(4 + 3) cycloadditions of allenyl ether-derived oxygen-stabilized oxyallyls with furans

Xian Huang a, Waygen Thor b, Xiangyu Feng a, Liangliang Kang a, Min Yang a, Chi-Sing Lee *b, Yuen-Kit Cheng *b and Shuzhong He *a
aSchool of Pharmaceutical Sciences, and Guizhou Engineering Laboratory for Synthetic Drugs, Guizhou University, Guiyang, Guizhou 550025, China. E-mail: szhe@gzu.edu.cn
bDepartment of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR

Received 6th November 2019 , Accepted 6th December 2019

First published on 9th December 2019


(4 + 3) cycloadditions between allenyl ethers and furans are described. The reaction features an in situ formation of oxygen-stabilized oxyallyls via epoxidations of allenyl ethers in the presence of H2PO4. The multiple interactions between the oxygen-stabilized oxyallyl species and H2PO4 were studied using DFT calculations for rationalization of the regio- and diastereoselectivity of this cycloaddition. The utilities of this cycloaddition have been demonstrated by converting the (4 + 3) cycloadduct into the cyclohepta[b]benzofuran skeleton of frondosin B in two steps.


Introduction

As a powerful synthetic method for the construction of seven-membered carbocycles, (4 + 3) cycloaddition has been extensively investigated for over five decades.1 Specifically, heteroatom-stabilized oxyallyls have attracted much attention and have become increasingly important in the forefront of (4 + 3) cycloadditions (Scheme 1).2 Significant advances have been made by employing heteroatoms such as halogens,3 oxygen,4 sulfur,5 and nitrogen6,7 to modify the stability and reactivity of oxyallyl species as well as to introduce an electronic bias in these intermediates that can lead to high regioselectivity and stereoselectivity.
image file: c9qo01349k-s1.tif
Scheme 1 Strategies for (4 + 3) cycloadditions.

From 2001 to 2014, Hsung and coworkers developed a series of (4 + 3) cycloadditions based on nitrogen-stabilized oxyallyl intermediates derived from epoxidations of N-sulfonyl- or N-acyl-substituted allenamides (Scheme 1).7–9 This elegant approach provided convenient access to seven-membered ring containing alkaloids.10 However, given the fact that carbocyclic seven-membered rings are found predominately in the terpenoid family,11 there is a practical interest to extend the nitrogen-based methodology to allenyl ethers. Allenyl ethers12 are isosteres of allenamides that could undergo a similar epoxidation/ring-opening process to generate oxygen-stabilized oxyallyls (7), which could undergo (4 + 3) reaction with dienes to afford alkoxy-substituted cycloheptanones. We reasoned that the introduction of an oxygen functionality to the cycloadducts would broaden the application of allene-based (4 + 3) cycloadditions and pave the road toward the synthesis of complex terpenoids. Epoxidations of allenyl ethers have been reported by Shimizu and Frontier (see Scheme 2);13 however, (4 + 3) cycloadditions of allenyl ethers remain unexplored. We herein report the development of (4 + 3) cycloaddition of allenyl ether-derived oxygen-stabilized oxyallyls with furans.


image file: c9qo01349k-s2.tif
Scheme 2 Previous reports on epoxidations of allenyl ethers.

Results and discussion

Considering the structural similarity between allenamides and allenyl ethers, we first applied the well-developed conditions of allenamide (4 + 3) reactions (ZnCl2, furan, dimethyldioxirane, and 4 Å MS in CH2Cl2) to simple allenyl ethers. As shown in Scheme 3, both 1-methoxyallene (5a) and 1-phenoxyallene (5b) readily reacted with furan to afford (4 + 3) cycloadducts 8a and 8b as single diastereomers in 39% and 53% yields respectively.14 Since the phenyl group of 5b provided extra stability for experimental handling, allenyl ether 5b was chosen as the substrate for optimizing the reaction conditions of the (4 + 3) cycloadditions.
image file: c9qo01349k-s3.tif
Scheme 3 Cycloaddition of simple allenyl ethers.

As shown in Table 1, a brief survey of the reaction temperature with ZnCl2 as the Lewis acid in CH2Cl2 showed that the highest yield (70%) was obtained at −30 °C (entries 1–3). At this temperature, several other Lewis acids were screened, and ZnCl2 remained as the optimal Lewis acid for promoting the allenyl ether (4 + 3) cycloadditions (entries 4–9). Inspired by Uria and Vicario's phosphoramide catalyzed asymmetric (4 + 3) cycloadditions of allenamides,15 the effects of phosphoric acid derivatives on this reaction were investigated. Phosphoric acid, diphenyl phosphate and K2HPO4 gave moderate yields for this cycloaddition (entries 10 and 11).16 To our delight, we found that KH2PO4 provided the best yield (81%) among all the conditions (entry 13). Further optimization using KH2PO4 with different solvents and reaction temperatures (ESI Table 2) indicated that optimal yield was obtained in CH2Cl2 at −30 °C.

