Nazarov Cyclization and Tandem [4 + 2]-Cycloaddition Reactions of

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Nazarov Cyclization and Tandem [4 + 2]-Cycloaddition Reactions of Donor−Acceptor Cyclopropanes Gangarajula Sudhakar,*,† S. K. Mahesh,† S. Phani Babu Vemulapalli,‡ and Jagadeesh Babu Nanubolu§ †

Department of CPC (Organic Chemistry-II), ‡Centre for NMR and Structural Chemistry, §Centre for X-ray Crystallography, CSIR-Indian Institute of Chemical Technology, Hyderabad-500007, India S Supporting Information *

ABSTRACT: The development of aryl vinyl/divinyl donor− acceptor cyclopropanes (DACs) as novel Nazarov cyclization (NC) precursors is described. The 1,3-zwitterion, generated from DACs embedded in the divinyl framework, acts as a pentadienyl cation, a requisite for Nazarov cyclization. A cyclic allyl cation in the course of NC was trapped with external nucleophiles to provide an interrupted NC product. Indeed, an allyl cation in this reaction is analogous to a 1,4-zwitterion that on reaction with dipolarophiles provided an easy access to substituted pyrans with excellent yield and diastereoselectivity via NC followed by a formal [4 + 2] cycloaddition.

D

Scheme 1. Our Previous and Present Plan of Work

onor−acceptor cyclopropanes (DACs) are distinctive and versatile building blocks in organic synthesis.1 Most of the studied DACs have a common substitution pattern, i.e. electronacceptor groups (usually an ester or a carbonyl group) vicinally positioned adjacent to an electron-donor substituent on the cyclopropane backbone. Polarizability of the highly π character 1,2 C−C bond is amplified by the presence of a 1,1-diester/ dicarbonyl as an electron-withdrawing group (EWG) and electron-rich aromatic group in a three-membered ring with an inherent angle and torsional strains. This intrinsic character of DACs was utilized in various reactions such as ring opening,2 cycloaddition/annulation,3 and rearrangement4 and was also employed in the synthesis of natural products and their analogs.5 Although different types of DAC reactivity are known, only the generation of 1,3-zwitterions was extensively exploited, and other possible DAC reactivity has been underexplored thus far. In this direction, Tamilov et al. reported the generation of 1,2- and 1,4dipoles as a new type of DAC reactivity.6 Nonetheless, the reactivity phenomenon of DACs combined with a framework suitable for another consecutive reaction would be of great importance in organic synthesis to assemble many carbo- and heterocyclic compounds. Research groups of Kerr1e,i,3l,o and Waser1d,h,3b,i,5b contributed significantly in this area of research. Besides, some examples of DAC reactivity combined with other reactions exist;7 Vicario et al. reported organocatalytically generated DACs in domino reactions,7a Alaniz et al. reported aza-Pianacatelli rearrangement initiated by ring opening of DACs,7b and the Yadav,7e,f France,7g−j and Waser7k research groups reported homo-Nazarov cyclization from DACs. Because of our interest in Nazarov cyclization,8 particularly in the generation of alternate Nazarov cyclization precursors,9 we recently developed dienyl oxyranes8a and aziridines8b as novel NC precursors (Scheme 1A). Continuing our interest, we envisaged the possibility of generating a pentadienyl cation, the requisite for Nazarov cyclization, by initiating ring opening of DACs. In this © 2017 American Chemical Society

direction, we pondered whether the dienyl framework strategically amalgamated in place of a donor group could be an endowed DAC for this purpose as shown in Scheme 1B. When two EWGs and two EDGs are placed vicinally on the cyclopropane (1,1-EWGs and 2,2-dienyl as EDGs system), the envisioned 1,2 C−C bond ring opening could be much faster than the archetypal single donor group system. This extra EDG (vinyl/ aryl) could also allow further elaboration in the products as an additional functional group handle. Dienyl DAC 1 does serve as a source of 1,3-zwitterion 1A compared with traditional reactions of DACs. But in this case, 1 would be a latent pentadienyl cation to undergo 4π-electrocyclization. Moreover, the formation of concomitant cyclic allyl cation IB in 4π-electrocyclization could readily be trapped by various nucleophiles to provide interrupted Nazarov cyclization product 3 or undergo elimination to furnish Received: July 6, 2017 Published: August 18, 2017 4500

