Reaction Discovery Employing Macrocycles: Transannular Cyclizations of Macrocyclic Bis-Lactams

Abstract

Macrocyclic bis-lactams have been synthesized by cyclodimerization of homoallylic amino esters employing a Zr(IV)-catalyzed ester-amide exchange protocol. Base-mediated transannular cyclizations have been identified to access both bicyclic [5-11] and tricyclic [5-8-5] frameworks in good yield and diastereoselectivity. Preliminary mechanistic studies support an olefin isomerization-intramolecular conjugate addition pathway.

Macrocyclic natural products often exhibit important biological activities and have thus inspired a number of studies involving diversity-oriented synthesis of macrocyclic frameworks. ¹ Recent studies have also highlighted elegant examples of transannular cyclizations enroute to complex natural products. ² As part of our studies, we considered preparation of macrocycles as substrates for reaction discovery ³ and potential complexity-generating transannular cyclizations.^4,5 In this communication, we report the preparation of 14-membered ring bis-lactams ⁶ and their conversion to polycyclic frameworks by divergent, transannular reaction processes as well as preliminary computational studies to probe the reaction mechanism.

In order to access macrocyclic bis-lactam substrates, we utilized cyclodimerization of stereochemically well-defined homoallylic amino esters⁷ using Zr(IV)-catalyzed ester-amide exchange.⁸ Alloc-protected amino esters 1a–d were prepared using asymmetric crotylation⁷ of the iminium species derived from condensation of allyl carbamate with aromatic aldehydes (Scheme 1). Subsequent alloc removal⁹ using a polymer-bound Pd (0) reagent (PS-PPh₃-Pd) ¹⁰ simplified product purification and afforded amino ester monomers 2a–d.

Scheme 1.

Preparation of Homoallylic Amino Ester Monomers.

As cyclodimerization of 2 involves consecutive intermolecular and intramolecular amidations, we anticipated that concentration may play an important role in reaction efficiency. Therefore, a range of concentrations (0.10–0.80 M) were examined for cyclodimerization of 2a using Zr(Ot-Bu)₄-2-hydroxypyridine (HYP)⁸ as catalyst (Scheme 2). Based on these studies, an optimal concentration for production of 14-membered bis-macrolactam 3a was found to be 0.60 M. Macrocyclic bis-lactams 3b–d were also prepared in moderate to good yield using the optimized conditions. The structure and stereochemistry of bis-lactam 3b was confirmed by single x-ray crystal structure analysis (one conformer shown).¹⁰

Scheme 2.

Synthesis of Bis-lactams via Cyclodimerization.

With the target macrolactams in hand, we focused on reaction discovery to identify complexity-generating transannular cyclizations. Initial attempted intramolecular hydroamidation¹¹ of bis-lactam 3b utilizing carbophilic late-transition metal catalysts including Pd(II), Ag(I), Pt(II), Au(I), and Au(III)¹² failed to afford any cyclized products. After reaction screening, use of NaH as base¹³ in DMF at 60 °C was found to provide tricyclic [5-8-5] product 5 (dr= 5:1:1) (Scheme 3, entry 1). Reaction at room temperature using NaH gave no conversion indicating a high activation energy for transannular cyclization (Scheme 3, entry 2). Further optimization was conducted by evaluating different bases and solvents. When DMF was used as the solvent, use of Cs₂CO₃ as base showed no conversion, while NaOt-Bu afforded full conversion (entries 3 and 4). It is noteworthy that a 64% conversion to a mixture of 4 and 5 could be obtained using a catalytic amount of NaOt-Bu (20 mol %) (entry 5). Bases were also evaluated using THF as solvent; NaOt-Bu yielded bicyclic product 4 (dr > 20:1) exclusively in 70% yield (entry 6).

Scheme 3.

Base Evaluation for Transannular Cyclization.^a

^aCombined yields of all the diasteromers. Diastereomer ratio determined by ¹H NMR integration. ^b0.20 equiv of base employed.

The relative stereochemistries of 4 and 5 were determined using nOe studies (Figure 1).¹⁰ Furthermore, a single x-ray crystal structure for major diastereomer 5 was obtained to fully support the structure and stereochemical assignment.¹⁰ In order to understand the potential for epimerization, either compound 5 or an inseparable mixture of diastereomers 6 and 7 were treated with NaOt-Bu in DMF at 60 °C which resulted in an approximate 5:1:1 diastereomeric ratio of 5:6:7 indicating a reversible, thermodynamically-controlled cyclization process.

Figure 1.

Determination of Relative Stereochemistry.

