Diastereoselective Construction of Novel Sugars Containing Variously

Chapter 11

Diastereoselective Construction of Novel Sugars Containing Variously Fluorinated Methyl Groups as Intermediates for Fluorinated Aldols Takashi Yamazaki, Shuichi Hiraoka, and Tomoya Kitazume

Downloaded by NORTH CAROLINA STATE UNIV on July 28, 2013 | http://pubs.acs.org Publication Date: December 29, 1999 | doi: 10.1021/bk-2000-0746.ch011

Department of Bioengineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8501, Japan

Highly diastereoselective introduction of an appropriately fluorinated methyl group into D-glucose has been realized by heterogeneous catalytic hydrogenation with the aid of relatively rigid cyclic conformations, and an exo-difluoromethylene group played an pivotal role in the preparation of mono-, di-, and trifluoromethylated materials via the divergent pathways.

Substitution of an appropriate hydrogen atom or a hydroxy moiety for fluorine has recently been one of the major strategies for realizing the effective modification or enhancement of the inherent biological activities of organic compounds (1-3). From such a point of view, aldol structures, widely found in the important naturally occurring biologically active substances, would be one of the most intriguing targets for the fluorine modification. However, as long as the authors concern, only a few examples (4-7) have been reported in the literature on the formation of this specific structure with introducing fluorine(s) to a methyl group at an appropriate position (812), and especially the corresponding nonracemic materials seem difficult to be obtained (13,14). This might be mainly because of the less ready availability (15-20) as well as the inherent instability of the requisite 3-fluorinated propionyl derivatives (Figure 1). Thus, in the case of β-fluorinated carbonyl compounds, hydrogen α to a carbonyl group becomes sufficiently acidic by the double activation by this moiety and fluorine(s), but once anionic species is formed during the course of deprotonation, then a fluorine atom is ready to depart from the original molecule to furnish α , β unsaturated materials usually in an irreversible manner (21). On the other hand, application of the traditionalfluorinationtechniques (22,23)

142

In Asymmetric Fluoroorganic Chemistry; Ramachandran, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


143

OH

OH

QMet -HF

CH . F 3

n: 0 - 3

n

n

CH3. F n

n

CH3„ F n

Χ

Ο

n

X.Y: Η or F

Figure 1 A Retrosynthetic Scheme to the Target Aldol Structures


144

CHF

CH F 2


[H]

CF

3

CF

3

CF

3

Figure 2 Synthetic Plans to Access the Targets or the direct introduction methods of fluorine-containing groups (24-30) would not be recognized as the alternative solution for this purpose in view of the still unsolved problems on the regio-, stereo-, and/or chemoselectivities. In our continuing work on the preparation of various types of optically active fluorinated building blocks (31-33) we have recently started the new synthetic study of fluorine-possessing aldol structures for opening a new route to access such interesting substances by way of the synthetic plans depicted in Figure 2. The key intermediate is the £*0-difluoromethylene compound which, starting from optically active 1,2,3triols with appropriate protections both at 1 and 3 positions, seems to be conveniently synthesized by oxidation of the free O H moiety at the central carbon atom, followed by difluoromethylenation (34,35). Moreover, this intermediate, on treatment with hydride (36,37) or fluoride (38) according to the Tellier and Sauvêtre's reports, would be further transformed into the corresponding exo-fluoromethylene compound or trifluoromethylated endo-oM'm which would be eventually converted to mono- or trifluoromethylated aldols, respectively. For this purpose, D-glucose was selected as the precursor of chiral 1,2,3-triols because, in addition to its ready availability as an optically pure form, i) regioselective protection of O H moieties at various positions has been extensively studied, and ii) usage of the different site would lead to the possible formation of diastereoisomeric structures.

