Nitrogen-Bridged, Natural Product-Like Octahydrobenzofurans and Octahydroindoles: Scope and Mechanism of Bridge-Forming Reductive Amination via Caged Heteroadamantanes

The biological significance of sp-rich synthetic scaffolds with natural product-like features yet distinct global frameworks is being increasingly recognised in medicinal chemistry and biochemistry. Taking inspiration from the vast array of bioactive, bridged alkaloids, we report the synthesis of unique, densely functionalised tricyclic scaffolds based on nitrogen-bridged, octahydrobenzofurans and octahydroindoles. These heterocycle-rich frameworks were assembled by a one-pot, two-step bridge-forming reductive amination process, which was shown to proceed via caged, heteroadamantane intermediates that thermodynamically drive an exo–endo epimerisation, enabling intramolecular azaMichael addition over the concave face of the fused bicyclic precursors. In addition to evaluating the scope of this aza bridge-forming reaction, further stereochemical complexity was introduced by subsequent diastereoselective ketone reductions and other manipulations. Finally, strategic diversity points (amino, carboxy) were decorated with common medicinal chemistry fragments, providing a set of exemplar derivatives with Lipinski compliant physicochemical properties. [a] Dr. S. M. Wales, Dr. W. Lewis, Prof. Dr. C. J. Moody School of Chemistry University Park, University of Nottingham Nottingham, NG7 2RD (United Kingdom) E-mail: c.j.moody@nottingham.ac.uk https://www.nottingham.ac.uk/~pczcm3/index.php [b] Dr. H. V. Adcock, Dr. D. Hamza Sygnature Discovery Ltd, Biocity Pennyfoot Street, Nottingham, NG1 1GF (United Kingdom) E-mail: D.Hamza@sygnaturediscovery.com


Introduction
Humanity has a constant need for the development of new small molecule therapeutics that provide improved target selectivity, overcome developing resistance and/or act on new biological targets. In the modern era, the increasing complexity of emerging drug targets (e.g., protein-protein interactions [1] ) underscores the need for high quality, lead compounds in the drug development process. In the crucial lead optimisation stages and beyond, molecules with increased stereochemical complexity, saturation, functionality and rigidity have a distinct advantage, [2] such that there is greater potential for highly specific target binding with minimal entropic penalty and for improving potency and selectivity through exploration of discrete vectors around a three-dimensional-like surface. [3][4][5] These desirable structural features are often manifested in polycyclic natural products and their derivatives, which have not only found application as pharmaceutical drugs [6] but have also informed the design of synthetically tractable, non-natural biological tools and medicines with diverse biological activities. [7][8][9] Polycyclic alkaloids in which nitrogen is incorporated into a bridged ring system represent a vast and important subset of biologically active natural products, [10,11] many of which are included on the World Health Organisation's Model List of Essential Medicines. [12] Prominent examples include the central nervous system drugs morphine (1, Figure 1) and atropine (2) and the antimalarial agent quinine (3). [13a] Non-natural, nitrogen polyheterocycles such as the antiemetic granisetron (4) and the benzomorphan analgesic pentazocine (5) are also among FDA-approved medications based on aza-bridged frameworks. [13]  In our ongoing efforts [14] to develop innovative scaffolds for the European Lead Factory (ELF) drug discovery initiative, [15] we became interested in work by Johnson [16] and separately, Coeffard and Greck, [17] describing enantio-and diastereoselective organocatalysed annulations of quinols and quinamines (6) with α,β-unsaturated aldehydes (Scheme 1). In a compelling application of the bicyclic products 7, Johnson reported one example of a reductive amination to give 8a, in which the existing rings were bridged via an aza-Michael addition, enabled by an uncommon exo-endo epimerisation at C-3 proven by deuterium incorporation. [16,18] While a small its C-3 epimer. [21] Accordingly, the bridge-forming reductive amination was investigated using the diastereoenriched bicyclic adducts 7a-c (Scheme 3). Reaction conditions for the one-pot process were directly adapted from Johnson's seminal example, [16] although reaction times for the initial condensation and subsequent reduction varied depending on the substrate (7a-c) and amine (RNH2) used (see the Supporting Information). Products (8) epimeric at C-2 were not observed in the crude reaction mixtures, indicating that any minor diastereomers in precursor 7 epimeric at C-2 reacted via other pathways. In fact, the desired tricycle 8 was generally the sole product observed after work-up by NMR spectroscopic analysis, which suggested oligomerisation as a competing pathway to account for the overall mass balance. [22] This facilitated the straightforward isolation of all bridged products (8) as single diastereomers following flash chromatography.
