Amino Acids (1996) 11:397-408

Amino Acids

© Springer-Verlag 1996 Printed in Austria

Stereoselective synthesis of t - a m i n o acids via conjugate addition of nitrogen nucleophiles to a,/3-unsaturated esters Recent advances

Review Article N. Sewald Institut ftir Organische Chemie der Universitfit Leipzig, Leipzig, Federal Republic of Germany Accepted March 29, 1996

Summary. The latest results concerning the asymmetric synthesis of/J-amino acids are reviewed, focussing on methodology involving 1,4-addition of nitrogen nucleophiles to a,/J-unsaturated esters. Approaches using both homochiral auxiliaries bound to the enoate and homochiral ammonia equivalents are included as well as alkylations and aldol reactions of enolates derived from homochiral/J-amino acids. Keywords: /J-Amino acids - a-Deuterio-/J-arnino acids - a-Alkyl-fi-amino acids - a-Hydroxy-/J-amino acids - Conjugate addition - Nitrogen nucleophiles - Homochiral ammonia equivalents - a,/J-Unsaturated esters During the last few years the stereoselective synthesis of/J-amino acids has gained increasing attention. Although being less abundant than the corresponding a-amino acids, /J-amino acids occur in nature both in free form and bound in peptides. A /J-amino acid is, for example, a constituent of Jasplakinolide: a cyclodepsipeptide of marine origin, consisting of a polypropionate and a peptide moiety containing (R)-/J-tyrosine and bromoabrine (Chu et al., 1991). Astins A, B, or C from Aster tataricus, which is widely used in traditional Chinese medicine because of its content of terpenoids and saponins, contain (R)-/J-phenylalanine (Jiang et al., 1994). (S)-/J-Phenylalanine is also found in the alkatoid dihydroperiphylline (Kaseda et al., 1989) and in the peptide antibiotic andrimid (Fredenhagen et al., 1987). /J-Amino acids are stronger bases and weaker acids compared to a-amino acids (pKa/J-Ala: 3.6, Gly 2.34//J-Ala: 10.36, Gly 9.6) and peptides containing fi-amino acids have a different skeleton atom pattern. The incorporation of/3amino acids into biologically active peptides may enhance the activity. Additionally, for instance [/J-AspI,VaP]-Hypertensin II exhibits considerable resistance towards degradation by aminopeptidases (Riniker et al., 1964).

398

N. Sewald

fi-Amino acids sometimes show interesting pharmacological properties and serve as precursors e.g. for the synthesis of fi-lactams. Stereoselective synthetic pathways to fi-amino acids have been summarized in reviews (Cole, 1994; Juaristi et al., 1994), covering references up to 1993. There is a variety of retrosynthetic approaches for these target molecules. Besides homologation of a-amino acids (A), enolate additions to imines (B), and hydrogenation reactions (C), the addition of nitrogen nucleophiles to a,fi-unsaturated esters (D) is an important strategy. R4 Ra.T~OR'

R4 R 3 @ @ OR5

RlfN.R20 R4 H R~OR

A

R4

C

R ~~ 0 Rs I II R~JN.R2 0

~B RIJN.R20 D ~

s

R1-N'R~H 2 O

R4 R3~-~OR s RIJN~R20

The addition of benzyl amine to the a,/~-unsaturated 7-1actone 1 obtained in a multistep procedure from D-mannitol proceeds highly diastereoselective (Collis et al., 1995). The lactone enolate derived from 2 undergoes alkylation (dr 3:1) and aldol reactions. Etl, DMPU. R 1 0 ~ R 1 0 " ~ _ _ O BnNH2. R I O ~ Bn-NH 2

1

Bn-NH

:~O LiHMDS

3

R2(C=O)R3

R'O'~O-~o 4

Likewise, the addition of hydroxylamine derivatives to the a,fi-unsaturated lactone 5 derived from a carbohydrate gives the isoxazolin-5-one 6 as precursor for the antibiotic negamycin (Socha et al., 1995). . OAc -

-

5

R~ HOR2N-o 6

Conjugate addition of homochiral nitrogen nucleophiles to a,fi-unsaturated esters represents a versatile methodology for the asymmetric synthesis of/~amino acids. However, as shown in early examples (Furukawa et al., 1977), the addition of scalemic 1-phenylethylamine 7 to enoates proceeds with poor stereoselectivity (9: 2-19% ee) and 20-40% overall yield.

