN AGLYCONE

The chiral synthetic building block 5,6-dimethoxy-1-tetralone-Cr(CO)(3 ) (7; >99% e.e.) was converted in only nine steps and with high regio- and diastereocontrol into the 18-nor-seco-pseudopterosin aglycone 3 ( 50% overall yield). The synthesis is centrally based on the specific r eactivity of the arene-Cr(CO)(3) substructure, especially on the stabi lization of negative charge in benzylic position. The trans-configurat ion of the two benzylic substituents is secured by diastereoselective protonation of an anionic intermediate generated by conjugate addition of 4-methyl-3-pentenyl-lithium to complex 9, prepared from 7 via Pete rson olefination, ortho-silylation and benzylic deprotonation/methylat ion. (C) 1997 Elsevier Science Ltd.