A highly enantioselective , organocatalytic [ 3 + 2 ]-cycloannulation reaction towards the de novo-synthesis of 1-cyclopentenyl-a-keto esters †

Asymmetric organocatalytic domino transformations have recently emerged as a powerful tool to rapidly assemble complex organic structures with defined absolute configuration. This strategy can now be applied to a wide range of carbon–carbon bondforming reactions employing different activation modes and organocatalysts. Very prominent among such processes stands the coupled iminium-enamine activation mode using a,b-unsaturated carbonyls in concert with chiral amines in which an initial conjugate addition of a nucleophilic component is directly followed by electrophilic trapping of the in situ-generated enamine to generate an a,b-difunctionalized carbonyl compound. Much less explored albeit very attractive as well is a different scenario in which the enal acts as a double 1,3-electrophile via initial conjugate and then direct 1,2-addition of two nucleophilic components. This concept, however, has thus far been largely limited to formal [3+3]-cycloaddition reactions of 1,3-dinucleophiles resulting in the synthesis of 6-membered rings. We have recently introduced a novel 1,2-dinucleophile for organic synthesis. The bis-silyl-1,3-dienediolate 1 smoothly underwent Lewis acid-catalyzed, domino-type vinylogous MannichN,O-acetalization reactions to produce pyrrolo[2,1-b]benzoxazoles and pyrrolo[1,2-a] benzoxazinones and was also employed in the direct and highly flexible synthesis of pyrrolo[3,2-c]quinolines via a domino-vinylogous Mannich-Mannich-Pictet-Spengler reaction. Given the capacity of 1 to readily participate in sequential Lewis acid-catalyzed Mannich processes we reasoned that we could exploit the reactivity of 1 in other important carbon–carbon bond-forming reactions, namely the vinylogous Michael reaction. We now report the amine-catalyzed, enantioselective [3+2]cycloannulation of 1 with a,b-unsaturated aldehydes 2 to furnish chiral 1-cyclopentenyl-a-keto esters 4 directly with exceptional enantioselectivity (Scheme 1). In this process, bis-silyl-1,3dienediolate 1 engages the aldehyde in an organocatalyzed vinylogous Michael reaction directly followed by an intramolecular Knoevenagel-type condensation of the silyl enol ether toward the intermediate iminium ion resulting both in ring-closure and regeneration of the chiral catalyst 3. We have previously established the first organocatalytic, enantioselective, vinylogous Michael reaction of acyclic dienol silyl ethers and enals producing highly versatile 1,7-dioxo compounds with exceptional enantioand regiocontrol. As chiral organocatalyst we employed diphenylprolinol silyl ether 3b which Hayashi and Jorgensen had established independently for iminium ion-activated conjugate addition reactions. Based upon this precedence we started our investigations with the model reaction shown in Table 1. Thus, we treated bis-silyl-1,3-dienediolate 1 (2.0 equiv.) with cinnamaldehyde (2a) (1.0 equiv.), Jørgensen catalyst 3a (20 mol%) and 2,4-dinitrobenzoic acid (1.0 equiv.) as cocatalyst

Asymmetric organocatalytic domino transformations have recently emerged as a powerful tool to rapidly assemble complex organic structures with defined absolute configuration. 1 This strategy can now be applied to a wide range of carbon-carbon bondforming reactions employing different activation modes and organocatalysts. 2 Very prominent among such processes stands the coupled iminium-enamine activation mode using a,b-unsaturated carbonyls in concert with chiral amines in which an initial conjugate addition of a nucleophilic component is directly followed by electrophilic trapping of the in situ-generated enamine to generate an a,b-difunctionalized carbonyl compound. 3Much less explored albeit very attractive as well is a different scenario in which the enal acts as a double 1,3-electrophile via initial conjugate and then direct 1,2-addition of two nucleophilic components.This concept, however, has thus far been largely limited to formal [3+3]-cycloaddition reactions of 1,3-dinucleophiles resulting in the synthesis of 6-membered rings. 4,5e have recently introduced a novel 1,2-dinucleophile for organic synthesis.The bis-silyl-1,3-dienediolate 1 smoothly underwent Lewis acid-catalyzed, domino-type vinylogous Mannich-N,O-acetalization reactions to produce pyrrolo [2,1-b]benzoxazoles 6 and pyrrolo[1,2-a] benzoxazinones 7 and was also employed in the direct and highly flexible synthesis of pyrrolo [3,2-c]quinolines via a domino-vinylogous Mannich-Mannich-Pictet-Spengler reaction. 8Given the capacity of 1 to readily participate in sequential Lewis acid-catalyzed Mannich processes we reasoned that we could exploit the reactivity of 1 in other important carbon-carbon bond-forming reactions, namely the vinylogous Michael reaction.
