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Multistep tandem reactions (or cascade reactions) are a sequence of chemical transformations (usually more than two steps) that happens consecutively to convert a starting material to a complex product. [28] This kind of organic reactions are designed to construct difficult structures encountered in natural product total synthesis.
The Diels-Alder reaction is perhaps the most important and commonly taught cycloaddition reaction. Formally it is a [4+2] cycloaddition reaction and exists in a huge range of forms, including the inverse electron-demand Diels–Alder reaction, hexadehydro Diels–Alder reaction and the related alkyne trimerisation.
The proposed reaction mechanism involves an initial [2+2] cycloaddition between the vinylcyclopropane and TCNE, followed by rearrangement to furnish the seven-membered ring in the product. Evidences have shown that depending on the reaction conditions, the rearrangement step can occur via either radical or ionic intermediates. [7]
The Diels–Alder reaction was one step in an early preparation of the steroids cortisone and cholesterol. [75] The reaction involved the addition of butadiene to a quinone. Diels-Alder in the total synthesis of cortisone by R. B. Woodward. Diels–Alder reactions were used in the original synthesis of prostaglandins F2α and E2. [76]
Ketene [2 + 2] cycloaddition reactions have been used in many total syntheses since Corey's use of the [2 + 2] cyclization in synthesizing the prostaglandins. [35] Robert Ireland's synthesis of (±)-aphidicolin uses the Wolff rearrangement to do a tandem ring-contraction, and [2 + 2] cycloaddition. [38] Synthesis of (±)-aphidicolin
Trimethylenemethane cycloaddition is the formal (3+2) annulation of trimethylenemethane (TMM) derivatives to two-atom pi systems. Although TMM itself is too reactive and unstable to be stored, reagents which can generate TMM or TMM synthons in situ can be used to effect cycloaddition reactions with appropriate electron acceptors .
Iodonium ylides undergo cycloaddition with alkene acceptors in low yields. [10] In the presence of nucleophiles, substitution of the iodonium group occurs. [11] (9) Reactions with electrophiles yield iodonium salts, which may be quenched in situ by nucleophilic counteranions. In the presence of non-nucleophilic counteranions, the substituted ...
The 1,3-dipolar cycloaddition is a chemical reaction between a 1,3-dipole and a dipolarophile to form a five-membered ring. The earliest 1,3-dipolar cycloadditions were described in the late 19th century to the early 20th century, following the discovery of 1,3-dipoles.