In organic chemistry, the Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene termed the dienophile, to form a substituted cyclohexene derivative. It is the prototypical example of a pericyclic reaction with a concerted mechanism. More it is classified as a thermally-allowed cycloaddition with Woodward–Hoffmann symbol, it was first described by Otto Diels and Kurt Alder in 1928. For the discovery of this reaction, they were awarded the Nobel Prize in Chemistry in 1950. Through the simultaneous construction of two new carbon–carbon bonds, the Diels–Alder reaction provides a reliable way to form six-membered rings with good control over the regio- and stereochemical outcomes, it has served as a powerful and applied tool for the introduction of chemical complexity in the synthesis of natural products and new materials. The underlying concept has been applied to π-systems involving heteroatoms, such as carbonyls and imines, which furnish the corresponding heterocycles.

The reaction has been generalized to other ring sizes, although none of these generalizations have matched the formation of six-membered rings in terms of scope or versatility. Because of the negative values of ΔH° and ΔS° for a typical Diels–Alder reaction, the microscopic reverse of a Diels–Alder reactions becomes favorable at high temperatures, although this is of synthetic importance for only a limited range of Diels-Alder adducts with some special structural features; the reaction is an example of a concerted pericyclic reaction. It is believed to occur via a single, cyclic transition state, with no intermediates generated during the course of the reaction; as such, the Diels–Alder reaction is governed by orbital symmetry considerations: it is classified as a cycloaddition, indicating that it proceeds through the suprafacial/suprafacial interaction of a 4π electron system with a 2π electron system, an interaction that leads to a transition state without an additional orbital symmetry-imposed energetic barrier and allows the Diels-Alder reaction to take place with relative ease.

A consideration of the reactants' frontier molecular orbitals makes plain. For the more common "normal" electron demand Diels–Alder reaction, the more important of the two HOMO/LUMO interactions is that between the electron-rich diene's ψ2 as the highest occupied molecular orbital with the electron-deficient dienophile's π* as the lowest unoccupied molecular orbital. However, the HOMO–LUMO energy gap is close enough that the roles can be reversed by switching electronic effects of the substituents on the two components. In an inverse electron-demand Diels–Alder reaction, electron-withdrawing substituents on the diene lower the energy of its empty ψ3 orbital and electron-donating substituents on the dienophile raise the energy of its filled π orbital sufficiently that the interaction between these two orbitals becomes the most energetically significant stabilizing orbital interaction. Regardless of which situation pertains, the HOMO and LUMO of the components are in phase and a bonding interaction results as can be seen in the diagram below.

Since the reactants are in their ground state, the reaction is initiated thermally and does not require activation by light. The "prevailing opinion" is that most Diels–Alder reactions proceed through a concerted mechanism. Despite the fact that the vast majority of Diels–Alder reactions exhibit stereospecific, syn addition of the two components, a diradical intermediate has been postulated on the grounds that the observed stereospecificity does not rule out a two-step addition involving an intermediate that collapses to product faster than it can rotate to allow for inversion of stereochemistry. There is a notable rate enhancement when certain Diels–Alder reactions are carried out in polar organic solvents such as dimethylformamide and ethylene glycol. and in water. The reaction of cyclopentadiene and butenone for example is 700 times faster in water relative to 2,2,4-trimethylpentane as solvent. Several explanations for this effect have been proposed, such as an increase in effective concentration due to hydrophobic packing or hydrogen-bond stabilization of the transition state.

The geometry of the diene and dienophile components each propagate into stereochemical details of the product. For intermolecular reactions the preferred positional and stereochemical relationship of subtituents of the two components compared to each other are controlled by electronic effects. However, for intramolecular Diels–Alder cycloaddition reactions, the conformational stability of the structure the transition state can be an overwhelming influence. Frontier molecular orbital theory has been used to explain the regioselectivity patterns observed in Diels–Alder reactions of substituted systems. Calculation of the energy and orbital coefficients of the components' frontier orbitals provides a picture, in good accord with the more straightforward analysis of the substituents' resonance effects, as illustrated below. In general, the regioselectivity found for both normal and inverse electron-demand Diels–Alder reaction follows the ortho-para rule, so named, because the cyclohexene product bears substituents in positions that are analogous to the ortho and para positions of disubstituted arenes.

