α-Lithiation-electrophile trapping of N-thiopivaloylazetidin-3-ol: stereoselective synthesis of 2-substituted 3-hydroxyazetidines

Visible light-mediated aza Paternò-Büchi reaction of acyclic oximes and alkenes to azetidines

The aza Paternò-Büchi reaction is a [2+2]-cycloaddition reaction between imines and alkenes that produces azetidines, four-membered nitrogen-containing heterocycles. Currently, successful examples rely primarily on either intramolecular variants or cyclic imine equivalents. To unlock the full synthetic potential of aza Paternò-Büchi reactions, it is essential to extend the reaction to acyclic imine equivalents. Here, we report that matching of the frontier molecular orbital energies of alkenes with those of acyclic oximes enables visible light-mediated aza Paternò-Büchi reactions through triplet energy transfer catalysis. The utility of this reaction is further showcased in the synthesis of epi-penaresidin B. Density functional theory computations reveal that a competition between the desired [2+2]-cycloaddition and alkene dimerization determines the success of the reaction. Frontier orbital energy matching between the reactive components lowers transition-state energy (ΔGǂ) values and ultimately promotes reactivity.

Fluorinated 2-Azetines: Synthesis, Applications, Biological Tests, and Development of a Catalytic Process

An efficient and straightforward phosphine-promoted tandem aza-Michael addition/intramolecular Wittig reaction was developed for the synthesis of polyfunctionalized 2-azetines. After demonstrating that this transformation could be made catalytic in phosphine through in situ reduction of phosphine oxide with phenylsilane, different post-transformation steps have been demonstrated, including an original [2 + 2] photodimerization. Preliminary biological tests highlighted that these fluorinated 1,2-dihydroazete-2,3-dicarboxylates exhibited significant cytotoxicity against the human tumor cell line.

Azetidine-Borane Complexes: Synthesis, Reactivity, and Stereoselective Functionalization

The present study reports, for the first time, the synthesis and structural features of azetidine-borane complexes, as well as their reactivity in lithiation reactions. A temperature-dependent stereoselectivity has been disclosed in the reaction of borane with N-alkyl-2-arylazetidines, allowing for a stereoselective preparation of azetidine-borane complexes 2 and 3. A regioselective hydrogen/lithium permutation, at the benzylic position, was observed in lithiation reactions of complexes possessing a syn relationship, between the ring proton and the BH₃ group. In contrast, scarce or no reactivity was noticed in complexes lacking such a stereochemical requirement. The configurational stability of the lithiated intermediates has also been investigated, in order to shed some light on the stereoselectivity of the lithiation/electrophile trapping sequence. Calculations helped in supporting experimental observations, concerning structure and reactivity of these azetidine-borane complexes. Data suggest that the BH₃ group could promote the lithiation reaction likely by an electrostatic complex induced proximity effect. Interestingly, a new synthetic strategy for the synthesis of N-alkyl-2,2-disubstituted azetidines has been developed.

Recent advances in the chemistry of metallated azetidines

The almost unexplored four-membered azetidines represent a particularly interesting class of molecules, among the family of saturated nitrogen heterocycles. This review reports recent developments in direct metal-based functionalization of the azetidine ring, focusing on the regio- and stereoselectivity of these reactions.

Exploiting structural and conformational effects for a site-selective lithiation of azetidines

Interest in molecular structures bearing four-membered heterocycles (FMHs) is growing due to the possibility to explore new regions of the chemical space and get new lead molecules. Our interest in the development of divergent synthesis of functionalized FMHs, prompted us to disclose factors affecting the reactivity of nitrogen-bearing FMHs towards metalating agents. Our investigations demonstrated that structural factors and conformational preferences need to be considered in planning a site-selective functionalization of azetidines. It will be showed how such factors could have pivotal importance in the reactivity of FMHs.

Mechanistic interrogation of the asymmetric lithiation-trapping of N-thiopivaloyl azetidine and pyrrolidine

A fundamental mechanistic study of the s-BuLi/chiral diamine-mediated lithiation-trapping of N-thiopivaloyl azetidine and pyrrolidine is reported. We show that lithiated thiopivalamides are configurationally unstable at -78 °C. Reaction then proceeds via a dynamic resolution of diastereomeric lithiated intermediates and this accounts for the variable sense and degree of asymmetric induction observed compared to N-Boc heterocycles.

α- and α'-Lithiation-Electrophile Trapping of N-Thiopivaloyl and N-tert-Butoxythiocarbonyl α-Substituted Azetidines: Rationalization of the Regiodivergence Using NMR and Computation

(1)H NMR and computational analyses provide insight into the regiodivergent (α- and α'-) lithiation-electrophile trapping of N-thiopivaloyl- and N-(tert-butoxythiocarbonyl)-α-alkylazetidines. The magnitudes of the rotation barriers in these azetidines indicate that rotamer interconversions do not occur at the temperature and on the time scale of the lithiations. The NMR and computational studies support the origin of regioselectivity as being thiocarbonyl-directed lithiation from the lowest energy amide-like rotameric forms (cis for N-thiopivaloyl and trans for N-tert-butoxythiocarbonyl).

α-Arylation of Saturated Azacycles and N-Methylamines via Palladium(II)-Catalyzed C(sp(3))-H Coupling

Pd(II)-catalyzed α-C(sp(3))-H arylation of pyrrolidines, piperidines, azepanes, and N-methylamines with arylboronic acids has been developed for the first time. This transformation is applicable to wide arrays of pyrrolidines and boronic acids, including heteroaromatic boronic acids. A diastereoselective one-pot heterodiarylation of pyrrolidines has also been achieved.

Easy access to constrained peptidomimetics and 2,2-disubstituted azetidines by the unexpected reactivity profile of α-lithiated N-Boc-azetidines

The unprecedented reactivity profile of lithiatedN-Boc-2-arylazetidines gave access either to new azetidine-based peptidomimetics or to the regioselective functionalization of the azetidine ring.

Amine protection/α-activation with the tert-butoxythiocarbonyl group: application to azetidine lithiation-electrophilic substitution

tert-Butoxythiocarbonyl (Botc), the long-neglected thiocarbonyl analogue of Boc, facilitates (unlike its alkoxycarbonyl cousin) α-lithiation and electrophile incorporation on N-Botc-azetidine. N,N,N',N'-endo,endo-Tetramethyl-2,5-diaminonorbornane proved optimal as a chiral ligand, generating adducts with er up to 92:8. Facile deprotection, under conditions that left the corresponding N-Boc systems intact, was achieved using either TFA or via thermolysis in ethanol.

Generation and electrophile trapping of N-Boc-2-lithio-2-azetine: synthesis of 2-substituted 2-azetines

s-BuLi-induced α-lithiation–elimination of LiOMe from N-Boc-3-methoxyazetidine and further in situ α-lithiation generates N-Boc-2-lithio-2-azetine which can be trapped with electrophiles, either directly (carbonyl or heteroatom electrophiles) or after transmetalation to copper (allowing allylations and propargylations), providing a concise access to 2-substituted 2-azetines.