Computational investigation of mechanistic aspects of the alkylation of chiral enamines

Journal of Molecular Structure: THEOCHEM 814 (2007) 75–84 www.elsevier.com/locate/theochem

Computational investigation of mechanistic aspects of the alkylation of chiral enamines Leandro Greff da Silveira, Doriane Sacheto, Paulo Augusto Netz *, Eduardo Rolim de Oliveira * Instituto de Quı´mica, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil Received 8 January 2007; received in revised form 26 February 2007; accepted 27 February 2007 Available online 4 March 2007

Abstract We studied the Michael reaction between the imines derived from the (5R)-dihydrocarvone and the homochiral 1-phenylethylamine (PEA), using semi-empirical AM1 method for conformational analysis and ab initio RHF/6-31G* for transition state calculations. Experimental results for this reaction show that the proportion of products depend strongly on the chirality of the amine moiety. With the conformational analysis, we found a pyramidal geometry of the nitrogen and could rationalize the significant role played by the sterical hindrance. We were able to explain the formation of the only product in the S amine diastereoisomer (matched case), but the analysis was not conclusive regarding the R one, where two products are formed (mismatched case). The results of our transition state calculations, however, helped us to understand the proportion of the products. In the matched case, the axial attack in the most favored conformer has a energy barrier significantly lower than the equatorial attack and we can explain the formation of only one product. On the other side, in the mismatched case, there is only a small difference between the most favorable attacks leading to either products, making plausible the formation of a mixture of products. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Michael reaction; Chiral imines; Diastereoselectivity; Transition state calculation; Conformational analysis

1. Introduction The Michael reaction of chiral imines described in 1985 by Pfau et al. [1] is a very important tool to the construction of a-carbonyl quaternary chiral centers. Its mechanistic trends [2] and synthetic applications [3] were reviewed by d’Angelo’s group and some attempts to stablish a theoretical model accounting to its stereochemical course appeared in the literature [4–7]. All of them are able to explain both the high degree of regio and diasteroselectivity observed in this reaction when the enamine cycle is unsubstituted, but they cannot answer why the diastereoisomeric excess (de) drops out when substituted cyclic enamines are *

Corresponding authors. Tel.: +5551 33167194 (P.A. Netz). E-mail addresses: [email protected] (P.A. Netz), [email protected] (E.R. de Oliveira). 0166-1280/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2007.02.044

employed as nucleophilic species. This lack of diastereoselectivity was shown in our pioneer study of double asymmetric induction starting from (5R)-(+)-dihydrocarvone (1), as seen in Scheme 1 [8]. We reported a marked difference of selectivity depending on the configuration of the amine moiety. Imines 3 (in which the carbon of the amine is R) and 4 (the carbon is S) are prepared by condensation of 1 and both enantiomers of homochiral 1-phenylethylamine (PEA) (2). With imine 3, the mismatched situation, the adduct 6a was obtained in only 58% de contrasting with the expected result of >95% de for 7 in the matched alkylation of imine 4. The diastereoisomeric excess of Michael reactions of the chiral imine 3 is strongly dependent on the nature of electrophilic partner as seen in Scheme 2 and Table 1. Entries 2–4 could reveal presumable solvent and temperature effects over de. This kind of solvent-dependence is

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R- or S -2

5a

R3 R4

N

O

O 1

H R

1R

2

6a R3= Me; R4= CH3CO(CH2)2- from 3 3 4 3 R1= Me; R2= Ph 7 R = CH3CO(CH2)2-; R = Me from 4 1 2 4 R = Ph; R = Me

Scheme 1.

X X 5a-c N

O

H MePh

6a-c 3

Scheme 2.