For example, in a normal-demand scenario

Kazuo Hatoyama was the patriarchal head of the prominent Japanese Hatoyama political family, called "Japan's Kennedy family." Hatoyama was born to a samurai family of the Katsuyama clan in Tokyo. He graduated from the Tokyo Kaisei School in 1875, he was selected for a government-sponsored study abroad program and attended Columbia University and Yale University Law School. When he returned to Tokyo in 1880, Hatoyama opened a law practice, while lecturing at the University of Tokyo, he thereafter joined the Rikken Kaishintō political party founded by Ōkuma Shigenobu and became active in politics. In 1890, at Okuma's urging, he was appointed president of the Tokyo Semmon Gakko, which shortly thereafter became Waseda University, he headed this institution until 1907, although his title was honorary in nature. In 1901, he was invited to Yale for its 200th anniversary celebration, awarded an honorary doctorate in law, he was elected to the House of Representatives in the 1892 general election and was re-elected eight times thereafter.

He became House Speaker in 1896. However, a rift developed between Okuma. Although Hatoyama angled to become foreign minister in Okuma's first cabinet, he was passed over for the post and only served as Vice Minister in 1898. In April 1907, he was demoted to board member status, he left the Rikken Kaishinto in January 1908 to join the rival Rikken Seiyukai party. He was elected to the Tokyo Municipal Assembly in 1908. In 1910, he was elected President of the Tokyo Bar Association, his wife, Haruko Hatoyama, was a co-founder of. His son is former Prime Minister Ichirō Hatoyama, who founded and was the first president of the Liberal Democratic Party, his grandson was former Foreign Minister Iichirō Hatoyama. His younger great-grandson Kunio Hatoyama served as Minister of Internal Affairs and Communications under Prime Minister Taro Aso until June 12, 2009, his older great-grandson Yukio Hatoyama is the leader of the Democratic Party of Japan and represents the 9th district of Hokkaidō in the House of Representatives.

Yukio became Prime Minister on September 16, 2009, following a win by the opposition coalition in the 2009 elections. His son-in-law was Suzuki Kisaburō, a judge, prosecutor and Minister of Justice and Home Minister. Hatoyama and his family resided in the Otowa neighborhood of Bunkyo, Tokyo in 1891. Following the Great Kanto Earthquake, his son Ichiro commissioned a new Western-style mansion on the site, now known as Hatoyama Hall. Itoh, Mayumi; the Hatoyama Dynasty: Japanese Political Leadership through the Generations. New York: Palgrave Macmillan. ISBN 1-403-96331-2, ISBN 978-1-403-96331-4. OCLC 248918078

De Cicco v. Schweizer, 117 N. E. 807, is a notable contract law case concerning privity of consideration. The case examined whether there was consideration in a contract where person A makes a promise to person B, in exchange person B promises to perform a previous contract obligation to person C. Additionally, the case looked at the general class of prenuptial agreements. On January 20, 1902, Count Oberto Gulinelli of Italy married Blanche Schweizer, of Lincoln Square, Manhattan. Joseph and Ernestine Teresa Schweizer, Blanche's parents, had signed documents providing a substantial dowry to the betrothed; the Schweizers were to pay $2,500 every January 20 for the rest of their lives, leave half their estate—estimated at well over $1M in 1912—to the pair. For ten years, the Schweizers paid on time, but payments ceased in 1912. Gulinelli assigned the $2,500 payment to one Attilio De Cicco, who brought suit in New York to recover the payment. According to testimony at trial, the Gulinellis may have sought to separate in 1911.

Blanche came to New York, where she asked for and received money from her father, which he argued was an advance on the 1912 allowance. The plaintiff filed suit in the summer of 1913, naming both Joseph and Ernestine Schweizer as defendants, requesting damages in the amount of $2500. Trial was held in the Supreme Court in Manhattan under Justice Erlanger. Judgment was rendered for the plaintiff in the amount of $99.70. Both parties submitted motions for a new trial. Both parties appealed; the Appellate Division rendered judgment on January 22, 1915. Presiding was Justice Ingraham, with Justices McLaughlin, Scott and Laughlin present. Ruling unanimously, the court modified the award of the trial court, awarding the plaintiff $2,500, with interest and costs for a total of $3,030.77. Joseph Schweizer appealed from this judgment; the court unanimously affirmed the Appellate Division. In a majority opinion by Judge Cardozo, the court held that there was a sufficient consideration for the promise. Manz, William H. ed..

"DeCicco v. Schweitzer". Records and Briefs of Landmark Benjamin Cardozo Opinions. 3. Buffalo, New York: William S. Hein & Co. ISBN 1575882388 – via HeinOnline. "De Cicco v. Schweizer". Northeastern Reporter. 117: 807. Retrieved 9 January 2016 – via HathiTrust. "De Cicco v. Schweizer". New York Reports. 221: 431. Retrieved 9 January 2016 – via HathiTrust. "De Cicco v. Schweizer". Lawyer's Reports Annotated. 1918E: 1004. Retrieved 10 January 2016 – via HathiTrust. "De Cicco v. Schweizer". Annotated Cases. 1918C: 816. Retrieved 10 January 2016 – via HathiTrust