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Gabriel–Colman rearrangement
The Gabriel–Colman rearrangement〔Gabriel, S.; Colman, J. "Ueber eine Umlagerung der Phtalimidoketone" ''Chem. Ber.'' 1900, ''33'', 2630. (())〕 is the chemical reaction of a saccharin or phthalimido ester with a strong base, such as an alkoxide, to form substituted isoquinolines.〔Koelsch, C. F.; Lindquist, R. M. "Some Attempts to Prepare Derivatives of Benz()isoquinoline and a Synthesis of Benz()isoquinoline" ''J. Org. Chem.'' 1956, ''21'', 657–659. (())〕 This rearrangement, a ring expansion, is seen to be general if there is an enolizable hydrogen on the group attached to the nitrogen,〔Allen, C. F. H. "The Naphthyridines" ''Chem. Rev.'' 1950, ''47'', 275-305. ((Review ))〕 since it is necessary for the nitrogen to abstract a hydrogen to form the carbanion that will close the ring.〔Hauser, C.R. and Kantor, S. W. "Rearrangement of Benzyl Ethers to Carbinols by Potassium Amide. Mechanism of Isomerization of Carbanions Involving 1,2-Shifts" ''Chem. Rev.'' 1951, ''73'', 1437–1441. (())〕 As shown in the case of the general example below, X is either CO or SO2.
==Mechanism==
The reaction mechanism〔Hill, J. H. M. "Mechanism of the Gabriel-Colman Rearrangement" ''J. Org. Chem.'' 1965, ''30'', 620-622. ((Mechanism ))〕〔Pflum, D. A.; Li, J. J.; Corey, E. J., Eds "Gabriel-Colman Rearrangement In Name Reactions in Heterocyclic Chemistry" ''Wiley & Sons: Hoboken, NJ'' 2005, 416-422. ((Review ))〕 starts with an attack on the carbonyl group by a strong base, such as methoxide ion. The ring is then opened, forming an imide anion. This is then followed by a rapid isomerization of the imide anion to the carbanion. This is facilitated by the electron withdrawing effect of the substituent, which allows for greater stabilization of the adjacent carbanion with respect to the imide anion. The reaction is then completed when the methoxide is displaced by the ring closing, which results in a ring expansion. The rate determining step of this reaction is the attack of the carbanion on the carbomethoxy group.
The displacement of the methoxide is analogous to the displacement seen in the Dieckman condensation, as it is also a result of a ring closure.
Furthermore, tautomerization can occur on both of the carbonyl groups on the ring, with interconversion of the keto form to the enol form and the amide form to the imidic acid form.
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