Alkynylation of chiral aldehydes: alkoxy-, amino-, and thio-substituted aldehydes

A recyclable polyester library from reversible alternating copolymerization of aldehyde and cyclic anhydride

Our society is pursuing chemically recyclable polymers to accelerate the green revolution in plastics. Here, we develop a recyclable polyester library from the alternating copolymerization of aldehyde and cyclic anhydride. Although these two monomer sets have little or no thermodynamic driving force for homopolymerization, their copolymerization demonstrates the unexpected alternating characteristics. In addition to readily available monomers, the method is performed under mild conditions, uses common Lewis/Brønsted acids as catalysts, achieves the facile tuning of polyester structure using two distinct monomer sets, and yields 60 polyesters. Interestingly, the copolymerization exhibits the chemical reversibility attributed to its relatively low enthalpy, which makes the resulting polyesters perform closed-loop recycling to monomers at high temperatures. This study provides a modular, efficient, and facile synthesis of recyclable polyesters using sustainable monomers.

Crystal structure of 4-methoxyphenyl-3-phenylpropiolate, C16H12O3

C16H12O3, orthorhombic, Pbca (no. 19), a = 3.9935(16) Å, b = 16.629(7) Å, c = 19.406(8) Å, V = 1288.7(9) Å3, Z = 4, Rgt(F) = 0.0387, wRref(F2) = 0.1084, T = 296(2) K.

Total Synthesis and Biological Activity of the Arachidonic Acid Metabolite Hemiketal E2

The total synthesis of hemiketal E₂ (HKE₂) has been accomplished using a gold(I)-mediated cycloisomerization followed by oxidation of the enol ether product to introduce a unique keto-hemiketal, the core structure of HKE₂. Synthetic hemiketal E₂ reproduced biosynthetically derived HKE₂ in the inhibition of human platelet aggregation.

Redox neutral [4+2] benzannulation of dienals and tertiary enaminones for benzaldehyde synthesis

The [4+2] benzannulation reactions between tertiary enaminones and dienals have been developed for the synthesis of polyfunctionalized benzaldehydes, providing a new access to benzaldehydes alongside the classical formylation strategy.

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.

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.

Aminodiols via stereocontrolled oxidation of methyleneaziridines

A highly diastereoselective Ru-catalyzed oxidation/reduction sequence of bicyclic methyleneaziridines provides a facile route to complex 1-amino-2,3-diol motifs. The relative anti stereochemistry between the amine and the vicinal alcohol are proposed to result from 1,3-bischelation in the transition state by the C1 and C3 heteroatoms.

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).

Highly diastereoselective chelation-controlled additions to α-silyloxy ketones

The polar Felkin-Anh, Cornforth-Evans, and Cram-chelation models predict that the addition of organometallic reagents to silyl-protected α-hydroxy ketones proceeds via a nonchelation pathway to give anti-diol addition products. This prediction has held true for the vast majority of additions reported in the literature, and few methods for chelation-controlled additions of organometallic reagents to silyl-protected α-hydroxy ketones have been introduced. Herein, we present a general and highly diastereoselective method for the addition of dialkylzincs and (E)-di-, (E)-tri-, and (Z)-disubstituted vinylzinc reagents to α-silyloxy ketones using alkyl zinc halide Lewis acids, RZnX, to give chelation-controlled products (dr ≥18:1). The compatibility of organozinc reagents with other functional groups makes this method potentially very useful in complex molecule synthesis.

Overriding Felkin control: a general method for highly diastereoselective chelation-controlled additions to alpha-silyloxy aldehydes

According to the Felkin-Anh and Cram-chelation models, nucleophilic additions to alpha-silyloxy aldehydes proceed through a nonchelation pathway due to the steric and electronic properties of the silyl group, giving rise to Felkin addition products. Herein we describe a general method to promote chelation-control in additions to alpha-silyloxy aldehydes. Dialkylzincs, functionalized dialkylzincs, and (E)-disubstituted, (E)-trisubstituted, and (Z)-disubstituted vinylzinc reagents add to silyl-protected alpha-hydroxy aldehydes with high selectivity for chelation-controlled products (dr of 10:1 to >20:1) in the presence of alkylzinc halides or triflates, RZnX. With the high functional group tolerance of organozinc reagents, the mild Lewis acidity of RZnX, and the excellent diastereoselectivities favoring the chelation-controlled products, this method will be useful in the synthesis of natural products. A mechanism involving chelation is supported by (1) NMR studies of a model substrate, (2) a dramatic increase in reaction rate in the presence of an alkylzinc halide, and (3) higher diastereoselectivity with larger alkyl substituents on the alpha-carbon of the aldehyde. This method provides access to chelation-controlled addition products with high diastereoselectivity previously unavailable using achiral organometallic reagents.

The Enantioselective Addition of Alkyne Nucleophiles to Carbonyl Groups

Over the past decade, large strides have been achieved in the invention of methods for the direct enantioselective addition of alkynes and metal alkynylide nucleophiles into prochiral aldehydes, ketones, and imines. This review highlights and compares the available methods for these transformations.