A one-pot synthesis for the preparation of 1,4-disubstituted 2-oxoazetidine-3-carboxylic acid amides was developed. 5-[(N-substituted-amino)(hydroxy)methylene] Meldrum’s acids act as a source of ketenes that react with aldimines in boiling toluene to give β-lactams with a retro-amide side chain.
The ability to undergo [4 + 2] versus [2 + 2] cycloaddition was under investigation for ketenes thermally generated from carbamoyl Meldrum's acid. Usually, 1,3‐oxazino‐5‐carbamoylo‐4,6‐diones are formed when carbamoyl Meldrum's acid reacts with imine. However, in some cases, a reaction takes an unexpected course, leading to the formation of tetraponerines alkaloids derivatives or cyclic iminoethers.
One-pot synthesis of optically active 1,4-disubstituted-3-carbamoyl-azetidinones from 5-[(N-arylamino)(hydroxyl)methylene]-2,2-dimethyl-1,3-dioxa-4,6-diones and chiral aldimines is achieved via thermal generation of carbamoyl ketenes and subsequent [2+2] cycloaddition. Three possible chiral induction approaches were tested and (R)-(+)-1-phenylethylamine was confirmed as the best chiral auxiliary. Among
analogue 11 reacted with three diazomethane molecules and underwent a ring expansion and O-methylation to the 3-anilido-1,4-dimethoxynaphthalene. It is suggested that the reaction initiates by protonation of the diazomethane by the enol and an approximate qualitative relationship exists between the acidity of the enol and K(enol) and the regioselectivity of the reaction.
Ketenes generated during the thermal decomposition of 5-[(N-aryl/alkylamino)(hydroxyl)methylene]-2,2-dimethyl-1,3-dioxa-4,6-diones react with highly electrophilic iminium ions. The initial [2+2] cycloaddition product rearranges to 2-arylidene malonamide. Mechanistic aspects of the process are presented.