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Enantioselective Organocatalytic Aldehyde–Aldehyde Cross-Aldol Couplings

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Enantioselective Organocatalytic Aldehyde–Aldehyde Cross-Aldol Couplings Tetrahedron 60 (2004) 7705–7714 Enantioselective organocatalytic aldehyde–aldehyde cross-aldol couplings. The broad utility of a-thioacetal aldehydes R. Ian Storer and David W. C. MacMillan* Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 E California Blvd; Pasadena, CA 91125, USA Received 6 April 2004; accepted 9 April 2004 Available online 15 July 2004 This manuscript is dedicated to Professor D. Seebach for his pioneering work in the area of asymmetric synthesis Abstract—An asymmetric proline catalyzed aldol reaction with a-thioacetal aldehydes has been developed. Thioacetal bearing aldehydes readily participate as electrophilic cross-aldol partners with a broad range of aldehyde and ketone donors. High levels of reaction efficiency as well as diastereo- and enantiocontrol are observed in the production of anti-aldol adducts. q 2004 Elsevier Ltd. All rights reserved. 1. Introduction enantioselective cross coupling allows access to highly oxidized, stereodefined synthons of broad versatility. More- The aldol reaction is widely considered to be one of the most over, the observed reactivity profile of ‘viable-electrophile, important technologies for carbon–carbon bond formation non-nucleophile’ has established a-thioacetal aldehydes as in chemical synthesis.1 Over the last thirty years, seminal research from the laboratories of Evans,2 Heathcock,3 Masamune4 and Mukaiyama5 have established this vener- able reaction as the principal chemical method for the stereoselective construction of complex polyol architecture. Recently, studies by Barbas,6 Evans,7 List,8 Shair,9 Shibasaki,10 and Trost11 have outlined the first examples of enantioselective direct aldol reactions, an important class of metal or proline catalyzed transformation that does not require the pregeneration of enolates or enolate equivalents. With these remarkable advances in place, a fundamental goal for asymmetric aldol technology has become the development of catalytic methods that would allow the direct coupling of aldehyde substrates.12 In this context, our laboratory has recently reported the first example of a direct enantioselective aldehyde–aldehyde cross-aldol reaction using small molecule catalysts (Scheme 1).13 Subsequently, we described the enantioselective dimerization and cross coupling of a-oxygenated aldehydes to provide erythrose architecture of broad utility for the construction of hexose carbohydrates.14 In this report, we further advance this enamine catalysis concept to describe a highly stereo- selective protocol for the cross coupling of aldehydes and ketones with a-thioacetal aldehydes. Importantly, the successful introduction of a-thioacetal functionality in this Keywords: Aldol; Catalysis; Diastereoselection; Enantioselective; Organocatalysis; Proline. * Corresponding author. Tel.: þ1-626-395-6044; fax: þ1-626-795-3658; e-mail address: [email protected] Scheme 1. Proline-catalyzed cross aldehyde aldol. 0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2004.04.089 7706 R. Ian Storer, D. W. C. MacMillan / Tetrahedron 60 (2004) 7705–7714 preeminent substrates for highly selective cross aldol upon the design of aldehydic reagents that would reactions with prototypical aldehyde and ketone donors. (i) selectively function as aldol donors or acceptors, (ii) engender high levels of diastereocontrol, and Traditionally, the enantioselective aldol coupling of non- (iii) contain chemical functionality that is readily elaborated equivalent aldehydes has been viewed as a formidable and synthetically versatile. In this context, we selected synthetic challenge on account of (i) the propensity of a-thioacetal aldehydes with the expectation that they would aldehydes to polymerize under metal catalyzed conditions be highly electrophilic yet sterically and electronically and (ii) the mechanistic requirement that non-equivalent deactivated towards enamine formation (Scheme 1 Eq. 3). aldehydes must selectively partition into two discrete Moreover, the substantial steric demand of the a-dithane components, a nucleophilic donor and an electrophilic was expected to enforce high levels of absolute and relative acceptor. In our recent studies, however, we have stereocontrol in the aldol event. Lastly, the synthetic utility established that enamine catalysis can provide good levels of thioacetals as latent carbonyl and alkyl equivalents has of chemo-differentiation and stereocontrol with non- been widely established.15 Indeed, a variety of conditions equivalent aldehydes, thereby establishing the aldehyde have been elucidated to transform dithianes to aldehydes, cross-aldol as an operationally useful transformation. More