We also probed the effect of aryl substitution in the transannular cyclization process (Scheme 4). Reaction of bis-lactam 3c with NaOt-Bu in DMA (condition a) afforded approximately a 1:1 mixture of monocyclized product 8c and bis-cyclized product 9c. The corresponding reaction of 3c in THF (condition b) afforded exclusively 8c, albeit in low yield. Reaction of 3d using DMA as solvent afforded a 1:4 mixture of 8d and tricyclic product 9d while reaction in THF afforded 8d exclusively in moderate yield. Transannular cyclization of both 3c and 3d also afforded slightly reduced diastereomeric ratios of the bis-cyclized products with diastereoselectivity of monocyclized products remaining high.

Scheme 4.

Preparation of Bicyclic and Tricyclic Products.

^a Conditions: a) NaOt-Bu (2.0 equiv), DMA, 60 °C, 12 h; b) NaOt-Bu (2.0 equiv) , THF, 60 °C, 24 h. ^b Diastereomeric ratios determined by 1H NMR integration.

In order to probe the reaction mechanism, we performed kinetic isotope effect experiments employing 3b and deuterated substrates 10 and 11 (Scheme 5). A significant kinetic isotope effect (k₁/k₂= 2.3) was observed when methylene-deuterated substrate 10 was utilized in comparison to 3b to afford monocyclized product 4. ¹⁴ These results suggest that the rate-determining step for production of 4 likely involves methylene deprotonation.¹⁵

Scheme 5.

Kinetic Isotope Effect Experiments.

^a Determined by HPLC analysis. See Supporting Information for details.

Based on our experimental results, we propose an olefin isomerization-conjugate addition mechanism for the base-mediated transannular cyclization (Scheme 6). ^{2b–c, 16 1}H NMR studies indicate that the initial step is likely deprotonation of bis-lactam 3b generating anionic species 12.¹⁰ Proton transfer may occur at the alpha position (red) or transannularly (blue) to afford olefin-isomerized intermediate 13. Subsequent conjugate addition affords the bicyclic product 4. The bicyclic product may undergo further deprotonation-isomerization to afford intermediate 14 which may be followed by conjugate addition to provide 5. In contrast to use of DMF as solvent, we speculate that the absence of a second cyclization in the less polar solvent THF¹⁷ may be due to diminished acidity of the methylene hydrogens of 4 which bears a neighboring tertiary amide relative to those in macrolactam 3b.

Scheme 6.

Proposed Reaction Pathway.

In order to understand the stereochemical outcome of the reaction, we carried out computational studies based on our proposed mechanism. The studies included conformational searches and M052X density functional calculations¹⁰ on bis-lactam 3b, key proposed intermediates (15,17, and 18), cyclized products (4–7, 16), and transition states. Scheme 7 summarizes relative energies for neutral species. Enone conjugation is endothermic, consistent with our failure to isolate conjugated intermediates. The two cyclization steps define an energy cascade with the experimentally observed products of lowest energy as might be expected for an equilibrium process.

Scheme 7.

DFT Relative Energies (kcal/mol) for Neutral Species.

Transition state searching for the conjugate addition steps posed a greater challenge. In macrocyclic rings, there are few systematic methods to simultaneously search both conformations and transition states. ¹⁸ Transition state conformational searches were carried out by Monte Carlo methods, with a constrained length (1.9 Å) for the nascent transannular bond. The 50–100 candidate structures of lowest energy were further optimized with the AM1 method, followed by single point HF/3-21G calculation. Finally, the lowest energy collection of structures was subjected to M052X/6-31G(d) optimization and frequency analysis.¹⁰ The predicted transition state energy barriers of 6–15 kcal/mol are consistent with the facile nature of these reactions. Each anionic cyclization step is endothermic ¹⁰ and as a consequence, the product distribution is thermodynamically controlled consistent with the relative energies shown in Scheme 7.

Further functionalization of the tricyclic core structure of 5 was conducted in an effort to generate further complexity. Stereoselective bis-alkylations were achieved in good yield by treatment of 5 with LiHMDS at −78 °C and subsequent quenching with alkyl halides (Scheme 8) to afford alkylated products (20–22) as single diastereomers. Stereoselectivity may be derived by approach of the electrophile from the convex face of the dienolate intermediate 19. The structure and stereochemistry of bis-alkylated product 20 was confirmed by single crystal x-ray analysis.

Scheme 8.

Diastereoselective Alkylation.

In conclusion, macrocyclic bis-lactams have been efficiently synthesized by cyclodimerization of homoallylic amino ester monomers employing a Zr(IV)-catalyzed ester-amide exchange. Reaction discovery has led to the identification of transannular cyclizations to provide bicyclic [5-11] or tricyclic [5-8-5] frameworks in good yield and diastereoselectivity. Diastereoselective C-alkylation was developed to further elaborate structures. Preliminary mechanistic studies including computational analyses support an olefin isomerization-intramolecular conjugate addition pathway. Further studies involving reaction discovery of macrocyclic frameworks are in progress and will be reported in due course.

Supplementary Material

1-si-001. Supporting Information Available.

Experimental procedures and characterization data for all new compounds and summary energetics for stationary points from density functional calculations. X-ray crystal structure coordinates and files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.