Peptides containing γ,δ,-dihydroxy-L-leucine

Article abstract of DOI:10.1021/jo900459f

(Chemical Equation Presented) (±)-Dehydroleucine was prepared and resolved by porcine kidney acylase. Under the conditions of the Sharpless asymmetric dihydroxylation (SAD), employing AD-mix-α, Nα- carbobenzyloxy-(2S)-4,5-dehydroleucine methyl ester (16)

Full text of DOI:10.1021/jo900459f

Peptides Containing γ,δ-Dihydroxy-L-leucine
Benson J. Edagwa and Carol M. Taylor*
Department of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803
cmtaylor@lsu.edu
ReceiVed March 4, 2009
(()-Dehydroleucine was prepared and resolved by porcine kidney acylase. Under the conditions of the Sharpless
asymmetric dihydroxylation (SAD), employing AD-mix-R, NR-carbobenzyloxy-(2S)-4,5-dehydroleucine methyl
ester (16) gave rise to a 6.5:1.0 mixture of γ-lactones 17, favoring the 4R configuration. Such carbamate-
protected R-amino-γ-hydroxylactones are not recommended as intermediates for peptide synthesis, since model
studies showed that lactone 13 was unreactive toward amines. Moreover, the lactone ring could not be opened
hydrolytically without epimerization at CR. NR-Carbobenzyloxy-(2S)-4,5-dehydroleucine (22) was condensed
with valine ethyl ester (19) to give dipeptide 23. Treatment of 23 with AD-mix-ꢀ, under the SAD conditions,
converted the dehydroleucine residue to γ,δ-dihydroxyleucine with 4S configuration, as occurs in alloviroidin
(3), a natural product isolated from Amanita suballiacea.
Introduction
γ,δ-Dihydroxyleucine occurs at position 7 of several members
of the cyclic peptide toxins isolated from Amanita mushrooms.1
The most well-studied of these is phalloidin (1, Figure 1), the
prototypical phallotoxin that binds tightly to F-actin. The (2S,4R)-
γ,δ-dihydroxyamino acid also occurs in phallacidin, secophalloidin
and some virotoxins (e.g., viroidin, 2).2 The 2S stereochemistry
was demonstrated by degradation to L-aspartic acid.3 Alloviroidin
(3) contains the (2S,4S) diastereoisomer of 4,5-dihydroxyleucine
and has equal affinity for actin.4
A single synthesis of γ,δ-dihydroxyleucine has appeared in
the literature. In 1957, Wieland and Weiberg reported bromi-
nation of compound 4 and subsequent treatment with acid and
silver salts that led to a mixture of all four stereoisomers of 7
(Scheme 1).5 The two racemates were separated by crystalliza-
tion and then the enantiomers separated by crystallization of
diastereoisomeric salts with ditoluoyl tartaric acid.6
FIGURE 1. Cyclic peptide toxins from Amanita.
analogs.7 Much of the early work by Wieland and co-workers
involved partial syntheses employing the natural product as starting
material.8 Total syntheses have invariably resorted to substitution
of the dihydroxyleucine residue.9
Considerable effort has been directed toward understanding
SARs of the phalloidins and there have been two reports of viroidin
Our aim was to produce a stereoisomerically pure γ,δ-dihy-
droxyleucine, for incorporation into virotoxins. We hoped that a
diastereoselective dihydroxylation of an (S)-dehydroleucine deriva-
(1) For an overview see: Wieland, T. Peptides of poisonous Amanita
mushrooms; Springer-Verlag: New York, 1986.
(2) Faulstich, H.; Buku, A.; Bodenmu¨ller, H.; Wieland, T. Biochemistry 1980,
19, 3334–3343.
(3) (a) Wieland, T.; Scho¨pf, A. Liebigs Ann. Chem. 1959, 626, 174–184. (b)
Pfleiderer, G.; Gruber, W.; Wieland, T. Biochem. Z. 1955, 326, 446–450.
(4) (a) Little, M. C.; Preston, J. F., III. J. Nat. Prod. 1984, 47, 93–99. (b)
Little, M. C.; Preston, J. F., III; Jackson, C.; Bonetti, S.; King, R. W.; Taylor,
L. C. Biochemistry 1986, 25, 2867–2872.
(5) Wieland, T.; Weiberg, O. Liebigs Ann. Chem. 1957, 607, 168–174.
(6) Wieland, T.; Krantz, H. Chem. Ber. 1958, 91, 2619–2629.
(7) (a) Kahl, J. U.; Vlasov, G. P.; Seeliger, A.; Wieland, T. Int. J. Pept.
Protein Res. 1984, 23, 543–550. (b) Zanotti, G.; Kobayashi, N.; Munekata, E.;
Zobeley, S.; Faulstich, H. Biochemistry 1999, 38, 10723–10729.
