Ketene equivalents, review

See. for example, Martin Gott Goodlett Hasek /. Org. Chem. 1965,30, 4175 Brady O Neal /. Org. Chem. 1967,32, 2704 Huisgen Feiler Otto Tetrahedron Lett. 1968,4491, Chem. Ber. 1969,102, 3475. For indirect methods of the 1,4 addition of the elements of ketene to a diene see Freeman Balls Brown /. Org. Chem. 1968, 33, 2211 Corey Ravindranathan Terashima /. Am. Chem. Soc. 1971, 93, 4326. For a review of ketene equivalents see Ranganathan Ranganathan Mehrotra Synthesis 1977, 289-296. 

The use of ketene equivalents in 4- -2-cycloaddition reactions for organic synthesis has been reviewed. " a-Carbonyl ketenes behave as dienes in Diels-Alder reactions with 4-aryl-2-methyl-2,3-dihydro-l,5-benzothiadiazepines to yield regiospecific cycloadducts. The reactions of diphenylketene with cyclic ( -cw)-l,3-dienes such as cyclopentadiene and cyclohexadiene initially yield the Diels-Alder adducts, which are converted into the Staudinger cyclobutanones by [3,3]-sigmatropic rearrangementsl-Benzyl-l,3-diazabuta-l,3-dienes react with ketenes to produce the 4 - - 2-cycloadducts, substituted l-benzyl-4-(benzylidenimino)-4-phenylazetidin-2-ones, which rearrange into the more stable 5,6-dihydro-3//-pyrimidin-4-ones. The formal 4 - - 2-cycloaddition reaction of ketenimines has been used to synthesize benzoimidazo[l,2-Z ]isoquinolines. ... 

Before proceeding, it is important to note that it is the conversion of the a-chloronitrile to a ketone (a functional group transformation) that establishes the a-chloroacrylonitrile as a ketene equivalent in Diels-Alder chemistry. a-Chloroacrylonitrile is not the only ketene equivalent that has been used in Diels-Alder chemistry, nor in this strategy for prostaglandin synthesis. Reviews have been written on this topic. What is perhaps most important is the development of the concept of equivalencies that was occuring at the time this synthesis was undertaken. We will see this concept again. 

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