Diketene: 2-Oxetanone, 4-methylene

The theoretical infrared (IR) spectra of diketene 4-methylene-2-oxetanone isomers (e.g., 2) and mono- and disulfur analogues such as 3 have been calculated at the MP2/6-311+- -G level to provide data on whether new analogues could potentially be prepared <2001JST433>. 

DimeriZa.tlon. A special case of the [2 + 2] cyclo additions is the dimerization of ketenes. Of the six possible isomeric stmctures, only the 1,3-cyclobutanediones and the 2-oxetanones (P-lactones) are usually formed. Ketene itself gives predominandy (80—90%) the lactone dimer, 4-methylene-2-oxetanone (3), called diketene [674-82-8], approximately 5% is converted to the symmetrical dimer, 1,3-cyclobutanedione [15506-53-3] (4) which undergoes enol-acetylation to so-called triketene [38425-52-4] (5) (44). 

Methylene-2-oxetanone, or diketene (37), does not decarboxylate on heating but cleaves to form ketene at 250 °C. A thermal rearrangement of 3-aryl-4-benzal-2-oxetanones (38) to form naphthalene derivatives appears to involve this type of pyrolysis, followed by a [ 2 + 4] cycloaddition (78JOC1146, 72LA(765)15>. 

A similar type of acid-catalyzed condensation of aldehydes with 4-methylene-2-oxetanone (diketene), giving 4-oxo-6-methyl-l,3-dioxins, has been patented (73GEP2149650). However, other work has established that <5-hydroxy-/3-keto acids or unsaturated keto acids are formed as the principal products (equation 24) (78CPB3877, 78CL409). The latter reaction probably involves electrophilic attack of the protonated aldehyde on the nucleophilic exocyclic methylene carbon atom of the diketone. A closely related reaction of acetals with diketene, catalyzed by titanium tetrachloride, gives the corresponding <5-alkoxy-/3-keto esters (74CL1189). 

Methylenemagnesium, 0401 f 4-Methylene-2-oxetanone, see Diketene, 1437 f Methyl ethanoate, see Methyl acetate, 1228... 

The chemistry of methylene-substituted 2-oxetanones, and in particular diketene, has attracted a vast amount of research over the years, so other alkylene-substituted oxetanes and oxetan-2-ones, including methylene oxetanes, are discussed separately in Section 2.05.7.4. Diketene is also known as 4-methylene-2-oxetanone and a notable difference between this and other 2-oxetanones is that it undergoes thermal decomposition by a cycloreversion reaction to give ketene, rather than forming allene and C02. CHEC-II(1996) refers to a series of comprehensive reviews of the chemistry of this compound <1996CHEC-II(1)721>. 

While diketene remains a very important synthetic precursor, there has been increasing interest in the chemistry of a-methylene-/3-lactones, 3-methylene-2-oxetanones. However, unlike diketene, which can be readily synthesized by the dimerization of aldehydic ketenes, there are few methods for the synthesis of a-methylene-/3-lactones in the literature. Recent strategies for the preparation of the compounds are discussed in Section 2.05.9.2. The kinetic resolution of racemates of alkyl-substituted a-methylene-/3-lactones has been carried out via a lipase-catalyzed transesterification reaction with benzyl alcohol (Equation 21) <1997TA833>. The most efficient lipase tested for this reaction was CAL-B (from Candida antarctica), which selectively transesterifies the (A)-lactone. At 51% conversion, the (R)-f3-lactone, (R)-74, and (A)-/3-hydroxy ester, (S)-75, were formed in very high enantio-selectivities (up to 99% ee). 

Methylene-substituted 2-oxetanones, and in particular diketene, have attracted a vast amount of work Schemes 49... 

SYNS 3-BUTENO-p-LACTONE DIKETENE DIKETENE, inhibited (DOT) 4-METHYLENE-2-OXETANONE... 

Beilstein Handbook Reference) Acetyl ketene BRN 0104541 But-3-en-3-olide 3-Buteno-p-lactone 3-Butenoic acid, 3-hydroxy-, p-lactone Diketene EINECS 211-617-1 Ethenone, dimer HSDB 2063 Ketene, dimer 4-Methylene-2-oxetanone NSC 93783 2-Oxetanone, 4-methylene- UN2521 Vinyl-aceto-p-lactone. Liquid mp = -6.5 bp = 126.1 d = 1.0877. 

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