Formaldehyde cycloaddition, with ketene

Oxetan-2-one can be formed by treating 3-iodopropanoic acid with moist silver oxide (Scheme 8.14a), and commercially it is obtained by the cycloaddition of formaldehyde (methanal) and ketene (ethenone). At 0-20 °C, ketene undergoes self-dimerization to give 4-methyleneoxetan-2-one (Scheme 8.14b). 

The cycloaddition of ketenes to carbonyl compounds also shows the expected regioselectivity. In this case, both HOMO(ketone)/LUMO(ketene) and LUMO(ketone)/HOMO(ketene) interactions may be important, but they lead to the same conclusions about regioselectivity, with the carbonyl oxygen atom bonding to the carbonyl carbon atom of the ketene as in the reactions of the quinone 6.384 and formaldehyde... 

Ab initio calculations indicate that in the gas phase the reaction of ketene inline and formaldehyde is concerted but asynchronous whereas in dichloromethane it is a two-step zwitterionic reaction.38 The 2 + 2-cycloadditions of keteniminium triflates with imines yields 2-azetidiniminium salts with cis stereoselectivity.39 The intramolecular 2 + 2-cycloaddition of ketenimines with imines (24) provides a novel synthesis of azeto[2,l-Z>]quinazolines (25) (Scheme 9).40... 

Ab initio MO calculations for the uncatalyzed [2 + 2]cycloaddition reaction between ketenes e.g. chloroketene) and carbonyl compounds e.g. formaldehyde) to yield oxetan-2-ones (y9-lactones) predict a relatively synchronous, concerted process, with a [ 2s + 2s +, i2s)] arrangement of the reactants in the activated complex and a negligible solvent effect [813],... 

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