A temporary-bridge strategy for enantioselective organocatalyzed synthesis of aza-seven-membered rings †

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. A temporary-bridge strategy for enantioselective organocatalyzed synthesis of aza-seven-membered rings † Sébastien Goudedranche, David Pierrot, Thierry Constantieux, Damien Bonne, Jean Rodriguez

The design of new efficient enantioselective organocatalyzed domino transformations is a very active field of research especially for synthesizing complex cyclic enantiopure molecules from easily accessible acyclic starting materials in an environmentally friendly way. 1 In this context, enantioselective organocascades of five-2 and six-membered cycles 3 are well documented, while accessing the parent seven-membered cycles still constitutes a real synthetic challenge 4,5 due to the need to overcome a negative entropic factor during the ring closing step. 6As rare examples of success in this field, in the benzoxepinone series, two closely related enantioselective carbene-catalyzed formal [4+3] heterocycloadditions have been proposed (Scheme 1a). 7Additionally, a very recent chiral phosphoric acid (CPA)-catalyzed enantioselective intramolecular Mannich reaction has been known to give access to dibenzo [1,4]diazepine scaffolds (Scheme 1b). 8Hence, an efficient complementary organocatalyzed domino reaction to synthesize challenging mediumsized ring systems in optically active forms is highly desirable and of relevance for synthetic purposes.Herein, we report our new organocatalytic approach to synthesize seven-membered rings based on a temporary-bridge strategy, 9 which circumvents difficulties associated with the construction of medium-sized rings by direct cyclization routes (Scheme 1c).The designed domino approach is based on organocatalyzed domino formation of bicyclo[3.2.1]octane derivative 3. The overall cascade is thermodynamically driven by the final bridging step, when properly functionalized 1,4-bis-nucleophile 1 bearing an additional electrophilic center reacts with 1,3-biselectrophile 2 possessing a latent nucleophile that results in the concomitant formation of three covalent bonds.In a subsequent step, the bridge could be selectively cleaved (revealing step), allowing the formation of a highly functionalized optically active seven-membered ring system 4.
To pursue this new strategy, the selection of the two bridging units is crucial in terms of ready availability and compatibility with the mild reaction conditions required to reach a high level of stereocontrol.We propose the use of a-ketoamides 1, 10 possessing multiple reactive sites, 11 as potential 1,4-bis-nucleophiles that could be combined with 1,3-bis-electrophilic enals 2 following a Michaelhemiaminalization sequence affording intermediate 5 (Scheme 2). 12his step would be potentially highly reversible and contrathermodynamic but the possible temporary formation of an internal oxygen tether by a final intramolecular hemiacetalization step of the highly electrophilic ketone could drive the reaction toward the formation of 3 possibly stabilized by an intramolecular hydrogen bond.The overall thermodynamically driven process allows direct access to a 7-aza-8-oxa-bicyclo[3.2.1]octane with concomitant formation of three covalent bonds and the creation of up to four stereogenic centers.This molecular scaffold is found in several This journal is © The Royal Society of Chemistry 2014 natural products such as the neurotoxin samandarine, 13 the antimalarial ribasine 14 or deglycocadambine, a new alkaloid recently isolated from the twigs and the leaves of Emmenopterys henryi. 15oreover, taking advantage of the ring strain release by selective cleavage of the oxa-bridged system will provide a straightforward access to highly functionalized optically active azepane derivatives.
We began to test our designed system for the synthesis of bicyclic products 3.We first investigated this reaction with ketoamide 1a and cinnamaldehyde (2a) as starting materials using Hayashi-Jørgensen catalyst 6 (Table 1). 16The reaction conducted in toluene did not proceed at all after 15 h (entry 1).Dichloromethane as well as acetonitrile proved to be unsuitable solvents for this reaction, as only small quantities of Michael adducts were recovered after similar reaction times (entries 2 and 3).Gratifyingly, the use of ethanol afforded the desired product 3a in very good yield, as only two diastereomers (dr = 3 : 1) and with excellent enantioselectivity for both diastereomers (entry 4).Slightly more acidic trichloroethanol allowed the formation of 3a with a better diastereoselectivity while high yield and high enantioselectivity were maintained (entry 5).Finally the reaction temperature was lowered to À7 1C, 17 which resulted in an increase of the diastereoselectivity to 8 : 1 even if a prolonged reaction time was required to drive the reaction to completion (entry 6).
With this final set of optimized conditions in hand, 18 the scope of the three bond-forming process was next explored with a variety of 3-aryl a,b-unsaturated aldehydes (Scheme 3).The reaction worked well with aromatic rings bearing either electron-donating or electron-withdrawing groups including sterically demanding ortho functionalization or an anthracenyl substituent, as well as a heteroaromatic ring.The corresponding products 3b-e were obtained in good to high yields with acceptable to very high diastereoselectivities from 4 : 1 to 420 : 1 dr, but with uniformly high enantioselectivities in the range 95-97% ee.Similarly, it was found that a broad range of ketoamides 1 could readily participate in this reaction.Increasing the size of the R 1 group resulted in generally higher diastereoselectivities (420 : 1 dr for R 1 = i-Pr in 3c, 3i, 3n, and 3q).Interestingly, the use of 2-oxo-Nphenylpropanamide (1c, R 1 = H) resulted in the formation of the desired bicyclic products 3f and 3g as single diastereomers with excellent enantioselectivities (ee = 97%).These results indicate a highly diastereoselective intramolecular hemiacetalization with the stereoselective creation of two stereogenic centers, which is Scheme 2 Use of a-ketoamides 1 for the synthesis of nitrogen-containing seven-membered cycles.consistent with the fact that only two diastereomers were produced when a-ketoamides with R 1 not equal to H were involved.
