Natural products enone chiral compounds

The chiral bicyclic enones, levoglucosenone, isolevoglucosenone, and new functionalized L-arabinose enone possess excellent reactivity and functionality. Their properties mid application as convenient precursors in the synthesis of many attractive templates or intermediates of complex natural products are reviewed. These compounds are attracting increasing interest due to their structural rigidity and ability for stereoselective functionalization without protection, deprotection sequences necessary in many synthetic organic methodologies. 

The enantioselective lithiation of anisolechroimum tricarbonyl was used by Schmalz in a route towards the natural product (+)-ptilocaulin [88,89]. In situ hthiation of 125 with ent-83 gave enf-126 in an optimised 91% ee (reaction carried out at -100°C over 10 min). A second, substrate-directed lithiation with BuLi alone, formation of the copper derivative, and a quench with crotyl bromide, gave 135. The planar chirality and reactivity of the chromium complex was then exploited in a nucleophilic addition of dithiane, which generated the ptilocaulin precursor 136 (Scheme 35). The stereochemistry of compound 126 has also been used to direct dearomatising additions, yielding other classes of enones [90]. 

The catalytic asymmetric Michael (AM) reaction of malonates to enones, in which a new C-C bond is formed at the p-position of the enones, is an important organic transformation, since the resulting compounds are used in the synthesis of several natural products and drug molecules. Many catalyst systems have been reported in recent years using a variety of metal catalysts with suitable chiral ligand combinations under homogeneous reaction conditions. 

The importance of chiral epoxy-ketones is becoming increasingly recognized as physiologically active natural products, as metabolic intermediates, and as chiral synthons. Cyclohex-2-enones (1) have been transformed into optically active epoxycyclohexanones (2) using t-butylhydroperoxide in toluene, to which catalytic quantities of solid sodium hydroxide and the chiral catalyst quininium benzyl chloride were added, under phase-transfer conditions. In the unsubstituted case the yield is 60% with enantiomeric excess of 20% as determined by n.m.r. Substituents at C(2), C(3), and C(4) block the epoxidation reaction but compounds with gem-dimethyl groups at C(5) and C(6) are readily converted. 

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