alcohols or carboxylic acids. Alternatively, the reductive specifically, we have found that aldehydes containing a extrusion of sulfur using transition metals is commonly methylene carbon (CH2R2) adjacent to the carbonyl have a utilized for the conversion of thioacetals to saturated alkyl strong propensity to dimerize, via participation as both substituents (Scheme 2).15 With regard to operational nucleophile and electrophile (Scheme 1, Eq. 1).13 In advantages, it is important to note that a-thioacetal contrast, aldehydes that incorporate an a-methine carbon aldehydes are readily accessible on large scale and can be 16 (CHR3) generally do not homo-dimerize when exposed to easily handled and stored. proline, instead behaving as electrophilic acceptors exclusively (Scheme 1, Eq. 2). In this latter case, it is presumed that the kinetic inaccessabilty of the a-methine proton and the thermodynamic instability of the correspond- ing enamine effectively prohibit nucleophile formation. As a consequence, chemoselective partitioning of a-methylene and a-methine aldehydes into nucleophilic and electrophilic partners respectively can be accomplished via slow addition of the enamine precursor to the a-methine aldehyde in the presence of catalyst. With respect to stereoselectivity, it should be noted that the bulky a,a-disubstituted aldehyde acceptors consistently provide superior levels of anti- diastereocontrol in crossed aldehyde aldol reactions in Scheme 2. Manipulation of dithianes. comparison to their a-methylene counterparts (Scheme 1, cf. Eqs. 1 and 3). This improvement in diastereocontrol as a 2. Results and discussion function of increasing steric demand of the electrophile is in accord with a chair-like Zimmerman–Traxler transition The proposed organocatalytic cross aldol was first examined state that has been proposed for enamine aldol reactions. using equimolar quantities of propanal and [1,3]-dithane-2- carbaldehyde16 in the presence of catalytic proline (Table 1, Basedonourknowledgeofthechemo-andstereo- entry 1). Preliminary studies revealed that this aldol reaction differentiating features of the enamine cross aldol, we was indeed possible to provide the cross aldehyde adduct in recently sought to devise a new and generally useful class of 99% ee, however small quantities of the propanal dimer aldehyde cross coupling partner. Specifically we focused were observed. Gratifyingly, the aldol union can be Table 1. Enantioselective direct cross-aldol reaction: preliminary studies Entry Donor (equiv.a) Acceptor (equiv.) % Crossb (desired) % Dimerb (undesired) % ee Cross 1 1.0 1.0 90 10 99 2 2.0 1.0 65 35 99 3 1.0 2.0 100 0 99 4 1.0 4.0 100 0 99 5c 1.0 2.0 90 10 99 6 0 1.0 no thioacetal dimer — — a Donor added by syringe pump over 24 h. b Ratio of products determined by 1H NMR integration of reaction crude. c Donor and acceptor premixed. R. Ian Storer, D. W. C. MacMillan / Tetrahedron 60 (2004) 7705–7714 7707 comprehensively partitioned to a cross aldol mechanism via metric induction and efficiency obtained via slow addition the slow addition of propanal to an excess of the of the donor with excess dithiane acceptor (2 equiv.) electrophilic aldehyde (entries 3 and 4). prompted us to select these catalytic conditions for further exploration. In these cases the corresponding cross aldol adduct was obtained with excellent levels of anti diastereoselectivity Experiments that probed the scope of the dithiane moiety and enantiocontrol (entry 3, 16:1 anti–syn, 99% ee) without and the donor aldehyde are summarized in Table 2. In all production of dimeric propanal. As revealed in entries 3 and cases, slow addition of the donor aldehyde to a series of 5, the slow addition of the donor component is critical to the thioacetal aldehydes in the presence of proline effectively kinetic circumvention of the undesired dimerization path- suppressed homodimerization, whilst providing good to way. The superior levels of cross-aldol selectivity, asym- excellent yields of the desired cross-aldol products (entries Table 2. Enantioselective direct cross-aldol reaction: scope studies Entry Temperature (8C) Mol% cat. Product % Yield anti–syna %cc 14 10 85 16:1 .99b 24 10 77 8:1 99b 32310 70 10:1 97b 42310 41 8:1 98b 5c 410 75 .20:1 97b 62310 73 .20:1 97b 72320 52 13:1 70b 8d 23 20 91 — 96e 9d,f 23 10 88 .20:1 .99e a Ratio determined by 1H NMR analysis of the crude reaction mixtures. b Enantiomeric excess determined by chiral HPLC analysis of the 2,2-dimethyl-1,3-propanediol acetal derivatives. c Ratio of donor–acceptor was 1:1.5. d An excess of ketone donor was used. e Enantiomeric excess determined by chiral HPLC analysis. f Relative and absolute stereochemistry was confirmed by X-ray analysis.17 7708 R. Ian Storer, D. W. C. MacMillan / Tetrahedron 60 (2004) 7705–7714 1–7). Considerable variation in
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