DOI: 10.1021/acs.orglett.7b02061 Org. Lett. 2017, 19, 4500−4503

Letter

Organic Letters

and 2j has one quaternary and three tertiary carbons. Furthermore, 2j (2D NMR confirmed the structure, SI) was obtained as a single isomer from (−)-myrtenal containing aryl vinyl DAC 1j. This may be due to the torquoselective conrotatory electrocyclization (see the SI). To broaden the substrate scope, we next examined various aromatic groups. In general, more activated DACs bearing electron-donating functionalities in the vicinal phenyl ring participated in the reaction more efficiently. The substrate without an EDG on the aryl moiety (1k) also underwent the reaction successfully (2k), but with a relatively lower yield (78%). It is worth mentioning that the fluoro atom was erroneously assumed to have electron-withdrawing character on the reactivity of the aromatic group, but indeed it has not affected the reactivity to a great extent.11 Under standard conditions, 2l was obtained in 76% yield, comparable with a 2k yield.12 A naphthalene substrate was also suitable for this reaction, thus producing the desired product 2m in 96% yield. Heteroarenes were then employed to probe the generality of transformations further. Substrate with a pyridine moiety required an excess amount of triflic acid (see the SI) and a longer reaction time to furnish the desired product (2n) with a yield of 78%. The slow reactivity of this substrate is presumably due to coordination of acid with the Lewis basic nitrogen. However, a thiophene moiety containing product 2o was obtained in a yield of 89% under standard reaction conditions (Scheme 2). Having synthesized a wide variety of indenes from aryl vinyl DACs, we subsequently became interested in evaluating divinyl DAC. To our delight, under optimized reaction conditions, 2p was obtained in almost quantitative yield from a symmetrical dienyl DAC 1p (see the SI for its synthesis and X-ray crystal structure).13 With the ability to form these indenes and cyclopentadiene in hand, we next questioned if the cyclic allyl cation III (Scheme 3)

classical NC product 2 (Scheme 1B). On the other hand, the cyclic allyl cation is analogous to the 1,4-zwitterion, and this complementary function (1,4-zwitterion) could be employed in reacting with dipolarophiles to provide a facile access to [4 + 2]cycloaddition product 4 (Scheme 1B). Herein, we communicate the development and potential synthetic exploration of aryl vinyl/ divinyl donor−acceptor cyclopropanes as novel Nazarov cyclization precursors for the first time. To begin the proposed study, we employed aryl vinyl DAC 1a with several Lewis acids and trifluoroacetic acid (see the Supporting Information (SI) for the synthesis of 1a and for optimization of reaction conditions). When 1a was subjected to 0.1 equiv of BF3·OEt2, the formation of 2a was observed in almost quantitative yield (Scheme 2). With BF3·Et2O, dienyl DACs with Scheme 2. Nazarov Cyclization of Dienyl DACsa

Scheme 3. Interrupted Nazarov Cyclizationa

a Reaction conditions: 1 (0.1 mmol), CH2Cl2 (0.1 M) with respect to 1 at −78 °C were added BF3·OEt2.