(8) (a) Munekata, E.; Faulstich, H.; Wieland, T. J. Am. Chem. Soc. 1977,
99, 6151–6153. (b) Munekata, E.; Faulstich, H.; Wieland, T. Liebigs Ann. Chem.
1979, 1020–1027.
4132 J. Org. Chem. 2009, 74, 4132–4136
10.1021/jo900459f CCC: $40.75 2009 American Chemical Society
Published on Web 05/04/2009

Peptides Containing γ,δ-Dihydroxy-L-leucine
SCHEME 1. Wieland’s Synthesis of γ,δ-Dihydroxyleucine
SCHEME 2. Synthesis and Resolution of
(()-Dehydroleucine
tive would enable us to achieve this goal. The 2S configuration of
γ,δ-dihydroxyleucine has been shown to be vital for biological
activity in the phalloidins.10 Since both viroidin (2) and alloviroidin
(3) bind actin with equal affinity,4 either configuration at C4 would
be acceptable.
Results and Discussion
Our preparation of dehydroleucine began with the condensation
of the anion of ethyl acetamidomalonate (8) with methallyl chloride
(9)11 (Scheme 2). We then followed the work of Schmidt and
Schmidt who utilized (S)-dehydroleucine in their synthesis of
eponemycin.12 Hydrolysis of the esters, acidification and decar-
boxylation resulted in racemic N-acetyl-4,5-dehydroleucine. The
enzymatic resolution of (()-10 was best conducted at room
temperature, with strict control of pH in the 7-8 range, for no
more than 24 h. The isolation of (S)-dehydroleucine in high optical
purity followed the recommendations of Chenault et al.13 Specif-
ically, the acidic layer was applied directly to a column of Dowex-
50 (H+). Attempts to concentrate the solution, even by freezedrying,
were accompanied by partial lactonization.14 Treatment of (+)-10
with acid led to deacetylation and concomitant lactonization.
Hydrochloride salt 12 could be converted to the Cbz-derivative
13 that was used in model studies (Vide infra).
SCHEME 3. Functionalization of the (-)-Dehydroleucine
The free L-3,4-dehydroleucine, (-)-11, was readily converted
to its methyl ester 14. Formation of the Mosher amide 15 (Scheme
3) and 19F NMR analysis indicated a 99:1 ratio in favor of the
S-configuration at CR. The carbobenzyloxy group was introduced
to protect NR; we hoped that the aromatic ring would provide an
“anchor” to bind in the so-called southwest binding pocket of the
ligand during the asymmetric dihydroxylation (Vide infra).
We were reluctant to predict which of the AD-mixes15 would
give the desired diastereomer. There have been several cases
reported where the Sharpless mnemonic has failed to predict the
SCHEME 4. Diastereoselective Dihydroxylation
outcome in the dihydroxylation of 1,1-disubstituted olefins.16 We
were also uncertain of the impact the existing CR stereocenter
would have on the stereochemical course of the reaction. It turned
out that under the standard conditions of the Sharpless asymmetric
dihydroxylation (SAD) reaction with AD-mix-R, an inseparable
mixture of diastereomers of 17 was obtained in a 6.5:1.0 ratio
(Scheme 4). Considerable effort was directed toward determining
the stereochemistry at C4. Standard NMR experiments (NOESY
and ROESY) failed to show any crosspeaks that would have
established the relative stereochemistry of the substituents on the
five-membered ring. This suggests that the γ-lactone is very
conformationally mobile. Attempts to separate and characterize the
diastereomers were unsuccessful, although the configuration was
later demonstrated to be 4R as shown. AD-mix-ꢀ gave a 1:1
mixture of diastereomers of 17.
(9) Two recent examples: (a) Anderson, M. O.; Shelat, A. A.; Guy, R. K. J.
Org. Chem. 2005, 70, 4578–4584. (b) Schuresko, L. A.; Lokey, R. S. Angew.
Chem., Int. Ed. 2007, 46, 3547–3549.
(10) Munekata, E.; Faulstich, H.; Wieland, T. Liebigs Ann. Chem. 1979,
1020–1027.
(11) Albertson, N. F.; Archer, S. J. Am. Chem. Soc. 1945, 67, 308–310.
(12) (a) Schmidt, U.; Schmidt, J. J. Chem. Soc., Chem. Commun. 1992, 52,
9–530. (b) Schmidt, U.; Schmidt, J. Synthesis 1994, 300–304.
(13) Chenault, H. K.; Dahmer, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989,
111, 6354–6364.
(14) (a) Fillman, J.; Albertson, N. F. J. Am. Chem. Soc. 1948, 70, 171–174.
(b) Goering, H. L.; Cristol, S. J.; Dittmer, K. J. Am. Chem. Soc. 1948, 70, 3310–
3313.