Stereoselective synthesis of ~-amino acids via conjugate addition

PhINH2 (E)-RCH=CHC%Et R ~ O E t (R)-7

~,~N~ H O Ph

i)H30 +

399

R~OH

ii) Pd(OH)2/C H2

NH2 0

8

9

Addition of homochiral hydroxylamine derivatives 10 represents a variant with increased stereoselectivity (38-72% de). The intermediate isoxazolin-5ones l l can be cleaved hydrogenolytically (Baldwin et al., 1987). P h I N -OH (E)-RCH=CHCO2M% R N ~ O H "~',,~ ~ "O Ph 11 (R)-10

Several metallated amines readily available in both enantiomeric forms are reported to be useful homochiral ammonia equivalents (12, 13: Davies et al., 1991; 14: Rico et al., 1993; Sewald et al., 1995; 15: Davies et al., 1995; 16: Enders et al., 1994; 17: Hawkins et al., 1994):

P h A N ~Bn P h / I " N ~ P h H H (R)-t2

(R,R)-13

Ph/I""N "SiMe3 Ph/~"N/""[I H (R)-14

, ~ ~''"";NH HN ~ OMe "~iMe~

(R)-I 5

(S)-16

(S)-17

Diastereoselective addition of homochiral N-benzyl-phenylethylamine 12 or bis(phenylethyl)amine 13 to cqfi-unsaturated esters gives fi-amino acids with excellent stereoselectivity (Davies et al., 1991). (R)-12

i) BuLi R __y.-,,.~ OH R. /-.. .OtBu. ii) (E)-RCH=CHCO2~Bu _._.v --]]/ I) H2, Pd/C _ iii) H3O÷ *~r/N~BnO ii) hydrolysis NH2 O Ph /

18

9

Cuprate reagents are known to suppress the competing 1,2-addition, which gives the corresponding a,fi-unsaturated amide, even in the case of a,fi-unsaturated methyl or ethyl esters (Yamamoto et al., 1992; Sewald et al., 1995). i) BuLi R~OEt ii) Cul, (EtO)3P " (R)-14 iii) (E)-RCH=CHCO2Et --'~N'H O iv) H30+ Ph 8

Although the C2-symmetric auxiliary 13 provides excellent results under these conditions, the conjugate addition requires longer reaction times and higher reaction temperatures because of the steric hindrance. Furthermore, the synthesis of 12 or 14 is more straightforward compared to 13. The diastereoselectivity observed for auxiliaries of the 1-phenylethylamine type (e.g. 12, 14) seems to be very high in most cases. Slightly lower asymmetric

400

N. Sewald

induction may originate from insufficient enantiomeric purity (->94% ee) of commercially available 1-phenylethylamine 7 (Rico et al., 1993). The high stereoselectivity observed in the conjugate addition of 12 is, according to theoretical calculations, a consequence of the preferred transition state geometry 19: the enoate reacts from its s-cis conformation, lithium is chelated between the carbonyl oxygen and the nitrogen lonepair and the phenyl rings are oriented nearly in parallel. All other transition state geometries have higher energy (Costello et al., 1994).

% ~LH

'Bu ~ N"'-"~C"---H 0 .". . . . . . . . . ,,H, " Me

O~.'..,.,,Ph H 19

The stereoselectivity of the conjugate addition to a,/J-unsaturated esters as well as the stereochemical outcome of the enolate trapping reaction strongly depends on the conformational preference of the enoate. The s-cis and s-trans conformers of methyl cinnamate are nearly equally populated in the gas phase at 4K, whereas in solution a slight preference of uncomplexed methyl cinnamate for the s-cis conformer exists (Shida et al., 1994). The Michael addition of metal amides to uncomplexed aft-unsaturated esters at low temperature is supposed to proceed through this conformation giving perferentially the intermediate Z-enolate (vide infra), as shown by several examples for lithium amides (Asao et al., 1990; Costello et al., 1994; Rico et al., 1993) and lithium amidocuprates (Shida et al., 1992; Sewald et al., 1995). The homochiral binaphthyl based nitrogen nucleophile 17 can also be used for this type of transformation (Hawkins et al., 1994). While anti a-methyl/3amino butanoate 20 is obtained on ester enolate trapping with methyl iodide, the corresponding C2-epimer 21 is formed on addition of the homochiral nitrogen nucleophile to tiglate (2-methylcrotonate) and subsequent hydrolysis. Me Me ~

