Highly diastereoselective chelation-controlled additions to α-silyloxy ketones

Genome-based discovery and total synthesis of janustatins, potent cytotoxins from a plant-associated bacterium

Host-associated bacteria are increasingly being recognized as underexplored sources of bioactive natural products with unprecedented chemical scaffolds. A recently identified example is the plant-root-associated marine bacterium Gynuella sunshinyii of the chemically underexplored order Oceanospirillales. Its genome contains at least 22 biosynthetic gene clusters, suggesting a rich and mostly uncharacterized specialized metabolism. Here, in silico chemical prediction of a non-canonical polyketide synthase cluster has led to the discovery of janustatins, structurally unprecedented polyketide alkaloids with potent cytotoxicity that are produced in minute quantities. A combination of MS and two-dimensional NMR experiments, density functional theory calculations of 13C chemical shifts and semiquantitative interpretation of transverse rotating-frame Overhauser effect spectroscopy data were conducted to determine the relative configuration, which enabled the total synthesis of both enantiomers and assignment of the absolute configuration. Janustatins feature a previously unknown pyridodihydropyranone heterocycle and an unusual biological activity consisting of delayed, synchronized cell death at subnanomolar concentrations.

Accessing Diverse Cross-Benzoin and α-Siloxy Ketone Products via Acyl Substitution Chemistry

An approach to diverse cross-benzoin and α-siloxy ketone products which leverages a simple yet underutilized C-C bond disconnection strategy is reported. Acyl substitution of readily accessible α-siloxy Weinreb amides with organolithium compounds enables access to a broad scope of aryl, heteroaryl, alkyl, alkenyl, and alkynyl derivatives. Enantiopure benzoins can be accessed via a chiral pool approach, and the utility of accessible cross-benzoins and α-siloxy ketones is highlighted in a suite of downstream synthetic applications.

Catalytic Reductive Cross-Coupling between Aromatic Aldehydes and Arylnitriles

A reductive cross-coupling reaction between aromatic aldehydes and arylnitriles using a copper catalyst and a silylboronate as a reductant is reported. This protocol represents an unprecedented approach to the chemoselective synthesis of α-hydroxy ketones by electrophile-electrophile cross-coupling.

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

Chelation-Controlled Additions to Chiral α- and β-Silyloxy, α-Halo, and β-Vinyl Carbonyl Compounds

The science and art of preventing and managing disease and prolonging life is dependent on advances in medicine, biology, and biochemistry. Many of these advances will involve interactions of small molecules with biological entities. As such, they will rely on the efficient synthesis of active compounds with very high stereochemical purity. Although enantioselective reactions are important in this regard, most stereocenters in complex molecule synthesis are installed in diastereoselective reactions. Perhaps the most well-known diastereoselective C-C bond-forming reaction is the addition of nucleophiles to carbonyl groups with α- or β-stereogenic centers. Diastereoselective additions of organometallic reagents to protected chiral α- and β-hydroxy aldehydes and ketones are described by either Cram chelation or Felkin-Anh models, which are protecting group (PG)-dependent. Small PGs (X = OMOM, OBn, etc.) favor Cram chelation, wherein both the carbonyl group and the O-PG bind to the Lewis acidic metal, providing syn diol motifs. In contrast, silyl PGs, with the OSiR₃ moiety being both bulky and weakly coordinating, provide anti diols (Felkin addition). It is well-known that exceptions to this paradigm are scarce. Therefore, the choice of PG is based on the desired stereochemical outcome in the addition step and is often inappropriate for the global protection strategy. Thus, it is critical to develop general methods for chelation-controlled additions of organometallics to chiral silyloxy aldehydes and ketones. Once the challenge of developing chelation-controlled additions to silyloxy carbonyl compounds can be met, the next question is what other pendant functional groups can chelate? Herein we introduce the first general methods for the chelation-controlled addition of organometallics to chiral silyloxy aldehydes and ketones. A wide variety of organozinc reagents have been used in these addition reactions, including dialkylzinc reagents that are commercially available or generated using Knochel's methods. Existing protocols for the generation of (E)-di- and -trisubstituted vinylzinc reagents have been employed, and new methods for the generation of (Z)-di- and -trisubstituted vinylzinc reagents have been developed. The generation of 1,1-heterobimetallic reagents based on boron and zinc has been advanced, and the addition of these reagents to silyloxy aldehydes via chelation-control is included. We will first describe the initial discovery and a model to explain the observed diastereoselectivities. A wide array of chelation-controlled additions to chiral α- and β-silyloxy aldehydes and ketones will then be presented. We next describe other functional groups that undergo chelation-controlled additions. α-Halo aldehyde derivatives are well-known to favor Felkin addition (via the Cornforth-Evans model). We introduce a general method for chelation-controlled additions to α-halo aldimines that provides useful precursors to aziridines. Finally, we provide preliminary evidence that even C═C bonds can play the role of chelating groups in additions to β,γ-unsaturated ketones. The results outlined in this Account redefine the commonly held idea that chiral silyloxy- and halo-substituted carbonyl compounds only give Felkin addition products. The key to achieving chelation control in these reactions is the use of weakly coordinating solvents (dichloromethane and toluene) that do not readily bind to the zinc Lewis acids RZnX.