(15) AD-mixes are reagent mixtures for the SAD reaction, as defined in:
Kolb, H. C.; Van Nieuwenhze, M. S.; Sharpless, K. B. Chem. ReV. 1994, 2483–
2547.
(16) (a) Hale, K. J.; Manaviazar, S.; Peak, S. A. Tetrahedron Lett. 1994, 35,
425–428. (b) Gardiner, J. M.; Bruce, S. E. Tetrahedron Lett. 1998, 39, 1029–
1032.
With diastereomerically enriched lactone 17 in-hand, we began
to consider how to incorporate such a residue into a peptide
J. Org. Chem. Vol. 74, No. 11, 2009 4133

Edagwa and Taylor
SCHEME 5. Amide Formation with Lactones 18 and 13
SCHEME 6. Dihydroxylation of Dipeptide 23
FIGURE 2. ORTEP diagram for (2S,4S)-26.
extensive study by Michl showed that this is a general phenomenon
for γ-lactones bearing an N-acyl or N-carbamate substituent.21
Dihydroxylation of ꢀ,γ-unsaturated ester 16 led to isolation of
γ-lactone (2S,4R)-17 that was unlikely to be a useful synthetic
intermediate for peptide synthesis, based on the precedents in
Scheme 5. During dihydroxylation of γ,δ-unsaturated esters,
concomitant lactonization is prevented by sterically hindered
esters.22 We reasoned that an amide bond would also be resistant
to uncatalyzed lactonization. Thus we prepared dipeptide 23,
bearing the alkene as a masked diol (Scheme 6). Dihydroxylation
of 23 with AD-mix-ꢀ gave a single peptide, 24. The diol was
protected as a silyl acetal according to Corey and Hopkins,23 but
this functionality was unstable to silica gel. The primary alcohol
in 24 was protected as a TBDMS ether to improve solubility. Other
attempts to derivatize diol 24, to produce crystals suitable for X-ray
analysis, were unsuccessful. However, cleavage of the peptide bond
under acidic conditions gave lactone (2S,4S)-17, the diastereomer
of the major product in Scheme 4. Hydrogenolysis, in the presence
of hydrochloric acid, gave the hydrochloride salt 26, a compound
that Wieland had previously described as crystalline. The crystal
structure of 26 revealed that the -NH3+ and -CH2OH substituents
were on the same face of the ring (Figure 2). Thus the configuration
of 26 is (2S,4S), as occurs in alloviroidin (3). Treatment of dipeptide
olefin 23 with AD-mix-R led to a mixture of diastereomers that
was not synthetically useful.
backbone. There are scattered reports of the direct reaction of
lactones with amines under neutral conditions,17 Sn(OAc)2 cataly-
sis,18 catalysis by 2-hydroxypyridine19 and via modification of the
Weinreb method.20 We decided to explore this strategy, to minimize
the number of steps and protecting group manipulations associated
with the dihydroxyleucine residue. We screened a number of
methods using (()-valerolactone (18) and valine ethyl ester (19)
and found that the conditions of Hansen et al. gave the best results
(Scheme 5). Using lactone 13 as a better model system, without
the complication of the additional, unassigned stereocenter at C4,
we were unable to form any dipeptide. Indeed, none of the
examples cited above involve a γ-lactone bearing a carbamate-
protected amine at CR; CR substituents were invariably small (e.g.,
H, Me, OMe, CN). We presume that this group prevents nucleo-
philic attack at the lactone carbonyl for steric reasons. The
δ-disubstitution was also unprecedented and undoubtedly contrib-
utes to the low reactivity of 13 toward nucleophiles. Our recourse
seemed obvious: we would hydrolytically open the lactone ring
and subsequently perform a conventional peptide coupling. Un-
fortunately, CR was epimerized under mildly basic conditions. An
Conclusion
In summary, we have found an effective method for the
preparation of a dipeptide containing (2S,4S)-4,5-dihydroxyleucine.
We found that compound 17, the dihydroxyleucine in its γ-lactone
form, was not a useful building block, since it cannot be opened
directly by an amine and it undergoes CR-epimerization on
hydrolytic opening of the lactone ring. We therefore incorporated
dehydroleucine into a dipeptide and introduced the diol stereose-
lectively via a Sharpless asymmetric dihydroxylation. We were
unable to produce the (2S,4R) diastereomer in an analogous manner.
Studies on the incorporation of dipeptide 25 into larger peptides
will be reported in due course.
(17) Ersmark, K.; Feierberg, I.; Bjelic, S.; Hulte´n, J.; Samuelsson, B.; Åqvist,
J.; Hallberg, A. Bioorg. Med. Chem. 2003, 11, 3723–2733.
(18) Hansen, K. K.; Grosch, B.; Greiveldinger-Poenaru, S.; Bartlett, P. A. J.
Org. Chem. 2003, 68, 8465–8470.
