Biocatalytic Approaches to Hetero-Diels-Alder Adducts of Carbonyl Compounds

Stereoselective Synthesis of Oxacycles via Ruthenium-Catalyzed Atom-Economic Coupling of Propargyl Alcohols and Michael Acceptors

Synthesis of β-hydroxyenones and its application toward development of tetrahydro-4H-pyran-4-one in an atom-economic fashion is limited. This manuscript describes a ruthenium-catalyzed atom-economic coupling of pent-2-yne-1,5-diols and Michael acceptors as an efficient route for the synthesis of β-hydroxyenones with excellent yields and high regioselectivity. The β-hydroxyenones further undergo a 6-endo trig cyclization under acid-catalyzed conditions to deliver the tetrahydro-4H-pyran-4-ones with high diastereoselectivity. An intramolecular aldol condensation under mild basic conditions and palladium-catalyzed oxidative aromatization was developed for the synthesis of hexahydro-6H-isochromen-6-ones and isochromanols, respectively, from highly substituted tetrahydro-4H-pyran-4-ones with excellent yield and diastereoselectivity. Overall, this work demonstrates the synthetic potential toward the synthesis of oxacycles like tetrahydro-4H-pyran-4-ones, hexahydro-6H-isochromen-6-ones, and isochromanols via an atom-economic catalysis.

Chiral disulfonimides: a versatile template for asymmetric catalysis

Since the emergence of pseudo-C2-symmetric chiral phosphoric acids (CPA), much work has been done to utilize these systems in stereoselective, organocatalytic processes. Despite the success in this field, reasonably basic substrates such as imines are often required to achieve appreciable activation. In order to access a wider variety of potential reaction partners, many related organocatalysts with enhanced Brønsted acidity have since been developed. Chiral disulfonimides (DSIs) have materialized as one such powerful class of organocatalysts and have been shown to expand the list of potential substrates to include aldehydes and ketones via Brønsted, Lewis, or bifunctional acid activation. This versatility renders DSIs amenable to an impressive scope of reaction types, typically with remarkable stereoselectivity induced by asymmetric counteranion-directed catalysis (ACDC). This review serves to provide a complete analysis of the successful applications, mechanistic insights, and unmet challenges exhibited to date in DSI-catalyzed and -assisted processes.

Simple organocatalyst component system for asymmetric hetero Diels-Alder reaction of isatins with enones

A simple two catalyst component system consisting of primary β-amino alcohols as a catalyst and amino acids as a co-catalyst put together works as an efficient organocatalyst system in the hetero Diels-Alder reaction of isatins with enones to afford the chiral spirooxindole-tetrahydropyranones in good chemical yields and stereoselectivities (up to 86%, up to 85 : 15 dr., up to 95% ee).

Total synthesis of (−)-aritasone via the ultra-high pressure hetero-Diels–Alder dimerisation of (−)-pinocarvone

The total synthesis of aritasone via the proposed biosyntheic hetero-Diels–Alder [4 + 2] cyclodimerisation of pinocarvove, has been achieved under ultra-high pressure (19.9 kbar) conditions.

Direct construction of chiral quaternary dihydropyranones through highly enantioselective organocatalytic hetero-Diels–Alder reactions of olefinic azlactones

The first organocatalytic hetero-Diels–Alder reaction of an in situ generated diene with α-keto esters was reported for the construction of chiral dihydropyranones.

The asymmetric hetero-Diels-Alder reaction in the syntheses of biologically relevant compounds

The hetero-Diels-Alder reaction is one of the most powerful transformations in the chemistry toolbox for the synthesis of aza- and oxa-heterocycles embodying multiple stereogenic centers. However, as compared to other cycloadditions, in particular the dipolar cycloadditions and the Diels-Alder reaction, the hetero-Diels-Alder reaction has been much less explored and exploited in organic synthesis. Nevertheless, this powerful transformation has opened up efficient and creative routes to biologically relevant small molecules and different natural products which contain six-membered oxygen or nitrogen ring systems. Recent developments in this field, in particular in the establishment of enantioselectively catalyzed hetero-Diels-Alder cycloadditions steered by a plethora of different catalysts and the application of the resulting small molecules in chemical biology and medicinal chemistry research, are highlighted in this Minireview.

Chiral metal phosphate catalysis: highly asymmetric hetero-Diels–Alder reactions

A highly enantioselective hetero-Diels–Alder reaction of α-keto esters and isatins catalyzed by chiral calcium BINOL-derived phosphates (X-ray) is reported.

Chiral gold complex-catalyzed hetero-Diels-Alder reaction of diazenes: highly enantioselective and general for dienes

A chiral gold(I) complex-catalyzed highly regio- and enantioselective azo hetero-Diels-Alder reaction has been developed. The chiral gold(I) complex acting as a Lewis acid exhibits high efficiency in the activation of urea-based diazene dienophiles. Moreover, this chiral gold catalyst also rendered a cascade intramolecular enyne cycloisomerization/asymmetric azo-HDA reaction.

Catalytic enantioselective hetero-Diels-Alder reactions of an azo compound

This communication describes studies in which an azo hetero-Diels-Alder adduct was furnished in high regio- and enantioselectivity using azopyridine as a reagent and silver as a catalyst. The obtained hetero-Diels-Alder adduct was easily converted to the corresponding chiral 1,4-diamino alcohol.

Probing the Limits of Rate Acceleration Mediated by Hydrogen Bonds

[Structure: see text] A simple receptor and substrate are used to probe the relationship between transition-state charge and the level of rate acceleration that can be created by stabilizing the transition state through hydrogen bonding. Pericyclic reactions are accelerated less than 2-fold by the receptor, whereas a conjugate addition reaction is accelerated more than 30-fold. Therefore, substrate polarization by hydrogen bonding would only appear to be effective for reactions that generate significant charge at the transition state.

Intramolecular Hetero-Michael Addition of β-Hydroxyenones for the Preparation of Highly Substituted Tetrahydropyranones

Structurally diverse beta-hydroxyenones are shown to undergo nonoxidative 6-endo-trig ring closure to form highly substituted tetrahydropyranones. Amberlyst-15, Al(ClO(4))(3) x 9 H(2)O and [Pd(MeCN)(4)](BF(4))(2) were found to be suitable catalysts for these intramolecular conjugate additions, preventing side reactions, such as dehydration or retroaldolisation. The use of [Pd(MeCN)(4)](BF(4))(2) is particularly effective, as this palladium-mediated reaction is under kinetic control and generates tri- and tetrasubstituted tetrahydropyranones with high levels of diastereocontrol. In the presence of the Lewis acid Al(ClO(4))(3) x 9 H(2)O, the reaction proceeded with a similar level of diastereocontrol; however, in contrast to [Pd(MeCN)(4)](BF(4))(2), this catalyst can promote enolisation. The palladium-mediated reaction was also found to be compatible with an enantioenriched beta-hydroxyenone substrate, giving no loss of enantiopurity upon ring closure. The most distinctive synthetic development to emerge from this new chemistry is the possibility to access tri- and tetrasubstituted 2,6-anti-tetrahydropyranones from anti-aldol precursors. These compounds are particularly difficult to access by using alternative methodologies. Two modes of activation were envisaged for the ring closure, involving metal coordination to either the C=C or C=O functional groups. Experimental results suggest that C=O coordination was the preferred mode of activation for reactions performed in the presence of Al(ClO(4))(3) x 9 H(2)O or [Pd(MeCN)(4)](BF(4))(2).