1-Substituted 2-Azaspiro[3.3]heptanes: Overlooked Motifs for Drug Discovery

3-Substituted 6-Azabicyclo[3.1.1]heptanes: Nonclassical Piperidine Isosteres for Drug Discovery

Advanced analogs of piperidine and smaller homologues of tropane─3-substituted 6-azabicyclo[3.1.1]heptanes─were synthesized on a large scale using readily available bulk reagents. The key step of the approach involved the double alkylation reaction of malonate with cis-2,4-bis(mesyloxymethyl)azetidine-1-carboxylate, in turn easily prepared on up to 1 kg scale. After hydrolysis, N-Boc-6-azabicyclo[3.1.1]heptane-3,3-dicarboxylic acid was obtained (up to 400 g in a single run), which was used as a common intermediate for the preparation of all the title building blocks. In particular, Pb(OAc)4-mediated oxidative decarboxylation of this intermediate gave 2,6-methanopiperidone derivative (up to 400 g scale), while monodecarboxylation gave N-Boc-6-azabicyclo[3.1.1]heptane-3-carboxylic acids as an easily separatable mixture of cis and trans diastereomers (up to 100 g scale). Further functional group transformations gave diastereopure cis- and trans-N-Boc-monoprotected diamines and amino alcohols. Molecular structure analysis using exit vector parameters (EVP) revealed that cis isomers of 3-substituted 6-azabicyclo[3.1.1]heptanes are three-dimensional analogs of common 1,4-disubstituted piperidine chair conformer, whereas trans isomers can be considered as unusual "boat" piperidines.

exo'-Selective Construction of Spirobipyrrolidines by the Silver-catalyzed Asymmetric [3+2] Cycloaddition of Imino Esters with 4-Benzylidene-2,3-dioxopyrrolidines

The unprecedented Ag-catalyzed exo ' -selective [3+2] cycloaddition of imino esters with 4-benzylidene-2,3-dioxopyrrolidines is described. The reaction was efficiently catalyzed by AgOAc/( R , S p )-ThioClickFerrophos (TCF) leading to the construction of the corresponding spirobipyrrolidine scaffolds in excellent enantio- and diastereoselectivities. This reaction is the first example of a silver-catalyzed exo ' -selective asymmetric [3+2] cycloaddition, as well as the first exo ' -selective spirobipyrrolidine construction via a [3+2] cycloaddition process using imino esters. The wide substrate scope of this reaction enabled the preparation of structurally diverse spirobipyrrolidine derivatives, which are attracting attention as targets for drug discovery. Mechanistic studies suggested that the unusual exo ' -selectivity of this reaction is not due to epimerization following the common exo - or endo -selective cycloaddition, but instead is due to a stepwise Michael addition/Mannich sequence with bond rotation.

Stereoselective Diboration of Spirocyclobutenes: A Platform for the Synthesis of Spirocycles with Orthogonal Exit Vectors

The diastereo- and enantioselective diboration of spirocyclobutenes provides a platform for the rapid preparation of a wide variety of chiral spirocyclic building blocks. The chemoselective functionalization of the carbon-boron bond in the products, including a stereospecific sp³ -sp² Suzuki-Miyaura cross-coupling reaction, provides a powerful tool to control the directionality and the nature of the exit vectors in the spirocyclic framework.

Strain-Release Driven Spirocyclization of Azabicyclo[1.1.0]butyl Ketones

Due to their intrinsic rigidity, three-dimensionality and structural novelty, spirocyclic molecules have become increasingly sought-after moieties in drug discovery. Herein, we report a strain-release driven synthesis of azetidine-containing spirocycles by harnessing the inherent ring strain of the azabicyclo[1.1.0]butane (ABB) fragment. Novel ABB-ketone precursors bearing silyl-protected alcohols were synthesized in a single step and shown to engage in electrophile-induced spirocyclization-desilylation reactions. Primary, secondary and tertiary silyl ethers were effectively transformed into a library of new spiro-azetidines, with a range of substituents and ring sizes. In addition, the products are generated with synthetically useful ketone and protected-amine functional groups, which provides the potential for further elaboration and for this chemistry to be utilized in the rapid assembly of medicinally relevant compounds.

Saturated Bioisosteres of ortho-Substituted Benzenes

Saturated bioisosteres of ortho-disubstituted benzenes (bicyclo[2.1.1]hexanes) were synthesized, characterized and validated. These cores were incorporated into the bioactive compounds Valsartan, Boskalid and Fluxapyroxad instead of the benzene ring. The saturated analogues showed a similar level of antifungal activity compared to that of Boskalid and Fluxapyroxad.

Water-Soluble Non-Classical Benzene Mimetics

A new generation of saturated benzene mimetics, 2-oxabicyclo[2.1.1]hexanes, was developed. These compounds were designed as analogues of bicyclo[1.1.1]pentane with an improved water solubility. Crystallographic analysis of 2-oxabicyclo[2.1.1]hexanes revealed that they occupy a novel chemical space, but, at the same time, resemble the motif of meta-disubstituted benzenes.

Synthesis of 3-Aryl-3-Sulfanyl Azetidines by Iron-Catalyzed Thiol Alkylation with N-Cbz Azetidinols

New small-ring derivatives can provide valuable motifs in new chemical space for drug design. 3-Aryl-3-sulfanyl azetidines are synthesized directly from azetidine-3-ols in excellent yield by a mild Fe-catalyzed thiol alkylation. A broad range of thiols and azetidinols bearing electron-donating aromatics are successful, proceeding via an azetidine carbocation. The N-carboxybenzyl group is a requirement for good reactivity and enables the NH-azetidine to be revealed. Further reactions of the azetidine sulfides demonstrate their potential for incorporation in drug discovery programs.

Synthesis of Multifunctional Spirocyclic Azetidines and Their Application in Drug Discovery

The synthesis of multifunctional spirocycles was achieved from common cyclic carboxylic acids (cyclobutane carboxylate, cyclopentane carboxylate, l-proline, etc.). The whole sequence included only two chemical steps-synthesis of azetidinones, and reduction into azetidines. The obtained spirocyclic amino acids were incorporated into a structure of the known anesthetic drug Bupivacaine. The obtained analogues were more active and less toxic than the original drug. We believe that this discovery will lead to a wide use of spirocyclic building blocks in drug discovery in the near future.

Synthesis of Spirocyclic Pyrrolidines: Advanced Building Blocks for Drug Discovery

In the context of drug discovery, novel spirocyclic pyrrolidines have been synthesized in two steps from common three- to seven-membered-ring (hetero)alicyclic ketones. The key transformation is a reaction between an electron-deficient exocyclic alkene and an in situ generated N-benzyl azomethine ylide. The developed method has been used to synthesize the central diamine core of the known antibacterial agents Sitafloxacin and Olamufloxacin.