(19) (a) Pyring, D.; Lindberg, J.; Rosenquist, Å.; Zuccarello, G.; Kvarnstro¨m,
I.; Zhang, H.; Vrang, L.; Unge, T.; Classon, B.; Hallberg, A.; Samuelsson, B.
J. Med. Chem. 2001, 44, 3083–3091. (b) Rojo, I.; Martin, J. A.; Broughton, H.;
Timm, D.; Erickson, J.; Yang, H.-C.; McCarthy, J. R. Bioorg. Med. Chem. Lett.
2006, 16, 191–195.
(20) (a) Ciapetti, P.; Taddei, M.; Ulivi, P. Tetrahedron Lett. 1994, 35, 3183–
3186. (b) Marshall, J. A.; Luke, G. P. J. Org. Chem. 1993, 58, 6229–6234.
(21) Michl, K. Liebigs Ann. Chem. 1981, 33–39.
(22) Lipshutz, B. H.; Buzard, D. J.; Olsson, C.; Noson, K. Tetrahedron 2004,
60, 4443–4449.
(23) Corey, E. J.; Hopkins, P. B. Tetrahedron Lett. 1982, 23, 4871–4874.
(24) The racemic free amine has been reported previously: (a) Ferroud, D.;
Genet, J. P.; Kiolle, R. Tetrahedron Lett. 1986, 27, 23–26. (b) Blaauw, R.;
Kingma, I. E.; Laan, J. H.; van der Baan, J. L.; Balt, S.; de Bolster, M. W. G.;
Klumpp, G. W.; Smeets, W. J. J.; Spek, A. L. J. Chem. Soc., Perkin Trans. 1
2000, 119, 9–1210.
4134 J. Org. Chem. Vol. 74, No. 11, 2009

Peptides Containing γ,δ-Dihydroxy-L-leucine
Lactone (2S,4R)-17. AD-mix-R (1.833 g) was added to a mixture
of tBuOH (6.5 mL) and H2O (6.5 mL) at rt. The clear orange solution
was cooled to 0 °C and Cbz-dehydroleucine-OMe, (+)-16, (363 mg,
1.31 mmol) was added. The reaction mixture was stirred at 0 °C for
24 h, quenched with Na2SO3 (1.964 g), stirred for an additional 1 h at
rt, and extracted with CH2Cl2 (6 × 30 mL). The organic layers were
combined, filtered through MgSO4 and concentrated. The residue was
purified by flash chromatography, eluting with 95:5 CH2Cl2/MeOH,
to give 17 as a 6.5:1.0 mixture of diastereomers (215 mg, 59%). Rf
Experimental Section
Dehydroleucine Methyl Ester Hydrochloride (14).24 Thionyl
chloride (112 µL, 184 mg, 1.5 mmol, 2.0 equiv.) was added dropwise
to a solution of dehydroleucine (-)-11 (100 mg, 0.77 mmol, 1.0 equiv.)
in anhydrous methanol (3 mL) at -10 °C under N2. The solution was
warmed to rt and left to stir overnight. The mixture was concentrated
to give 14 as a colorless oil (136 mg, 98%). Rf 0.78 (6:4:1 CHCl3/
24
1
CH3OH/H2O); [R]D -5.1° (c 1.0, MeOH). H NMR (400 MHz,
CD3OD): δ 1.81 (s, 3H), 2.60 (dd, J ) 14.2, 8.4 Hz, 1H), 2.70 (dd, J
) 14.2, 5.6 Hz, 1H), 3.84 (s, 3H), 4.23 (dd, J ) 8.0, 6.2 Hz, 1H), 4.94
(br, 1H) 5.00 (t, J ) 1.3 Hz, 1H); 13C NMR (100 MHz, CD3OD) δ
19.6, 37.9, 50.1, 51.7, 115.1, 137.9, 168.8; HRMS (+TOF) calcd for
C7H14NO2 (M + H)+ 144.1019; obsd: 144.1008.
26
0.55 (9:1 CH2Cl2/CH3OH); [R]D -6.3 (c 0.95, CHCl3). NMR data
reported are for the major diastereomer: 1H NMR (CDCl3) δ 1.41 (s,
3H), 2.08 (t, J ) 11.7 Hz, 1H), 2.35 (br, 1H), 2.77 (t, J ) 11.2 Hz,
1H), 3.54 (d, J ) 12.0 Hz, 1H), 3.70 (d, J ) 12.0 Hz, 1H), 4.69 (dd,
J ) 17.1, 9.6 Hz, 1H), 5.10 (s, 2H), 5.49 (d, J ) 6.3 Hz, 1H),
7.29-7.38 (m, 5H); 13C NMR (100 MHz, CDCl3) δ 23.4, 35.8, 38.0,
52.3, 67.2, 68.6, 84.9, 128.1, 128.2, 128.5, 136.0, 156.1, 174.9; HRMS
(+TOF) calcd for C15H14N5O (M + H)+: 280.1179; obsd: 280.1183.
