Editing Diels–Alder reaction (section)

===The dienophile===
In a normal demand Diels–Alder reaction, the dienophile has an electron-withdrawing group in conjugation with the alkene; in an inverse-demand scenario, the dienophile is conjugated with an electron-donating group.<ref name=ca839/> Dienophiles can be chosen to contain a "masked functionality". The dienophile undergoes Diels–Alder reaction with a diene introducing such a functionality onto the product molecule. A series of reactions then follow to transform the functionality into a desirable group. The end product cannot be made in a single DA step because equivalent dienophile is either unreactive or inaccessible. An example of such approach is the use of [[α-chloroacrylonitrile]] (CH<sub>2</sub>=CClCN). When reacted with a diene, this dienophile will introduce α-chloronitrile functionality onto the product molecule. This is a "masked functionality" which can be then hydrolyzed to form a [[ketone]]. α-Chloroacrylonitrile dienophile is an equivalent of [[ketene]] dienophile (CH<sub>2</sub>=C=O), which would produce same product in one DA step. The problem is that ketene itself cannot be used in Diels–Alder reactions because it reacts with dienes in unwanted manner (by [2+2] cycloaddition), and therefore "masked functionality" approach has to be used.<ref name="RanganathanRanganathan1977">{{cite journal |last1=Ranganathan |first1=S. |last2=Ranganathan |first2=D. |last3=Mehrotra |first3=A. K. |year=1977 |title=Ketene Equivalents |journal=Synthesis |volume=1977 |issue=5 |pages=289–296 |doi=10.1055/s-1977-24362|s2cid=260335918 }}</ref> Other such functionalities are [[phosphonium]] substituents (yielding exocyclic double bonds after [[Wittig reaction]]), various [[sulfoxide]] and [[sulfonyl]] functionalities (both are acetylene equivalents), and [[nitro group]]s (ketene equivalents).<ref name=cb474/>