Preparation of Difluoromethyl Compounds The requisite key intermediates 4, 6a, and 8 were prepared as described in Figure 3 from easily accessible methyl 4,6-O-benzylidene-a-D-glucopyranoside 1 (39) only by two steps from D-glucose. Reaction of 1 with i^-bufyldimethylsilyl (TBS) chloride, already performed by the Fraser-Reid's group (40), produced an easily separable regioisomeric mixture of 2a and 2b almost in a 1:1 ratio and an O H


145

ΌΒζ

Ph^Ô^ y ' O R CF 6a: R=Bz 6b: R=TBS


2

OMe

a) 'OR OR

2

1

2

r 2a: R =H, R =TBS 2 a :

b )

R

1

2

^2b: R =TBS, R =H

TBS

c), d)^ T B S O - ^ V

OBz

0

V

o

0Me J

OBz 8

a) TBSCI.imidazole, b) K C0 /MeOH, e) PDC, A C 0 , d) CBr F , (Me N) P, MS 4À, e) n-Bu SnO/MeOH, f) BzCI, g) BzCI, pyr., h) cat. p-TsOH/MeOH 2

2

2

2

3

3

2

2

Figure 3 Preparation of S u g a r s Containing a Difluoromethylene G r o u p at 2 , 3 , a n d 4 Positions group at 2 position of the former was oxidized with PDC in acetic anhydride. Conversion of 3 to 4 was first tried with the well-known Ishikawa-Burton protocol {34A1) and then its modified method (35) but 4 was obtained only in moderate yields (32 and 45%, respectively). However, our investigation of the reaction conditions in detail demonstrated that employment of MS 4 Â guaranteed the much higher yield (91%) as well as reproducibility. The regioisomer 2 b was utilized for the formation of 6 b or partial isomerization to 2 a under base-catalyzed conditions (2a:2b=65:35). The 3-difluoromethylenated counterpart 6 a , on the other hand, was accessible by way of tin acetal-mediated regioselective benzoylation at 2 position (42,43), followed by the above-depicted PDC oxidation-difluoromethylene Wittig reaction processes. For introduction of a C F = C group at 4 position, after benzylidene acetal was cleaved and the resultant diol was regioselectively protected at 6 position by TBSC1, the routine oxidation-difluoromethylenation procedure yielded the desired compound 8. Three types of the key exo-difluoromethylenated materials in hand, they were at the next stage subjected to hydrogénation conditions in the presence of 10% Pd/C 2


146 0

- ^ γ Ο γ Ο Μ θ

Pd/C, Η

2

9 9 % yield

"CHF

2

ÔTBS

OTBS

9 (single isomer)

0 ^ γ ° χ ^


Rt' ^ O * "

Ο

Μ

Pd/C, H

θ

y ^ Q B z CF

/ N A O .

0

2

9 5 % yield

P r f ^ O 4 X "'OBz X

ÔHF

2

2

10 (single isomer)

6a T B S O ^ ° y

VOMe

D

M

e

F^^^Y'-OBz

Pd/C, H

2

TBSO^-°j -°

M

e

2

9 2 % yield

F H C 4 Y ""OBz r

2

OBz

OBz

11 (64:36 mixture)

Figure 4 Hydrogénation of Difluoromethylenated Materials under atmospheric pressure of hydrogen at room temperature (Figure 4). Substrates 4 and 6 a with an exo-difluoromethylene group at 2 and 3 positions, respectively, were smoothly transformed into the corresponding saturated forms 9 and 10 almost in quantitative yields, both as single stereoisomers with the benzylidene acetal part intact. The stereochemistries at the newly created chiral sp carbon atoms were deduced from their H Ν MR. Thus, the coupling constant of 10.0 Hz between H and H in 9 allowed us to assume that these protons have an antiperiplcmar relationship and the C H F moiety at 2 position should occupy the equatorial position. On the other hand, NMR analysis of 10 revealed the coupling constants of H - H and H - H as 4.9 and 10.5 Hz, respectively. Since the latter value unambiguously indicated that the H proton was axially disposed, the 4.9 Hz J coupling between H and H should support the equatorial-axial relationship of these protons, and thus the C H F 2 moiety of 10 was suggested to be located at the axial position. Stereochemistry could not be determined for the compound 11 due to overlap and complexity of NMR peaks. For clarification of the origin of the excellent diastereofacial selectivity on hydrogénation obtained as above, conformational analysis for the compound 6a was performed by use of MOP A C AMI (44) with some modification (BzO-»AcO and Ph-»Me) for the convenience of computation. The most stable structure of the model compound 12 calculated is depicted in Figure 5 in two ways, one by the ball and stick model on the left and the other by the CPK model so as to place the F-C-F atoms in the X - Z plane (horizontally perpendicular to the paper) on the right hand. One would easily understand that attack at the C F = C moiety preferentially occurs from the direction of the more vacant upper si face. Although an acetoxy group at 2 position and the ethylidene acetal part are not likely to exert the significant steric hindrance towards incoming reagents (C=C-C-0Ac: 13.3°, C=C-C-0-C(O)Me: 161.4°; C=C-COCHMe: 5.8°, C=C-C-0-CHMe: 178.0°), a palladium catalyst would suffer from the 3

