Phosphine-catalyzed enantioselective intramolecular [3+2] annulations to generate fused ring systems.

Substantial progress has been described in the development of asymmetric variants of the phosphine-catalyzed intermolecular [3+2] annulation of allenes with alkenes; however, there have not been corresponding advances for the intramolecular process, which can generate a higher level of complexity (an additional ring and stereocenter(s)). In this study, we describe the application of chiral phosphepine catalysts to address this challenge, thereby providing access to useful scaffolds that are found in bioactive compounds, including diquinane and quinolin-2-one derivatives, with very good stereoselectivity. The products of the [3+2] annulation can be readily transformed into structures that are even more stereochemically rich. Mechanistic studies are consistent with β addition of the phosphepine to the allene being the turnover-limiting step of the catalytic cycle, followed by a concerted [3+2] cycloaddition to the pendant olefin.

Introduction

During the past decade, the use of chiral tertiary phosphines as enantioselective nucleophilic catalysts has expanded rapidly.[1] The phosphine-catalyzed [3+2] annulation of electron-poor allenes (or alkynes) with olefins, originally described by Lu,[2] represents a powerful approach to the synthesis of functionalized cyclopentenes (eq 1).[3] Zhang reported the first example of an asymmetric variant of this method,[4] and we[5] and others[6] have since enlarged the scope of such processes.[7]

Krische[8] and Kwon[9] have established that the intramolecular version of the Lu [3+2] annulation can circumvent the formation of isomeric products and generate an array of useful, stereochemically rich fused ring systems, including diquinane and coumarin derivatives.[10,11] To date, despite the considerable progress that has been described for asymmetric catalysis of the intermolecular Lu annulation,[4−6] there has been no corresponding success with intramolecular reactions, wherein the conformation of the stereochemistry-determining transition state is relatively constrained. In this Article, we establish that such catalytic enantioselective intramolecular [3+2] annulations can indeed be achieved, thereby affording functionalized, fused bicyclic ring systems that bear multiple contiguous stereocenters (eqs 2 and 3).

Results and Discussion

In our initial studies, we chose to focus on the enantioselective synthesis of diquinanes, which Krische has established can be generated effectively in racemic form with P(n-Bu)3 as the catalyst.[8] We have reported earlier that phosphepines can serve as useful chiral nucleophilic catalysts,[12,13] and we have now determined that previously described phosphines 1–3 do indeed furnish promising results in the target annulation (eq 4). Furthermore, new chiral phosphepine 4,[14] which bears a 3,5-dimethoxyphenyl group on phosphorus, catalyzes the formation of the desired diquinane in excellent ee and high yield (eq 4; single diastereomer).[15,16]

Phosphepine 4 serves as an effective catalyst for the highly enantioselective synthesis of an array of diastereomerically pure diquinanes (and related structures) from acyclic precursors, generating two rings and three contiguous tertiary stereocenters in the process (Table 1).[17] The electron-withdrawing group of the allene can be any of an array of esters, and the electron-withdrawing substituent of the alkene can be an ester, thioester, or amide (e.g., entries 1–6).[18] Significantly, good enantioselectivity and yield are observed with a variety of tethers between the allene and the alkene (entries 7–10), thereby furnishing a fused pyrrolidine as well as a benzannulated diquinane. These annulations are stereoconvergent processes wherein both enantiomers of the racemic allene are being converted by the chiral catalyst into the same stereoisomer of the product with good selectivity.

Table 1

Phosphine-Catalyzed Enantioselective Intramolecular [3+2] Annulationsa

All data are the average of two experiments.

Yield of purified product; only one diastereomer was observed (analysis of the unpurified mixture by 1H NMR spectroscopy).

Reaction temperature: 40 °C.

Catalyst (S)-2 (20%) was employed instead of (S)-4.

Next, we turned our attention to the challenge of achieving enantioselective intramolecular [3+2] annulations of substrates that include a trisubstituted olefin.[19] We have determined that chiral phosphepines can indeed catalyze the desired annulations with good enantioselectivities and yields, as well as complete diastereoselectivity (Table 2). Any of a variety of linkers between the allene and the alkene are tolerated, as are different classes of trisubstituted olefins, thereby affording an array of products that include a quaternary stereocenter.[20]

Table 2

Catalytic Enantioselective Intramolecular [3+2] Annulations: Trisubstituted Olefins as a Reaction Partnera

All data are the average of two experiments.

Yield of purified product; only one diastereomer was observed (analysis of the unpurified mixture by 1H NMR spectroscopy).

Catalyst loading: 20%.

We sought to demonstrate that the scope of this new method for catalytic enantioselective intramolecular [3+2] cycloadditions extends beyond the synthesis of diquinane-like structures. Building on Kwon’s report of P(n-Bu)3-catalyzed, diastereoselective [3+2] annulations to furnish fused dihydrocoumarins,[9] we have developed an enantioselective variant of this reaction that proceeds in very good ee and yield (eq 5; a single diastereomer).