Table 1 Influence of nature of electrophile on the diastereoselectivity of alkylation of enamine R-3 Entry

Reagent

X

Conditions

Product de (ratio)

1 2 3 4 5

5a 5b 5b 5b 5c

COMe COEt COEt COEt SO2Ph

THF/r.t./3 days THF/r.t./3 days THF/40 °C/3 days PhMe/40 °C/24 h 35 °Ca

6a 6b 6b 6b 6c

a

53 60 66 71 90

(3.76:1) (4:1) (5:1) (6:1) (9:1)

Ref. [8] [9] [10] [11] [12]

Solvent and reaction time are not reported by authors.

somewhat surprising because strong evidences supporting a concerted transition state in aprotic medium were early pointed out [13,14]. Despite its moderate diastereoselectivity, our methodology seems to be a very good improvement to earlier procedures [15] for preparation of the important building block 8. As it can be seen in Scheme 3, a selective aldol condensation leads to a easily separable mixture of 8 and the ketol 9 [16]. From this way, our group prepared the natural (+)-acyperone (10) [9], and our route to 10 was used as the

6a + 7

KOH EtOH/Et2O 0ºC

+

O

O

OH

8

9

Scheme 3.

key step for many synthetic applications. Li and co-workers synthesized the same molecule and some derivatives [11] as well several natural products like (+)-eudesma-3,11(13)dien-12-oic acid (11) [17], ()-13-hydroxy-a-eudesmol (12) [18] and others [19]. Finally, allohedycaryol (13) was prepared by de Groot [10]. Similarly, Chiu adds imine 3 to phenylvinylsulfone in the preparation of a bicyclo[4.3.0] skeleton [12]. The regioselectivity of alkylation of 2-substituted cyclohexanones, on their imine derivative, can be explained by the participation of the tautomeric more substituted enamine as the effective nucleophile in the reaction [2]. The de is normally very high in favour of 2,2-disubstituted product, decreases using very reactive electrophiles as nitroalkenes [20] or 1,1-bis(phenylsulfonyl)ethylene [21]. However, abnormal results were observed with electrophiles such as vinyl ketones [22] or acrylates [23] when a resident chiral center is present in the imine cycle (Fig. 1). These unexpected low regio and diastereoselectivity for substituted enamines could reveal an influence of conformational factors in the enamine cycle during the transition state of addition step [24]. Based in preliminary results of Sevin et al. [4] an empirical model was designed accounting for the stereoselection of alkylation, shown in Fig. 2. The more substituted enamines, obtained from both enantiomers of PEA (2), attacks the electrophile by the less hindered face, on their conformers 14 or 15, yielding the corresponding adducts in a diastereoselective process. As we can see in the picture, the model predicts a Si attack when the chirality of amine is S and the opposite with the enantiomer, in accordance with the observed results using unsubstituted enamines [2]. But the model cannot explain the mismatched situation presented in Scheme 1 [8] because Re product is not obtained exclusively with (R)-PEA as we can see in Table 1 [8–12]. In a more detailed work, Sevin et al. [5] showed the importance to consider the hybridization of nitrogen atom in the conformation of enamines. Excepting vinylamine,

CO2H O

H 10

OH OH

H 11

12

OH 13

Fig. 1. Natural products prepared by our synthetic route shown in Scheme 3.

L.G. da Silveira et al. / Journal of Molecular Structure: THEOCHEM 814 (2007) 75–84 Si attack

Me Ph

77 H

H

H N

Ph

H 14

Me

from (S)-2

Ph

H N

O Me

H 15

Re attack

from (R)-2

axial Re attack

N H O H HMe 18'

O Me

O

)(

N H

H

Ph

Me

H

18''

Scheme 4.

Fig. 2. Empirical model for the stereoselection of alkylation in unsubstituted enamines.

the standard geometry of nitrogen atom is pyramidal, so the use of an planar enamine model could not represent the real structure, making difficult the statement of a theoretical model to explain the stereochemical course of reaction. More recently, Tran-Huu-Dau and co-workers [7], studying structures like enaminone 16 demonstrated the previous considerations of an anti attack with respect to the phenyl group, as shown in Fig. 3. The results are supported by crystallographic data of enaminoesters [25], in which the conformation around C*–N bond is nearly the same calculated for a Re approach from enaminone 16. In such extended conjugated system the nitrogen atom assumes a planar geometry and the cyclopentene ring is not so flexible as a cyclohexene one. Therefore, there are less degrees of freedom and the calculations are easier. Another attempt to explain the origin of diastereoselectivity was proposed by Lucero and Houk in 1997 [6]. The authors using ab initio methods, estimated in 2.0 kcal/mol the difference of activation energy between axial and equatorial attacks of N-methylaminocyclohexene (17) to acrylonitrile, as we can see in Fig. 4, in accordance with known stereoelectronic preference for the alkylation of enolates and enolethers [26].

