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Molecules 2012, 17(12), 14249-14259; doi:10.3390/molecules171214249

Review
Recent Developments in the Reformatsky-Claisen Rearrangement
Jun Ishihara * and Susumi Hatakeyama
Graduate School of Biomedical Sciences, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan; E-Mail: susumi@nagasaki-u.ac.jp
*
Author to whom correspondence should be addressed; E-Mail: jishi@nagasaki-u.ac.jp; Tel.: +81-95-819-2427; Fax: +81-95-819-2426.
Received: 8 November 2012; in revised form: 21 November 2012 / Accepted: 27 November 2012 /
Published: 30 November 2012

Abstract

: The rearrangement of allyl α-bromoacetates with Zn dust is known as the Reformatsky-Claisen rearrangement. Whereas the Ireland-Claisen rearrangement has been widely used in the synthesis of a diverse range of natural products, the Zn-mediated Reformatsky-Claisen rearrangement has not been utilized so often. In this article, we will provide an overview of recent advances in the Reformatsky-Claisen rearrangement field, including the In-mediated Reformatsky-Claisen rearrangement we have recently developed.
Keywords:
Reformatsky-Claisen rearrangement; Reformatsky reaction; Claisen rearrangement; zinc; indium

1. Introduction

The development of new methods for stereoselective carbon-carbon bond formation has been important in the creation of useful molecules such as drugs and other chemical entities. [3,3]-Sigmatropic rearrangements are reliable reactions for selective carbon-carbon bond formation, in particular, the Claisen rearrangement is one of the most competent methods to provide useful building blocks for the synthesis of natural products [1,2,3,4]. The synthetic utility of this reaction has prompted the development of a considerable number of variants of the classical Claisen rearrangement [5,6,7,8,9]. For instance, zinc-mediated [3,3]-sigmatropic rearrangement of α-haloesters, proceeding through Zn enolates, are referred to as the Reformatsky-Claisen rearrangement. Compared to the Ireland-Claisen rearrangement which was widely used in the synthesis of a diverse range of natural products [10,11,12,13,14], the Reformatsky-Claisen rearrangement has the advantage of being performed under non-basic conditions. This review article focuses on the chemistry of the Reformatsky-Claisen rearrangement and its applications, as well as the recent development of the In-mediated Reformatsky-Claisen rearrangement.

2. Pioneering Works of the Reformatsky-Claisen Rearrangement

In 1973, Baldwin and Walker reported a synthetically useful sigmatropic rearrangement of α-halogenated allyl esters [15]. Zinc enolates, generated by Reformatsky-type reactions of α-halogenated allyl esters with zinc dust, provided the corresponding rearranged products. For instance, when allyl α-bromoisobutyrate 1a was added to a refluxing suspension of an excess amount of Zn dust it afforded acid 2a in excellent yield (Table 1). It should be noted that the rearrangement of 1a can readily install a quaternary center into the product. The reaction of α-bromopropionate 1b also proceeded smoothly to give a rearranged product 2b. In contrast, the rearrangement of simple allyl ester 1c and secondary ester 1d were found to be fruitless. Baldwin stated that one of the reasons for the low yield would be the generation of 1,3-dicarbonyl products by intermolecular Claisen condensation, which had been known as byproducts of classical Reformatsky reactions. Another reason would arise from the decomposion of Zn enolate 4 to afford 5 and allyl bromide (Scheme 1). The resulting allyl bromide would undergo Friedel-Crafts reaction with the aromatic solvent catalyzed by the zinc bromide generated in the reaction.

Table Table 1. Baldwin and Walker’s pioneering work of the Reformatsky-Claisen rearrangement.

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Table 1. Baldwin and Walker’s pioneering work of the Reformatsky-Claisen rearrangement.
Molecules 17 14249 i001
SubstrateR1R2R3R4SolventTemp (°C)Yield of 2a–d
1aMeMeHHPhH80100%
1bMeHHMePhMe11096%
1cHHHHXylene140<15%
1dMeHPhHPhMe11016%
Molecules 17 14249 g001 1024
Scheme 1. Possible pathway to generate byproduct.

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Scheme 1. Possible pathway to generate byproduct.
Molecules 17 14249 g001 1024

2.1. Reformatsky-Claisen Rearrangement in the Presence of Zinc and a Silylating Reagent

The Reformatsky-Claisen rearrangement also proceeds in the presence of a silylating agent, in which a silyl ketene acetal is the most likely intermediate. Ireland and co-workers demonstrated that upon heating a mixture of α-bromo ester 6, Zn dust, and TBSCl in THF and HMPA under reflux, the carboxylic acid 7 was obtained in 73% yield (Scheme 2) [11]. This reaction would be evaluated as a base-free reaction complementary to the ester enolate Claisen rearrangement.

