Claisen rearrangement transform

The retron for the Claisen rearrangement transform (see above) is easily established by the application of a Wittig disconnection at each of the equivalent terminal double bonds of 57... 

A prominent structural feature of 21 and its precursor 22 is the trans C16-C17 trisubstituted double bond. The particular relationship between the ethoxycarbonyl function and the A16 17 double bond in 22 is significant because it satisfies the structural prerequisite for the Johnson ortho ester Claisen rearrangement transform.2130 Mixed ketene acetal 23 thus emerges as the immediate... 

One example of the Claisen rearrangement transforms allyl phenol ethers to o-allylphenols through the redistribution of she electrons in a cyclic transition state. 

You have already seen that a carbon-heteroatom bond is easy to make, since we used such bonds as natural places for disconnections (frames 234 ft). It is good strategy therefore to make a carbon-heteroatom bond and then to transform it into a carbon-earbon bond. The Claisen rearrangement is one way to do this an ortho allyl phenol (B) made from an allyl ether (A) ... 

There are a number of powerful synthetic reactions which join two trigonal carbons to form a CC single bond in a stereocontrolled way under proper reaction conditions. Included in this group are the aldol, Michael, Claisen rearrangement, ene and metalloallyl-carbonyl addition reactions. The corresponding transforms are powerfully stereosimplifying, especially when rendered enantioselective as well as diastereoselective by the use of chiral controller groups. Some examples are listed in Chart 20. 

The cw-isobacteriochlorin 16A formed by the Claisen rearrangement has been utilized for the further transformation into porphyrin dp2311 0 the metal-free ligand of heme dr... 

Even if organocatalysis is a common activation process in biological transformations, this concept has only recently been developed for chemical applications. During the last decade, achiral ureas and thioureas have been used in allylation reactions [146], the Bayhs-Hillman reaction [147] and the Claisen rearrangement [148]. Chiral organocatalysis can be achieved with optically active ureas and thioureas for asymmetric C - C bond-forming reactions such as the Strecker reaction (Sect. 5.1), Mannich reactions (Sect. 5.2), phosphorylation reactions (Sect. 5.3), Michael reactions (Sect. 5.4) and Diels-Alder cyclisations (Sect. 5.6). Finally, deprotonated chiral thioureas were used as chiral bases (Sect. 5.7). 

Jasmonates are important odorant compounds. For the synthesis of new substances of this type, Giersch and Forris developed a domino Claisen/ene/retro-ene process which allows the acid-catalyzed transformation of sorbyl alcohol 4-285 and the cyclic acetals 4-286 into the cycloalkenone 4-292 with the proposed intermediates 4-287 to 4-291 (Scheme 4.61) [97]. A similar domino process had been described by Srikrishna and coworkers [98]. Acyclic acetals gave only the Claisen rearrangement products. 

A combination of a Claisen rearrangement, a Wittig rearrangement and a Wittig reaction was described by Mali and coworkers for the synthesis of 6-prenylcou-marins. In these transformations, 2-prenyloxybenzaldehydes was employed as substrate [105]. 

Quite recently, Wipf and coworkers developed a combination of a Claisen rearrangement with the addition of an organo aluminum species to the newly formed carbonyl moiety [108]. The new process is based on an already-discussed (see above) transformation by the same author [104]. 

A methylenation of cyclic carbonates such as 6/4-132 using dimethyltitanocene to give a ketene acetal, followed by a subsequent Claisen rearrangement, allowed the synthesis of medium-ring lactones such as 6/4-133 in good yields these are otherwise difficult to obtain. In this transformation, 6/4-133 is formed as a l l-mix-ture of the two atropisomers 6/4-133a and 6/4-133b (Scheme 6/4.33). The substrate... 

The C-H functionalization protocol is not limited to the development of surrogate chemistry to enolate transformations. The C-H activation at allylic C-H bonds readily generates 7,6-unsaturated esters, the products of the classic Claisen rearrangement (Figure 6). 

Important advances in propargylic etherification have come from the use of copper-based systems that achieve efficient, catalytic O-progargylation of phenols (Scheme 8).245,246 While the mechanism of this transformation remains unclear, the products of these reactions have been readily converted into chromenes through subsequent Claisen rearrangement,... 

A proposed reaction pathway is shown in Scheme 7.29, where either the aromatic carbon or oxygen atom of naphthol may work as a nucleophile. Thus, the first step is the nucleophilic attack of the carbon atom of 1 -position of 2-naphthol on the C. atom of an allenylidene complex A to give a vinylidene complex B, which is then transformed into an alkenyl complex C by nucleophilic attack of the oxygen atom of a hydroxy group upon the Co, atom of B. Another possibility is the nucleophilic attack ofthe oxygen of 2-naphthol upon the Co, atom of the complex A. In this case, the initial attack of the naphthol oxygen results in the formation of a ruthenium-carbene complex, which subsequently leads to the complex B via the Claisen rearrangement of the carbene complex. 

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