16 Me O Re approach

N H Me O

H

H Me

Ph ) θ=60° cycle H Re face

O Me

Fig. 3. Proposal of anti attack with respect to the phenyl group.

By MMX calculations, they estimated in 0.8 kcal/mol the difference of heat of formation of two limiting conformers of enaminoester 18, favoring 18 0 in which there were less severe non-ligand interactions between the allylic hydrogens and the substituents of chiral ligand [6] as shown in Scheme 4. According to Lucero and Houk [6], the preferential attack becomes an axial Re attack in the most stable conformer 18 0 , explaining experimental results with unsubstituted enamines [2]. The advantage of this model is the assumption of conformational effects in enamine ring, and the stereoelectronic effects to determine the diastereoselectivity. The presence of resident substituents in the cycle can strongly modify the equilibrium position of 18 analogs, then affecting both regio and stereoselectivity of Michael addition. An interesting point to be mentioned is that an aromatic ring is needed in the chiral auxiliary. d’Angelo’s group performed the reaction with a series of amines as chiral inductors and they described a dramatic loss of selectivity when a saturated cyclohexyl ring is present in the place of phenyl one (de = 45%) [25]. 2. Methods of calculations Considering the importance of the enamine alkylation and the fact that, in spite of the proposed models, the stereoselectivity of this reaction is not fully understood, we proposed a detailed mechanistic study, employing quantum mechanical (semiempirical and ab initio) calculations. The starting point was the conformational analysis of enanines 3 and 4. Using molecular mechanics we were able to map the most stable conformers, that were further optimized using semiempirical AM1 method.

Fig. 4. Transition states of axial (left) and equatorial (right) attacks of N-methylaminocyclohexene 17 to acrylonitrile. Reproduced from Ref. [6], with permission of the American Chemical Society.

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L.G. da Silveira et al. / Journal of Molecular Structure: THEOCHEM 814 (2007) 75–84 R1

equatorial Si attack

axial Si attack

R1 N R2

equatorial Re attack

H

S N R2

axial Re attack

19-26 pe

H 19-26 pa

N H 3 pe

Me H

Entry

Enamine

R1

R2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26

Me Me Et Et Et Et Et Et Et Et Et Et Et Et Et Et

H H H H Me Me Et Et (S)s-Bu (S)s-Bu (R)s-Bu (R)s-Bu (S) Ph(Me)CH– (S) Ph(Me)CH– (R) Ph(Me)CH– (R) Ph(Me)CH–

pe pa pe pa pe pa pe pa pe pa pe pa pe pa pe pa

After this analysis we carried out transition state calculations for the reaction between model enamines 19–26 and ethylene in both axial and equatorial attacks in a RHF/631G* level of calculation. These structures were built from the most stable structures obtained in the previous conformational analysis, with a pseudo-equatorial (pe) or pseudoaxial (pa) conformations for R1, as shown in Fig. 5 and Table 2. The conformational analysis was made using SPARTAN PLUS (version 1.5) [27]. For the transition state analysis we used the program JAGUAR (version 3.5) [28], using the pseudo-spectral method [29]. This method reduces the time dependence on the number of base functions from N4 to N3. Both programs were licensed to the Laborato´rio de Quı´mica Teo´rica of the Instituto de Quı´mica, UFRGS. 3. Results and discussion 3.1. Conformational analysis The results of the conformational analysis of the four conformers (3 pe, 3 pa, 4 pe and 4 pa, Scheme 5) are shown in Figs. 6 and 7. They show the most stable (least energetic) structures of these conformers, calculated using molecular mechanics and semiempirical AM1. In the enamine conformer 3a (DHf,AM1 = 22.57 kcal/ mol), we can observe that the axial attack (Si face) is sterically hindered, whereas in the conformer 3b

N

Ph

Me H Ph

H 3 pa

R

Fig. 5. Attacks Si and Re between ethylene and model enamines 19–26.

Table 2 Enamines studied in the ab initio transition states calculations

S

N H 4 pe

R

Ph H

N

Me

H

Ph H Me

4 pa Scheme 5.