We now report the amine-catalyzed, enantioselective [3+2]cycloannulation of 1 with a,b-unsaturated aldehydes 2 to furnish chiral 1-cyclopentenyl-a-keto esters 4 directly with exceptional enantioselectivity (Scheme 1).In this process, bis-silyl-1,3dienediolate 1 engages the aldehyde in an organocatalyzed vinylogous Michael reaction directly followed by an intramolecular Knoevenagel-type condensation of the silyl enol ether toward the intermediate iminium ion resulting both in ring-closure and regeneration of the chiral catalyst 3.
We have previously established the first organocatalytic, enantioselective, vinylogous Michael reaction of acyclic dienol silyl ethers and enals producing highly versatile 1,7-dioxo compounds with exceptional enantio-and regiocontrol. 9As chiral organocatalyst we employed diphenylprolinol silyl ether 3b which Hayashi and Jorgensen had established independently for iminium ion-activated conjugate addition reactions. 10Based upon this precedence we started our investigations with the model reaction shown in Table 1.
in the solvent mixture THF/H 2 O (1/1) for 48 h at rt. 11 Under these conditions, a mixture of the desired g-product 4a and the regioisomeric a-product 5a (4 : 1 of g : a-ratio) was obtained with low yield, but with very high enantioselectivity for the g-product (Table 1, entry 1).Increasing the acidity of the aqueous phase by using a solvent mixture of THF/pH-4 buffer solution (1 : 1) helped to turnover the reaction and gave rise to 4a and 5a (4 : 1 of g : a-ratio) with 50% combined yield and with exceptional enantioselectivity for 4a (entry 2).
Encouraged by this result, some more etheral solvents were evaluated in combination with pH-4 buffer solution in this transformation (entries 3-6).Whereas MTBE, DME, and 1,4dioxane did not improve the yield significantly, the reaction conducted in Et 2 O/pH-4 buffer solution (1 : 1) delivered the mixture of gand a-products with 69% combined yield, from which the desired pure g-product 4a could be separated with 57% yield and 98 : 2 er by silica gel chromatography after treatment with 1 N HCl-solution (entry 6, see ESI † for more details).Under identical reaction conditions, the Hayashi catalyst 3b furnished an increased yield of 78% for the mixture of regioisomers from which the desired pure g-product 4a was isolated with 63% yield and 99 : 1-enantioselectivity suggesting this catalyst for further studies (entry 7).In addition we tested other acidic cocatalysts in combination with 3b which led, however, to inferior results (entries 8 and 9).
