Stereoselective Synthesis of the Spirocyclic γ-Lactam Core of the Ansalactams

Abstract

Ansalactam A is an ansa macrolide natural product that contains a densely functionalized spiro-γ-lactam core containing three contiguous stereocenters. This unusual motif distinguishes it from other members of this family and represents a significant synthetic challenge. Herein, we report the development of a stereoselective formal [3+2] cycloaddition reaction to construct this key spiro-γ-lactam motif for the first time, thereby enabling access to the northern domain of ansalactam A.

Graphical Abstract

Ansalactams A-D represent a sub-class of ansamycin polyketides that were isolated by Moore and Fenical from a Streptomyces sp. strain CNH-189 isolated from marine sediments.^1,2 These natural products are members of the broader ansamycin family that includes biologically potent molecules such as rifamycin SV and geldanamycin.^3,4 In addition to their biological activity, the ansamycins have complex biosynthetic origins and are collectively formed from a common aromatic acid 3-amino-5-hydroxybenzoate precursor.¹ The ansamycins are structurally differentiated by variations in a shared spiro-γ-lactam dihydronaphthoquinone core, further bridged by a polyketide chain forming a 16-membered macrocyclic ring (1-4, Figure 1).

Figure 1.

Structures of the ansalactam natural products (1–4).

As part of our broader program targeting bioactive marine natural products,⁵ we became interested in developing a synthetic route to the spiro-γ-lactam system unique to the ansalactams. This iso-butyryl substituted lactam has been hypothesized by Trauner to arise biosynthetically through either a 5-exo or 5-endo radical cyclization. However, preliminary synthetic studies were not successful in realizing this biomimetic logic to date (Figure 2a).⁶ We envisioned a [3+2] cycloaddition approach to prepare the spiro-γ-lactam core, leveraging the well-studied steric tolerance and stereocontrol of these reactions (Figure 2b).^7,8 While this approach was attractive, it was not clear at the outset of this work what the identity of the 1,3-dipole (represented by 8) or dipolarophile (represented by 9) would be, as the proposed disconnections required inverting the native reactivity of an oxy-azomethine ylide precursor such as 8. Attempted formation of 8 from a formamide precursor such as 10 may result in formation of an undesired α-isocyanoacetate derivative, precluding formation of the targeted spirocycle due to competitive [4+1] cycloaddition reactions that commonly provide an array of heterocycles from these motifs.⁹

Figure 2.

a) Biomimetic logic presented by Trauner; b) retrosynthetic approach to spirolactam core enabled by key [3+2] dipolar cycloaddition

To begin explorations into the reactivity of these systems, we sought to prepare formamide 10 (Scheme 1). Beginning with methyl ester 12, reduction and Appel chlorination afforded benzylic chloride 13 in 99% yield. Subsequent Heck coupling,¹⁰ followed by hydrogenation and saponification allowed for the preparation of carboxylic acid 15 in excellent yield over 3 steps. Friedel-Crafts acylation proceeded smoothly to generate ketone 11.¹¹ Sequential bromination, nucleophilic azide installation, and hydrogenation afforded the desired 2-aminotetralone 16 in 60% yield. Lastly, N-formylation of 16 provided the targeted formamide 10 in 75% yield. The presently described sequenced proved highly scalable and has allowed for scalable preparation of 10 that we envision further streamlining and leveraging for gram-scale quantities in future efforts.

Scheme 1. Synthesis of formamide 10.

PTAB = Phenyltrimethylammonium tribromide

With 10 in hand, we envisioned activation of the formamide group with triflic anhydride, triggering formation of either an oxy-azomethine-type 1,3-dipole or isocyanide. Subsequent reaction with an α,β-unsaturated carbonyl compound, in either a concerted or stepwise fashion, would then forge the desired spiro-γ-lactam through a formal [3+2] process. To explore this approach, we first employed methyl acrylate as the reaction partner in the presence of Et₃N (not rigorously dried), resulting in the formation of a 5-membered spirocyclic product 17 that possessed the desired regiochemical and stereochemical outcome, yet had lost the oxygen atom from the formamide group (Scheme 2). This result suggested that the reaction was proceeding through an isocyanide intermediate, but questions remained regarding mechanistic aspects of this process. Given the stoichiometric quantities of acid present, it is likely that the added amine base is only functioning as an addition proton shuttle and source of trace water (addition of small quantities of water alone were not as effective).

Scheme 2.

Initial exploration of proposed [3+2] cycloaddition.