Stereoselective Arylation of Amino Aldehydes: Overriding Natural Substrate Control through Chelation

The chelation-controlled arylation reaction of chiral, enantiopure acyclic α-amino aldehydes enabled by a B/Zn exchange reaction between arylboronic acids and Et₂Zn is reported. The presence of dibenzyl substituents at the nitrogen plays a key role in the stereochemical outcome of the reaction, and chelation is favored over the natural tendency of this type of substrate to undergo Felkin-Anh controlled additions with organomagnesium and organolithium reagents.

A Highly Convergent Total Synthesis of Leustroducsin B

Leustroducsin B exhibits a large variety of biological activities and unique structural features. An efficient and highly convergent total synthesis of Leustroducsin B was achieved in 17 longest linear and 39 total steps by disconnecting the molecule into three fragments having similar levels of complexity. These pieces were connected via a highly efficient chelate-controlled addition of a vinyl zincate to an α-hydroxy ketone and a silicon-mediated cross-coupling. The stereochemistry of the central and western fragments was set catalytically in high yields and excellent de by a zinc-ProPhenol-catalyzed aldol reaction and a palladium-catalyzed asymmetric allylic alkylation.

A useful methoxyvinyl cation equivalent: α-t-butyldimethylsilyl-α-methoxyacetaldehyde

Described are the synthesis and application of α-t-butyldimethylsilyl-α-methoxyacetaldehyde as a formal methoxyvinyl cation equivalent. Addition of Grignard reagents to the title aldehyde, followed by treatment of the intermediate β-hydroxysilanes with KH, gives good yields of large Z-methoxyvinylated products. Assuming a Peterson-like elimination mechanism, one can infer that the Grignard addition proceeds with high syn selectivity. These results are consistent with a chelation control model involving coordination to the α-methoxy group in the title aldehyde rather than an alternative stereoelectronic Felkin-Anh-type model. It must be noted that a steric Felkin-Anh model also accounts for the observed stereochemistry. All told, the title reagent can be employed to efficiently append a Z-configured methoxyvinyl group to an appropriate R-M species, in two steps.

Alkenes as Chelating Groups in Diastereoselective Additions of Organometallics to Ketones

Alkenes have been discovered to be chelating groups to Zn(II), enforcing highly stereoselective additions of organozincs to β,γ-unsaturated ketones. ¹H NMR studies and DFT calculations provide support for this surprising chelation mode. The results expand the range of coordinating groups for chelation-controlled carbonyl additions from heteroatom Lewis bases to simple C–C double bonds, broadening the 60 year old paradigm.

Chelation-controlled additions to α-silyloxy aldehydes: an autocatalytic approach

The Felkin-Anh model has been widely accepted to describe stereochemical outcomes in nucleophilic additions to α-silyloxy carbonyl compounds. Herein, it is demonstrated that chelation-controlled additions can be performed using dialkylzinc reagents in the presence of chlorotrimethylsilane with good to excellent diastereoselectivities. Ethyl zinc chloride, the Lewis acid responsible for promoting chelation, is generated in situ in an autocatalytic fashion. This approach circumvents its use in stoichiometric amounts.

Diastereoselective chelation-controlled additions to β-silyloxy aldehydes

A general diastereoselective method for the addition of dialkylzincs and (E)-di- and (E)-trisubstituted vinylzinc reagents to β-silyloxy aldehydes is presented. This method employs alkyl zinc triflate and nonaflate Lewis acids and affords chelation-controlled products (6:1 to > 20:1 dr).