Gold-catalyzed regioselective oxidation of propargylic carboxylates: a reliable access to α-carboxy-α,β-unsaturated ketones/aldehydes

Abstract

SummaryGold-catalyzed intermolecular oxidation of carboxylates of primary or secondary propargylic alcohols are realized with excellent regioselectivity, which is ascribed to inductive polarization of the C–C triple bond by the electron-withdrawing carboxy group. The gold carbene intermediates thus generated undergo selective 1,2-acyloxy migration over a 1,2-C–H insertion, and the selectivities could be dramatically improved by the use of a P,S-bidentate ligand, which is proposed to enable the formation of tris-coordinated and hence less electrophilic gold carbene species. α-Carboxy α,β-unsaturated ketones/aldehydes can be obtained with fair to excellent yields.

Gold‐Catalyzed Synthesis of Quinolines from Propargyl Silyl Ethers and Anthranils through the Umpolung of a Gold Carbene Carbon

“…Preliminary mechanistic studies were also conducted. The reaction of anthranil and a propargylic ester afforded the 1,2‐acyloxy migration product, which suggests a gold carbene intermediate formed through ring opening of the anthranil, which is trapped in a subsequent step by the adjacent acetoxy group (Scheme a). The cyclization process of the silyl propargyl ether substrate follows a Mukaiyama‐type process, which was verified by a 18 O‐labelling experiment (Scheme ).…”

Gold‐Catalyzed Ring Expansion of Alkynyl Heterocycles through 1,2‐Migration of an Endocyclic Carbon–Heteroatom Bond

“…In this oxidative ring expansion, gold–carbene species 16 may not be involved, since if it were generated, an alkene product would possibly be formed through competing and highly favorable 1,2‐CH insertion from the ring side, as indicated in many gold‐catalyzed reactions involving alkyl‐substituted gold–carbene species 5e,i,l. 10a, 11 However, no such product was detected in our case. It is also unlikely to involve the formation of gold–carbene 18 in the course of oxidation.…”

Recent Developments in the Chemistry of Heteroaromatic N-Oxides

“…As shown in the scheme, the oxidation with 8-isopropylquinoline N-oxide could be performed at a temperature as low as -20 °C, indicating facile formation of the gold carbene intermediate; moreover, the oxidant attacks highly regioselectively at the less hindered end of a sterically biased carbon-carbon triple bond, thereby leading to selective formation of the enone products such as 7. In addition, electronic factors such as conjugation 27 and inductive effects can also impose good to excellent regioselectivity under the reaction conditions, as demonstrated in the cases of 8 and 9. This regioselective oxidation of alkynes into enones offers a synthetically useful strategy of masking enones as…”