By Alan R. Katritzky (Eds.)
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195 Although N ,N -bis[3,5-bis(trifluoromethyl)phenyl]thiourea is achiral, its addition to aldols catalysed by l-proline (61) enhances the diastereo- and enantio-selectivities (up to 94 and >99%), via formation of a 1 : 1 host:guest complex. The complex is proposed to involve hydrogen bonding between the proline oxygens and the thiourea N−Hs, driven by the use of non-polar solvent. 196 4-Substituted cyclohexanones have been desymmetrized with high diastereo- and enantio-selectivities via a simple direct aldol reaction catalysed by l-proline (61).
For example, ethyl benzoylacetate (PhCOCH2 CO2 Et) reacts with benzylideneacetone (PhCH=CHCOMe) to yield enone (89). 339 A diarylprolinol TMS ether with heavy aromatic buttressing catalyses a tandem Michael addition–Wittig reaction of trans-enals with (3-carboxy-2oxopropylidene)-triphenylphosphorane (Ph3 P=CHCOCH2 CO2 −t-Bu) to yield cyclohexenone esters with 98% de and 99% ee. 340 An intramolecular Rauhut–Currier reaction creates a C−C bond between appropriately tethered Michael acceptors. A new enantioselective version (90 → 91) uses proline-derived catalysts to form electron-rich dienamines as key intermediates.
66), from benzaldehyde]. 227 t-Butyldimethylsilyl triflate, in combination with triethylamine, provides dual activation catalysis for intramolecular alkynylogous Mukaiyama aldol reaction of bicyclic alkanones tethered to alkynyl esters. 229 BINAPO, a chiral phosphine oxide, catalyses enantioselective direct aldols. 231 Enantioselective addition of O,O-ketene silyl acetals (derived from simple esters) to α-keto esters has been achieved using copper(II)–bis(oxazoline)complexes, dispensing with the need for the thioesters used in the Evans aldol.
Aminomethylenemalonates and Their Use in Heterocyclic Synthesis by Alan R. Katritzky (Eds.)