different ester groups (1b and 1c) were tested to verify the effect of the ester group in the conversion of the product. Obtaining 2b in 96% yield and 2c in 93% yield (comparable with a yield of 2a) implies that there is no such profound effect of the ester group. With optimized conditions in hand for the reaction of aryl vinyl DAC, we decided to investigate the substrate scope, and the results are depicted in Scheme 2. At first, we examined a series of substrates incorporating the 3,5-dimethoxyphenyl moiety (1d−j) with varying substitutions on the vinyl group. Compounds with alkyl or aryl substitution at the β-position show similar reactivity, with ethyl 1d and with phenyl 1e proceeding smoothly under optimized conditions to give indenes 2d−e in excellent yield. A substrate with phenyl at the α-position and isopropyl group at the β-position 1f also provided 2f in 94% yield. Disubstitution at the β-position in the NC precursor slows down the reaction due to steric hindrance,10 but the formation of indene with quaternary carbon 2g in excellent yield from 1g is remarkable. Indenes with polycyclic 2h−j (95−97% yields) were obtained from 1h−j, respectively. Of note, compound 2i has two quaternary carbons

a

Reaction conditions: 1p (0.1 mmol), Nu (0.2 mmol), CH2Cl2 (0.1 M) with respect to 1p at −78 °C were added BF3·OEt2.

generated from symmetrical dienyl DAC 1p via pentadienyl cation II would allow its participation in the interrupted NC.14 First, we tested Et3SiH.14l Gratifyingly, we isolated 3a as an inseparable diastereomer mixture (dr 9:1) in 80% yield (Scheme 3). Next, allylTMS was examined, and we isolated 3b as a single diastereomer in 76% yield. With trimethylaluminum, 3c was obtained in very good yield. Further, the cyclic allyl cation was trapped with 1-methylindole to attain a corresponding product from a Nazarov cyclization/Friedel−Crafts reaction sequence (interrupted NC). Here, in the major product 3d (56%), the indole group and phenyl group on an adjacent carbon are in antirelation, whereas in the minor product 3d′ (41%), the indole group, and phenyl group are in syn-relation. Similarly, indole 4501

DOI: 10.1021/acs.orglett.7b02061 Org. Lett. 2017, 19, 4500−4503

Letter

Organic Letters without any protection on nitrogen also participated in the reaction, obtaining 3e (49%), and 3e′ (40%) in a similar yield and diastereomeric ratio. The structures of 3e and 3e′ were confirmed by 2D NMR (see the SI). The selectivity in this interrupted NC stands in line with a report by the West group.14d,e Typically, DACs were used to generate 1,3-zwitterions, but DACs embedded with divinyl offered a pentadienyl cation (II) and subsequently a cyclic allyl cation (III) via 4π-electrocyclization. As this cyclic allyl cation (III) is analogous to the 1,4zwitterion (IV, Scheme 4), we further questioned if the 1,4-

Indole-3-carboxaldehyde also reacted and produced 4f in 88% yield. Further, aliphatic aldehydes such as pivalaldehyde and isobutyraldehyde also reacted, providing 4g (94%) and 4h (95%). The relative configuration of 4h was further confirmed by 2D NMR (SI). Similar to 4a, the major isomer 4b−h is also associated with the inseparable isomer ranging from ∼10−30% (based on the 1H NMR; see the SI for the structures s4b−h, assigned based on the analogy to s4a). The previous reports14d,e and our interpretation based on the relative configurations of 4a, 4a′, and 4e (single crystal X-ray analysis), 4a, 4a′, and 4h (2D NMR), we propose the most probable six-membered cyclic transition state V, in which dipolarophile attacks from the opposite side to the phenyl group (α to carbocation). And, the upcoming alkyl/aryl group in the dipolarophile attains a pseudoequatorial position, which accounts for the diastereoselectivity of the major product (Scheme 4). Unfortunately, 1p with the application of the standard conditions to styrene and imines as dipolarophiles failed to afford the desired product. The unsuccessful reaction could be attributed to the low reaction temperature. At rt or above, only standard NC product 2p was observed along with a small amount of exomethylene containing cyclopentene s2p (see the SI for its structure). This implies that the transient cyclic allyl cation is perhaps trapped only if the dipolarophile is reactive enough at a lower temperature. Otherwise, the cyclic allyl cation leads to the elimination of a proton to form 2p. Furthermore, on addition of maleic anhydride to 1p, the tricyclic product 5a was obtained. This formation of 5a can be explained from the initially formed cyclopentadiene 2p which reacted with the dienophile to provide a Diels−Alder product. This was further ascertained by adding maleic anhydride to preformed 2p (1p was first stirred with BF3·Et2O until the complete formation of 2p), which provided 5a in 96% yield and with complete endo selectivity (2D NMR confirmed the structure, SI). Similarly, dimethyl acetylenedicarboxylate was also reacted to afford 5b in 82% yield. Notably, highly substituted polycyclic systems with additional functional groups in 5a−b can provide the possibility of further functionalization. In summary, 1,3-zwitterions generated from DACs embedded with dienyl groups were explored to undergo 4π-electrocyclization, and the concomitant cyclic allyl cation was trapped to provide an interrupted Nazarov cyclization product. Furthermore, we found that these new dienyl DACs are not only capable of providing a dienyl cation (1,3-zwitterions) for 4πelectrocyclization but also capable of generating complementary functionality, the 1,4-zwitterion/cyclic allyl cation. This was successfully employed in various [4 + 2]-cycloaddition reactions to provide bi- and tricyclic products in excellent yield and diastereoselectivity. The formation of C−C bonds is a fundamental reaction; up to 3 C−C bonds or 2 C−C and 1 C− O bonds and up to five contiguous stereocenters (5a) were achieved with this method. Our current research efforts are directed toward functionalized donor−acceptor cyclopropanes and expanding the method to other heterocyclic compounds; the results will be reported in due course.

