Asymmetric synthesis of optically active spiroacetals from alkynyl glycols catalyzed by gold complex/Brønsted acid binary system

Catalytic Asymmetric Hydroalkoxylation of C–C Multiple Bonds

“…A later example by Rexit and Mailikezati adopted a modified approach to successfully catalyze an asymmetric dihydroxylation to yield spiroacetals in good yields and enantioselectivities. 101 As will be discussed in section 5.1, highly enantioselective spiroacetalizations have been achieved under the influence of confined organic Brønsted acid catalysts. 102 While it is very prominent within Au(I) catalysis, the use of chiral counteranions to induce enantioselectivity has also been extended to other π-Lewis acidic metals.…”

“…The intermediacy of a putative enol ether has been verified by a control experiment, in which subjecting the separately prepared intermediate to the reaction condition leads to the experimentally observed product with identical selectivity. A later example by Rexit and Mailikezati adopted a modified approach to successfully catalyze an asymmetric dihydroxylation to yield spiroacetals in good yields and enantioselectivities . As will be discussed in section , highly enantioselective spiroacetalizations have been achieved under the influence of confined organic Brønsted acid catalysts…”

Preparation of benzannulated spiroketals by gold(III) catalyzed spirocyclization of alkynyl diols

“…The formation of a black solution immediately after silver salt addition indicated decomposition of the gold catalyst and that the reaction, in this case, might be gold(0) catalysed. As gold catalysed dihydroalkoxylation of alkynyl diols with chiral phosphoric acid co-catalysts has been successful 30,38,39 , a reaction with 10 mol% S-camphorsulfonic acid was performed. The addition of acid sped up the reaction up significantly and gave excellent yield of product 14, but did not afford stereoselectivity (93%, 1h, entry 8).…”

“…Only a few asymmetric approaches are known for the preparation of chiral spiroketals. [26][27][28][29][30][31][32] One successful strategy has demonstrated that chiral phosphoric acids (CPAs) may serve as effective catalysts for enantioselective spiroketalizations. 43 Despite the chiral nature of the Au(III)-BOX and Au(III)-PYR-OX catalysts 11 and 13, no enantioselectivity was obtained in our previous catalytic studies of alkoxycyclizations or in cyclopropanation reactions.…”

“…25 Despite the fact that several approaches have been developed for spiroketal synthesis, methods for direct catalytic enantioselective synthesis of chiral spiroketals by transition-metal catalysis or organocatalysis are still limited. [26][27][28][29][30][31][32] Highly enantioselective condensation reactions of unsaturated ketones to afford spiroketals (>99% ee) have been performed with Ir(I) complexes of chiral phosphine-oxazoline ligands 29 , while catalytic enantioselective spiroketalizations by single OH attack on cyclic enol substrates 27 are reported with both BINOL-derived chiral phosphoric acids (92% ee) 26 and chiral BINOL-based imidodiphosphoric acid (>99% ee). 32 Benzannulated spiroacetals are successfully formed by intramolecular tandem dihydroalkoxylation of alkynyl diols by binary systems of both gold(I) complexes with chiral Brønsted acids (up to 93% ee) 30 and chiral gold(I)-phosphine complexes with chiral silver phosphate (74% ee).…”