Relevance of Conformational Constraints to the Regioselective Lithiation of Aromatic Diethers. Application to the Convenient Construction of the DEF Tricyclic Subunit of the Austalides
作者:Leo A. Paquette、Matthias M. Schulze、David G. Bolin
DOI:10.1021/jo00087a018
日期:1994.4
The lithiation of 29 and 30 is shown to occur at all three sites with a dissimilar kinetic preference. For the dihydrofuran, reaction at the proton labeled H-beta, operates predominantly; in the dihydropyran example, H-alpha is the favored site of deprotonation. These protons represent those that are the most deshielded in the respective H-1 NMR spectra. The same is true for 9 and 19, both of which undergo metalation adjacent to the ring oxygen. No crossover in regioselectivity is observed, presumably because the methoxy substituent is sterically precluded from rotating freely. Mixed complexes (dimers, etc.) or mixed aggregates in low equilibrium concentration are key to understanding the acidification phenomenon of ortho hydrogens. As a consequence of the dominance of regiocontrol by the ring oxygen in 9, a convenient means has been developed for elaboration of the tricyclic eastern sector of the austalide mycotoxins.