synthesis of morphine. Mechanism: Ireland–Claisen rearrangement The Ireland–Claisen rearrangement is the reaction of an allylic carboxylate with a strong base (such as lithium diisopropylamide) to give a γ,δ-unsaturated carboxylic acid. The rearrangement proceeds via silylketene acetal, which is formed by trapping the lithium enolate with chlorotrimethylsilane. Like the Bellus-Claisen (above), Ireland-Claisen rearrangement can take place at room temperature and above. The E- and Z-configured silylketene acetals lead to anti and syn rearranged products, respectively. There are numerous examples of enantioselective Ireland-Claisen rearrangements found in literature to include chiral boron reagents and the use of chiral auxiliaries. Johnson–Claisen rearrangement The Johnson–Claisen rearrangement is the reaction of an allylic alcohol with an orthoester to yield a ester. Weak acids, such as propionic acid, have been used to catalyze this reaction. This rearrangement often requires high temperatures (100–200 °C) and can take anywhere from 10 to 120 hours to complete. However, microwave assisted heating in the presence of KSF-clay or propionic acid have demonstrated dramatic increases in reaction rate and yields. Mechanism: Kazmaier–Claisen rearrangement The Kazmaier-Claisen rearrangement is the reaction of an unsaturated amino acid ester with a strong base (such as lithium diisopropylamide) and a metal salt at –78 °C to give a chelated enolate as intermediate. While different metal salts can be used to form the enolate, the use of zinc chloride results in the highest yield and gives the best stereospecificity. The enolate species rearranges at –20 °C to form an amino acid with an allylic side chain in α-position. This method was described by Uli Kazmaier in 1993. Photo-Claisen rearrangement The Claisen rearrangement of aryl ethers can also be performed as a photochemical reaction. In addition to the traditional ortho product obtained under thermal conditions (the [3,3] rearrangement product), the photochemical variation also gives the para product ([3,5] product), alternate isomers of the allyl group (for example, [1,3] and [1,5] products), and simple loss of the ether group, and even can rearrange alkyl ethers in addition to allyl ethers. The photochemical reaction occurs via a stepwise process of radical-cleavage followed by bond-formation rather than as a concerted pericyclic reaction, which therefore allows the opportunity for the greater variety of possible substrates and product isomers. The [1,3] and [1,5] results of the photo-Claisen rearrangement are analogous to the photo-Fries rearrangement of aryl esters and related acyl compounds. Hetero-Claisens Aza–Claisen An iminium can serve as one of the pi-bonded moieties in