Propargyl Claisen

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... 

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... 

The Kirsch group studied the propargyl Claisen reaction, which consists of an initial [3,3]-sigmatropic reaction and a subsequent ring closure (Scheme 12.11).26... 

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>. 

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>. 

SYNTHESIS OF FURANS AND PYRROLES VIA PROPARGYL CLAISEN REARRANGEMENT... 

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

Scheme 5 Cascade reaction consisting of propargyl Claisen rearrangement and cyclocondensation.

Scheme 14 Synthesis of pyrroles via propargyl Claisen rearrangement.

Scheme 20 2H-Pyrans synthesized by propargyl Claisen rearrangement/isomerization cascade.

We attempted the synthesis of 1,2-dihydropyridines from propargyl vinyl ethers in an independent study (11JOC2145). In our early experiments, we studied propargyl vinyl ethers 79 with additional alkyl substituents at the vinylic position. The propargyl Claisen rearrangement was effected by catalytic amounts of silver(I) hexafluoroantimonate, and the addition of an amine to the aUenylcarbonyl intermediate was expected to provide the 1,2-dihydropyridine product 80. Unfortunately, the major product was, in aU cases, the 2ff-pyran 81, and the desired dihydropyridine 80 was only formed in yields ranging from 10% to 44% when aniline derivatives were employed (Scheme 27).With aliphatic amines, the formation of 1,2-dihydropyridines was not observed. 

Propargyl Claisen rearrangement AUene synthesis and beyond 13CSR458. Smiles rearrangement in synthetic, mainly, heterocycHc chemistry 13COS935. 

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. 

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