With these reaction conditions in hand the generality of this new organocatalytic [3+2]-cycloannulation reaction of bis-silyl-1,3-dienediolate 1 and a,b-unsaturated aldehydes was investigated and the results are summarised in Table 2.A wide array of a,b-unsaturated aldehydes bearing electrondonating and electronwithdrawing groups on the aromatic ring could be coupled with bis-silyl-1,3-dienediolate 1, and the corresponding products were obtained with generally moderate to good yields and excellent enantioselectivities. Alkyl-, halogen-, and nitro-groups were readily tolerated as aromatic substituents within the cinnamaldehydes (entries 1-10).Heteroaromatic substituents were found to be equally effective in furnishing the corresponding products with excellent enantioselectivities (entries 11 and 12).The ratio between gand a-product typically varied between 5 : 1 and 10 : 1, and the pure g-regioisomers could be isolated in all cases studied.Remarkably, b-silyl-substituted aldehyde 2m was successfully coupled with 1 and delivered the desired g-product 4m as a single regioisomer with good yield and excellent enantioselectivity (entry 13).Likewise, aliphatic a,b-unsaturated aldehydes 2n-o gave rise to single g-regioisomers 4n-o again with very high enantioselectivities (entries 14 and 15).In addition, the estersubstituted enal 2p furnished cyclopentene 4p with good isolated yield, but with somewhat diminished enantioselectivity (entry 16).
To probe the applicability and robustness of this new process we carried out the large-scale synthesis of two new cyclopentenes.In the presence of 20 mol% of Hayashi catalyst 3b the domino vinylogous Michael-intramolecular Knoevenagel-type condensation of 1 and the a,b-unsaturated aldehydes 2a and 2g proceeded to completion within three days at room temperature delivering the corresponding 1-cyclopentenyl-a-keto esters 4a and 4g with good isolated yields and excellent enantioselectivity (Scheme 2).
To demonstrate the synthetic utility of the highly functionalized cyclopentene products 4a (R = Ph) and 4g (R = p-Br-Ph) were taken as representative examples and converted into diversely   functionalized products via a series of simple transformations (Scheme 3).Thus, Sc(OTf) 3 -catalyzed hetero-Diels Alder (HDA) 12 reaction of 4g and 3,4-dihydro-2H-pyran ( 6) afforded the tricyclic a,b-unsaturated ester 7 as a single endo-diastereomer in good yield.Remarkably, only a single stereoisomer was formed in this reaction that created three new contiguous stereogenic centers documenting the large inherent substrate selectivity.The absolute and relative configuration of hetero Diels-Alder adduct 7 was unambiguously determined by an X-ray diffraction analysis 13 of the corresponding alcohol 8 (Fig. 1) which at the same time established the absolute configuration of the 1-cyclopentenyl-a-keto esters 4 as well.
In addition, b,g-unsaturated a-keto ester 4g underwent facile base-catalyzed 1,4-conjugate addition with dimethyl malonate (9) to afford the highly functionalized cyclopentane 10 with 65% yield (d.r. 9 : 1). 14Moreover, cyclopentene 4a and nitromethane reacted in a selective 1,2-addition 15 delivering the corresponding nitroaldol product 11 with 60% yield as a 1 : 1-mixture of diastereomers.Finally, we were delighted to find that cyclopentene 4a could also be converted into pyrazole 12 with moderate yield in a two step-sequence comprising hydrazone formation and oxidative cyclization (Scheme 3). 16n conclusion, we have developed a highly enantioselective, organocatalytic domino reaction of bis-silyl-1,3-dienediolate 1 and a,b-unsaturated aldehydes 2 furnishing 1-cyclopentenyl-aketo esters 4 in moderate to good yields and excellent enantioselectivities of up to 499 : 1 er.Using an operationally very simple protocol this process comprises an initial vinylogous Michael reaction followed by a Knoevenagel-type condensation reaction with concomitant ring-closure and regeneration of the chiral catalyst.The reaction has been shown to be quite general with respect to the enal component.The versatility of the cyclopentene products has been demonstrated by a series of synthetic transformations involving either the a-keto ester or the a,b-unsaturated carbonyl moiety.Studies to further improve and extend this process to other Michael acceptors are currently underway in our laboratory and will be reported in due course.
Generous financial support provided by the DFG (SCHN 441/ 10-1) is gratefully acknowledged.We thank K. Stockhausen (University of Leipzig) for preliminary experiments, Dr P. Lo ¨nnecke (University of Leipzig) for the crystal structure analysis and Evonik and BASF for the donation of chemicals.

Table 2
Substrate scope a