Table 1 Optimization of (4 + 3) cycloaddition conditions

image file: c9qo01349k-u1.tif

Entrya Additive Temp (°C) Yieldb (%)
a All reactions were carried out with 0.2 mmol 5b, 0.4 mmol furan, 0.6 mmol DMDO (as a solution in CH2Cl2, added by syringe pump within 1 hour), and 0.4 mmol of the indicated. b Isolated yields.
1 ZnCl2 −78 53
2 ZnCl2 −30 70
3 ZnCl2 0 18
4 None −30 21
5 BF3·OEt2 −30 32
6 In(OTf)3 −30 41
7 MgBr2 −30 62
8 ZnBr2 −30 23
9 Zn(OTf)3 −30 50
10 H3PO4 −30 52
11 (PhO)2P(O)OH −30 44
12 K2HPO4 −30 61
13 KH2PO4 −30 81


With the optimized conditions in hand, the scope of substrates for the (4 + 3) cycloadditions was explored. As shown in Fig. 1, the yield of cycloadduct 8a was increased to 52% under the optimal conditions. Other aliphatic allenyl ethers such as n-butoxyallene and i-propoxyallene did not give the desired products (ESI Table 5). In contrast, aryl allenyl ethers with different substituents on the phenyl ring readily reacted with furan and afforded cycloadducts 8c–l in moderate to good yields. Para-substituted phenoxyallenes with electron-withdrawing groups (EWG) (8c and 8h–j) generally gave higher yields than those with electron-donating groups (EDG) (8k and 8l). The results of the halophenoxyallenes (8d–g) also gave lower yields. Phenyloxyallenes with an extended π-systems resulted in only moderate yields of the cycloadducts 8m and 8n. Benzyloxyallenes also worked well, especially with a strong EWG at the para-position (8q). When a chiral benzyloxy group was introduced in the allene substrate, a promising level of asymmetric induction was observed since only two diastereoisomers of 8r were obtained. Interestingly, the cycloadducts of prenyloxyallene with furan was further oxidized under the reaction conditions and gave a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 diastereomeric mixture of epoxides 8s. Moreover, protected pyrrole reacted well as the diene moiety to afford 8t in 76% yield. The structure of 8d was unambiguously characterized by its X-ray single-crystal structure (ESI Fig. 1).17


image file: c9qo01349k-f1.tif
Fig. 1 Scope of the allenyl ether (4 + 3) cycloaddition.

To further study the substrate scope and selectivities of the (4 + 3) cycloadditions, different substituted furans were employed for reaction with 5b. As shown in Table 2, cycloaddition of 3-methylfuran 9 with 5b provided cycloadducts 17 with moderate regioselectivity slightly favoring the syn-product, in which the phenoxy group and furan substituents are on the same side of the cycloadduct.18 Using 3-bromofuran as the substrate led to a mixture of unidentifiable isomers and side-products. A much better isolated yield (80%) was obtained by using methyl 3-furoate (10) as the diene, albeit still with low regioselectivity (1.6[thin space (1/6-em)]:[thin space (1/6-em)]1). In a comparison study, (4 + 3) cycloaddition of 5b with 10 using ZnCl2, MgBr2 or no acid additive led to only 20–29% yields of cycloadduct 18 as a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 syn/anti isomer. These results indicated that KH2PO4 may play a specific role in this type of cycloaddition. Using benzyl 3-furoate 11 gave an equimolar mixture of regioisomers in 73% yield, while using tert-butyl 3-furoate 12 afforded a similar yield with the anti-cycloadduct as the major product.

Table 2 Cycloaddition of 5b with substituted furans

image file: c9qo01349k-u2.tif

Furan-R Products Yielda (rr)
a Total isolated yield (%) of all isomers. b Ratio determined by 1H NMR. c Ratio determined by isolated yields.
3-Me (9) image file: c9qo01349k-u3.tif image file: c9qo01349k-u4.tif 39 (1.7[thin space (1/6-em)]:[thin space (1/6-em)]1)b
3-CO2Me (10) image file: c9qo01349k-u5.tif image file: c9qo01349k-u6.tif 80 (1.6[thin space (1/6-em)]:[thin space (1/6-em)]1)c
3-CO2Bn (11) image file: c9qo01349k-u7.tif image file: c9qo01349k-u8.tif 73 (1[thin space (1/6-em)]:[thin space (1/6-em)]1)c
3-CO2tBu (12) image file: c9qo01349k-u9.tif image file: c9qo01349k-u10.tif 75 (1[thin space (1/6-em)]:[thin space (1/6-em)]1.5)c
2-Me (13) image file: c9qo01349k-u11.tif image file: c9qo01349k-u12.tif image file: c9qo01349k-u13.tif 41 (2[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]3)b
2-Br (14) image file: c9qo01349k-u14.tif image file: c9qo01349k-u15.tif image file: c9qo01349k-u16.tif 37 (4[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]1)b
2-CO2Me (15) image file: c9qo01349k-u17.tif 71
2-CO2tBu (16) image file: c9qo01349k-u18.tif image file: c9qo01349k-u19.tif 64 (1[thin space (1/6-em)]:[thin space (1/6-em)]1)c