l

2

3

2

3

4

4

5

4

3

3

2


4


147

Figure 5 The Most Stable Conformer 12 Obtained from AM1 Calculations sterically repulsive interaction with the anomeric methoxy moiety (the angle between F C = C and C-OMe of 69.4°) when it approaches from the bottom face. The C P K model also explicitly illustrates the exclusive si face selection trend at the olefinic reaction site. 2

Preparation of Mono- and Trifluoromethylated Materials The corresponding monofluoromethylated materials were also prepared in a similar manner (Figure 6). In this case, the partial reduction of an aco-difluoromethylene group was required before palladium-mediated hydrogénation. We first used Red-Al (34) for this purpose, but the reaction actually occurred was the unexpected S 2 ' type hydride attack to furnish 13. In quite sharp contrast, DIB A H was proved to be effective only for the reductive cleavage of the benzoyl group at 2 position, and the Red-Al reduction of the resultant intermediary alcohol 15 (a 61:39 mixture, stereochemistry was not clarified yet), followed by hydrogénation smoothly produced the desired monofluoromethylated substance 16 again as a single isomer. From the *H N M R analysis, the stereochemistry at 3 position was manifested to be the same as the case of the corresponding C H F form 10. On the other hand, conversion of a difluoromethylene group in the substrate 4 to the corresponding exofluoromethylene moiety was facilitated by Red-Al, and a single stereoisomer was obtained by the subsequent hydrogénation. The same explanation towards C H F materials would be also operative here for the realization of the high level of facial selectivity because, on the basis of the most stable conformation of 12 displayed in Figure 5, substitution of one of two fluorine atoms for hydrogen seems not to significantly alter the conformational preference. It is interesting to mention that the substrate 6c was successfully transformed into 15 by Red-Al reduction in a direct fashion and such reactivity difference between 6 a and 6c caused by a slight change N

2

2


148 b) for 6c

Ο

^ γ ° γ ·

0

RfA

0

Μ

β β

V

-

0

)

forte

R

—

b)

CHF 15(61:39)

CF 6a (R=Bz) 6c (R=Ac) Downloaded by NORTH CAROLINA STATE UNIV on July 28, 2013 | http://pubs.acs.org Publication Date: December 29, 1999 | doi: 10.1021/bk-2000-0746.ch011

2

c) [83% total yield

K*Sr

ΌΗ

ÔH F 2

16

a) DIBAH,

b) Red-Al,

c) Pd/C, H /AcOEt 2

Figure 6 Hydrogénation of exo-Monofluoromethylenated Materials would be, at least qualitatively, elucidated in terms of the leaving group ability (pKa values of benzoic and acetic acids in H 0 at 25 of 4.20 and 4.76 (45), respectively). At this stage, ab initio calculation of 1,1-difluoroethylene as well as its monoand non-fiuorinated counterparts for comparison was carried out by Gaussian 94 (46) at the HF/6-311++G** level of theory for better understanding of the above reactivity difference highly dependent on the nature of hydride reagents used (Figure 7). It is 2

-0.457

CH =CH 2

2

LUMO : 1.34 eV HOMO : -10.34 eV

CHF=CH

2

1.25 eV LUMO HOMO : -10.55 eV

0.893

CF =CH 2

2

LUMO : 1.28 eV HOMO : -10.76 eV

Figure 7 N B O C h a r g e s a n d Frontier Orbital E n e r g y L e v e l s of E t h y l e n e s O b t a i n e d by Ab initio (6-311++G**) Calculation


