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Cite This: Org. Lett. 2018, 20, 6730−6735
Stereoselective Homocrotylation of Aldehydes: Enantioselective Synthesis of Allylic-Substituted Z/E‑Alkenes Leiming Tian and Isaac J. Krauss* Department of Chemistry, Brandeis University, MS 015, Waltham, Massachusetts 02454-9110, United States
Org. Lett. 2018.20:6730-6735. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 11/02/18. For personal use only.
S Supporting Information *
ABSTRACT: Cyclopropanated allyl- and crotylboron reagents participate in homoallylation and homocrotylation reactions that enable enantioselective access to motifs that otherwise require many steps to synthesize. In this study, we investigated the effect of substituents α- to boron, predicted either to counteract or reinforce the 1,3- selectivity of the parent reagents. We then investigated the transformation of the substituted homocrotylation products in intramolecular photocycloadditions to produce stereochemically complex natural-product-like scaffolds, finding that flow conditions enhanced the regioselectivity and yield.
A
required in these reagents because it is sufficiently labile for ligand exchange activation by PhBCl2 in the homoallylation reaction.3 Thus, the analogous cyclopropylboronate precursors 9 (cis-9b shown, Scheme 2, eq 4) were prepared in multigram scale and high optical purity (93−97% ee) according to our previously published methods.3d We then planned to use Aggarwal’s stereoselective lithiation−borylation to insert the required chiral carbon fragment.6 Although propanediol boronate cis-9b undergoes clean methylene insertion reactions with chloromethyllithium,3,7 we were unable to achieve consistently high yield, conversion, or diastereoselectivity when utilizing propanediol boronates. We hypothesize that, compared to the pinacol- or neopentyl diol boronates utilized in Aggarwal’s reports, the 1,3-propanediol boronate (9) gives rise to an -ate complex 13 that may be more prone to B−O bond cleavage (14) or epimerization (through reversion to 8).8 Alternatively, 9 may be prone to deprotonation (19) or elimination (20) (Scheme 2, eq 5). Although we did not determine which of these pathways led to low yields of cis-12b, we did achieve full conversion and good to excellent diastereoselectivity when pinacolboronate esters 10 were used (Scheme 2, eq 6). After homologation of all possible stereoisomers of 10, oxidative hydrolysis and workup with propanediol afforded the desired propanediolboronate reagents 12 (Scheme 2, eq 6). With the required reagents in hand, we first tested the reactivity of trans-12a and cis-12a, which provided very high
llylboration of carbonyl compounds to produce allylic alcohols was first described in 1964,1 and numerous stoichiometric and catalytic methods have been developed.2 However, comparatively little is known about homoallylation reactions. Our group has shown that cyclopropanated allylboration reagents can be used for this transformation,3 with stereoselectivity that can be predicted by Zimmerman− Traxler models, similar to allylboration (Scheme 1, eq 1).4 We recently reported the scope of this transformation in enantioselective homoallylation and homocrotylation of aldehydes.3d Given that these reactions have consistently proceeded through chair transition states, with homocrotylation yielding 1,3-substituted products (Scheme 1, eq 1), we were interested in whether E/Z selectivity or 1,2-substitution could be achieved by introduction of new substituents α- to boron.5 Scheme 1b depicts the expected consequences of an α-boron substituent in transition states for homocrotylation with transsubstituted cyclopropane reagents 4 and 5. For stereoreinforcing reagent 4, transition state TS 1 is expected to be destabilized both by an unfavorable gauche interaction between substituents at positions 4 and 5 and by a destabilizing interaction due to the axial methyl group at position 1. By contrast, both of these interactions are avoided in pseudoenantiomeric chair TS 2, so that 1,3-substituted E alkene is expected. With nonstereoreinforcing reagent 5, both possible chair transition states contain an unfavorable interaction and the preferred result is not obvious. To answer these questions, we began our study with the preparation of methyl substituted cyclopropylcarbinyl reagents 12 (Scheme 2, top). The propanediol boronate ester group is © 2018 American Chemical Society
Received: September 5, 2018 Published: October 23, 2018 6730