Encouraged by this result, we decided to pursue the application of our method to the catalytic asymmetric synthesis of fused quinolin-2-ones, a motif found in an array of bioactive compounds[21] and which to our knowledge has not previously been generated via such a [3+2] annulation. We have determined that phosphepine 4 does indeed catalyze the desired transformation with moderate-to-good enantioselectivity and in high yield, thereby affording two rings and two adjacent stereocenters (Table 3). The substrates may include either a di- or a trisubstituted olefin.

Table 3

Catalytic Enantioselective Intramolecular [3+2] Annulations To Generate Quinolin-2-one Derivativesa

All data are the average of two experiments.

Yield of purified product; only one diastereomer was observed (analysis of the unpurified mixture by 1H NMR spectroscopy).

Catalyst loading: 20% (S)-4.

Catalyst loading: 10% (S)-4

Diastereoselective functionalization of the various annulation products can provide additional stereocenters. For example, hydrogenation and epoxidation proceed with high stereoselectivity and yield (eqs 6 and 7).

An outline of a possible mechanism for the phosphine-catalyzed enantioselective intramolecular [3+2] annulation of allenes with alkenes is depicted in Figure 1.[7] Nucleophilic addition of the phosphine to the β position of the allene generates zwitterion A, which undergoes a cycloaddition with the alkene, furnishing a new zwitterion (B), which tautomerizes to C. Fragmentation then affords the bicyclic product and regenerates the phosphine catalyst.

Figure 1

An outline of a possible mechanism for phosphine-catalyzed enantioselective intramolecular [3+2] annulations of allenes with alkenes.

When an asymmetric intramolecular [3+2] annulation is stopped at partial conversion, modest enantiomeric enrichment of the unreacted allene is observed (eq 8; s ∼ 2.7), indicating that the chiral catalyst can discriminate between the enantiomers of the racemic substrate.[22,23] Taking into account our observation that the predominant resting state of the catalyst during the reaction is the free phosphine (31P NMR spectroscopy), we hypothesize that the first step of the catalytic cycle is likely the turnover-limiting step (Figure 1).

Finally, to gain insight into whether ring formation (A → B in Figure 1) occurs through a concerted or through a stepwise pathway, we have examined the configuration of the product as a function of the configuration of the olefin of the starting material. The stereochemical outcomes for all of the reactions described above are consistent with a concerted mechanism; however, there are no direct comparisons between corresponding E and Z olefins. We therefore investigated the [3+2] annulation of the Z isomer of the substrate depicted in entry 1 of Table 1, and we have determined that it leads to the epimeric diquinane as a single isomer (eq 9), consistent with formation of the five-membered ring via a concerted pathway.[24]

Conclusions

We have developed the first phosphine-catalyzed enantioselective intramolecular [3+2] cycloadditions of allenes with olefins, a process that generates two new rings and multiple stereogenic centers. Thus, an array of diasteromerically pure fused ring systems that are found in bioactive compounds, including diquinane and quinolin-2-one derivatives, are produced in generally good ee and yield with the aid of chiral phosphepine catalysts; furthermore, the bicyclic reaction products are well-suited for further diastereoselective functionalizations. Mechanistic studies establish that the chiral catalyst has a modest preference for reaction with one of the enantiomers of the racemic substrate and that the rate-determining step for the overall process is likely the initial addition of the phosphepine to the allene; the resulting adduct then undergoes a concerted intramolecular [3+2] cycloaddition with the pendant olefin. Further studies of enantioselective nucleophile-catalyzed processes are underway.

Experimental Section

General Procedure

The phosphine catalyst (0.10 equiv) was added to an oven-dried 20 mL vial equipped with a stir bar. This vial was capped with a septum-lined cap, the joint was covered with electrical tape, and then the vial was evacuated and backfilled with nitrogen (three cycles). The substrate (1.00 equiv) was added to a separate vial, which was then capped with a septum-lined cap, the joint was covered with electrical tape, and then the vial was evacuated and backfilled with nitrogen (three cycles). Next, toluene (anhydrous) was added to the vial that contained the substrate. This solution was added via syringe to the vial that contained the catalyst (under a positive pressure of nitrogen). Next, the reaction vial was detached from the nitrogen manifold, and grease was applied to the puncture hole in the septum in order to impede moisture/air from entering the vial. The reaction mixture was stirred at room temperature for 24 h, and then an aqueous solution of hydrogen peroxide (10%; 1.0 mL) was added. The resulting mixture was stirred for 10 min, and then an aqueous solution of Na2S2O3 (saturated; 2.0 mL) was added. The mixture was stirred for 10 min, and then the aqueous layer was extracted with EtOAc (5 mL × 3), and the combined organic layers were dried over MgSO4 and then concentrated under reduced pressure. The resulting residue was purified by column chromatography.