Controlling Regiochemistry in the Gold-Catalyzed Synthesis of Unsaturated Spiroketals

Regioselective Synthesis of Benzannulated [5,6]-Oxaspirolactones via Cu(II)-Catalyzed Cycloisomerization of 2-(5-Hydroxyalkynyl)benzoates

Spiroketals and oxaspirolactones are widely found in biologically active natural products, serving as important structural motifs. In this study, we present a Cu(II)-catalyzed cascade cycloisomerization of 2-(5-hydroxyalkynyl)benzoates, enabling the regioselective synthesis of benzannulated [5,6]-oxaspirolactones containing an isochromen-1-one moiety. This strategy offers a rapid and efficient approach to access a diverse array of benzannulated [5,6]-oxaspirolactones. The methodology presented here showcases a broad substrate scope, delivering good yields and scalability up to gram scale. The structures of the oxaspirolactones were unequivocally confirmed through single-crystal X-ray analysis and by analogy using 1H and 13C{1H} NMR data.

Gold(I)-Catalyzed Synthesis of Heterocycles via Allene Oxide from Propargylic Alcohols

A new mechanistic pathway of propargylic alcohol activation by gold(I) catalysis has been proposed toward the efficient synthesis of N-protected pyrroles, 5,6-dihydropyridin-3(4H)-ones from N-protected 5-aminopent-2-yn-1-ol, and 5-aminopent-2-yn-1-ol. Control experiments support that the reaction proceeded via the neighboring group participation of the oxygen atom of propargylic alcohol to form an allene oxide intermediate where the nucleophilic heteroatom attacks intramolecularly. Further, this methodology is successfully extrapolated toward the atom-economic synthesis of hydroxyalkyl indoles and benzofurans. The short reaction time of 30 s, low catalyst loading of 0.5 mol %, high yield, variation in the substrate scope, and procedurally simple open-flask reaction conditions make this methodology highly applied.

Mechanistic Study on Gold(I)-Catalyzed Unsaturated Spiroketalization Reaction

Abstract:

The mechanism of metal-catalyzed spiroketalization of propargyl acetonide is explored by employing DFT with the B3LYP/6-31+G(d) method. Acetonide is used as a regioselective regulator in the formation of monounsaturated spiroketal. The energies of transition states, intermediates, reactants and products are calculated to provide new insight into the mechanism of the reaction. The energetic features, validation of the observed trends in regioselectivity are conferred in terms of electronic indices via FMO analysis. The presence of acetonide facilitates a stepwise spiroketalization as it masks the competing nucleophile, and thus hydroxyl group present, exclusively acts as a nucleophile. The vinyl gold intermediate 3 is formed from 2 via activation barrier TS1. This is the first ring formation, which is 6-exo-dig cyclization. The intermediate 3 is converted into allenyl ether 4, which isomerizes to the intermediate oxocarbenium ion 5 via activation barrier TS2. The intermediate 5 cyclizes to 6 via TS3. This is the second ring formation. The intermediate 6 on protodeauration turns into 6,6-monounsaturated spiroketal 7. It is concluded that acetonide as a protecting group serves the purpose, and thus a wide range of spiroketals can be prepared, regioselectivity.

A silver-catalyzed [3 + 3]-annulation cascade of alkynyl alcohols and α,β-unsaturated ketones for the regioselective assembly of chromanes

An unprecedented Ag(i)-catalyzed [3 + 3]-annulation of alkynyl alcohols (5-hexyn-1-ols) and α,β-unsaturated ketones is reported to construct simple to complex chromanes.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

The synthesis of biologically active indolocarbazole natural products

Covering: up to 2020The indolocarbazoles, in particular indolo[2,3-a]pyrrolo[3,4-c]carbazole derivatives, are an important class of natural products that exhibit a wide range of biological activities. There has been a plethora of synthetic approaches to this family of natural products, leading to advances in chemical methodology, as well as affording access to molecular scaffolds central to protein kinase drug discovery programmes. In this review, we compile and summarise the synthetic approaches to the indolo[2,3-a]pyrrolo[3,4-c]carbazole derivatives, spanning the period from their isolation in 1980 up to 2020. The selected natural products include indolocarbazoles not functionalised at indolic nitrogen, pyranosylated indolocarbazoles, furanosylated indolocarbazoles and disaccharideindolocarbazoles.

