Catalytic Addition Methods for the Synthesis of Functionalized Diazoacetoacetates and Application to the Construction of Highly Substituted Cyclobutanones

[reaction: see text] Methyl 3-(trialkylsilanyloxy)-2-diazo-3-butenoate undergoes Lewis acid-catalyzed Mukaiyama aldol addition with aromatic and aliphatic aldehydes in the presence of low catalytic amounts of Lewis acids in nearly quantitative yields. Scandium(III) triflate is the preferred catalyst and, notably, addition proceeds without decomposition of the diazo moiety. Diazoacetoacetate products from reactions with aromatic aldehydes undergo rhodium(II)-catalyzed ring closure to cyclobutanones with high diastereocontrol. Examples of complimentary Mannich-type addition reactions with imines are reported.