Convenient Methods for the Preparation of Unsymmetrical Double Aldols

Double aldol reaction proceeded stereoselectively at one α-carbon of ketones to give α-(1-hydroxyalkyl)-β-hydroxyalkyl ketones (double aldols) in good to high yields by the following three methods: i) tin(II) trifluoromethanesulfonate-mediated aldol reaction of aldehydes with β-hydroxy ketones (mono-aldols) in the presence of tertiary amines, ii) samarium(II) iodide-mediated aldol reaction of aldehydes with alkyl or aryl oxiranyl ketones, iii) Sn(OTf)2-promoted aldol reaction of α-bromo ketones with aldehydes giving α-bromo-β-stannyloxy ketones which were then converted to titanium enolates on treatment with low-valent titanium. Double aldols were formed by subsequent reaction of the above titanium enolates with aldehydes in one-pot procedure. Further, small and medium-sized carbocyclic compounds whose ring skeletons were composed of double aldol structure were synthesized by SmI2-mediated intramolecular cyclization between oxiranyl ketone and aldehyde functions.