Catalyzed propargyl-Claisen

The silver-catalyzed, propargyl Claisen rearrangement has also been combined with cyclization reactions. Propargyl ethers derived from [3-ketoesters were rearranged by silver hexafluoroantimonate and the resulting allenic [3-ketoesters cyclized on treatment with base, leading to the formation of 2/7-pyrans in moderate to excellent yields (Scheme 3.48).75 The cyclization process was described as a base-catalyzed... 

In much the same way, but with improved yields, a gold(l)-catalyzed propargyl Claisen rearrangement affords tri-and tetrasubstituted furans in good to excellent yields (Equation 32) <2005OL3925>. 

Scheme 4 Furans synthesized via gold(l)-catalyzed propargyl Claisen rearrangement.

In a convenient one-pot process, easily accessed propargyl vinyl ethers and aromatic amines are effectively converted into tetra- and pentasubstituted 5-methylpyrroles, which can fnrther be transformed into 5-formylpyrroles via 2-iodoxybenzoic acid (IBX)-mediated oxidation (Binder and Kirsch 2006). The cascade reaction proceeds through a silver(l)-catalyzed propargyl Claisen rearrangement, an amine condensation, and a gold(l)-catalyzed 5-exo-dig heterocyclization, as shown in Schane 11.4. 

Kirsch has reported the conversion of propargyl vinyl ethers to form pyrroles via a multi-step transformation involving the silver-catalyzed propargyl Claisen rearrangement to form the a-allenyl P-keto ester, condensation with a primary aryl amine. 

Toste and coworkers have reported the diastereoselective preparation of 3,6-dihydro-2//-pyran 93 via a gold(I)-catalyzed propargyl Claisen rearrangement from propargyl vinyl ether 92 possessing a pendant-free alcohol (Scheme 9.26) [47]. It is interesting to note that a total chirality transfer was observed when an enantiomer-ically enriched (81 % ee) propargyl ether was used in the process (dr > 14 1,81 % ee). 

In another related and well-known [3,3]-sigmatropic shift usually performed under thermal conditions, the propargyl-Claisen rearrangement,62 silver salts were also able to catalyze the reaction. Silver tetrafluoroborate and hexafluoroantimonate proved to be the best catalysts for this reaction, leading quantitatively to allenic p-ketoesters when starting from propargyl ethers derived from p-ketoesters (Scheme 3.41).63 64... 

Au-Catalyzed reaction of propargyl vinyl ethers furnished tri- and tetrasubstituted furans in high yields. The reaction proceeded through cyclization of 2-allenyl-13-dicarbonyl intermediates produced from propargyl-Claisen rearrangement <05OL3925>. 

Representative procedure for the gold-catalyzed Gosteli-Claisen rearrangement/ cycloisomerization of alkoxycarbonyl-substituted propargyl vinyl ethers. Isopropyl 4,5-dimethyl-3-(4-nitrophenyl)juran-2-carboxylat ... 

More recently, Saito and Hanzawa demonstrated that a variety of fused, tri-, tetra-and penta-substituted indoles 190 could be efficiently accessed via the Rh(I)-catalyzed cydoisomerization of N-propargyl anilines 188 (Scheme 9.68) [225, 226]. This transformation presumably involves the initial Rh(I)-catalyzed amino-Claisen rearrangement of 188 to produce a putative reactive allenyl aniline intermediate 189, which is smoothly converted into the indole core, as exemplified by the cycloisomerization of an independently prepared allene 191 (Scheme 9.69). On the other hand, the latter transformation of allenes can be achieved under thermal reaction conditions, albeit with a diminished effidency. 

Saito and Hanzawa have reported a cationic rhodium(I)-catalyzed aza-Claisen rearrangement of 7V-propargyl aniline 380. The un-catalyzed thermal variant required extremely high reaction temperatures and produced poor conversions. The mild protocol is an efficient entry for the synthesis of various indole derivatives. The reaction proceeds through the o-allenylaniline intermediate 381 and a subsequent cyclization results in 382. ... 

A mercury-free route to allyl vinyl ethers that relies on the Michael addition of allyl alcohols to unsubstituted alkenyl sulfoxides, followed by thermal loss of sulfenic acid and concurrent Claisen rearrangement has been described [145]. This methodology has been applied to the synthesis of isocar-bacyclin [146]. Posner reported an acid-catalyzed protocol that produces conjugated dienoate esters from allylic alcohols and a sulfinyl orthoester [147]. Additionally, the use of propargyl alcoholates and a chloro alkenyl sulfox-... 

For the design of organometallic catalytic reactions, one can apply stereoelectronic effects that are specific to the metal centers, such as the trans-effect. An interesting manifestation of this effect in the Rh-catalyzed Claisen rearrangement of propargyl vinyl ethers (Figure 11.13) was found in the computational analysis of this process. ... 

Figure 11.13 Rh-catalyzed Claisen rearrangement of propargyl vinyl ethers.

Figure 11.14 Energy profile of the Rh-catalyzed Claisen rearrangement of propargyl vinyl ethers initiated by coordination at the alkyne. Red and gray depict the cis and the trans orientations, respectively, of CO relative to substrate at the Rh center. Energies were calculated relative to the most stable Rh(l)-vinyl ether complex at the M05-2X/LANL2DZ level.

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