We also employed 2-substituted furans for the (4 + 3) cycloadditions. While 2-methylfuran 13 provided 41% yield of a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 syn[thin space (1/6-em)]:[thin space (1/6-em)]anti mixture of regioisomers, 2-bromofuran 14 gave a high syn selectivity (5[thin space (1/6-em)]:[thin space (1/6-em)]1). In both reactions, the syn-products were found to be a mixture of equatorial/axial isomers implying the existence of stepwise reactions in these cases. Finally, when methyl 2-furoate 15 was employed, syn-23 was obtained as the only regioisomer in 71% yield. However, switching to tert-butyl ester 16 eliminates the regioselectivity.

To understand the herein unparalleled experimental observations compared with the analogous allenamide system,19a DFT calculations were performed to address the observed regioselectivities specifically. It turns out that the exact electronic feature of the allenyl ether oxyallyl generated during the reaction, and the use of the protic KH2PO4 instead of Lewis acids led to the unique regioselectivities observed. Firstly, the ethereal oxygen atom in 7 is not as electropositive as the nitrogen atom in intermediate 3 determined at the same level of calculation (ESI Fig. 2). In fact, it tends to be weakly electronegative based on ChelpG partial atomic charge (−0.2719e for 7b). This probably is due to the intrinsic electronegativity of oxygen and the vicinal relatively polarizable phenyl ring. Various allenyl ethers show variation in the partial atomic charge of this ethereal oxygen (ESI Fig. 3). In general, the more negative of the partial charge the higher chemical yield of the corresponding cycloadducts. Similar charge effect can be observed in benzyloxyallenes 8o and 8p. A reasonable yield of 8o retains even though the magnitude of the negative partial charge of the ethereal oxygen drops significantly in 7o (ESI Fig. 3). The added torsional degrees of freedom of the extra methylene may reduce the steric crowding during the concerted reaction (inferred from Fig. 2vide infra). In view of the similar cycloaddition system with the acid additive ZnCl2 modelled singly coordinated to the terminal anionic oxyallyl oxygen atom,19b both the hydrogen-bonding (HB) donors –OH of the H2PO4 were also initially modelled close to this same oxygen of the allenyl ether transition-state (TS) structures in the current work. Fig. 2 displays the syn- versus anti-TSs leading to products 18 and 23 after optimizations. In the current experimental conditions, H2PO4 should remain undissociated in organic solvent based on the second and third pKa values (7.2 and 12.4 respectively) of aqueous phosphoric acid (H3PO4). This rationalizes KH2PO4 as the optimized acid additive in our model. H3PO4 has a dissociable proton which could affect the intended reaction undesirably, while either K2HPO4 or diphenyl phosphate has only one possible HB donor and the latter is sterically bulky in addition.


image file: c9qo01349k-f2.tif
Fig. 2 Proposed transition structures leading to cycloadducts 18 and 23. The possible steric repulsion discussed of the syn-TSs of 10, 11 and 12 has a better view displayed after rotation in the inset. ΔG in kcal mol−1 at 298.15 K. Distances in Å (those of HBs underlined). C: grey, H: white, O: red, P: orange. Optimized and energies calculated using DFT at B3LYP-D3, 6-31+G(d,p). Full computational details are described in ESI.

Based on the optimized TS structures in Fig. 2, the H2PO4 can further interact electrostatically with the allenyl ether or furan via one of the negatively charged phosphate oxygen atoms in addition to the originally modelled hydrogen bonds. The calculated ΔG values agree with the trend of syn- versus anti-regioselectivities, especially the relatively more significant difference (ΔΔG = 1.35 kcal mol−1) in the case of 23 possibly leads to the observed absolute syn-selectivity. This might be caused by the presumably extra CH–π interaction in the TS-23-syn (Fig. 2) as also observed in the recent analogous allenamide system,19b but which should be eliminated by using a tert-butyl ester in agreement with the experimental result of using the diene 16 instead of 15 (Table 2). For the 3-furoates, the syn-/anti-product ratio shifted from 1.6[thin space (1/6-em)]:[thin space (1/6-em)]1 to 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5 according to the bulkiness of the ester (101112). In this case, the CH–π interaction is absent but the increasing steric repulsion of the furoate ester with the closest hydrogen atom on the allene phenyl ring (Fig. 2, TS-18-syn) is likely the cause of the product ratio shift. Two syn products were obtained each from the dienes 13 and 14 but not from 9. This observation can be inferred from the syn TSs of Fig. 2. By replacing the 2-methyl ester with either 2-Me or 2-Br, the phenyl ring of the allenyl ether will experience increased steric repulsion leading possibly to a stepwise TS (and likely to a lower yield) which allows the phenoxy end of the allene to sample more conformational space via the central dihedral rotation. This in turn may lead to two possible syn products as observed (Table 2). For 3-Me, there is no such steric repulsion with the phenyl ring of the allenyl ether and hence only one syn product is observed. In this case, however, the 3-Me group (also 3-Br, see ESI) will sterically hinder the stabilizing HB interaction of phosphoric acid and again leads to the low chemical yield as observed (Table 2).

