Synthesis of Polypropionate Fragments Containing Tertiary‐Alcohol Moieties. Cross‐aldolisations with lithium enolates of 7‐oxabicyclo[2.2.1]heptan‐2‐one derivatives

Abstract

The Diels-Alder adduct of 2,4-dimethylfuran to I-cyanovinyl (1'R)-camphanate ( ( + ) cyano-l,5-dimethyl-7-oxabicyclo[2.2.1]hept-5-en-2-endo-yl (1'S)-camphanate ((-)-3)).All these cross-aldolisations are highly em-face selective for the bicyclic ketones. The best stereochemical matching is obtained when the lithium enolates and (x -methyI-substituted aldehydes can realize a 'chelated transition state' that obeys the Cmrn and Fefkin-Anh models (steric effects). Polypropionate fragments containing eleven contiguous stereogenic centres and tertiary-alcohol moieties are thus prepared with high stereoselectivity in a convergent fashion. The chiral auxiliaries ((1R)-and (1s)-camphanic acid) are recovered at the beginning of the syntheses.A large variety of natural products of biological interest contain polypropionate fragments (chain with alternating OH and Me substituents) and analogues involving tertiary-alcoholic centres [2]. Several methods and strategies have been developed to provide access to these systems which possess a high density of stereochemical information [3]. The control of the stereochemistry can rely on pericyclic reactions such as the Claisen rearrangement [4]

First Asymmetric Diels−Alder Reactions of Furan and Chiral Acrylates. Usefulness of Acid Heterogeneous Catalysts

“…2-Substituted 7-oxabicyclo[2.2.1]hept-5-ene derivatives are powerful intermediates in the preparation of carbohydrates and other biologically active compounds. − These products can be obtained, via Diels−Alder reactions, from furan, an inexpensive compound obtained from agricultural leftovers . Given their relationship with biologically active compounds, there is a noticeable interest in the asymmetric preparation of these bicyclic compounds and excellent results have been described using chiral derivatives of acrylonitrile, such as 1-(cyanovinyl)-(1‘ S )-camphanate, as a chiral dienophile.…”