Stereo flexible synthesis of the C8-C23 fragment of antarlides, androgen receptor antagonists.

A practical and efficient synthesis of the C8-C23 fragment of antarlides A-H, incorporating six stereocenters and a conjugated diene, is reported. A strategic combination of synthetic methods, including CBS reduction, Evans' aldol reaction, Keck-Maruoka allylation, and enzymatic resolution, enabled the selective introduction of these stereocenters. Furthermore, the pivotal coupling of key fragments is successfully executed through a Julia-Kocienski olefination reaction, connecting the C8-C14 and C15-C23 subunits.

Organocatalytic Enantiospecific Total Synthesis of Butenolides.

Biologically important, chiral natural products of butenolides, (-)-blastmycinolactol, (+)-blastmycinone, (-)-NFX-2, (+)-antimycinone, lipid metabolites, (+)-ancepsenolide, (+)-homoancepsenolide, mosquito larvicidal butenolide and their analogues were synthesized in very good yields in a sequential one-pot manner by using an organocatalytic reductive coupling and palladium-mediated reductive deoxygenation or organocatalytic reductive coupling and silica-mediated reductive deamination as the key steps.

Highly Diastereoselective Strain-Increase Allylborations: Rapid Access to Alkylidenecyclopropanes and Alkylidenecyclobutanes.

Allylboration of carbonyl compounds is one of the most widely used methods in the stereoselective synthesis of natural products. However, these powerful transformations are so far limited to allyl- or crotylboron reagents; ring-strained substituents in the α-position have not been investigated. Such substrates would lead to an increase in strain energy upon allylboration and as such cause a significant increase in the activation barrier of the reaction. Indeed, no reaction was observed between an α-cyclopropyl allylboronic ester and an aldehyde. However, by converting the boronic ester into the much more reactive borinic ester, the allylboration proceeded well giving alkylidenecyclopropanes in high yield. This process was highly diastereoselective and gives rapid access to versatile alkylidenecyclopropanes and alkylidenecyclobutanes. The chemistry shows a broad substrate scope in terms of both the range of vinylcycloalkyl boronic esters and aldehydes that can be employed. The intermediate boronate complexes were also found to be potent nucleophiles, reacting with a range of non-carbonyl-based electrophiles and radicals, leading to an even broader range of alkylidenecyclopropanes and alkylidenecyclobutanes. Using 11B NMR experiments, we were able to track the intermediates involved, and DFT calculations supported the experimental findings.

Photoinduced Fragmentation Borylation of Cyclic Alcohols and Hemiacetals.

A visible-light photoinduced fragmentation borylation of O-phthalimido cycloalkanols with bis(catecholato)diboron is described. Structurally diverse keto and formyloxy alkyl boronic esters are shown to be conveniently prepared by radical-mediated ring opening of cyclic alcohols and hemiacetals, respectively. The reactions proceed under mild conditions in the absence of additives or photocatalysts, display excellent functional group tolerance, and are shown to allow cleavage of 4-, 5-, 6-, and 7-membered ring substrates. The mechanism proceeds via sequential homolytic N-O and C-C bond cleavages, the latter of which involves ß-scission of an alkoxy radical, generating a carbonyl and an alkyl radical that is trapped by the diboron reagent. Spectroscopic studies suggest direct photoexcitation of either the phthalimide or diboron substrates with blue light can initiate a radical chain mechanism.

Synthesis of Highly Substituted Imidazole Uracil Containing Molecules via Ugi-4CR and Passerini-3CR.

The synthesis of uracil/thymine containing tetra/trisubstituted imidazole derivatives was demonstrated using Ugi/Passerini-reaction followed by a postcyclization reaction sequence. The approach enables the one-pot facile construction of diverse compounds in moderate to excellent yields (47-82%). The 5-fluorouracil and 5-methyluracil moieties afford potentially bioactive molecules with drug-like properties. These scaffolds are currently being utilized in the screening deck of the European Lead Factory.

ß-Carbolinone Analogues from the Ugi Silver Mine.

Here we describe a facile, tandem synthetic route for ß-carbolinones, a class of natural products of high biological significance. Commercially available building blocks yield highly diverse analogues in just two simple steps.

Ugi Multicomponent Reaction Based Synthesis of Medium-Sized Rings.

An Ugi multicomponent reaction based two-step strategy was applied to generate medium-sized rings. In the first linear expansion phase, a series of diamines reacted with cyclic anhydrides to produce different lengths of terminal synthetic amino acids as the starting material for the second phase. The Ugi-4-center 3-component reaction was utilized to construct complex medium-sized rings (8-11) by the addition of isocyanides and oxo components. This method features mild conditions and a broad substrate scope.

With unprotected amino acids to tetrazolo peptidomimetics.

Here we describe the direct usage of C,N-unprotected amino acids in Ugi-tetrazole reactions to produce a novel class of acid-tetrazole compounds. Surprisingly, only the tetrazole Ugi product is found and not traces of other possible Ugi type reactions. Based on this reaction pathway we have designed the synthesis of novel tetrazole-peptidomimetics. A high level of structural diversity can be achieved using this isocyanide based multicomponent reaction (IMCR), providing a platform for the production of functionalized building blocks for novel bioactive molecules and nontraditional scaffolds which previously were not accessible.

Two-Step Synthesis of Complex Artificial Macrocyclic Compounds.

The design and synthesis of head-to-tail linked artificial macrocycles using the Ugi-reaction has been developed. This synthetic approach of just two steps is unprecedented, short, efficient and works over a wide range of medium (8-11) and macrocyclic (≥12) loop sizes. The substrate scope and functional group tolerance is exceptional. Using this approach, we have synthesized 39 novel macrocycles by two or even one single synthetic operation. The properties of our macrocycles are discussed with respect to their potential to bind to biological targets that are not druggable by conventional, drug-like compounds. As an application of these artificial macrocycles we highlight potent p53-MDM2 antagonism.

De Novo Assembly of Highly Substituted Morpholines and Piperazines.

The morpholine and piperazine with their remarkable physical and biochemical properties are popular heterocycles in organic and medicinal chemistry used in rational property design. However, in the majority of cases these rings are added to an existing molecule in a building block approach thus limiting their substitution pattern and diversity. Here we introduce a versatile de novo synthesis of the morpholine and piperazine rings using multicomponent reaction chemistry. The large scale amenable building blocks can be further substituted at up to four positions, making this a very versatile scaffold synthesis strategy. Our methods thus fulfill the increasing demand for novel building block design and nontraditional scaffolds which previously were not accessible.