Relevance of Conformational Constraints to the Regioselective Lithiation of Aromatic Diethers. Application to the Convenient Construction of the DEF Tricyclic Subunit of the Austalides

Autor:	David G. Bolin, Leo A. Paquette, Matthias M. Schulze
Rok vydání:	1994
Předmět:	Steric effects chemistry.chemical_compound Deprotonation Chemistry Dihydropyran Stereochemistry Organic Chemistry Proton NMR Substituent Regioselectivity Ring (chemistry) Formylation
Zdroj:	The Journal of Organic Chemistry. 59:2043-2051
ISSN:	1520-6904 0022-3263
DOI:	10.1021/jo00087a018
Popis:	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 β' operates predominantly; in the dihydropyran example, H α is the favored site of deprotonation. These protons repre- sent those that are the most deshielded in the respective 1 H NMR spec- tra. The same is true for (9) and (19), both of which undergo mediation adjacent to the ring oxygen. No crossover in regioselectivity is obser- ved, 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 acidifi- cation 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 austa- lide mycotoxins
Databáze:	OpenAIRE
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