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Scheme 2. Reformatsky-Claisen rearrangement with zinc and a silylating reagent.

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Scheme 2. Reformatsky-Claisen rearrangement with zinc and a silylating reagent.
Molecules 17 14249 g002 1024

An example of a reactions performed under these conditions was illustrated by Akiba, which involves preparation of a carboxylic acid bearing a silyl group [16]. A thermal reaction of α-bromoacetate 8a with Zn and TBSCl in THF and HMPA provided carboxylic acid 9, LiAlH4 reduction of which afforded alcohol 10 in 68% overall yield. On the other hand, application of the Ireland-Claisen protocol to acetate 8b resulted in the production of the same carboxylic acid 9 in lower yield (Scheme 3).

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Scheme 3. Reformatsky-Claisen rearrangement of 8a.

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Scheme 3. Reformatsky-Claisen rearrangement of 8a.
Molecules 17 14249 g003 1024

Narasaka and co-workers employed the Reformatsky-Claisen rearrangement for the preparation of highly functionalized carboxylic acid 12. Exposure of α-bromoisobutyrate 11 to Zn and TMSCl furnished branched acid 12, having a quaternary carbon. The product 12 was transformed to a dienyl oxime 13, which was a precursor in a palladium-catalyzed domino cyclization (Scheme 4) [17].

Molecules 17 14249 g004 1024
Scheme 4. Preparation of functionalized carboxylic acid 12.

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Scheme 4. Preparation of functionalized carboxylic acid 12.
Molecules 17 14249 g004 1024

2.2. Application of Reformatsky-Claisen Rearrangement with Zinc and a Silylating Reagent

The most frequently reported Reformatsky-Claisen protocol involves heating a substrate with Zn dust and a silylating reagent in an aprotic polar solvent. Several additional applications are described below.

2.2.1. Preparation of α-Fluorocarboxylic Acid by Reformatsky-Claisen Rearrangement

Fluorinated ketones have been successfully employed as enzyme inhibitors in modern bioorganic chemistry. Therefore, the synthesis of selectively fluorinated molecules which have fluorine substituents adjacent to a carbonyl group became a major target in fluoroorganic chemistry. One of the earliest applications of Reformatsky-Claisen rearrangement of fluorinated substrates, reported by Lang and co-workers, was the conversion of allyl chlorodifluoroacetate 14 to difluoroacid 15 (Scheme 5). Various allyl chlorodifluoroacetates can undergo a Reformatsky-Claisen protocol to give 2,2-difluoro-4-pentenoic acid derivatives [18].

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Scheme 5. Reformatsky-Claisen reaction of fluorinated substrate 14.

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Scheme 5. Reformatsky-Claisen reaction of fluorinated substrate 14.
Molecules 17 14249 g005 1024

2.2.2. Application of α-Fluorocarboxylic Acid Induced by Reformatsky-Claisen Rearrangement to Biological Active Compounds

An intriguing extension of this methodology would be found in the synthesis of a key building block for a number of second-generation HIV protease inhibitors reported by Chen’s group at Pfizer. They performed the reaction of chlorodifluoro derivative 16 with Zn and TMSCl in 1,3-dimethyl-imidazolidin-2-one (DMI) for the preparation of difluorocarboxylic acid 17. The product 17 was then converted to amide 18, from which 4,4-difluoro-3,3-dimethylproline derivative 19, a core part of HIV protease inhibitors such as 20 and 21 was synthesized (Scheme 6) [19].

Molecules 17 14249 g006 1024
Scheme 6. Synthesis of 2,2-Difluoroproline Derivative by Reformatsky-Claisen Rearrangement.

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Scheme 6. Synthesis of 2,2-Difluoroproline Derivative by Reformatsky-Claisen Rearrangement.
Molecules 17 14249 g006 1024

Qing and co-workers employed a Reformatsky-Claisen rearrangement to synthesize fluorinated thionucleosides. The replacement of a carbohydrate moiety of naturally occurring nucleosides with other five membered rings is one of the promising approaches for exerting a significant effect on the biologic activity. In the course of the studies of nucleoside analogues, they targeted difluoromethylene containing thionucleosides, such as 22 and 23, which were the modified analogs of highly bioactive (−)-2'-deoxy-3'-thiacytidine (3TC) and (+)-2'-deoxy-3'-oxacytidine (L-OddC) (Scheme 7) [20]. When Reformatsky-Claisen rearrangement of 24 was carried out under conventional conditions (Zn, and TMSCl), none of desired product was obtained. On the other hand, the addition of pyridine promoted this rearrangement effectively. Thus, treatment of 24 with Zn and TMSCl in the presence of pyridine at 120 °C in a sealed tube afforded the desired product 25 in 43% yield. Compound 25 was then transformed to thiofuranose 26 in 4 steps, which was condensed with pyrimidine bases by regioselective Pummerer reaction to afford nucleosides 27a and 27b (Scheme 7).