Scheme 4. Nazarov Cyclization and [4 + 2]-Cycloadditiona

a Reaction conditions: 1p (0.1 mmol), CH2Cl2 (0.1 M) with respect to 1p, aldehyde (0.4 mmol) at −78 °C were added BF3·OEt2.

zwitterion (IV) would allow its participation in a formal [4 + 2]cycloaddition reaction. The successful reaction can provide a novel method to build up polycyclic molecules of high diversity in structure. For instance, aldehyde participation in the proposed cycloaddition would afford a highly functionalized tetrahydropyran ring containing a product which is an important structural motif present in a wide range of antibiotic natural products, pheromones, and marine toxins.15 Under optimized reaction conditions, when 1p was treated with benzaldehyde, as expected, a formal [4 + 2]-cycloaddition product was obtained in column chromatographically separable 4a (*2R) and 4a′ (*2S) (dr 8:2) in 98% combined yield (Scheme 4). The major isomer 4a is associated with a small amount of inseparable isomer (s4a (*2S and *7aR); see the SI for its structure and dr 95:5 based on the 1H NMR). The structures of 4a, s4a, and 4a′ were deduced by using 2D NMR (see the SI). Further, the structures of 4a and 4a′ were confirmed by single crystal X-ray analysis.13 Next, by addition of electron-rich p-methoxy benzaldehyde, 4b and 4b′ were isolated in 96% yield with 7:3 dr. Further, 2-naphthaldehyde and pbromobenzaldehyde also provided 4c and 4c′ (96%) and 4d and 4d′ (94%) with 7:3 dr. Notably, an aryl group bearing an electronwithdrawing substrate (p-nitrobenzaldehyde) provided 4e in 92% yield and no separable minor isomer was observed. The structure of 4e was further confirmed using single-crystal X-ray analysis.13

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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.7b02061. 4502

DOI: 10.1021/acs.orglett.7b02061 Org. Lett. 2017, 19, 4500−4503

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Organic Letters

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Supplementary schemes, tables, experimental procedures, 2D NMR, 1H and 13C NMR spectra, X-ray crystallographic information (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gangarajula Sudhakar: 0000-0003-3498-5584 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Science and Engineering Research Board (SERB), DST, New Delhi, India for a research grant (EMR/2016/002289). We also thank the UGC, New Delhi, India for the award of a Senior Research Fellowship (S.K.M.) and Dr. B. Jagadeesh, Head, Centre for NMR, for NMR analytical support.

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DOI: 10.1021/acs.orglett.7b02061 Org. Lett. 2017, 19, 4500−4503