To demonstrate the synthetic potential of this (4 + 3) reaction, the possibility of removing the O-protecting groups on the cycloadducts was explored. Under typical conditions, the p-methoxyphenyl group, benzyl group, and p-methoxybenzyl group in cycloadducts 8l, 8o, and 8p can be readily removed to afford α-hydroxyl cycloheptanone 25 respectively (Scheme 4). Moreover, the aryloxy groups on the cycloadducts can also be recruited directly in synthesis. For instance, following a two-step reaction sequence we have reported,10b cycloadduct 8f was readily converted to cyclohepta[b]benzofuran 27 (Scheme 5), which shares the same tricyclic core as frondosin B.20


image file: c9qo01349k-s4.tif
Scheme 4 Removal of O-protecting groups on the cycloadducts.

image file: c9qo01349k-s5.tif
Scheme 5 Synthesis of cyclohepta[b]benzofuran skeleton.

Conclusions

We have developed an allenyl ether (4 + 3) cycloaddition for the synthesis of substituted cycloheptanones. The reaction conditions were optimized by using H2PO4 (in CH2Cl2 at −30 °C). As indicated in a theoretical study, this HB-capable Brønsted acid can have multiple interactions with the oxygen-stabilized oxyallyls that were derived from epoxidations of allenyl ethers, and lead to interesting regioselectivities that could possibly for further development. Efforts in applying this method in natural product syntheses are underway.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant No. 21562011, 21861010 and 21871017), the Science and Technology Foundation of Guizhou Province (Qian Ke He J Zi [2015]2047, Qian Ke He platform talents [2018]5781), the Case Base Construction Project for Guizhou Graduate Education (KCALK2017012), Guizhou University Talent Project (No. [2014]36) and Hong Kong Baptist University (RC-SGT2/18-19/SCI/005 and RC-ICRS-18-19-01A) for their financial support.

Notes and references

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  14. 8b could also be obtained by using m-CPBA as the oxidant, albeit with a lower yield (41%).
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  16. For an example of using K2HPO4 as an additive in (4 + 3) cycloaddition, see: M. Topinka, K. Zawatzky, C. L. Barnes, C. L. Welch and M. Harmata, An asymmetric, catalytic (4+3) cycloaddition reaction of cyclopentenyl oxyallylic cations, Org. Lett., 2017, 19, 4106–4109 CrossRef CAS PubMed.
  17. Single crystal data of 8d could be found in the Cambridge Crystallographic Data Centre using CCDC number 1915744..
  18. According to the IUPAC definition, “syn” and “anti” descriptors are used to denote the relative stereochemistry in a ring system. However, for consistency with previous studies (see ref. 7) and for the convenience of description, here we use “syn” and “anti” to describe different regioisomers of the (4 + 3) cycloadducts.
  19. (a) A. G. Lohse, E. H. Krenske, J. E. Antoline, K. N. Houk and R. P. Hsung, Regioselectivities of (4+3) cycloadditions between furans and oxazolidinone-substituted oxyallyls, Org. Lett., 2010, 12, 5506–5509 CrossRef CAS PubMed; (b) J. E. Antoline, E. H. Krenske, A. G. Lohse, K. N. Houk and R. P. Hsung, Stereoselectivities and regioselectivities of (4+3) cycloadditions between allenamide-derived chiral oxazolidinone-stabilized oxyallyls and furans: Experiment and theory, J. Am. Chem. Soc., 2011, 133, 14443–14452 CrossRef CAS PubMed.
  20. A. D. Patil, A. J. Freyer, L. Killmer, P. Offen, B. Carte, A. J. Jurewicz and R. K. Johnson, Frondosins, five new sesquiterpene hydroquinone derivatives with novel skeletons from the sponge Dysidea frondosa: Inhibitors of interleukin-8 receptors, Tetrahedron, 1997, 53, 5047–5060 CrossRef CAS.

Footnote

Electronic supplementary information (ESI) available. CCDC 1915744. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qo01349k

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