Our opening experiment to assess the reaction scope was carried out with benzofuran 7a and 4-methoxybenzylamine to give 8b in 59% yield (Scheme 3). [23] Next, reactions with 4-methoxybenzylamine were successfully extended to sulfonamides 7b and 7c to give octahydroindole-containing tricycles 8c and 8d in 72% and 54% yields, respectively. The structure of 8c was confirmed by X-ray crystallography. Amines of biological relevance and/or downstream synthetic utility were then evaluated and were found to be well tolerated including methylamine, glycine esters and allylamine to give 8f-i in moderate yields (46-62%). The N-unsubstituted cyclic amine 8e was also directly accessible from 7a and ammonium acetate using this methodology, however a decrease in yield was observed (38%). [24] A notable feature of this method is its practical simplicity: all reactions were performed under air at room temperature with standard grade solvents and reagents. This aided the straightforward preparation of selected polycycles 8b and 8e-g on synthetically useful scales (5 mmol) which were isolated in 0.5-1.0 g quantities (percentage yields shown in Scheme 3). Amines that did not form the desired product 8 when combined with 7a under the standard conditions included N-Boc-ethylenediamine, triphenylmethylamine and O-benzylhydroxylamine. In these cases, the reactions proceeded with complete consumption of 7a, but gave intractable mixtures containing, at most, traces of the desired products as ascertained by 1 H NMR spectroscopic analysis. Yields are isolated yields as single diastereomers and are uncorrected for any diastereomers in 7a-c epimeric at C-2. c) Diastereomeric ratio of precursor 7a = 2.1:1.0. d) The X-ray crystal structure of 8c is depicted as the opposite enantiomer to drawn. e) Yield uncorrected for impurity (19 mol %) in precursor 7c. f) Reaction performed with NH4OAc (3 equiv) as the ammonia source. g) Reaction performed with the corresponding amine hydrochloride salt (2 equiv) with NEt3 (2 equiv) as an additive.
With a collection of tricyclic scaffolds 8b-i prepared, we proceeded to investigate the mechanism of the reductive amination process. To gain further insight into the C-3 exo-endo epimerisation, we attempted to isolate the reaction intermediates from amine condensation by omitting the subsequent reduction step. Accordingly, 7a was treated with 4-methoxybenzylamine under the acidic conditions and the reaction was quenched after complete consumption of 7a (Scheme 4). Interestingly, analysis of the organic extract by NMR spectroscopy revealed complete disappearance of both the aldehyde and enone (alkene) functional groups of 7a, indicating that an aza-Michael addition had likely taken place in the absence of the reducing agent. Although several unidentified products were formed, the major product 9 was isolated by flash chromatography in the form of separable acetal 9a and hemiacetal 9b (48% combined yield). It should be noted that the isolated ratio of 9a/9b (0.8:1.0) was significantly decreased from that originally observed in the crude mixture (≥5:1), indicating that partial acetal-hemiacetal exchange occurred during silica gel chromatography. [25] Otherwise, both forms of 9 were stable and could be fully characterised and stored without any special precautions. This enabled confirmation of the caged structure of 9b by X-ray crystallography, which is reminiscent of natural products such as fusidilactone C, [26] tetrodotoxin, [27] daphnezomine A [28] and caloundrin B [29] that contain heteroadamantane cores with O-C-O or O-C-N linkages. Scheme 4. Isolation of key intermediate 9 in the bridge-forming reductive amination. a) Isolated yield after silica gel chromatography as a single diastereomer, uncorrected for any diastereomers in 7a epimeric at C-2. b) Ratio 9a/9b before silica gel chromatography ≥5:1. c) Isolated with 12 mol % of a minor product, tentatively assigned as the corresponding caged aminal (see the Supporting Information). d) Yield determined by 1 H NMR with mesitylene as internal standard.
When the isolated samples of 9a and 9b were separately subjected to the standard reduction conditions with NaBH3CN (Scheme 4), clean formation of tricyclic product 8b was observed in both cases (72% and 77% NMR yields, respectively).
Based on the above findings (Scheme 4), a dynamic mechanism is proposed for the bridge-forming process, whereby aza-Michael addition occurs prior to reduction (Scheme 5). [30,31] Initial condensation of aldehyde 7 and the amine (RNH2) would give imine (3-exo)-10, which could undergo reversible epimerisation to diastereomer (3-endo)-10 under the acidic conditions. Addition of water or methanol to (3-endo)-10 would provide hemiaminal (ether) 11, which places the nitrogen in the required orientation to undergo an intramolecular aza-Michael addition to the concave face of the enone. [32] Subsequent hemiacetal formation at the ketone of tricycle 12 (via 13) would enable intramolecular trapping of the derived iminium ion 14 to produce the observed, stable intermediate 9, which presumably drives the overall equilibrium and the initial epimerisation of 10. Upon addition of the hydride source (step 2), the irreversible reduction of iminium ions 14 or 15 perturbs the equilibrium and promotes cleavage of the heteroadamantane core of 9 by sequential hemiaminal ether fragmentation and acetal collapse, ultimately giving the half-caged product 8. Scheme 5. Proposed mechanism of the bridge-forming reductive amination.