DOI: 10.1021/acs.orglett.8b02837 Org. Lett. 2018, 20, 6730−6735
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Organic Letters
Scheme 2. Synthesis of α-Methyl Homocrotylaion Reagents
Scheme 1. Substituent Effects on Homocrotylation Regioand Stereoselectivity
selectivity for E alkenes and syn- and anti-products (13/14), respectively (Scheme 3).9 This is consistent with a large apparent energy difference between alternative Zimmerman− Traxler chair transition states TS 1 and TS 2 (Scheme 1, eq 2), of which TS 1 contains two destabilizing interactions. This reagent gave high yields with linear and branched aliphatic aldehydes, including enolization-prone phenylacetaldehyde (96% yield). Isobutyraldehyde and pivaldehyde reacted in similarly high yields, as measured by NMR integration, though isolated yields were lower due to product volatility. We next tested the reactivity of nonstereoreinforcing reagents trans-12b and cis-12b, for which the preferred homocrotylation transition state was not obvious (Scheme 1, TS 3 vs TS 4). Reagent trans-12b reacted cleanly to give exclusively 1,3-substituted products in good (>10:1) Z- and high syn-selectivity with a range of aliphatic aldehydes (Scheme 4, products 15a−g).9 Apparently, the axial orientation of the methyl group in TS 4 is less destabilizing than the presence of a methyl group at the cylclopropane carbon attacking the aldehyde (TS 3) that would lead to the 1,2-substituted product. The minor isomers produced in these reactions match spectroscopically the 1,3-syn E products 13a−g of Scheme 3 (vide infra) and, thus, likely result from reaction of the minor diastereomer present in 12. Given that optically active Z alkenes such as 15a−g are difficult to synthesize by other methods,10 we further explored the scope of this homocrotylation with aromatic aldehyde substrates. Aromatic aldehydes are more reactive in PhBCl2promoted homoallylation than are aliphatic aldehydes, and the reaction time is significantly shortened.3d However, decomposition of the product through benzylic ionization is observed when these reactions are allowed to proceed indefinitely; thus
a
The dr was measured by 1H NMR, where the minor isomer differs in the configuration of the methyl α to boron.
we quenched these reactions immediately after full conversion. As we have previously observed,3d electron-deficient aromatic aldehydes reacted in high yields, while moderately electronrich aromatic aldehydes gave acceptable yields (Scheme 5). We next explored the reactivity of cis-cyclopropyl nonstereoreinforcing reagent cis-12b. In contrast to trans-12b, cis12b reacted sluggishly to afford a mixture of products (Scheme 6). The anti Z product 16a, analogous to 15a, was obtained, albeit in very low (10%) yield.9 Additionally, anti E product ent-14a was isolated in low (13%) yield, presumably derived from the minor but much more reactive (i.e., stereoreinforcing) diastereomeric reagent (ent-cis-12a) present in a 6:94 ratio. The balance of remaining aldehyde-derived product was primarily polar material derived from oligomerization, with no 1,2-substituted product 18 observed. The low reactivity of cis-12b compared with its trans analog can be rationalized by comparing the pair of transition states TS 5/TS 6 (Scheme 6) with TS 3/TS 4 (Scheme 1, eq 3). Whereas the axial methyl in TS 4 is well tolerated, providing high selectivity over TS 3 and a reasonable reaction rate (complete in hours), the analogous axial methyl in TS 6 is destabilized by an additional A1,3-like clash with the cyclopropane methyl. Although this slows the formation of 16a to a time scale of days, alternative TS 5, leading to 1,2-substituted product 18, is still not competitive, with oligomerization pathways (aldol or polyacetal) responsible for the majority of the remaining product. The low reactivity to form Z anti products is reminiscent of situations 6731
DOI: 10.1021/acs.orglett.8b02837 Org. Lett. 2018, 20, 6730−6735
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Organic Letters Scheme 3. E-Selective Homocrotylation of Aliphatic Aldehydes
Scheme 5. Z-Selective Homocrotylation of Aromatic Aldehydes
a
Z/E ratio measured by 1H NMR. bFor yields in parentheses: trans12b (1.0 equiv) (0.4 M), PhBCl2 (1.0 equiv), aldehyde 1 (3.0 equiv), and K2CO3 (s) (6.0 equiv) in pentane at room temperature for 3 h, yield based on boronate.
a
E/Z ratio measured by 1H NMR. bIsolated yields, with NMR yields in parentheses in the case of volatile products. c40 h.