149 quite intriguing to note that the carbon-carbon double bond of this material was highly polarized due to the electronic repulsion between lone pairs of two fluorine atoms and the olefin π electron, and the fluorine-possessing carbon atom was charged almost like a usual cation (the N B O charge of 0.89 at this site). On the other hand, its L U M O was calculated to be basically at the same level as the one of nonfluorinated ethylene. So, the S 2' type attack of hydride to 6a by Red-Al was occurred not because the F C = C part was activated by decrease of the L U M O energy level but merely because the "strongly cationic" carbon increased the ability to accept the attack by a nucleophilic ate complex Red-Al. Mechanism of Lewis acidic DIBAH reduction would be inter preted as the exclusive complexation with the benzoyl carbonyl group at the first stage due to difficult coordination at the electron-deficient olefinic site, and then delivery of hydride would be occurred to affect the reductive deprotection of the BzO moiety. In the following Figures 8 and 9 was described the preparation of the C F containing substances. As was already discussed, DAST plays a pivotal role in converting an exo-difluoromethylene group into the corresponding endo-tnflnovomethylated olefin via S 2 ' type attack of fluoride (38). Because 6a includes two latent terminally difluorinated allylic alcohol structures two different types of saturated N


2

3

N

,OMe Λ

ο

^0 *

1 „OMe

a), b)

OMe

Η

67% total yield

''ΌΒζ CF

c), d)

2

6a c), e), b) 83% yield ,Ο ,ΟΜθ ν

d) 9 1 %

"OBZ

y

i e , d

CF

3

21 (80:20)

ο Ο Λ Ο Μ θ

OMe 0

I

RTÔ*

0

I

Î 1?

f), b)

Y ^ C F

3

total yield

2

OTBS

^ ^ ' " C F

3

22 23 (single isomer)

a) DIBAH, b) DAST, c) p-TsOH/MeOH, d) Pd/C, H /AcOEt, e) TrCI, Et N, f) TBAF 2

3

Figure 8 Hydrogénation of 2 - a n d 3 - C F Olefins 3


150 2

3

C F compounds would be obtained. For example, the ' type olefin 18 was isolated by reductive cleavage of a benzoyl moiety with DIBAH, followed by DAST fluorination, and was further hydrogenated to give 19 as a 77:23 diastereomer mixture. On the other hand, the regioisomeric > olefin 2 0 was constructed by the successive conversion of 6a by deprotection of the benzylidene acetal moiety, the regioselective protection of the primary alcohol at 6 position, and S 2 ' type DAST fluorination, and was then subjected to hydrogénation conditions to produce the corresponding saturated material 21 as an 80:20 diastereomerie mixture. A trifluoromethyl moiety of both major products, 19 and 21, were determined to have the equatorial location by NMR coupling constants of H -H =13.4 Hz and H - H = 11.2 Hz, respectively. The compound 23 with a C F 3 moiety at 2 position was efficiently prepared by the three-step sequence as shown in Figure 8, and the newly created sp center was proved to possess R stereochemistry (an equatorial C F 3 group) on the basis of the strong NOE observed between H and H . An interesting reaction was encountered during the same transformation starting from 8. Thus, benzoyl deprotection with DIBAH and benzoylation allowed us to obtain a requisite terminally difluorinated allylic alcohol structure 24, which, on 3

3

4

N


2ax

3

2

3

2

TBSO^V

α

4

r>Me γ f-BuMe Si ρ 2

F C ^ Y "OBz

• path to 25

2

» path to 26

OBz

OMe S: SNEt

2

a), b)

O^ . 0Me o

+ OBz

OBz

OBz

F3

OH 24

Q 26 67% total yield

25 18% total yield

d), e), c) 95% total yield

20

TrO'

OÔMe

5QJ,OMe

d),f) 86% yield

OBz

F^'f^

OBz

28 (93:7)

27

a) DIBAH, b) BzCI, pyr., c) DAST, d) p-TsOH/MeOH, e) TrCI, Et N, f) Pd/C, H /AcOEt 3

