A Formal Synthesis of (−)-Englerin A by Relay Ring Closing Metathesis and Transannular Etherification

A bicyclization approach to englerin A has culminated in a formal asymmetric total synthesis. Key transformations in the 10-step sequence are a regiospecific epoxide opening and a relay ene-yne-ene metathesis that converts linear substrates specifically to Δ^4,6-guaiadiene-9,10 diol derivatives. Regiospecific functionalization of the diene moiety installs the oxygen bridge required for the englerin tricyclic core

Five Easy Pieces. The Total Synthesis of Phosphoiodyn A (and Placotylene A)

The convergent total synthesis of the marine natural product phosphoiodyn A, a nanomolar agonist of human peroxisome proliferator-activated receptor delta (hPPARδ), was achieved in five steps total from commercially available and inexpensive starting materials. The synthesis relies on the unprecedented regioselective hydrozirconation of a terminal acetylene in the presence of a conjugated 1,3-diyne and on ammonolysis of a β-chlorophosphonic acid monoester. The scheme also provides the newly isolated placotylene A, an inhibitor of bone marrow-derived macrophage (BMM) differentiation

Intermolecular Radical Cation Diels–Alder (RCDA) Reaction of Bicyclooctadienes: Biomimetic Formal Total Synthesis of Kingianin A and Total Syntheses of Kingianins D, F, H, and J

Three endo bicyclooctadienol dimers corresponding to kingianins A and H, D, and F and J were obtained by the intermolecular radical cation Diels–Alder (RCDA) reaction. Each isomer was cleanly isolated without the aid of preparative HPLC. Kingianins D, F, H, and J were prepared by way of these intermediates from commercially available materials in 10, 13, 9, and 17 steps, respectively. Kingianin A has already been prepared from one of these compounds. Completion of the synthesis of kingianin H relied on Manchand’s one-step, three-carbon homologation

Total Synthesis of Kingianin A

A 12-step synthesis of kingianin A, an inhibitor of the antiapoptotic protein Bcl-xL, is based on a radical cation Diels–Alder reaction (RCDA). This approach is thought to be biomimetic. The use of a tether in the key RCDA step controls the regiochemistry of the cycloaddition, leading to the desired core structure and a separable diastereomer

Total Synthesis of Kingianin A

A 12-step synthesis of kingianin A, an inhibitor of the antiapoptotic protein Bcl-xL, is based on a radical cation Diels–Alder reaction (RCDA). This approach is thought to be biomimetic. The use of a tether in the key RCDA step controls the regiochemistry of the cycloaddition, leading to the desired core structure and a separable diastereomer

Asymmetric Induction in 8π Electrocyclizations. Design of a Removable Chiral Auxiliary

The pseudo <i>C</i>₂ symmetric <i>trans</i> diphenyl oxazoline group acts as an effective chiral auxiliary in the 8π, 6π tandem electrocyclization of a substituted tetraene 1-carboxylic acid. Assignment of absolute stereochemistry to the [4.2.0] bicyclooctadiene product supports a model in which both <i>s-cis</i> and <i>s-trans</i> conformations favor the transition states with the same helical twist. This assignment prefaces the development of analogs of SNF4435 C and D. These natural products demonstrate activity as androgen receptor antagonists and as multidrug resistance (mdr) reversal agents

Asymmetric Induction in 8π Electrocyclizations. Design of a Removable Chiral Auxiliary

The pseudo <i>C</i>₂ symmetric <i>trans</i> diphenyl oxazoline group acts as an effective chiral auxiliary in the 8π, 6π tandem electrocyclization of a substituted tetraene 1-carboxylic acid. Assignment of absolute stereochemistry to the [4.2.0] bicyclooctadiene product supports a model in which both <i>s-cis</i> and <i>s-trans</i> conformations favor the transition states with the same helical twist. This assignment prefaces the development of analogs of SNF4435 C and D. These natural products demonstrate activity as androgen receptor antagonists and as multidrug resistance (mdr) reversal agents

A Bicyclo[4.2.0]octene-Derived Monomer Provides Completely Linear Alternating Copolymers via Alternating Ring-Opening Metathesis Polymerization (AROMP)

Strained bicyclic carbomethoxy olefins were utilized as substrates in alternating ring-opening metathesis polymerization and found to provide low-dispersity polymers with novel backbones. The polymerization of methyl bicyclo[4.2.0]­oct-7-ene-7-carboxylate with cyclohexene in the presence of the fast-initiating Grubbs catalyst (H₂IMes)­(3-Br-Pyr)₂Cl₂RuCHPh leads to a completely linear as well as alternating copolymer, as demonstrated by NMR spectroscopy, isotopic labeling, and gel permeation chromatography. In contrast, intramolecular chain-transfer reactions were observed with [5.2.0] and [3.2.0] bicyclic carbomethoxy olefins, although to a lesser extent than with the previously reported monocyclic cyclobutenecarboxylic ester monomers [Song, A.; Parker, K. A.; Sampson, N. S. J. Am. Chem. Soc. 2009, 131, 3444]. Inclusion of cyclohexyl rings fused to the copolymer backbone minimizes intramolecular chain-transfer reactions and provides a framework for creating alternating functionality in a one-step polymerization

Total Synthesis of (±)-Bisabosqual A

The synthesis of the novel squalene synthase inhibitor, bisabosqual A, was completed in 14 steps (longest linear sequence) from commercially available starting materials. The doubly convergent route employs a tandem 5-exo, 6-exo radical cyclization as the key step. This reaction assembles the fully functionalized tetracyclic core and introduces three stereogenic centers. Other effective transformations are the regioselective deoxygenation of an advanced enone intermediate and the chemo- and diastereoselective addition of trimethylaluminum to a ketone in the presence of esters

Importance of a 4‑Alkyl Substituent for Activity in the Englerin Series

The ring closing metathesis/transannular etherification approach to the englerin nucleus was adapted to provide two key intermediates for analogue synthesis: the 4-desmethyl Δ^5,6 tricycle and the 4-oxo Δ^5,6 tricycle. The former was elaborated to 4-desmethyl englerin A and the latter served as a common precursor for englerin A, 4-ethyl englerin A, and 4-isopropyl englerin A. 4-Desmethyl englerin A was less active than the natural product by an order of magnitude, but the 4-ethyl and 4-isopropyl analogues were comparable in activity to englerin A. These results are consistent with the premise that the 4-alkyl group enforces the binding conformation of the cinnamoyl ester substituent. Furthermore, they suggest that 4-alkyl englerin structures may prove to be useful tool compounds