(DHf,AM1 = 24.26 kcal/mol) the axial attack via Re face is hindered, leading to the formation of 7 as the only product. In the case of Fig. 7, in both conformers 4a (DHf,AM1 = 23.20 kcal/mol) and 4b (DHf,AM1 = 25.14 kcal/mol), there is no sterical hindrance to axial attacks (from Si face in the former case and Re face in the later case), leading to the formation of products 6a and 7. The most stable conformation predicted by Sevin et al. [5] (according to Fig. 2) was not found. However, we could confirm a pyramidal geometry of the nitrogen, experimentally found by crystallography [30]. These results lead to the consideration that the chiral auxiliar may play the role proposed by Lucero and Houk [6], that is, the conformational transmission of chirality. The quantitative aspects, however, remain to be explained, because of the proportion of 3.76 to 1 in the products of the reaction involving the enamine 4 [8], considering that the major product is derived from the least stable conformer according Houk’s interpretation. In order to solve this paradox, we could postulate a kinetic control and therefore we need to determine the activation energies for the enamines 3 and 4. 3.2. Transition state analysis According to our transition state calculations, the hydrogen transfer does not occur with a zwitterionic intermediate. The distances between the electrophile carbon atom and the nucleophile carbon atom in the double bond ˚ , which, together were found to be between 2.03 and 2.14 A with the normal vibration modes analysis, support a concerted mechanism for the hydrogen transfer. Considering, however, that we are using a simplified model for the electrophile, extrapolation of this particular result for the attack with vinyl ketones could not be made. The activation energies for enamines 19–26 (entries 1–16 in Table 2), obtained with ab initio (RHF/6-31G*), are shown in Tables 3 and 4. We observe in Tables 3 and 4, that the axial attack is the energetically most favored. This axial attack preference in cyclohexenes, enolates and similar compounds is probably related to a torsional strain in the cycle in the transition state. The axial attack goes through a chair-like transition

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79

Fig. 6. Most stable conformers of the enamine (S,R), found by molecular mechanics and AM1.

Fig. 7. Most stable conformers of the enamine (R,R), found by molecular mechanics and AM1.

Table 3 Results of transition state RHF/6-31G* calculations, with substituent R1 in pseudo- equatorial conformation Enamine

R1

R2

Ab initio (RHF/6-31G*) DE (kcal/mol)

19 20 21 22 23 24 25 26

pe pe pe pe pe pe pe pe

Me Et Et Et Et Et Et Et

H H Me Et (S) s-Bu (R) s-Bu (S) Ph(Me)CH (R) Ph(Me)CH

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Equatorial attack

Axial attack

DEa (kcal/mol)

Ea (kcal/mol)

(N*)

Ea (kcal/mol)

(N*)

57.36 57.28 56.96 57.34 59.23 59.52 59.69 56.24

— — (S) (S) (S) (S) (S) (S)

54.54 54.54 54.05 54.49 56.10 57.93 54.27 55.12

— — (R) (R) (R) (R) (R) (R)

2.82 2.74 2.91 2.85 3.13 1.59 5.42 1.12

Ea, TS  free reactants (enamine + ethene); DEa, activation energy difference between Axial and Equatorial attacks; DE, energy difference between the least and the most stable conformers; N*, nitrogen absolute configuration.

state, whereas the equatorial attack goes through a twisted boat-like [26], as illustrated in Fig. 8. As we can see in this figure, the favored axial attack leads to the formation of the Si product in the case of R1 pseudo-equatorial and Re product otherwise. We note that, changing R1 from methyl to ethyl (enamines 19 and 20), there is no significant change in the activation energies, what means that these groups play no significant role in sterical effects. In the calculations involving enamines 19–22, the energies of axial attacks are similar in both R1 conformations (comparing Tables 3 and 4), but there is a difference of about 1.5 kcal/mol in the equatorial attacks because of

the sterical interactions between R1 and ethylene. However, with the insertion of a chiral group (starting from enamine 23 in Tables 3 and 4), this trend changes. The chiral group brings additional sterical repulsions, as we shall discuss next. Figs. 9 and 10 show the combined effect of the configuration of the nitrogen and the insertion of the chiral group. In Fig. 9 four cases are shown, considering the best hydrogen transfer geometry, with absolute nitrogen configuration S or R and both Si and Re faces. Note in each case the sterical repulsions. In a reaction in Si face, the hydrogen is oriented between 0° and 90° (cases (B) and (C)), whereas in a Re face reaction, the hydrogen is oriented