Scheme 6. Homocrotylation of 1a with cis-12b
Scheme 4. Z-Selective Homocrotylation of Aliphatic Aldehydes
photocycloaddition, pioneered by Bryce-Smith et al.12 and Wilzbach et al.,13 and used extensively in synthesis by Wender,14 which rapidly generates stereochemical complexity from this motif. However, this reaction generally exhibits low regioselectivity, affording both linear 1,3-adduct 17 or angular 1,3-adduct 17′, except with constrained substrates (Scheme 7).15 We first set up the photolysis under conditions similar to those previously employed by Wender,16 utilizing a substrate with the hydroxyl group unprotected (2a). Reactions were carried out in a quartz flask within a Rayonet reactor under 254 nm UV irradiation (three low pressure Hg lamps, each 35 W). As shown in Table 1, entries 1−3, a maximum of 12% yield was achieved with significant angular product evident only at low conversion. In order to increase the reaction rate and potentially modulate selectivity, we set the reaction up in flow conditions,17 pumping the substrate solution through a coil of fluorinated ethylenepolypropylene (FEP) tubing inside the Rayonet reactor (Supporting Information, Figure 1). As expected, we observed a trend of increasing conversion at lower flow rate (longer residence time), and this was
a
Z/E was measured by 1H NMR. bIsolated yields, with NMR yields in parentheses in the case of volatile products. c40 h.
arising in aldol or allylation chemistry, where the formation of anti, anti motifs is particularly difficult.11 With methods in hand for producing several different isomeric series of compounds 13, 14, and 15 in optically active form, we next investigated the conversion of these products into natural-product-like skeleta. We selected the arene−alkene 6732
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conversions and less selectivity for linear product. Next, the intensity of UV light was studied by increasing the number of lamps. With the flow rate fixed at 7.8 mL/h, we found that the use of more lamps led to higher conversion, yield, and selectivity (Table 1, entries 13−15 vs 6−8), with 7.8 mL/min as the optimum flow rate. A 300 nm wavelength was also tested, but no reaction was observed (Table 1, entry 16). As a control to verify that the higher yield and selectivity achieved in flow resulted from the flow format rather the light transmission properties of FEP, matched batch experiments were performed in FEP and quartz vessels (Table 1, entries 17−22). Batch reactions in FEP resulted in similar regioselectivity but lower conversion and yield compared to batch reactions in quartz, and both were worse than flow results. This indicates that the flow setup, with its penetration of light through the thin tubing, is crucial for reaction enhancement. The optimized flow conditions were then used preparatively to generate four differently substituted tetracyclic natural-product-like skeleta with differing substitution (Scheme 8). In summary, these studies have shown that, in homocrotylation with substituted cyclopropylcarbinyl boronates, the preference for 1,3- vs 1,2-substituted products overrides other stereochemical factors such as product alkene stereochemistry. Thus, either Z or E alkenes can be produced with good selectivity, though the yield is poor in the case of Z-antisubstituted products. These products can then be rapidly converted into synthetically interesting natural product-like skeleta.
Scheme 7. Arene−Alkene Photocycloaddition
accompanied by increased selectivity for the linear isomer. Increased selectivity for the linear isomer at longer residence time is consistent with batch data (Table 1, entries 1−3, 17− 19, 20−22) and likely results from more thorough interconversion of isomers under the photolysis conditions.18 We also tested the effect of altering tubing diameter (1/16 in. I.D., 1/8 in. O.D. vs 1/32 in. I.D., 1/16 in. O.D. FEP tubing) and observed greater conversion and still greater linear selectivity with the small-diameter tubing, which is consistent with greater light penetration (Table 1, entries 4−8). We then examined the use of several protecting groups under flow conditions (Table 1, entries 9−12), observing lower
Table 1. Optimization of Arene−Alkene Photocycloaddition Conditions
entry
R
lamps
I.D. × O.D.