To gain access to fully-saturated core derivatives, the prochiral ketone of selected polycycles 8b, 8e-g and 8i was subjected to diastereoselective reduction under substrate dependent conditions (Scheme 6). Treatment of 8b, 8e and 8f with L-selectride resulted in exclusive equatorial hydride delivery to give axial alcohols 16a-c in 69-78% yields. Again, this transformation was demonstrated on preparative scales, with up to 1.0 g of material isolated (for 16a). The relative stereochemistry of 16b was determined by X-ray crystallographic analysis of the dimesylate derivative 16f (Scheme 7), while 16c was independently prepared from 16b by reductive Nmethylation thus confirming the analogous stereochemistry (Scheme 7). The structure of 16a was assigned by analogy.
In general, the sterically encumbered ketone of scaffold 8 proved relatively slow to reduce, which introduced some chemoselectivity issues in the presence of the ester functionalities of 8g and 8i. For example, reduction of the methyl ester group in 8g occurred at a competitive rate to ketone reduction using both L-selectride and NaBH4 (in THF or i-PrOH). This necessitated 'protection' of the methyl ester as the lithium carboxylate salt prior to ketone reduction with L-selectride, allowing isolation of alcohol 16d (50%) over a three-step process (Scheme 6). An alternative synthesis of 16d via N-alkylation of 16b with methyl bromoacetate (78%) was also carried out to confirm the expected relative stereochemistry (Scheme 7). The tert-butyl ester of 8i was also incompatible with L-selectride but proved inert to NaBH4, allowing selective ketone reduction to give a mixture of diastereomers 16e (66%) and (5-epi)-16e (17%) which were separated by column chromatography (Scheme 6). Similar to the previous reductions with L-selectride, the major diastereomer 16e had the axial configuration at the hydroxy group as confirmed by X-ray crystallography. To introduce other useful functionality, representative alcohol 16c was converted into mesylate derivative 16g in high yield (94%, Scheme 8), which was subjected to nucleophilic SN2 displacement with sodium azide to give 16h (74%). Subsequent Staudinger reduction and treatment with Boc2O gave the protected primary amine 16i in 60% yield over the two steps. Scheme 8. Synthesis of Boc-protected amine 16i. a) Isolated with a minor alkene impurity (7% w/w) arising from elimination of MsOH. b) Yield uncorrected for alkene impurity (7% w/w) in azide 16h.
By design, several of the reduced scaffolds (16) were decorated with modifiable (protected) amino or carboxy groups, thus presenting opportunities to create larger compound libraries by further derivatisation. To demonstrate this capability, esters 16d and 16e and N-Boc-amine 16i were deprotected to give intermediates 16j-l, which, along with amine 16b, were elaborated in divergent fashion to an exemplar set of 14 final compounds 16m-z using standard transformations (Scheme 9). Except for 16v, these derivatives (16m-z) were prepared using high-throughput techniques (plate format/preparative HPLC purification) and the yields are unoptimised. Analysis of the 14 exemplar compounds 16m-z using the computational model LLAMA [33] predicts favourable pharmacokinetic properties [34] within Lipinski space; [35] specifically: an average molecular weight of 401, AlogP of 2.3, topological polar surface area of 61.1 Å 2 and low rotatable bond count of 3.4 (Table S1). The average "fraction sp 3 " of the molecules (16m-z), many of which have been decorated with (hetero)aromatic groups, is 0.55 (Table S1), which is above the average "fraction sp 3 " (0.47) of marketed drugs from 1980-2009. [2a] Further, the overall three-dimensional nature of the tricyclic scaffolds is supported by an average plane-of-best-fit (PBF) [36] deviation of 1.1 Å (Table S1), which compares favourably with that of the ChEMBL database [37] of published bioactive compounds (average PBF for ChEMBL compounds = 0.6 Å). [3] Taken collectively, these data reveal an encouraging physiochemical profile that would appear to make these scaffolds promising candidates for biological screening.

Conclusion
A total of thirty four derivatives (8b-i and 16a-z) of densely functionalised tricyclic scaffolds based on aminomethyl-bridged, octahydrobenzofurans and indoles have been prepared. Construction of the key aza-containing bridge was achieved by a one-pot, twostep reductive amination process, in which a formyl group on the convex face of the bicyclic precursor was linked to the concave face of a distal enone via a stereodynamic amino-condensation process, driven by the formation of stable heteroadamantane intermediates. This (reductive) bridge-forming reaction was amenable to a variety of amines with associated biological relevance and/or downstream synthetic utility and has provided preparative access to (octahydro)benzofuran and N-sulfonylindole-based tricycles, which were further elaborated by diastereoselecive reduction and other standard transformations, including to 14 drug-like examples (16m-z).
These synthetically tractable scaffolds (16) contain many attractive features for biological applications including a high heterocycle and sp 3 -content and provide numerous points of potential diversity to be explored in further synthetic and biological studies (i.e., X, Y, R, R 1 , R 2 of structures 8 and 16 in Scheme 1). Furthermore, the elucidation of the bridge-forming reductive amination pathway opens up intriguing opportunities to create even greater structural and stereochemical complexity by the diastereoselective addition of carbon nucleophiles to the half-caged iminium ions (14 or 15) in the second step of the bridge-forming process.