OtBu

Me Me~O tBu i) BuLi ~ ii) (E)-MeCH=CHC%tBu iii) Mei

"

20

(S)-17

i) BuLi ii) (E)-MeCH=C(Me)CO2tBu iii) NH4CI (aq) "

21

Likewise, stereoselective addition of the Davies auxiliary 12 to a-alkyl a,fiunsaturated esters and subsequent protonation of the intermediate ester enolate with a sterically hindered proton donor gives syn a-alkyl substituted /J-amino acids 22 with excellent stereocontrol (Davies et al., 1994b). This

Stereoselective synthesis of [3-amino acids via conjugate addition

401

strategy was successfully applied to the asymmetric synthesis of (-)-(1R,2S)2-aminocyclopentane-l-carboxylic acid (cispentacin) and its cyclohexane analogues (Davies et al., 1994c). i) BuLl ii)

Me

RI...~/OR a

O (R)-12 iii) Proton source

Me _ R1. ~ OR2 Me v _--.[~_!_ i) H2, Pd/C R I ~ O H _--II ii) hydrolysis ~/N~Bn O NH2 0 Ph 22 23

A similar approach utilizes TMS-SAMP/TMS-RAMP ((S)-16/(R)-16) as homochiral nitrogen nucleophiles forming 24 with de values >93%. Unsilylated SAMP or RAMP yields 1,2-adducts exclusively (Enders et al.,

1995b). MeO_ R I ~ OR2 i)H+ ,,- R'-...~--~OH i)BuLi (S)-16 iii)(E)-RICH=CHCO2R2" ~L...N O ~aney-Ni, N

'SiMe a

H2' H20

NH2 O

24

9

The addition of achiral lithium N-trimethylsilyl benzylamide 25 to methyl crotonate and subsequent trapping of the intermediate ester enolate with trimethylsilyl chloride predominantly gives the Z-configured O-silyl ketene acetal 26, whereas the E-ketene acetal 27 is formed upon deprotonation of a /3-amino acid ester with lithium diisopropylamide followed by O-silylation (Asao et al., 1990). Trapping of the Z- or E-ester enolate with carbon electrophiles, e.g. alkyl halides or aldehydes, yields the corresponding ct-alkyl or aldol adducts, respectively. Product 28 dominates in the aldol reaction of the Z-enolate 26 while 31 is preferentially formed from the E-enolate 27 via a skewed chair transition state. HO,,,,., R '°°r"

Me "-.S'., I - OMe --

MeaSi i) BuLi Me...S e A-~ ...~OM N-H Bn' ii)"-.///~CO2M e Bn/N~si OM 25

~

",f

Me

HtN.Bn 0 RCHO HO.

R

Me ~.,,,,,,,, ....OMe &

26 M= Li, SiM%

/

H--N-Bn O rac-29 j l. MeOH 2, LDA

Me3Si\ .L}. Bn-N'" ~ "'O MenORaH

HO....1/ R Me.~/[.~ ~OMe

OMe

HOMe R

~ L~

T

- -

H/N..BnO

Bn-N rac-30

Me ....]../.~ OM RCHO

H 'SiMe3

HO,,.. R Me...

27 M= Li, SiM%

OMe

rac-28

/1\

Bn/N.si OMe /1\

MesSi\ .Li. Bn-N'" , "'0

R OMe

H~N-Bn 0

Me,,,,,.,~-

Bn_N 4

~

rac-31

~O F

, H SiMea

," "O

402

N. Sewald

Conjugate addition of homochiral amidoeuprates based on N-trimethylsilyl (R)- or (S)-l-phenylethylamine 14, resp., to a,/3-unsaturated esters and subsequent trapping of the intermediate enolate with DtO proceeds with excellent stereoselectivity (dr > 95:5) providing efficient methodology for the asymmetric synthesis of anti configured a-deuterated/3-amino acids 33 (Sewald et al., 1995). D i) BuLl R~~OR 2 ii) Cul, (EtO)3P (R)-14 iii) (E)-RICH=CHCO2R2 L ~ , ~ N ~ H 0 iv) D20 (lh), then NH4CI Ph 33