Imine oxidation to the corresponding lactam 18 using mCBPA provided the target spiro-γ-lactam in 90% yield and as a single isolated diastereomer, the absolute stereochemistry of which was assigned though X-ray analysis. Unfortunately, all attempts to alkylate or otherwise functionalize this compound to install the isobutyl group failed. We then sought to incorporate this sterically demanding group in the dipolarophile component of the key [3+2] reaction. To probe this approach, formamide 10 was activated with triflic anhydride and reacted with enoate 9 (Scheme 2), ultimately providing 20 in a modest 28% yield over the 2 steps after oxidation of 19 with mCPBA. Only one regioisomer was isolated from this reaction, albeit the low yield makes the overall selectivity uncertain. Characterization of the product revealed that a skeletal rearrangement had occurred, and this was further corroborated by X-ray (Scheme 2). At this stage it is not clear what drives this ring expansion, but we hypothesize that the increased sterics of this system slows the desired addition allowing for alternate reaction pathways to dominate. While this transformation was not relevant to the synthesis this family of natural products, the structural complexity generated in this process may be of interest in other contexts.

At this it was clear that a [3+2] cycloaddition could be utilized to forge the desired spiro-lactam, but further optimization was required in order to incorporate the isobutyl sidechain contained in the natural product. Mechanistically, we originally envisioned this transformation proceeding via azomethine ylide- type reactivity (Figure 3a), but observation of deoxygenated imine product 17 suggests the formation of such an intermediate was unlikely in the presence of strong electrophilic activating agents (Tf₂O). Instead, we propose formation of protonated isocyanide intermediate (22) under the strongly acidic reaction conditions (Figure 3b). Activation of 10 in the absence of an electrophilic reaction partner yielded oxazole 21 (confirmed by X-ray), further suggesting the intermediacy of such an isocyanide species. Furthermore, this isolated compound is a competent reactant in the desired [3+2] reaction under the same conditions, supporting its formation during the course of the reaction. Control experiments have confirmed the requirement of strong acid, with no reaction observed under neutral or basic reaction conditions. We propose that the protonated isocyanide 21 is critical to drive the observed reactivity and regiochemical outcome as the compound can react as an extended enolate, enabling closure of the ester enol in a stepwise fashion; however, we cannot rule out the possibility of the protonated oxazole itself as an intermediate in the reaction. Additional mechanistic studies are underway to further probe this reaction and identify pathways to render it asymmetric.⁹

Figure 3.

a) Initially envisioned reactivity; b) Proposed reaction pathway to provide 17.

Given our lack of success utilizing substituted electrophilic alkenes as dipolarophiles, we sought to explore allenes as alternate reactants. It was hypothesized that the planar nature of these compounds would limit the impact that sterics may have played in our preliminary studies of the [3+2] cycloaddition. Our first effort in this regard employed allene 23 under conditions identical to those utilized previously for the alkene reactants (entry 1, Figure 4a). This reaction proceeded in low conversion after 16 hours but provided the desired extended imine product 24 in 15% isolated yield as a mixture of E/Z alkene isomers. We only isolate one diastereomer from these reactions, likely due to the proposed intramolecular sprio-center forming reaction. Further optimization identified dichloromethane as a more suitable solvent at increased concentrations and reaction times, ultimately providing 62% yield (entries 2–5, Figure 4a). As before, neutralizing the reaction by employing stoichiometric quantities of triethylamine inhibited the reaction, highlighting the importance of Bronsted acid in this transformation (entry 6, Figure 4a).

Figure 4.

a) Optimization of reaction with allene 23; b) Conversion of 24 to the northern fragment of ansalactam A (7).

With compound 24 in hand, we explored various sequences to reduce the exocyclic alkene and oxidize the imine to reveal the targeted spiro-γ-lactam 7 (Figure 4b). Hydrogenation of 24 in ethanol using Pd/C chemoselectively reduced the extended imine, forming isobutyl-substituted imine 25. Oxidation of this compound under Pinnick oxidation conditions resulted in the formation of lactam 7 in 63% yield over the 2 steps as a single isolated diastereomer.¹² Previously employed mCPBA conditions did not provide productive yields in this more sterically hindered substrate. Compound 7 represents the entire carbon skeleton of the northern domain of ansalactam A (1), lacking only the hemi-quinone carbonyl of the natural product. Efforts to accomplish this last oxidation and install the southern polyketide fragment are well underway.

In conclusion, we have developed a stereoselective approach to the core of the ansalactam natural products. Additionally, the methods presented herein are relevant for the synthesis of other natural product scaffolds and bioactive molecules. Efforts to further explore the mechanism and scope of the novel spirocyclic dipolar cycloaddition, optimize preparation of the starting materials, and complete the synthesis of ansalactam A are underway and will be reported in due course.