Gold(I)-Catalyzed [2 + 2 + 2] Cyclotrimerization of 1,3-Diarylpropargyl Acetals

Abstract

A gold-nitrone catalyzed [2 + 2 + 2] cyclotrimerization of 1,3-diarylpropargyl acetals into cyclohexylidene products (up to 74% yield) is reported. The trimerization is proposed to proceed through allenic intermediates via gold-catalyzed 1,3-alkoxy rearrangement. The presence of catalytic amounts of different nitrones, tuning of the Au(I) catalyst activity, was essential for controlled regio-/chemoselective cyclotrimerization. A linear nitrone-O-Au(I)-P coordination mode was shown (X-ray analysis) for a catalytic active phosphane-gold-nitrone complex, representing a group of Au(I) catalysts with specific properties.

Regio‐ and Site‐selective Molecular Rearrangements by Homogeneous Gold Catalysis

“…The selectivity of the overall process could be attributed to the 5‐ exo ‐ dig cyclization driven by electron deficient carboxyl group at the alkyne terminus. However, 1,3‐diarylpropargyl acetals 145 were reported to undergo 1,3‐alkoxy rearrangement by an in situ formed linearly coordinated Au(I)‐nitrone complex to form [2+2+2] cyclotrimerized product 146 (Scheme 35, eq 2) [192] . This tandem chemistry reported by Fiksdahl et al ., involves the Au(I)‐triggered selective 1,3‐alkoxy shift suitable for the formation of allenic intermediates which then subsequently undergoes trimerization to generate densely functionalized cyclohexylidene products 146 .…”

Asymmetric Total Syntheses of Kopsane Alkaloids via a PtCl2‐Catalyzed Intramolecular [3+2] Cycloaddition

“…Compound 17 reacted with NaH in DMF in the presence of 4‐methoxybenzyl bromide or methyl iodide to give the N ‐PMB derivative 18 a (85 % yield) or N ‐methylated indole 18 b (87 % yield), respectively. Compounds 18 a and 18 b could then be converted into the corresponding 1,3‐dipole precursors 9 a (61 % yield) and 9 b (58 % yield) by a three‐step sequence involving oxidative cleavage of the olefin, Grignard addition of the resultant aldehyde with ethynylmagnesium bromide, and subsequent PPTS‐promoted ketal formation …”

Gold(I)‐Catalysed Azepine Synthesis from Propargyl Acetals and Aryl Azides

“…Alkylpropargyl ethyl acetal 1e reacted only slowly with azide 2b to give 22 % of tetramethylazepine 3g after 4 d; most of the acetal (i.e., 1e; approx. [18] A separate gold-catalysed [4+3] cycloaddition between acetal 1a and 2-methoxy vinyl imine 4, prepared from propargyl acetal 1d and azide 2b as described above (Table 1, entry 6), was carried out, and trans-azepine product 3h (54 %; Scheme 3) was successfully obtained. Alkyl azide 2d hardly reacted at all ( Table 1, entry 8).…”