Amide 20. Diisopropylethylamine (182 µL, 142 mg, 1.10 mmol,
1.1 equiv), γ-valerolactone (95 µL, 100 mg, 1.00 mmol, 1 equiv) and
Sn(OAc)2 (47 mg, 0.20 mmol, 0.2 equiv) were added sequentially to
a solution of L-valine ethyl ester hydrochloride (272 mg, 1.50 mmol,
1.5 equiv) in DMF (3 mL) at 0 °C under N2. The mixture was warmed
to 80 °C and stirred for 44 h, concentrated, and the product isolated
by flash chromatography eluting with 95:5 CH2Cl2/MeOH to give 20
as a 1:1 mixture of diastereomers (159 mg, 65%). Rf 0.37 (95:5 CH2Cl2/
Mosher Amide 15. N-Methyl morpholine (67 µL, 62 mg, 0.61
mmol, 1.1 equiv) was added to a solution of dehydroleucine methyl
ester hydrochloride 14 (100 mg, 0.56 mmol, 1 equiv) in THF (2 mL)
at 0 °C under N2. (S)-(-)-Methoxy(trifluoromethyl)phenyl acetic acid
(MTPA) (143 mg, 0.61 mmol, 1.1 equiv) and N,N’-dicyclohexyl
carbodiimide (DCC) (138 mg, 0.67 mmol, 1.2 equiv) were added. The
mixture was stirred at 0 °C for 3 h and then at rt overnight. The
resulting N,N′-dicyclohexyl urea was removed by filtration and
the filtrate concentrated. The residue was dissolved in ethyl acetate
(15 mL) and washed successively with 10% citric acid (10 mL), 5%
NaHCO3 (10 mL) and brine (10 mL). The organic layer was filtered
through MgSO4 and concentrated. The residue was purified by flash
chromatography, eluting with 5:1 Hex/EtOAc to give 15 as an oil (123
1
MeOH); H NMR (400 MHz, CDCl3): δ 0.91 (dd, J ) 6.9, 0.6 Hz,
3H), 0.94 (d, J ) 6.9 Hz, 3H), 1.98 (d, J ) 1.2 Hz, 1.5H), 1.21 (d, J
) 1.2, Hz, 1.5H), 1.29 (t, J ) 7.1 Hz, 3H), 1.66-1.75 (m, 1H),
1.80-1.89 (m, 1H), 2.12-2.20 (m, 1H), 2.37-2.48 (m, 1H), 2.42 (t,
J ) 6.8 Hz, 1H), 2.43 (t, J ) 7.3 Hz, 1H), 3.27 (br, 1H), 3.81-3.87
(m, 1H), 4.14-4.26 (m, 2H), 4.53 (dd, J ) 8.7, 5.0 Hz, 1H), 6.48 (br,
1H); 13C NMR (100 MHz, CDCl3) δ 14.1, 17.7, 18.8, 23.5, 31.1, 33.0,
34.2, 57.0, 61.2, 67.1 and 67.2, 172.1 and 172.2, 173.6; HRMS
(+TOF) calcd for C12H24NO4 (M + H)+: 246.1699; obsd: 246.1701.
Cbz-L-dehydroleucine-OH (22). Aqueous NaOH (2M, 10 mL)
was added dropwise to a suspension of dehydroleucine (380 mg, 2.94
mmol, 1 equiv) in THF (5 mL) at 0 °C. Cbz-Cl (497 µL, 602 mg,
3.53 mmol, 1.2 equiv) was added dropwise over 30 min, with vigorous
stirring. The cloudy reaction mixture was left to stir overnight at rt
and concentrated to remove THF. The residue was diluted with H2O
(20 mL), extracted with ether (2 × 10 mL), acidified with 6 M HCl
to pH 1 and extracted with EtOAc (3 × 25 mL). The organic layers
were combined, washed with brine (25 mL) and concentrated to give
22 as a colorless oil (630 mg, 81%). Rf 0.35 (9:1 CH2Cl2/MeOH);
28
mg, 62%). Rf 0.53 (2:1 Hexanes/EtOAc); [R]D +16.5° (c 0.85,
CHCl3). 1H NMR (400 MHz, CDCl3): δ 1.67 (s, 3H), 2.39 (dq, J )
8.8, 5.2 Hz, 1H), 2.56 (dd, J ) 14.0, 5.1 Hz, 1H), 3.51 (dd, J ) 3.3,
1.6 Hz, 3H), 3.80 (s, 3H), 4.60 (app d, J ) 0.8 Hz, 1H), 4.70 (app. t,
J ) 1.5 Hz, 1H), 4.80 (dt, J ) 8.6, 4.4 Hz, 1H), 7.35-7.58 (m, 5H);
13C NMR (100 MHz, CDCl3) δ 21.6, 40.4, 50.1, 55.2, 83.7, 84.0, 114.8,
122.1, 125.0, 127.5, 128.4, 129.4, 132.7, 140.0, 166.2, 172.0; 19F NMR
(236 MHz, CDCl3) δ -69.35; HRMS (+TOF) calcd for C17H21NO4F3
(M + H)+: 360.1417; obsd: 360.1419.
