Synthesis of the C20–C32 Tetrahydropyran Core of the Phorboxazoles and the C22 Epimer via a Stereodivergent Michael Reaction

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.

Towards understanding oxygen heterocycle-forming biocatalysts: a selectivity study of the pyran synthase PedPS7

Intramolecular oxa-Michael addition-catalysing cyclases are widespread in polyketide biosynthetic pathways. Although they have significant potential in biotechnology and chemoenzymatic synthesis of chiral heterocycles, they have only scarcely been studied. Here, we present detailed investigations on the selectivity profile of the pyran synthase PedPS7 showing that it combines broad substrate tolerance with high selectivity for the formation of up to two new stereocentres and relaxed precursor stereoisomer discrimination. Two of the four possible tetrahydropyran stereoisomers are reliably accessible by this enzyme. The results indicate fundamental differences between the individual subtypes of intramolecular oxa-Michael addition-catalysing cyclases.

Direct Catalytic Asymmetric Synthesis of Oxindole-Derived δ-Hydroxy-β-ketoesters by Aldol Reactions

Direct asymmetric synthesis of δ-hydroxy-β-ketoesters was accomplished via regio- and enantioselective aldol reactions of β-ketoesters with isatins catalyzed by cinchona alkaloid thiourea derivatives. The C-C bond formation of the reactions occurred only at the γ-position of the β-ketoesters. Reaction progress monitoring and product stability analyses under the conditions that included the catalyst indicated that the γ-position reaction products were formed kinetically. Various δ-hydroxy-β-ketoesters bearing 3-alkyl-3-hydroxyoxindole cores relevant to the development of bioactive molecules were synthesized.

Muscarine, imidazole, oxazole and thiazole alkaloids

Covering: July 2012 to June 2015. Previous review: Nat. Prod. Rep., 2013, 30, 869-915The structurally diverse imidazole-, oxazole-, and thiazole-containing secondary metabolites are widely distributed in terrestrial and marine environments, and exhibit extensive pharmacological activities. In this review the latest progress involving the isolation, biological activities, and chemical and biogenetic synthesis studies on these natural products has been summarized.

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.

A Maitland-Japp inspired synthesis of dihydropyran-4-ones and their stereoselective conversion to functionalised tetrahydropyran-4-ones

The Maitland-Japp reaction has been extended to the synthesis of highly functionalised dihydropyran-4-ones. These dihydropyran-4-ones can in turn be converted stereoselectively into tetrahydropyran-4-ones with tertiary and quaternary stereocentres via the one-pot addition of hydride or carbon nucleophiles and trapping with carbon electrophiles. The utility of this method is demonstrated by providing access to the functionalised tetrahydropyran units present in a component of the Civet fragrance and the anticancer polyketide lasonolide A.

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.