Iph~--t'-M-O L

SiM% 32

The intermediate lithium/copper ester enolates 32 react only slowly with deuterium oxide. They do not react with carbon electrophiles under similar reaction conditions. However, transmetallation of the ester enolate with chlorotriisopropoxytitanium from copper to titanium (34) enhances the reactivity towards carbon electrophiles and trapping with carbon electrophiles is then possible in a one-pot reaction; the stereoselectivity varies from good to excellent values favouring the anti isomers (Sewald et al., 1996). _

i) BuLi ii) Cul (R)-14 iii) (E)-RICH=CHCO2R ~ iv) CITi(OiPr)3

I R1L

R21 34

R 3

i) R3X

,-

_._,

_~

35

Analogously, the application of TMS-SAMP (S)-16 or TMS-RAMP (R)-16 in a tandem Michael addition provides access to anti a-alkyl/3-amino acids 36 with excellent stereoselectivity (de between 63 and >96%) in moderate to good yields (26-68%). Phenyl substituents in position 3 (R 1 = Ph), however, are reduced to cyclohexyl residues under the reaction conditions necessary to cleave the N-N bond in 36 (Enders et al., 1994). R2 _=

R2 OtBu iv)H ÷ R~~OtBu i) BuLi ,. i e O _ R I ' ~ ' ~ (S)-16 ii) (E)-R~CH=CHCO2tBu ~../N ~ N O v) Raney-Ni, "SiMe3 H2' H20 NH2 O iii) R2X, HMPA 36

37

Intramolecular alkylation of the intermediate enolate provides access to trans 2-aminocycloalkane-l-carboxylates 39 via this route (Enders et al., 1995a). The concept of tandem conjugate addition also works well on Michael addition of the achiral nitrogen nucleophile 25 followed by carbocyclization via Michael addition of the intermediate ester enolate when a chiral auxiliary

Stereoselective synthesis of ~-amino acids via conjugate addition

403

H H

/"-('C'),~-1 i) BuLi (S)-16 ii) X""(C~

O OR

,,

H H

H H

MeO..~ ~'~',..,,,~.~OR i) SiO2 (H+) //~03n-1 O ~ j N ~N "SiM% ii) Raney-Ni, ~.~..,,,,,,,~]...OH H2' H20 NH2 O iii) 6N HCl 38 iv) Dowex 39

is bound to the substrate. The stereoselectivity of this process is highly dependent on the reagent (lithium amide, lithium amidocuprate, lithium amidozincate) and the presence of a bidentate chelating additive, e.g. ZnCI2 or MgBr2. However, according to the results observed, the enoate geometry in 40 is postulated to be s-cis even when a chelating additive is used, whereas s-trans geometry is involved in several other conjugate additions to complexed enoates. The products 41, 42 are obtained in 40-87% yield with diastereomeric ratios between 61:39 and 95:5 (Shida et al., 1992). i) BuLi Ph ~ " N "H Ph J~" N "H ~ ~..,,,C%R* ii) MXn(Cul°rZnCl2) r@CO2R* . + iii) 41), additive k"~"'.,,,/C02R ~ CO2R*

25

C%R*

41

42

CO2R* 40; R*= (-)-menthyl

a-Alkyl substituted fi-amino acids are also obtained via enolate trapping after Michael addition of metallated N-benzyl (R)- or (S)-phenylethylamine 12. However, direct trapping of the enolate in most cases gives moderate to good stereoselectivity (dr between 1.2:1 and 13:1). Higher anti selectivity is observed (dr between 1:1 and 30:1) in a two-step reaction where the corresponding E-configured ester enolate, formed with L D A starting from the fi-amino ester 18, is alkylated. Hence, the tandem and the stepwise protocol proceed via two different enolate geometries. O-Silylation of the intermediate lithium enolates involved in both methods affords two O-silyl ketene acetals with different double bond geometry (Davies et al., 1994a) as predicted by literature precedence (vide supra). R3 i) BuLi R1 ~OR2 ii) (E)-R1CH=CHC%R2 II iii) R3X ~..T~ N.Bn O