2

Figure 9 Hydrogénation of 4 - C F Olefin 3


3


151

treatment with DAST, afforded the desired trifluorinated endo-oM'm 25 only in 18% yield. On the other hand, the main product 2 6 was manifested to possess the bicyclic structure as depicted in Figure 9 and its formation would be considered as the result of the activation of ether oxygen at 6 position by intramolecular attack of fluoride to silicon, followed by cyclization in a 5-endo-trig manner known as the disfavored mode in terms of the Baldwin's rule (47-49). Due to such a reason, the endo-oMm 27 was synthesized by changing the protective group at 6 position of 24 to a trityl group (50), and the following hydrogénation after removal of a trityl group led to the formation of 28 as a 93:7 diastereomer mixture. The stereostructure of the major isomer at 4 position was deduced as S ( C F at an equatorial position) from the H - H coupling constant of 10.3 Hz, showing these protons in an antiperiplanar relationship. Different from the cases of 22 and 27 attaining highly diastereoselective hydrogénation, an acceptable level of olefinie facial discrimination was not observed for materials 18 and 20 with a C F group at 3 position, and thus the substituent effect at both 2 and 6 positions were briefly studied for the purpose of improving diastereoselectivity towards 20 (Table 1). Comparison of data in Entries 1-3 suggested larger R protective groups at 6 position decreased the preference of the CF -eq isomer 2 1 eq and the same tendency was noticed for the substituent at 2 position (Entry 2 vs 4). Because the non-protected hydroxy group afforded the best result at the both sites, we changed the solvent to a protic EtOH aiming at attainment of the higher proportion of 21b-eq even when the totally protected form 20b was employed, and the selectivity was conveniently increased to an acceptable level as our expectation (Entry 2 vs 5). in general, sterically demanding trityl (Tr, 20a) or fôri-butyldimethylsilyl (TBS, 20b) moieties at 6 position are likely to restrict free movement to fix molecules at specific conformations. If this is the case, effective discrimination between two olefinie faces would be anticipated and possibly lead to better diastereoselectivity on hydrogénation. We again performed AMI computation of the model material 22 (modified at a benzoyl group at 2 position of 20c with an acetoxy moiety) to obtain information on its spatial shape preference. The most stable conformer was, as described in Figure 10, revealed to possess the half-chair conformation with pseudoTable 1 Substituent Effect on the Hydrogénation Stereoselectivity 4

5

3

3

1

3

R i Q ^ ° > - '

0

M

"OR

e

2

Pd/C , h

2

ρ ΐ

0

- ^ γ ° γ ·

0

Y'"OR CF 21-eq 3

Μ

θ +

2

R i O - ^ V ° V

O

M

V""OR OF 21-ax 3

Diastereomeric Ratio R2 21-ax Solvent 21-eq 20 20a*) 80 Bz AcOEt 18 20b 82 AcOEt Bz 0 20c AcOEt 100 Bz 20d 6 94 Η AcOEt 3 20b 97 EtOH Bz a) The trityl group was deprotected during the course of hydrogénation. Entry 1 2 3 4 5

Substrate

Rl Tr TBS Η TBS TBS


e

2


152

Figure 10 T h e M o s t Stable C o n f o r m e r 22 Obtained byAM1 Calculations equatorially disposed acetoxy and hydroxymethyl groups at 2 and 6 positions, respectively (C=C(CF )-C-OAc: 133.2°, HOH C-C-C=C(CF ): 138.0°), and an anomeric methoxy group in an axial position. In consequence, the factor controlling the direction of incoming reagents was expected to be the 1-OMe group (C=C(CF )C-C-OMe: 81.7°, C=C(CF )-C-C-0Me: 101.0°) as clearly shown by the C P K model, and approach from the upper si face would be preferable to the one from the other side. However, this discussion anticipated the formation of the wrong diastereomer 213

