Biosynthesis-Inspired Intramolecular Oxa-Conjugate Cyclization of α,β-Unsaturated Thioesters: Stereoselective Synthesis of 2,6-cis-Substituted Tetrahydropyrans

Stereodivergent Assembly of 2,6-cis- and -trans-Tetrahydropyrans via Base-Mediated Oxa-Michael Cyclization: The Key Role of the TMEDA Additive

The stereodivergent synthesis of cis- and trans-2,6-disubstituted tetrahydropyrans (THPs) via sodium hexamethyldisilazide-promoted oxa-Michael cyclization of (E)-ζ-hydroxy α,β-unsaturated esters is presented. The cyclization affords the kinetically favored trans-THPs with high stereoselectivity (dr up to 93:7) at a low temperature (-78 °C), while the room-temperature reaction does not produce the thermodynamically preferred cis-THPs as major products and occurs with poor stereocontrol. The addition of tetramethylethylenediamine (TMEDA) significantly improves the stereochemical outcome of the room-temperature cyclization and allows attaining high cis-selectivity (dr up to 99:1). The remarkable effect of TMEDA indicates that the sodium cation plays an important role in controlling the stereoselectivity of the thermodynamically driven process, that is, complexation of the cation with the cyclization products results in diminished selectivity. DFT calculations support this conclusion, indicating a greater difference in Gibbs energies of sodium-free cis- and trans-enolates compared to the respective sodium chelate complexes. The synthetic utility of the method has been demonstrated by the formal syntheses of (+)-Neopeltolide and (-)-Diospongin B and the total synthesis of (-)-Diospongin A.

Nickel-Catalyzed Carbonylative Synthesis of α,β-Unsaturated Thioesters from Vinyl Triflates and Arylsulfonyl Chlorides

A straightforward and efficient synthesis of α,β-unsaturated thioesters has been developed via a nickel-catalyzed thiocarbonylation reaction of vinyl triflates with arylsulfonyl chlorides. With Mo(CO)₆ as both CO source and reductant, a variety of α,β-unsaturated thioesters were obtained in moderate to good yields with very good functional group compatibility. It is noteworthy that the present method is the first example on nickel-catalyzed carbonylative synthesis of α,β-unsaturated thioesters by using arylsulfonyl chlorides as the coupling partner.

Enantioselective "clip-cycle" synthesis of di-, tri- and spiro-substituted tetrahydropyrans

ω-Unsaturated alcohols were "clipped" via alkene metathesis to a thioester activating group, which was followed by a chiral phosphoric acid catalyzed intramolecular oxa-Michael cyclization to yield tetrahydropyrans and spiro-tetrahydropyrans with excellent enantioselectivity. The mechanism and origin of the enantioselectivity was probed by DFT calculations and kinetic isotope studies, where there was excellent correlation between the computational and synthetic investigations.

The stereodivergent formation of 2,6-cis and 2,6-trans-tetrahydropyrans: experimental and computational investigation of the mechanism of a thioester oxy-Michael cyclization

Computational and synthetic studies have elucidated the origins of stereodivergence in an oxy-Michael synthesis of 2,6-disubstituted tetrahydropyrans.

The origins of the stereodivergence in the thioester oxy-Michael cyclization for the formation of 4-hydroxy-2,6-cis- or 2,6-trans-substituted tetrahydropyran rings under different conditions was investigated both computationally and experimentally. Synthetic studies showed that the 4-hydroxyl group was essential for stereodivergence. When the 4-hydroxyl group was present, TBAF-mediated conditions gave the 2,6-trans-tetrahydropyran and trifluoroacetic acid-mediated conditions gave the 2,6-cis-tetrahydropyran. This stereodivergence vanished when the hydroxyl group was removed or protected. Computational studies revealed that: (i) the trifluoroacetic acid catalysed formation of 2,6-cis-tetrahydropyrans was mediated by a trifluoroacetate-hydroxonium bridge and proceeded via a chair-like transition state; (ii) the TBAF-mediated formation of 2,6-trans-tetrahydropyrans proceeded via a boat-like transition state, where the 4-hydroxyl group formed a crucial hydrogen bond to the cyclizing alkoxide; (iii) both reactions are under kinetic control. The utility of this stereodivergent approach for the formation of 4-hydroxy-2,6-substituted tetrahydropyran rings has been demonstrated by the total syntheses of the anti-osteoporotic natural products diospongin A and B.

Total synthesis of (+)-herboxidiene/GEX 1A

An efficient synthesis of herboxidiene is granted from highly stereoselective aldol reactions from two lactate-derived ketones and an oxa-Michael cyclization.

Total Synthesis of (+)-SCH 351448: Efficiency via Chemoselectivity and Redox-Economy Powered by Metal Catalysis

The polyketide natural product (+)-SCH 351448, a macrodiolide ionophore bearing 14 stereogenic centers, is prepared in 14 steps (LLS). In eight prior syntheses, 22-32 steps were required. Multiple chemoselective and redox-economic functional group interconversions collectively contribute to a step-change in efficiency.

Strategies for the construction of tetrahydropyran rings in the synthesis of natural products

This review focuses on the methodology used for the construction of tetrahydropyran (THP) rings in the synthesis of natural products over the last seven years. While methods like cyclisation onto oxocarbenium ions, reduction of cyclic hemi-ketals, Michael reactions, hetero-Diels-Alder cycloadditions and cyclisations onto epoxides continue to find application, several other strategies including metal-mediated cyclisations, ring-closing metathesis, radical cyclisations and carbocation cyclisations have also found use. This review is intended to provide an overview of the area for those who are unfamiliar, and to refresh and remind those who do work in the area of the exciting developments in the field.

Polyketide construction via hydrohydroxyalkylation and related alcohol C-H functionalizations: reinventing the chemistry of carbonyl addition

Despite the longstanding importance of polyketide natural products in human medicine, nearly all commercial polyketide-based drugs are prepared through fermentation or semi-synthesis. The paucity of manufacturing routes involving de novo chemical synthesis reflects the inability of current methods to concisely address the preparation of these complex structures. Direct alcohol C-H bond functionalization via"C-C bond forming transfer hydrogenation" provides a powerful, new means of constructing type I polyketides that bypasses stoichiometric use of chiral auxiliaries, premetallated C-nucleophiles, and discrete alcohol-to-aldehyde redox reactions. Using this emergent technology, total syntheses of 6-deoxyerythronolide B, bryostatin 7, trienomycins A and F, cyanolide A, roxaticin, and formal syntheses of rifamycin S and scytophycin C, were accomplished. These syntheses represent the most concise routes reported to any member of the respective natural product families.

A tandem isomerization/prins strategy: iridium(III)/Brønsted acid cooperative catalysis

Working together: A mild and efficient isomerization/protonation sequence generates pyran-fused indoles by cooperative catalysis between cationic iridium(III) and Bi(OTf)3 . Three distinct cyclization manifolds lead to the corresponding bioactive scaffolds in good yields. In addition, N-substituted indoles can be synthesized enantioselectively in the presence of a chiral phosphate.

Total synthesis of (+)-herboxidiene from two chiral lactate-derived ketones

A substrate-controlled synthesis of (+)-herboxidiene from two lactate-derived chiral ketones is described. Remarkably, most of the carbon backbone was constructed through highly stereoselective titanium-mediated aldol reactions and an Ireland-Claisen rearrangement. Furthermore, an oxa-Michael cyclization and a high-yield Suzuki coupling were used to assemble the pyran ring and the diene moiety respectively.