(R)-12

l

i) BuLl ii) (E)-R1CH=CHCO2R2 iii) H 3 0 +

R1c_:."'~'~ OR2 . iv) LDAor

..rN.BnO Ph

Ph

Ph anti-43

R3 R I ~ __,

K.MOS ..rN.B00 v) RaX Ph

18

R3 RigOR2 + _--" ~N.Bn O

anti-43

syn-43

OR2 +

R3 ~ ~ R1 ~ ii OR2

..r%oO Ph syn-43

404

N. Sewald

Surprisingly, the conjugate addition of achiral lithium dibenzylamide followed by tandem enolate alkylation proceeds highly diastereoselective, whereas application of the stepwise protocol is nearly non-selective in this case. This finding proves the influence of the remote N-l-phenylethyl group on chiral recognition during enolate alkylation. Tandem conjugate addition/methylation occurs with excellent syn selectivity on replacement of the lithium amide based on (R)-12 by the corresponding magnesium amide (Bunnage et al., 1994a). Me i) RMgX Ph~OtB ii) (E)-PhCH=CHCO2tBu (R)-12 iii) Mel ~..N.Bn O Ph

u

22

In summary, while the topicity of the Michael addition is identical for 1phenylethylamine derivatives with different metals, the diastereoselectivity of the ester enolate trapping reaction is strongly governed by both the enolate geometry and the counterion (Li, Cu, Mg). Asymmetric alkylations of enolates derived from perhydropyrimidin-4ones (cyclic aminals of fl-amino acid amides) are also described in the literature. For the preparation of the corresponding starting materials, the strategies rely on conjugate addition of nitrogen nucleophiles (Juaristi et al., 1992: (S)-phenylethylamine to methyl crotonate, 20% de; Braschi et al., 1994: hydroxylamine to aft-unsaturated acids), followed by ester to amide transformation with subsequent aminal formation using pivalaldehyde (Juaristi et al., 1992) or formaldehyde (Braschi et al., 1994) and finally diastereomeric separation by recrystallization or flash chromatography. Deprotonation of 45 using LDA and methylation or benzylation gives stereochemically homogeneous products. Hydrolysis is achieved by heating with 6N HC1 in a sealed tube to give anti a-alkyl substituted fl-amino acids (Juaristi et al., 1992). Stereoselective aldol reactions starting from 45 are also reported (Murer et al., 1994). i) (S)-7 ii) H2, Pd/C, EtOH, AcOH Me ~ . ~ ' ~ ''OMe iv) MeNH2 v) tBuCHO, NEt3 44 O vi) BzCl, DMAP vii) chromatography

~ .

R

Me %j--....~ O Me O ' Nr ~ NT" i) LDA NI ~ N I Bz" "Me ii) RX ~ Bz ~ "Me 45

46

The second type of perhydropyrimidin-4-ones described so far, e.g. 49, is deprotonated with LiHMDS prior to enolate alkylation. The stereoselectivity of the a-alkylation slightly depends on the steric bulk of the electrophile. While the trans/cis ratio for methylations of the enolate derived from 49 ranges from 77: 23 to 92: 8, no cis isomer can be detected on reaction with ethyl iodide or benzyl bromide (Braschi et al., 1994). Aldol reactions of dilithiated ester enolates derived from protected acyclic fl-amino acid derivatives 52 mainly proceed with the relative topicity lk under u/-1,2 induction clearly favouring the (/,u) product 53 (Ettmayer et al., 1994).

Stereoselective synthesis of ~-amino acids via conjugate addition

405

R1

O

Bzlo. .v..< i) SOCl2

RI~____/.~OH i) 2 eq. NH2OH, EtOH R1 ii) BzlOCOCI, N a O H ~ O H O O.~NH O 47

BzlO

R~

O

49

48

O R2

O

ii) R2CHO

R~NH"

52

O

+ R~jNH

53 (I,u)

49

Ph

RI,,.., / - . . . O

"r

T~

Bzlo.. .v..ff. O

50

Ph

O

''lr"

Ph

51

HO,,.,, Rz

"U R~.NH

O

BzlO-.~ N v N-..~

Ph HO.

R2 ~

R1

i) LiHMDS ii) R2X

BzIO ...~..Nv N~ , ~ O

i~) (S)-7 iii) (CH20)o, TsOH iv) chromatography

HO,,,., R2

HO.