2

3

3

3

ax. Hydrogénation Mechanism This type of heterogeneous catalytic hydrogénation is in general considered to be a multistep equilibration sequence (51), and taking the previous report on the ratedetermining step (52) into account, it is most likely that the observed stereoselectivity is the reflection of the thermodynamic stability difference of the intermediates just prior to proceed reductive elimination of the Pd catalyst. In Figure 11 was shown the possible scheme. As was pointed out, the upper face of the substrate 20 seems to have more vacancy than the opposite bottom side, and the palladium catalyst will preferentially approach from the " U " direction to furnish the π complex Int-U at the first stage. By way of the oxidative addition of Pd-related species (abbreviated as Pd in the Figure for clarity), this intermediate would be further converted to either regioisomeric pair of Int-U a or Int-U b, but both of them should suffer from the significantly destabilizing 1,3-diaxial type repulsive interactions between methoxy and trifluoromethyl moieties. So, it is anticipated that the energy barriers between I n t - U and I n t - U a or Int-Ub are rather high. On the other hand, because of the axially disposed anomeric methoxy group, the "B" side of 20 is more congested than the "U" side and reluctant to accept reagents. However, the energetically less favorable Int-B is transformed to the next inter mediates, Int-Ba and Int-Bb, at the oxidative addition step, and although Int-Ba is annoyed by the 1,3-diaxial repulsion similar to the case of Int-Ua or U b , all the



153

Int-Bb Figure 11 P o s s i b l e M e c h a n i s m o n Hydrogénation of 2 0 substituents in Int-Bb locate the favorable equatorial positions except for the methoxy moiety at the axial position, which, in turn, stabilized by the well-known anomeric effect (53-55). We at present believe that the present result was obtained on the basis of the higher energy barrier between Int-U and Int-Ua (or U b ) relative to the one between 20 and Int-B, leading to the exclusive formation of 21-eq via Int-Bb.

Ring Opening of the Cyclic Compounds Obtained Sugar derivatives with variously fluorinated methyl groups at specific positions synthesized as above were further transformed into the corresponding acyclic forms and the representative example was described in Figure 12. The glycosidic bond and



154

a) B B r , 3

b) ( H S C H ) , B F O E t , 2

2

3

2

c) T B S C I , imidazole,

d) AcCI, pyr.

Figure 12 Ring Opening of the C H F Compound 10 2

benzylidene acetal of 10 were simultaneously cleaved by B B r , followed by the successive protection of the resultant hemiacetal as 1,3-dithiolane and the primary alcohol at 6 position as a TBS ether to give diol 29. Interestingly, its acetylation exclusively occurred at the 5-OH moiety, which would be elucidated on the basis of the less nucleophilic ability of the hydroxy group at 4 position due to the proximity of the electron-withdrawing CHF2 group (5(5). Compounds 29 or 30 thus obtained would be recognized as the useful anti,anti-a\do\ unit 31 and because of its latent me so structure, they would be easily employed for the formation of the corresponding enantiomers. As described above, a novel preparation of homochiral mono-, di-, as well as trifluoromethylated aldol structures starting from the readily available chiral pool compound, D-glucose, has been demonstrated. Further synthetic utilization of optically active cyclic materials as well as investigation of the similar route with different sugars is under way in our laboratory. 3

Acknowledgment This work was financially supported by the Ministry of Education, Science and Culture of Japan [Grant-in-Aid No. 08651002 and 085269]. One of the authors, S. H., is grateful to JSPS Fellowship for Japanese Junior Scientists.

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156 37) 38) 39) 40) 41) 42)


43) 44)

45) 46)

47)

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C. J. Chem. Soc., Chem. Commun. 1976, 736. 48) Baldwin, J. E . J. Chem. Soc., Chem. Commun. 1976, 734. 49) The similar type of cyclization was independently found out. See, Ichikawa, J.; Wada, Y.; Okauchi,T.; Minami, T. Chem. Commun. 1997, 1537. 50) Treatment of 1 with TrCl and Et N instead of the combination of T B S C l and imidazole would be the alternative and direct route to the requisite compound 3

27. 51) Siegel, S. In Comprehensive Organic Synthesis: Trost, B. M . , Ed.; Pergamon Press, Inc.: New York, 1991, vol. 8, p. 417. 52) Gonzo, E. E.; Boudart, M . J. Catal. 1978, 52, 462. 53) Graczyk, P. P.; Mikolajczyk, M . Top. Stereochem. 1994, 21, 159. 54) Juaristi, E.; Cuevas, G . Tetrahedron 1992, 48, 5019.

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