Mosher Amide of (()-11. Compound (()-10 (100 mg, 0.58
mmol) was suspended in aqueous NaOH (2.5 N, 3 mL) and heated at
reflux for 4 h. The solution was neutralized to pH 7 (monitored with
UIP) by the addition of 6 M HCl. The solution was then loaded onto
a column (25 mm diameter, 30 mm high) of Dowex-50 (H+), rinsed
with water (∼150 mL), eluted with 1 N aqueous NH4OH. Fractions
were monitored by TLC, staining with nihydrin. Relevant fractions
were freezedried to give (()-11 as a colorless, amorphous powder in
quantitative yield. A portion of this material (50 mg) was derivatized
with MTPA, as described above, to give a 1:1 mixture of diastereomers
(75 mg, 54%). 19F NMR (236 MHz, CDCl3) δ -69.35, -69.49.
Cbz-dehydroleucine-OMe (16).25 Thionyl chloride (229 µL, 376
mg, 3.2 mmol, 2 equiv) was gradually added to a suspension of (-)-
11 (204 mg, 1.6 mmol, 1 equiv) in MeOH (4 mL) at -10 °C under
N2. The solution was gradually warmed to rt, stirred for 2 d, and
concentrated. The residue was dissolved in a biphasic mixture of
CH2Cl2 (3 mL) and H2O (1.5 mL) and cooled to 0 °C at which point
NaHCO3 (728 mg, 8.7 mmol, 6.6 equiv) and N-(benzyloxycarbonyl-
oxy)succinimide (393 mg, 1.6 mmol, 1.2 equiv) were added sequen-
tially. The reaction mixture was gradually warmed to rt overnight and
diluted with CH2Cl2 and H2O (20 mL each). The aqueous layer was
back-extracted with EtOAc (2 × 15 mL). The organic extracts were
combined, filtered through MgSO4 and concentrated. The residue was
purified by flash chromatography eluting with 2:1 Hex/EtOAc to give
16 as a colorless oil (266 mg, 73%). Rf 0.50 (3:1 Hexanes/EtOAc);
[R]D27 +7.3 (c 1.2, CHCl3). 1H NMR (400 MHz, CDCl3): δ 1.73 (s,
3H), 2.38 (dd, J ) 14.0, 8.4 Hz, 1H), 2.54 (dd, J ) 14.0, 5.4 Hz, 1H),
3.73 (s, 3H), 4.49 (td, J ) 8.1, 5.6 Hz, 1H), 4.75 (br, 1H), 4.84 (app.
t, J ) 1.5 Hz, 1H), 5.10 (s, 2H), 5.27 (d, J ) 7.8 Hz, 1H), 7.27-7.38
(m, 5 H); 13C NMR (100 MHz, CDCl3) δ 21.7, 40.6, 52.1, 52.2, 66.9,
114.6, 127.9, 128.0, 128.4, 136.2, 140.2, 155.7, 172.6; HRMS (+TOF)
calcd for C15H20NO4 (M + H)+: 278.1386; obsd: 278.1387.
[R]D +4.4° (c 1.2, CH3OH), Lit.26 ent-22 [R]D
-5.8 (c 1.18,
27
24.3
CH3OH); 1H NMR (400 MHz, CD3OD): δ 1.75 (s, 3H), 2.38 (dd, J
) 14.1, 10.0 Hz, 1H), 2.56 (dd, J ) 14.1, 4.7 Hz, 1H), 4.36 (dd, J )
10.0, 4.8 Hz, 1H), 4.78 (br, 1H), 4.81 (br, 1H), 4.95 (br, 1H), 5.07 (s,
2H), 7.26-7.35 (m, 5 H); 13C NMR (100 MHz, CD3OD) δ 20.0, 38.9,
51.7, 65.5, 112.3, 126.7, 126.9, 127.4, 136.2, 140.3, 156.5, 173.7;
HRMS (+TOF) calcd for C14H18NO4 (M + H)+: 264.1230; obsd:
264.1224.