flow rate (mL/h)
Timea (min)
linear: angularb
Yieldb (%)
Conversionb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16c 17 18 19 20 21 22
H H H H H H H H TBS TMS TFA Ac H H H H H H H H H H
3 3 3 3 3 3 3 3 3 3 3 3 8 8 8 8 8 8 8 8 8 8
Batch (quartz) Batch (quartz) Batch (quartz) 1/16 × 1/8 1/16 × 1/8 1/32 × 1/16 1/32 × 1/16 1/32 × 1/16 1/32 × 1/16 1/32 × 1/16 1/32 × 1/16 1/32 × 1/16 1/32 × 1/16 1/32 × 1/16 1/32 × 1/16 1/32 × 1/16 Batch (quartz) Batch (quartz) Batch (quartz) Batch (FEP) Batch (FEP) Batch (FEP)
N/A N/A N/A 2 6 2.6 5.2 7.8 2.6 2.6 2.6 2.6 5.2 7.8 15.6 7.8 N/A N/A N/A N/A N/A N/A
300 150 50 150 50 150 75 50 150 150 150 150 75 50 25 50 300 150 50 300 150 50
>20:1 8:1 2:1 16:1 5:1 >20:1 12:1 10:1 4:1 2:1 n.d. 3:1 >20:1 >20:1 9:1 n/a >20:1 >20:1 3:1 >20:1 19:1 9:1
11 12 4 31 39 57 57 35 35 29 99 96 70 0 >99 96 73 >99 68 34
a
Reaction time/residence time. bLinear/angular ratios, yields, and conversions were measured by GCMS. c300 nm. 6733
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Soc. 2013, 135, 82. (c) Dugas, G. J.; Lam, Y.-h.; Houk, K. N.; Krauss, I. J. J. Org. Chem. 2014, 79, 4277. (d) Lin, H.; Tian, L.; Krauss, I. J. J. Am. Chem. Soc. 2015, 137, 13176. (4) Zimmerman, H. E.; Traxler, M. D. J. Am. Chem. Soc. 1957, 79, 1920. (5) For synthesis and application of allylboration reagents with a ±-methyl group: (a) Hoffman, R. W.; Weidmann, U. J. Organomet. Chem. 1980, 195, 137. (b) Hoffmann, R. W.; Niel, G.; Schlapbach, A. Pure Appl. Chem. 1990, 62, 1993. (c) Hoffmann, R. W.; Sander, T. Chem. Ber. 1990, 123, 145. (d) Carosi, L.; Hall, D. G. Angew. Chem., Int. Ed. 2007, 46, 5913. (e) Althaus, M.; Mahmood, A.; Suárez, J. R.; Thomas, S. P.; Aggarwal, V. K. J. Am. Chem. Soc. 2010, 132, 4025. (f) Chakrabarti, A.; Konishi, H.; Yamaguchi, M.; Schneider, U.; Kobayashi, S. Angew. Chem., Int. Ed. 2010, 49, 1838. (g) Chen, M.; Roush, W. R. Org. Lett. 2010, 12, 2706. (h) Fandrick, K. R.; Fandrick, D. R.; Gao, J. J.; Reeves, J. T.; Tan, Z.; Li, W.; Song, J. J.; Lu, B.; Yee, N. K.; Senanayake, C. H. Org. Lett. 2010, 12, 3748. (i) Kobayashi, S.; Endo, T.; Ueno, M. Angew. Chem., Int. Ed. 2011, 50, 12262. (j) Chen, J. L.-Y.; Scott, H. K.; Hesse, M. J.; Willis, C. L.; Aggarwal, V. K. J. Am. Chem. Soc. 2013, 135, 5316. (k) Chen, J. L.-Y.; Aggarwal, V. K. Angew. Chem., Int. Ed. 2014, 53, 10992. (l) Millan, A.; Smith, J. R.; Chen, J. L.-Y.; Aggarwal, V. K. Angew. Chem., Int. Ed. 2016, 55, 2498. (m) Zhou, Q.; Srinivas, H. D.; Zhang, S.; Watson, M. P. J. Am. Chem. Soc. 2016, 138, 11989. (n) Diner, C.; Szabó, K. J. J. Am. Chem. Soc. 2017, 139, 2. (6) For reviews, see: (a) Thomas, S. P.; French, R. M.; Jheengut, V.; Aggarwal, V. K. Chem. Rec. 2009, 9, 24. (b) Scott, H. K.; Aggarwal, V. K. Chem. - Eur. J. 2011, 17, 13124. (c) Leonori, D.; Aggarwal, V. K. Acc. Chem. Res. 2014, 47, 3174. (d) Leonori, D.; Aggarwal, V. K. In Synthesis and Application of Organoboron Compounds; Fernández, E., Whiting, A., Eds.; Springer International Publishing: Cham, 2015; Vol. 9, p 271. (e) Collins, B. S. L.; Wilson, C. M.; Myers, E. L.; Aggarwal, V. K. Angew. Chem., Int. Ed. 2017, 56, 11700. (f) Sandford, C.; Aggarwal, V. K. Chem. Commun. 