Ketene cycloaddition

The first example of this phenomenon was observed in 1908,[2] and since then, cycloadditions of ketenes have expanded and gained synthetic utility.

Thus, the suprafacial- antarafacial geometry required for concerted, thermal [2+2] cycloaddition can be achieved in reactions of ketenes.

[4] This geometry has the interesting consequence that the bulkier substituent on the ketene will tend to end up on the more sterically hindered face of the cyclobutanone ring.

The use of chiral amine catalysts has allowed access to cycloaddition products in high enantiomeric excess.

Lewis acid catalysis is necessary for this process, unless the carbonyl compound possesses strongly electron- withdrawing substituents.

β-amino or -alkoxy unsaturated ketones, for instance, react with ketenes in a [4+2] sense to give synthetically useful yields of lactones.

[15] (9)Examples in which a vinylketene serves as the 4π partner are rare, but ketene-containing heterodienes such as acyl ketenes react with many heterodienophiles to give heterocyclic products in good yield.

The progress of the reaction can be estimated by disappearance of the characteristic yellow color of the ketene, by loss of the band at about 2100 cm−1 in the infrared spectrum, or by 1H NMR spectroscopy.

The mixture was then stirred at reflux for 4 hours and the residual zinc removed by filtration on a pad of Celite.

After removal of ether under reduced pressure, the product was purified by bulb-to-bulb distillation at 100° bath temperature (0.1 mm), to give 1.08 g (90%) of the title compound as a clear oil.