Cbz-L-Dehydroleucine-Val-OEt (23). Diisopropylethylamine (1.2
mL, 910 mg, 6.9 mmol, 3.0 equiv) was added to a solution of Cbz-
protected dehydroleucine 22 (618 mg, 2.3 mmol, 1.0 equiv), valine
ethyl ester hydrochloride (426 mg, 2.3 mmol, 1.0 equiv) and BOP
reagent (1.1 g, 2.6 mmol, 1.1 equiv) in acetonitrile (15 mL). The
mixture was stirred at 0 °C for 1 h and then at rt overnight. The mixture
was concentrated and the product isolated by flash chromatography
eluting with 2:1 Hex/EtOAc, to give 23 as a colorless solid (851 mg,
30
93%). Rf 0.53 (2:1 Hexanes/EtOAc); [R]D -2.4° (c 1.05, CHCl3);
1H NMR (400 MHz, CDCl3): δ 0.88 (d, J ) 6.9 Hz, 3H), 0.91 (d, J
) 6.9 Hz, 3H), 1.27 (t, J ) 7.1 Hz, 3H), 1.75 (s, 3H), 2.16 (app. pd,
J ) 6.9, 4.8 Hz, 1H), 2.39 (dd, J ) 14.2, 8.8 Hz, 1H), 2.56 (dd, J )
14.2, 5.8 Hz, 1H), 4.14-4.24 (m, 2H), 4.31 (br, 1H), 4.50 (dd, J )
(25) Racemic material has been prepared via another route, but no data
reported: Kazmaier, U.; Maier, S. Tetrahedron 1996, 52, 941–954.
(26) Qu, H.; Gu, X.; Liu, Z.; Min, B. J.; Hruby, V. J. Org. Lett. 2007, 9,
3997–4000.
J. Org. Chem. Vol. 74, No. 11, 2009 4135

Edagwa and Taylor
8.7, 4.8 Hz, 1H), 4.80 (br, 1H), 4.87 (t, J ) 1.4 Hz, 1H), 5.11 (s, 2H),
5.26 (br, 1H), 6.63 (d, J ) 7.8 Hz, 1H), 7.29-7.37 (m, 5 H); 13C
NMR (100 MHz, CDCl3) δ 14.1, 17.7, 18.8, 21.9, 31.3, 40.4, 53.2,
57.2, 61.2, 67.1, 114.5, 127.9, 128.1, 128.5, 136.2, 140.8, 156.1, 171.3,
171.5; HRMS (+TOF) calcd for C21H31N2O5 (M + H)+: 391.2227;
obsd: 391.2240.
with 95:5 CH2Cl2/MeOH to give (2S,4S)-17 (23 mg, 82%). Rf 0.53
25
1
(9:1 CH2Cl2/CH3OH); [R]D -5.7° (c 1.0, CHCl3); H NMR (400
MHz, CDCl3): δ 1.35 (s, 3H), 2.34 (d, J ) 9.5 Hz, 1H), 2.54 (br, 1H),
3.44 (d, J ) 8.0 Hz, 1H), 3.73 (d, J ) 12.2 Hz, 1H), 4.70 (dd, J )
17.1, 8.8 Hz, 1H), 5.11 (dd, J ) 18.5, 12.2 Hz, 2H), 5.79 (d, J ) 7.3
Hz, 1H), 7.30-7.36 (m, 5H); 13C NMR (100 MHz, CDCl3) δ 22.5,
29.7, 35.8, 51.1, 67.3, 67.4, 84.6, 128.1, 128.2, 128.5, 136.0, 156.1,
174.9; HRMS (+TOF) calcd for C15H14N5O (M + H)+: 280.1179;
obsd: 280.1183.
t
Dipeptide 24. AD-mix-ꢀ (563 mg) was dissolved in BuOH (2
mL) and H2O (2 mL) at rt. The clear orange solution was cooled to 0
°C and dipeptide olefin 23 (157 mg, 0.4 mmol) was added. The mixture
was stirred for 48 h at 0 °C, quenched with Na2SO3 (604 mg), stirred
for 1 h at rt, diluted with H2O (10 mL) and extracted with EtOAc (6
× 15 mL). The organic layers were combined, dried over MgSO4 and
concentrated. The crude product was purified by flash chromatography
eluting with 20:1 EtOAc/MeOH, to give 24 as a colorless solid (154
Hydrochloride Salt of Lactone (2S,4S)-26. Concentrated HCl
(48 µL) and 10% Pd/C (22 mg, 0.206 mmol) were added to a solution
of (2S,4S)-17 (70 mg, 0.25 mmol) in EtOH (2.4 mL) at rt under N2.
The mixture was hydrogenated for 4 h, then filtered through Celite,
and concentrated to give the hydrochloride salt as a colorless solid.