Bismuth(III)-catalyzed bis-cyclization of propargylic diol-esters: a unified approach for the synthesis of [5,5]- and [6,5]-oxaspirolactones

Herein we disclose an unprecedented intramolecular cascade strategy for the construction of α,β-unsaturated [5,5]- and [6,5]-oxaspirolactones that capitalizes on the π-electrophilic Lewis acid-catalyzed 5-exo-dig or 6-exo-dig mode of cyclization of propargylic diol esters, followed by dehydration and spirolactonization steps. Moreover, semi-protected substrates also delivered the respective oxaspirolactones with the same ease and in appreciable yields under optimal reaction conditions.

Synthesis and Biological Evaluation of the Southern Hemisphere of Spirastrellolide A and Analogues

The synthesis and biological evaluation of truncated spirastrellolide A analogues comprised of the southern hemisphere against protein phosphatase 2A are described. A convergent synthesis was designed featuring two gold-catalyzed cyclization reactions, specifically, a dehydrative cyclization of monoallylic diols for the synthesis of the tetrahydropyran (A-ring) and a regioselective spiroketalization for the efficient generation of the [6,6]-spiroketal (B, C-ring system). The synthesis of the southern hemisphere of spirastrellolide A was achieved involving the longest linear sequence of 19 steps. A total of eight spirastrellolide A analogues were synthesized, and preliminary PP2A enzyme assay inhibition studies were performed for the first time on analogues of the southern hemisphere. Several analogues showed inhibition, which is a positive indication and perhaps suggests that the unsaturated spiroketal fragment might be crucial to induce PP2A inhibition.

Palladium catalyzed synthesis of benzannulated steroid spiroketals

Nine cytotoxic [5/7] and [6/6] benzannulated steroid spiroketals were synthesized by palladium catalyzed spiroketalization of 5α and 5β-alkynediols derived from testosterone, diosgenin and cholesterol.

Catalytic asymmetric synthesis of geminal-dicarboxylates

Stereogenic acetals, spiroacetals and ketals are well-studied stereochemical features that bear two heteroatoms at a common carbon atom. These stereocenters are normally found in cyclic structures while linear (or acyclic) analogues bearing two heteroatoms are rare. Chiral geminal-dicarboxylates are illustrative, there is no current way to access this class of compounds while controlling the stereochemistry at the carbon center bound to two oxygen atoms. Here we report a rhodium-catalysed asymmetric carboxylation of ester-containing allylic bromides to form stereogenic carbon centers bearing two different carboxylates with high yields and enantioselectivities. The products, which are surprisingly stable to a variety of acidic and basic conditions, can be manipulated with no loss of enantiomeric purity as demonstrated by ring closing metathesis reactions to form chiral lactones, which have been extensively used as building blocks in asymmetric synthesis.

Catalytic Dehydrative Lactonization of Allylic Alcohols

A convenient strategy for the synthesis of phthalides and γ-butyrolactones is reported. The method utilizes readily prepared allylic alcohols in formal Au(I)- and Pd(II)-catalyzed S_N2' reactions. Using these catalysts, exclusive formation of the desired five-membered lactones is observed, completely avoiding the competing direct lactonization pathway that forms the undesired seven-membered ring with protic acids and alternative metal salts. This mild and operationally simple method notably tolerates exomethylene groups and should find use in both phthalide and terpene syntheses.

Gold catalysis: synthesis of spiro, bridged, and fused ketal natural products

This review summarises the application of gold catalysis for the syntheses of spiro, bridged and fused ketal natural products.

Synthesis of a C(1)-C(23) fragment for spirastrellolide E: development of a mechanistic rationale for spiroketalization

Synthetic analysis of spirastrellolide E envisioned to entail a cross-metathesis union of the northern and southern hemispheres followed by a Sharpless epoxidation/methylation sequence to achieve the C(22,23) stereogenicity leads to the design of a C(1)–C(23) advanced southern hemisphere exploiting a gold-catalyzed directed spiroketalization as a key step. Stereochemical analysis of this strategic transformation provides insight on the impact of the directing group carbinol stereogenicity on the reaction efficiency and, in turn, permits the conversion of the minor isomer of the spiroketal precursor to the requisite congener for successful spiroketalization.

Regioselectivity in the Au-catalyzed hydration and hydroalkoxylation of alkynes

The issue of regioselectivity in the Au-catalyzed hydration and hydroalkoxylation of alkynes is a perpetual challenge that essentially remains an unsolved problem with respect to general solutions.