R2

+ R~/NH

O

+

O

R~NH'

54 (u,u)

(3

55 (I,I)

56 (u,I)

The concept of Michael addition / enolate trapping used for the synthesis of a-alkyl substituted /3-amino acids has been extended to electrophilic hydroxylation reactions of the ester enolate. Based on a publication by Davis reporting on stereoselective oxaziridine mediated a-hydroxylation of N-benzoyl /3-amino ester enolates, an efficient protocol for the synthesis of isoserine derivatives anti-58 (a-hydroxy fi-amino acids) has been elaborated. Although the tandem addition/hydroxylation using the camphorsulfonyloxaziridine 57 proceeds with good diastereoselectivity in the case of cinnamates (R 1 = Ph), only medium yields are obtained in the case of methyl esters because of the competing 1,2 addition which can be excluded on application of tbutyl esters. Earlier literature precedence suggests the stereoselectivity of the hydroxylation to be governed by the 1,2-induction of the homochiral enolate and being less dependent on the absolute configuration of the oxaziridine (Davis et al., 1992). Regardless the fact that different enolate configurations are supposed to dominate in the tandem or in the stepwise protocol, respectively, only a slight divergence in diastereoselectivity

(R)-12

i) BuLl ii) (E)-RICH=CHCO2Rt iii) O2~S~/ (+)-5"1 N

OH RigOR2 = ~/N"Bn Ph

+

O

OH RigOR2 ~',~N-Bn O Ph

anti-58

syn-58

i) BuLi ii) (E)-RICH=CHCO2R2

iii) AcOH R~ ~

t--~l~2 ,..,r~

~T/N.Bn O Ph 18

iv) LDA v) (+)-57

OH R~_ v ~ O R ~,, ~,.T/N.Bn O Ph anti-58

2

+

R~

OH ~

~TN.Bn Ph

O

syn-58

OR2

406

N. Sewald

is observed in most cases with (+)-57 (R 1 = Ph, Me, Et) (Bunnage et al., 1994c). A higher degree of chirality recognition with the oxaziridine reagent 57 is reported in the case of R 1 = Bzl. Again, the stepwise protocol via 60 is reported to compare most favourably to the tandem protocol via 59 in the synthesis of the allophenylnorstatine derivative anti-61 as a consequence of mismatched reagent pairing in the latter case (59/(+)-57, Bunnage et al., 1994e). The matched pair ent-59/(+)-57 reacts highly stereoselective also in the tandem reaction. OH

q

ph o Bu

(R)-12 i) BuLi h ~ OrB (+)-67 ,,ii) (E)-PhCH2CH=CHCO2tBu" / ~ [ / N , Bn OLi / mismatched ~ N , B n O i) BuLi ii) (E)-R1CH=CHCO2R2 I ) AcOH

L

Ph

5

61

Ph

59

anti. syn 40:60

OH ~....N, Bn O

16

matched

L

Ph

60

J

~N,BnO

62

Ph

anti : syn > 98:2

OH i) BuLi

ii)

L

...

(+)-57

.B0

;matched

p h ~ ',,,,,.....[/N, Bn O i

Ph ent-89

OtBu

anti : syn

ent-62 22:1

Several biologically relevant derivatives like N-benzoyl (2S,3R)-3phenylisoserine (Bunnage et al., 1994d) and (2S,3R)-3-amino-2hydroxydecanoic acid, a fi-amino acid occurring in the pseudopentapeptide microginin (Bunnage et al., 1994b) were synthesized using this methodology in combination with Mitsunobu inversion at C-2.

References Asao N, Uyehara T, Yamamoto Y (1990) Stereodivergent synthesis of the enolates of a fi-amino ester by using lithium N-benzyltrimethylsilylamide. Tetrahedron 46: 45634572 Baldwin SW, Aub~ J (1987) Asymmetric synthesis with chiral hydroxylamines. Synthesis of optically pure 4-substituted azetidinones. Tetrahedron Lett 28:179-182 Braschi I, Cardillo G, Tomasini C, Venezia R (1994) Highly diastereoselective alkylation of perhydropyrimidin-4-ones directed toward the synthesis of a-substituted fi-amino acids. J Org Chem 59:7292-7298 Bunnage ME, Davies SG, Goodwin CJ, Walters IAS (1994a) Asymmetric Michael additions of homochiral magnesium amides. Tetrahedron: Asymmetry 5:35-36 Bunnage ME, Burke AJ, Davies SG, Goodwin CJ (1994b) Asymmetric synthesis of (2S,3R)-3-amino-2-hydroxydecanoic acid: the unknown amino acid component of microginin. Tetrahedron: Asymmetry 5:203-206

Stereoselective synthesis of [3-amino acids via conjugate addition

407

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408 N. Sewald: Stereoselective synthesis of fl-amino acids via conjugate addition

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Talstrage 35, D-04103 Leipzig, Federal Republic of Germany. Received February 20, 1996