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Table 4 Results of transition state RHF/6-31G* calculations, with substituent R1 in pseudo- axial conformation R1

Enamine

R2

Ab initio (RHF/6-31G*) DE (kcal/mol)

19 20 21 22 23 24 25 26

Me Et Et Et Et Et Et Et

pa pa pa pa pa pa pa pa

H H Me Et (S)s-Bu (R)s-Bu (S) Ph(Me)CH (R) Ph(Me)CH

1.97 1.99 1.48 1.58 1.27 1.36 1.56 1.46

Equatorial attack

Axial attack

DEa (kcal/mol)

Ea (kcal/mol)

(N*)

Ea (kcal/mol)

(N*)

58.67 58.67 58.60 58.74 59.77 61.52 58.86 60.02

— — (R) (R) (R) (R) (R) (R)

54.04 54.01 54.11 54.48 57.48 56.91 58.51 53.23

— — (S) (S) (S) (S) (S) (S)

4.63 4.66 4.49 4.26 2.29 4.61 0.35 6.79

Ea, TS  free reactants (enamine + ethene); DEa, Activation energy difference between Axial and Equatorial attacks; DE, energy difference between the least and the most stable conformers; N*, nitrogen absolute configuration.

H3 δ− Et R1 pseudo-equatorial

R1

H4

H3 EtH

Ha He

Et NH

3

R1

H4

He

δ+

H H

N

R2

Ha

4

R1

Ha

Si product

R2

Axial attack NH

He

R2

R2 H3

NH

δ+

Ha

Et R1

4

H

H3 R1

H4

Ha

He

He Et = Ethylene (Electrophile) Et

EtH

δ−

Re product

Equatorial attack

R1 pseudo-axial

δ−

Et

N

R1

R2

R1 Ha

R2

3

H

3

H

N

HEt

Et

He

H4

δ+

NH

H

He

1

R

R2

NH

Si product

Equatorial attack

Ha

H3

Ha

4

He 2

H4

R

R1

R2

3

H Et

NH

Et

δ−

R2 N

Ha H4

4

H

Et = Ethylene (Electrophile)

δ+

R1 H3

He

Axial attack

EtH

He

Re product

Fig. 8. Transition state conformations of the cycle in the axial and equatorial attack to enamines.

Ha

L.G. da Silveira et al. / Journal of Molecular Structure: THEOCHEM 814 (2007) 75–84

81

Fig. 9. Effect of the nitrogen configuration in the attacks.

Ha He R 2 = (S)Ph(Me)CH

H

R2 R1

H

H

H

Ha

N

C

H

Less steric effect

N CH3 C* H

S He

2

R = (R)Ph(Me)CH

H

C

H

H

Less steric effect

N

H

C* H H3 C

R

Fig. 10. Effect of the insertion of a chiral group.

between 0° and 90° (cases (D) and (E)). In the cases (B) and (E) the R2 has a larger sterical effect due to the proximity of the methyl group. For this reason, the cases (C) and (D) are preferred, and the reactions via Si face display an absolute configuration inversion of the nitrogen, but the reaction via Re face do not change this configuration. Fig. 10 shows the effect of a chiral group bound to the nitrogen. It is important to note that, as consequence of the preferential geometries in the transition state (Fig. 9, C and D), the group R2 lies almost in the cycle’s plane. Therefore, if the group R2 is chiral, less repulsion is achieved if the smallest atom bound to C* (hydrogen in this case) is located in (or near to) this plane. This would bring the largest group toward the face Si if the chiral

group configuration is R and toward the face Re if the chiral group configuration is S. In both cases, there is an increase of activation energy for the attack in the same side of the largest group. Based on the discussion above and on the results of Tables 3 and 4 we can propose a hierarchy of importance for the factors contributing to the activation energy. The most important is the type of attack (axial or equatorial), followed by the chiral auxiliary configuration (S or R) because of the orientation of the bulky group and the last factor is the nature of the substituent R1, which can increase the sterical effect in the Si face when its conformation is pseudo-axial (pa). It must be mentioned that all these factors depend on the pyramidal geometry of the nitrogen.

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Fig. 11. Transition state geometries for the enamine 25.