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Scheme 7. Synthesis of thionucleosides through Reformatsky-Claisen Rearrangement by Qing.

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Scheme 7. Synthesis of thionucleosides through Reformatsky-Claisen Rearrangement by Qing.
Molecules 17 14249 g007 1024

Qing’s group further targeted fluorinated carbocyclic nucleosides based on the similar strategy. Upon treatment of 28 with Zn and TMSCl at 105 °C in MeCN, the rearrangement took place to deliver difluorinated acids, esterification of which afforded ethyl ester 29 as a mixture (syn:anti = 3:1) in 72% yield [21]. On the other hand, the reaction of monofluoro ester 31 provided four isomers (dr = 8.7:3.4:1.8:1), major component of which was syn-anti-product 32. The resulting esters 29 and 32 were convertible to cyclopentenes 30a,b, and 33a–c after installation of nucleobases, respectively (Scheme 8) [22].

Molecules 17 14249 g008 1024
Scheme 8. Synthesis of carbocyclic nucleosides by Qing.

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Scheme 8. Synthesis of carbocyclic nucleosides by Qing.
Molecules 17 14249 g008 1024

3. Indium-Mediated Reformatsky-Claisen Rearrangement

Recently Ishihara and Hatakeyama reported the In-mediated Reformatsky-Claisen rearrangement, which is feasible for various α-bromoisobutyrate derivatives [23,24]. Initially, α-bromo-cyclohexanecarboxylate 34 was subjected to the conventional rearrangement conditions. However, when 34 was treated with Zn and TMSCl-Et3N in boiling THF, the protonated product 35 was obtained exclusively (Scheme 9). The scope of the Reformatsky reaction has been considerably extended by the use of metals other than Zn. For instance, in is known to react readily with α-halo esters to induce the Reformatsky-type reaction [25]. Recently Baba and co-workers demonstrated that In(I)X is effective for the Reformatsky-type reactions of ketones and esters to afford β-hydroxyketones and β-hydroxyesters diastereoselectively [26,27,28]. Gratifyingly, we found that treatment of 34 with In-InCl3 in the presence of TMSCl and Et3N in MeCN under ultrasonication conditions at 10–30 °C furnished 36 in 88% yield.

Molecules 17 14249 g009 1024
Scheme 9. Reformatsky-Claisen rearrangement of 34.

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Scheme 9. Reformatsky-Claisen rearrangement of 34.
Molecules 17 14249 g009 1024

3.1. Indium-Mediated Reformatsky-Claisen Rearrangement of α-Bromopropionates

To probe the generality of the In-mediated reaction, α-bromoesters 37ac were subjected to the optimized reaction conditions (Scheme 10). The benzyl and TBS ethers were also not affected at all under the conditions; however the THP group was susceptible owing to the Lewis acidity of InCl3.

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Scheme 10. Indium-mediated Reformatsky-Claisen rearrangement.

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Scheme 10. Indium-mediated Reformatsky-Claisen rearrangement.
Molecules 17 14249 g010 1024

3.2. Indium-Mediated Reformatsky-Claisen Rearrangement of Various Substrates

We next turned our attention to the reactions of various α-bromopropionate derivatives, which are readily prepared by acylation of the corresponding allylic alcohols with 2-bromoisobutyryl bromide or 2-bromopropionyl bromide (Table 2). Most reactions afforded the rearranged products along with the protonated compounds.

Table Table 2. Indium-mediated Reformatsky-Claisen Rearrangement of Various Substrates.

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Table 2. Indium-mediated Reformatsky-Claisen Rearrangement of Various Substrates.
SubstratesRMethodProducts Yield
Molecules 17 14249 i00239a: Mea Molecules 17 14249 i00740a96%
39b: Hb40b84% (1.5:1)
Molecules 17 14249 i00341a: Mea Molecules 17 14249 i00842a94%
41b: Hb42b61% (1.6:1)
Molecules 17 14249 i00443a: Mea Molecules 17 14249 i00944a71%
43b: Hb44b54% (1.4:1)
Molecules 17 14249 i00545a: Mea Molecules 17 14249 i01046a63%
45b: Hb46b62%
Molecules 17 14249 i00647a: Mea Molecules 17 14249 i01148a91%
47b: Hb48b34%

Method a: In (2 eq.), InCl3 (2 eq.), TMSCl (8 eq.), Et3N (8 eq.), MeCN, 10–30 °C; Method b: In (2 eq.), InCl3 (2 eq.), TMSCl (4 eq.), Et3N (4 eq.), THF-DMPU (1:1), 10–30 °C.

