Ketenes from cyclobutenones

Simple olefins do not usually add well to ketenes except to ketoketenes and halogenated ketenes. Mild Lewis acids as well as bases often increase the rate of the cyclo addition. The cycloaddition of ketenes to acetylenes yields cyclobutenones. The cycloaddition of ketenes to aldehydes and ketones yields oxetanones. The reaction can also be base-cataly2ed if the reactant contains electron-poor carbonyl bonds. Optically active bases lead to chiral lactones (41—43). The dimerization of the ketene itself is the main competing reaction. This process precludes the parent compound ketene from many [2 + 2] cyclo additions. Intramolecular cycloaddition reactions of ketenes are known and have been reviewed (7). 

Cycloaddition to alkynes, cyclobutenones. This ketene when formed in situ from CCI3COCI and Zn/Cu, reacts with alkynes to form 4,4-dichlorocyclobuten-ones,3 which can rearrange in part to 2,4-dichlorocyclobutenones.4 Both products are reduced to the same cyclobutenone by Zn(Cu) in HOAc/pyridine (4 1) or by zinc and acetic acid/TMEDA.5... 

Benzocvclobutenone was obtained from decomposition of 0-alkyl substituted benzoyl chlorides189. Thus, with 2-propylbenzoyl chloride at 600 °C, the 0-quinonoid ketene intermediate is stabilized by 1,5-hydrogen shift to give the aldehyde (equation 101). In this case, the 0-alkyl contains a /1-hydrogen. However, if the 0-substituent lacks a /1-hydrogen, i.e. it is a methyl group, then the cyclobutenone is obtained (equation 102). 

An unusual synthesis of ( )-septicine (866) from dimethyl squarate (878) exploits rearrangement of a 4-(l-pyrrolo)cyclobutenone 879 to the indolizine-5,8-dione 880, presumably via a ketene intermediate formed by electrocyclic ring opening of 879 (Scheme 113) (584). Partial reduction of 880 yielded the 4-hydroxypyridone 881, the triflate ester of which was deoxygenated with a palladium(II)-formic acid system to yield the l,2,3,5-tetrahyi oindolizin-5-one 882. Mild reduction with aluminum hydride completed the synthesis of ( )-866 in an excellent overall yield of 27% from 878. 

In another example from Moore, cyclobutenone 148, rearranges to give enyne-ketene 149. The resulting diradical was intercepted by a tethered alkyne, which in turn led to spirocycle 153 (Scheme 27) [49],... 

Photochemical ring-opening in benzene solution to the allenic keten (201) explains the conversion of the cyclobutenone (202) into the lactone (203 80%).118 A similar route explains the formation of (204) from (205). Such ring-opening... 

Cyclobutenones 177 have been prepared from the reaction of bis(trifluoromethyl)ketene with hexyne-1, butyne-2, phenylacetylene and... 

When the [2+2] cycloadducts derived from ketenes and vinyl ethers are treated with basic or neutral AI2O3, the cyclobutenones 203 are obtained in high yields ". Heating of 203 affords the naphthalene derivatives 205 in quantitative yields via the vinyl ketene intermediate 204 " . 

Diphenylketen reacts with di-t-butylacetylene to give the cyclobutenone (200) in 47% yield. The same product is obtained in the presence of iron pentacarbonyl in a reaction which is quite different from the carbonyl insertion which occurs between diphenylacetylene and diphenylketen. Ynamines have been shown to give substituted cyclobutenones with carbonyl-substituted ketens. The natures of the substituents of the reactants affect the extent of... 

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