With the combination of these factors, we can understand the diastereoselectivity of the reaction: the difference between axial Si and Re attacks is large (4.24 kcal/mol) for the enamine 25 (chiral auxiliar S) – with the least energetic attack occurring in the Si face in the most stable conformer 25 pe (Fig. 11) whereas for the enamine 26 (chiral auxiliar R) this difference is smaller (1.89 kcal/mol) – and the least energetic attack occurs in the Re face in the least stable conformer 26 pa (Fig. 12). The explanation for the small diastereoselectivity becomes more clear if we consider the difference in the stability of the conformers. The energy barrier to obtain products of Re attack from 26 pa (Table 4) is 53.23 kcal/mol, but if we sum the difference between conformers (1.46 kcal/mol starting from 26 pe), is 54.69 kcal/mol, very similar to the energy in the axial attack in the most stable conformer 26 pe (55.12 kcal/ mol, Table 3) attacked in the Si face. These very similar energies lead to the prediction that both products would be formed, in this case, with a predominance of the Re attack. This can explain the mismatched results of the Michael reaction [8]. With our results, we can explain the low diastereoselectivity for the enamines 23 and 24 (see DEa in Tables 3 and 4), because a sec-butyl substituent in R2 is less bulky and

has a higher flexibility regarding rotation, thus lowering the repulsion with the electrophile and the cycle. In this sense, we can understand why it is needed a bulky and not flexible substituent as the phenyl group in order to have a high diastereoselectivity, in agreement with experimental data [25]. 4. Conclusions Using computational methods we investigated the Michael reactions of chiral imines (Scheme 1) in particular the reaction between the imines derived from the (5R)-(+)dihydrocarvone and the homochiral 1-phenylethylamine (PEA), where a intriguing difference of selectivity depending on the configuration of the amine moiety was found. If the amine carbon is R, two products were formed (the mismatched case), whereas with the S amine essentially one product (the matched case). With a preliminary conformational analysis on both R and S (compounds 3 and 4) diastereoisomers we could understand, in terms of sterical hindrance, why the attack in the S diastereoisomer lead to the formation of only one product. For the attack in the R diastereoisomer, this analysis show the possibility of formation of both

L.G. da Silveira et al. / Journal of Molecular Structure: THEOCHEM 814 (2007) 75–84

83

Fig. 12. Transition state geometries for the enamine 26.

products, but the explanation of their proportion was not fully conclusive. Furthermore, we could find a pyramidal geometry of the nitrogen, that plays a significant role in the transmission of chirality. Our transition state calculations yield, for a simplified model for the electrophile, a concerted mechanism for the hydrogen transfer and a semi-quantitative explanation for the proportion of the products in the Michael reaction. The transition state calculation results can be rationalized taking into account several factors: depending on the geometry of attack (axial or equatorial), the geometrical requirements for hydrogen transfer lead to different chiralities in the nitrogen atom. The nitrogen’s chirality, on its turn, induces different conformations in the chiral auxiliary and this last interacts with the cyclohexene ring. As a consequence, regardless the initial geometry, the axial attack is preferred, but the energy differences between the attacks depend strongly on the chirality of the amine. In the matched case, the axial attack in the most favored conformer has a energy barrier significantly lower than the equatorial attack and we can explain the formation of only

one product. In the mismatched case, the difference between the most favorable attacks leading to either products is small, making plausible the formation of a mixture of products. Acknowledgements We thank GQT-UFRGS for the computer facilities and PROPESQ-UFRGS for financial support. References [1] M. Pfau, G. Revial, A. Guingant, J. d’Angelo, J. Am. Chem. Soc. 107 (1985) 273. [2] J. d’Angelo, D. Desmae¨le, F. Dumas, A. Guingant, Tetrahedron: Asymmetry 3 (1992) 459. [3] J. d’Angelo, C. Cave´, D. Desmae¨le, F. Dumas, in: S.G. Pandalai, (Ed.), Trends in Organic Chemistry, Trivandrum, India, 1993, pp. 555–615. [4] A. Sevin, J. Tortajada, M. Pfau, J. Org. Chem. 51 (1986) 2671. [5] A. Sevin, D. Masure, C. Giessner-Prettre, M. Pfau, Helv. Chim. Acta 73 (1990) 552. [6] M.J. Lucero, K.N. Houk, J. Am. Chem. Soc. 119 (1997) 826.

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