The aromatic compounds 39, 41, and 43 underwent rearrangement to carboxylic acids 40, 42, and 44 in moderate to excellent yields, although the diastereoselectivities were poor. In the case of 39a, 41a, and 43a, the rearrangement took place in MeCN rather smoothly, whereas the rearrangement of 39b, 41b, and 43b proceeded in THF-DMPU (1:1) but not in MeCN. On the other hand, the reactions of aliphatic substrates 45 and 47 brought about the Reformatsky-Claisen rearrangement successfully to give highly functionalized carboxylic acids 46 and 48. In fact, when the In-mediated reaction of bulky 2-methylbut-3-en-2-yl esters 45a and 45b were performed, compounds 46a and 46b were obtained in 63% and 62% yields, respectively. Notably, the rearrangement of 47a can install contiguous quaternary centers, giving compound 48a in 91% yield.

3.3. The Reaction of Base-Sensitive Compounds

The most intriguing feature of the Reformatsky-Claisen rearrangement is the feasibility of utilizing base-sensitive substrates. The reactions of α-bromoisobutyrates E-49 and Z-49 under the optimized conditions proceeded smoothly to afford 50 in 80% and 53% yields, respectively (Scheme 11). In the case of α-bromopropionate 51, a moderate diastereoselectivity was observed, although the yield was not satisfying. On the other hand, the reaction of 53 afforded carboxylic acid 54 having contiguous quaternary carbons in 66% yield. It should be noted that the acetoxy group could survive under the reaction conditions in stark contrast to the Ireland-Claisen rearrangement.

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Scheme 11. Investigation of the Reformatsky-Claisen rearrangement of acetoxy α-bromoesters.

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Scheme 11. Investigation of the Reformatsky-Claisen rearrangement of acetoxy α-bromoesters.
Molecules 17 14249 g011 1024

The results shown in Scheme 12 reveals a marked advantage over the Ireland-Claisen rearrangement. Thus, when the reaction of 55 was performed under the above mentioned In-mediated rearrangement conditions, the rearranged product 56 was obtained in 64% yield. In contrast, the reaction of 57 with KHMDS in the presence of TMSCl and Et3N afforded isomer 58 in place of 56 [29,30,31].

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Scheme 12. Attempted rearrangements of 55 and 57.

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Scheme 12. Attempted rearrangements of 55 and 57.
Molecules 17 14249 g012 1024

3.4. Proposed Mechanism of the Indium-Mediated Reformatsky-Claisen Rearrangement

As illustrated in Scheme 13, it has been reported that a mixture of In and InCl3 generates InCl (I) in situ, which readily reacts with α-bromoacetate 59 to afford α-In(III) intermediate 60 or α-In(I) 61 [27]. Both α-indium intermediates can be transformed to the In enolate 62, which is converted to silyl ketene acetal 63 by silylation. Since no rearrangement was observed without TMSCl and Et3N, the direct rearrangement of In enolates seems unlikely. Finally, the rearrangement of 63 proceeds to generate the corresponding carboxylic acid 64. The rearrangement of α-bromoisobutyrate derivatives (R3, R4 = Me) smoothly proceeded in MeCN, whereas the reaction of α-bromopropionates (R3 = Me, R4 = H) in MeCN afforded only the protonated products. These results can be rationalized as follows: compared to enolate 62 (R3, R4 = Me) derived from isobutyrate, the enolate 62 derived from the propionate derivative (R3 = Me, R4 = H) is more nucleophilic, so it easily undergoes protonation with MeCN to form a protonated product. The deuteration experiment in MeCN-d3 clearly supported that the rearrangement would proceed, provided that the protonation of an enolate is relatively slow.

Molecules 17 14249 g013 1024
Scheme 13. Proposed mechanism of Reformatsky-Claisen rearrangement.

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Scheme 13. Proposed mechanism of Reformatsky-Claisen rearrangement.
Molecules 17 14249 g013 1024

4. Conclusions

The recent studies show that Zn- and In-mediated Reformatsky-Claisen rearrangements of α-haloacetate derivatives are regarded as a useful variant of the classical Ireland-Claisen rearranegements. The feasibility of these methods for base-sensitive substrates makes it complementary to the Ireland-Claisen rearrangement, and allows simple access to valuable building blocks for the synthesis of complex natural products.

Acknowledgments

This work was financially supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan (24590011).

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