2017, 53, 5481. (7) Sadhu, K. M.; Matteson, D. S. Organometallics 1985, 4, 1687. (8) Bagutski, V.; French, R. M.; Aggarwal, V. K. Angew. Chem., Int. Ed. 2010, 49, 5142. (9) For structure confirmation, 13a−16a were subjected to alkene metathesis conditions under ethylene atmosphere and compared with previously published data;3d for example:
Scheme 8. Scope of Arene−Alkene Photocycloaddition
a
Flow rate 10.4 mL/h.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02837. Experimental procedures, characterization data, and 1H and 13C NMR spectra for new compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Isaac J. Krauss: 0000-0003-0984-4085 Notes
The authors declare no competing financial interest.
Mosher ester analysis was also performed to determine the absolute configuration of 13a (see Supporting Information).
ACKNOWLEDGMENTS Mr. Songchen Yu (Nankai University) is acknowledged for preliminary experiments. I.J.K. acknowledges the generous financial support of Brandeis University, the Donors of the American Chemical Society Petroleum Research Fund (51975DNI), and the National Science Foundation CAREER program (CHE-1253363).
(10) Although no methods generate this motif asymmetrically in one step, numerous syntheses can create the Z alkene and asymmetric center separately through multiple steps. For ±-substituted Z alkenes prepared by Still−Gennari modified Horner−Wadsworth−Emmons reaction: (a) Murakami, N.; Wang, W.; Aoki, M.; Tsutsui, Y.; Sugimoto, M.; Kobayashi, M. Tetrahedron Lett. 1998, 39, 2349. (b) Paterson, I.; Lyothier, I. J. Org. Chem. 2005, 70, 5494. (c) Menche, D.; Hassfeld, J.; Li, J.; Rudolph, S. J. Am. Chem. Soc. 2007, 129, 6100. For Wittig reaction: (d) Smith, A. B.; Beauchamp, T. J.; LaMarche, M. J.; Kaufman, M. D.; Qiu, Y.; Arimoto, H.; Jones, D. R.; Kobayashi, K. J. Am. Chem. Soc. 2000, 122, 8654. For metal-catalyzed coupling reaction: (e) Langille, N. F.; Panek, J. S. Org. Lett. 2004, 6, 3203. (f) Tan, Z.; Negishi, E. Angew. Chem., Int. Ed. 2006, 45, 762. (g) Huang, Z.; Negishi, E.-i J. Am. Chem. Soc. 2007, 129, 14788. (h) Belardi, J. K.; Micalizio, G. C. J. Am. Chem. Soc. 2008, 130, 16870. For alkene metathesis reaction: (i) Xu, C.; Shen, X.; Hoveyda, A. H. J. Am. Chem. Soc. 2017, 139, 10919. For Claisen rearrangement: (j) Nonoshita, K.; Banno, H.; Maruoka, K.; Yamamoto, H. J. Am. Chem. Soc. 1990, 112, 316. For Claisen-ene rearrangement: (k) Leclercq, C.; Markó, I. E. Tetrahedron Lett. 2005, 46, 7229. For Lindlar hydrogenation: (l) Shin, Y.; Fournier, J.-H.; Fukui, Y.; Brückner, A. M.; Curran, D. P. Angew. Chem., Int. Ed. 2004, 43, 4634. For Tamaru−Mori reaction: (m) Kimura, M.; Ezoe, A.; Shibata, K.;
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REFERENCES
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