In addition, a-ketoamides having various electron-donating or electron-withdrawing substituents on the phenyl ring or bearing a pyridine nucleus also proceeded very well in this cascade reaction, thus affording the azepane derivatives (3k-p) in good to excellent yields and high diastereo-and enantioselectivities. 19 Finally, the robustness of the transformation was shown by performing gram scale preparations of 3a, 3c and 3g, which proceeded without any difficulty in excellent yields with good stereoselectivities.Concerning the stereochemical assignment, 3p was crystallized and both the relative and absolute configurations of these bicyclic structures were unambiguously identified by X-ray diffraction as (3R,4S,5R,7S). 20cheme 4 shows our current understanding of the reaction pathway and its thermodynamically driven stereochemical outcome.The initial kinetically controlled Michael addition step involves (Z)-enolate 7 and iminium ion 8. 21 In the resulting conformationally restrained environment, the Re face of the thermodynamic (Z)-enolate 7 preferentially interacts with the Si face of iminium ion 8 accounting for the formation of the (3S,4R)-Michael adduct 9.Then, nucleophilic addition of the amide nitrogen atom to give azepane 5 could in theory occur on both diastereotopic faces of aldehyde 7 giving rise reversibly to the two resulting diastereomers 3 and 3 0 after an hemiacetalization step.However, the formation of bicyclic product 3 0 is hampered by the presence of strong 1,3-diaxial steric interactions between the R 2 substituent and the amide moiety.
Following our initial temporary-bridge strategy to synthesize azepane rings, we next moved to the development of synthetically useful transformations of bicyclic products 3 in order to release highly functionalized optically active azepanes.These heterocycles are present in numerous natural products and biologically interesting molecules such as (À)-balanol, 22 (À)-cobactin T, 23 and SQ-33351 (Fig. 1). 24Therefore, a general and efficient strategy to afford such azacyclic rings would be very useful to design new synthetic routes toward these bioactive compounds and their analogs.
First, bicyclic ring 3c was easily reduced using the borane-THF complex (1.05 equiv.) to afford azepan-3-one moiety 4a in good yield with no erosion of chiral information (Scheme 5).Also, using three equivalents of the BH 3 -THF complex, the ketone moiety could be diastereoselectively reduced yielding azepan-3-ol derivative 4b bearing three contiguous stereogenic centers in good yield and excellent stereoselectivity on a synthetically useful mmol scale.This transformation was accomplished on other bicyclic substrates 3 with similar success (see ESI †).Next, we found that Martin's sulfurane reagent 25 efficiently triggered a remote dehydration of 3c giving the corresponding unsaturated azepan-2,3-dione 4c in good yield with full retention of the stereochemical information.Interestingly, adding two equivalents of 1,1,1,3,3,3-hexafluoro-2phenylpropan-2-ol in the reaction mixture furnished the functionalized azepan-2,3-dione 4d bearing three stereogenic centers with an interesting 15 : 1 dr and a preserved 94% ee.This opens the way for the introduction of an additional point of diversity in the final azepane structure. 26e have demonstrated the feasibility and the efficiency of a new organocatalytic temporary-bridge approach to synthesize optically active nitrogen-containing seven-membered cycles.During this cascade, the synthetic power of a-ketoamides is fully exploited as three complementary reactive sites are involved, triggering an unprecedented thermodynamically driven formal [4+3] heterocyclization.Three chemical bonds (C-C, C-N and C-O) are assembled concomitantly and up to four stereogenic centers are created with very high stereoselectivity.Moreover this domino sequence represents a rare example of an organocatalyzed access to seven-membered ring systems in optically active forms and the first example in the azepanes series.These bicyclic molecules are then easily converted to synthetically valuable azepanone, azepanol or azepanedione derivatives by selective reduction or dehydration providing a complementary answer to the challenging synthesis of optically active sevenmembered cyclic nitrogen-containing compounds.
The Agence Nationale pour la Recherche (ANR-11-BS07-0014), the Direction Ge ´ne ´rale de l'Armement (DGA), the Re ´gion PACA, the Centre National de la Recherche Scientifique (CNRS) and the Aix-Marseille Universite ´are gratefully acknowledged for financial support.We also thank Dr N. Vanthuyne and M. Jean (ee measurements), and Dr M. Giorgi (X-ray diffraction analysis).

a
Reaction conditions: ketoamide 1a (0.2 mmol, 1.0 equiv.)and catalyst 6 (10 mol%) were dissolved in the solvent (2 mL) and stirred for 15 min (0 1C or À7 1C).Then, cinnamaldehyde (2a) (2 equiv.) was added and the mixture was stirred for the time and temperature indicated.b Isolated yields after flash chromatography.c Determined by 1 H NMR of the crude reaction products.d Determined by chiral HPLC analysis.e Values in brackets are for the minor diastereomer.f 10-15% of the Michael adduct was isolated.g The reaction was conducted at À7 1C.This journal is © The Royal Society of Chemistry 2014 Chem.Commun., 2014, 50, 15605--15608 | 15607

Fig. 1
Fig. 1 Examples of natural and non-natural products featuring the chiral azepane scaffold.

Table 1
Reaction optimisation a