Recrystallization from ethanol/ether yielded colorless crystals (46 mg,
29
mg, 90%). Rf 0.32 (9:1 CH2Cl2/MeOH); [R]D -25.8° (c 0.95,
n
CH3OH); 1H NMR (400 MHz, CD3OD): δ 0.93 (d, J ) 2.3 Hz, 3H),
0.95 (d, J ) 2.3 Hz, 3H), 1.19 (s, 3H), 1.26 (t, J ) 7.1 Hz, 3H), 1.77
(dd, J ) 14.8, 8.7 Hz, 1H), 2.03 (dd, J ) 14.6, 3.3 Hz, 1H), 2.15
(app. qd, J ) 13.1, 6.5 Hz, 1H), 3.38 (dd, J ) 15.9, 11.1 Hz, 2H),
4.10-4.21 (m, 2H), 4.30 (d, J ) 5.5 Hz, 1H), 4.39 (dd, J ) 8.4, 3.8
Hz, 1H), 5.09 (s, 2H), 7.25-7.36 (m, 5 H); 13C NMR (100 MHz,
CD3OD) δ 12.5, 16.3, 17.4, 22.3, 29.8, 39.0, 50.8, 57.2, 60.2, 65.7,
68.6, 71.1, 126.8, 127.0, 127.5, 136.2, 156.2, 170.9, 173.5; HRMS
(+TOF) calcd for C21H33N2O7 (M + H)+: 425.2282; obsd: 425.2280.
Dipeptide 25. Triethylamine (273 µL, 199 mg, 1.96 mmol, 2.4
equiv), DMAP (20 mg, 0.16 mmol, 0.2 equiv) and TBDMSOTf (206
µL, 238 mg, 0.90 mmol, 1.1 equiv) were added to a solution of 24
(347 mg, 0.82 mmol, 1 equiv) in CH2Cl2 (4 mL) at 0 °C under N2.
The mixture was warmed to rt overnight, concentrated, and the product
isolated by flash chromatography eluting with 2:1 Hex/EtOAc to give
100%); Rf 0.46 (3:3:3:1 BuOH/EtOH/NH3/H2O); mp 210-213 °C,
Lit.6 205-207 °C. [R]D23 -7.4° (c 0.75, 6 N HCl), Lit.6 [R]D20 -12°
(c 2, 6 N HCl). 1H NMR (400 MHz, D2O): δ 1.45 (s, 3H), 2.43 (dd,
J ) 13.0, 10.4 Hz, 1H), 2.60 (dd, J ) 13.2, 9.6 Hz, 1H), 3.61 (d, J )
12.7 Hz, 1H), 3.77 (d, J ) 12.7 Hz, 1H), 4.64 (t, J ) 9.9 Hz, 1H); 13
C
NMR (100 MHz, D2O) δ 20.9, 32.8, 49.4, 65.9, 87.5, 173.2; HRMS
(+TOF) calcd for C6H12NO3 (M - HCl)+: 146.0811; obsd: 146.0813.
Hydrochloride Salt of Lactone (2S,4R)-26. (2S,4R)-17 (32 mg)
was treated, as for the diastereomer, to cleave the Cbz group to give
the hydrochloride salt of aminolactone (2S,4R)-26 (18 mg, 86%) as a
6.5:1.0 mixture of diastereomers. mp 197-200 °C, Lit.6 199-200 °C.
[R]D -22° (c 0.45, 6 M HCl), Lit.6 [R]D -35.5° (c 2, 6N HCl).
NMR spectra are reported for the MAJOR (2S,4R) diastereomer. 1H
NMR (400 MHz, D2O) δ 1.46 (s, 3H), 2.24 (dd, J ) 13.4, 11.0 Hz,
1H), 2.88 (dd, J ) 13.4, 9.8 Hz, 1H), 3.65 (d, J ) 12.6 Hz, 1H), 3.71
(d, J ) 12.6 Hz, 1H), 4.64 (t, J ) 10.3 Hz, 1H); 13C NMR (100 MHz,
D2O) δ 22.1, 35.3, 50.0, 66.9, 87.9, 173.7.
23
20
26
25 (368 mg, 84%). Rf 0.42 (2:1 Hex/EtOAc); [R]D +3.3 (c 1.15,
1
CHCl3). H NMR (400 MHz, CDCl3): δ 0.07 (d, J ) 4.4 Hz, 6H),
0.88 (d, J ) 6.8 Hz, 3H), 0.89 (s, 9H), 0.93 (d, J ) 6.8 Hz, 3H), 1.26
(t, J ) 8.8 Hz, 3H), 1.29 (s, 3H), 1.76 (dd, J ) 14.8, 4.2 Hz, 1H),
1.84 (br, 1H), 2.13 (d, J ) 6.6 Hz, 1H), 2.14-2.23 (m, 1H), 3.29 (br,
1H), 3.43 (app.t, J ) 10.6 Hz, 2H), 4.08-4.24 (m, 3H), 4.42 (br, 1H),
4.48 (q, J ) 8.8 Hz, 1H), 5.12 (dd, J ) 10.9, 6.1 Hz, 2H), 6.12 (d, J
) 10.9, 5.4 Hz, 1H), 7.28-7.36 (m, 5 H); 13C NMR (100 MHz, CDCl3)
δ -5.5, 14.2, 17.5, 18.3, 19.0, 23.6, 25.8, 30.9, 41.1, 51.1, 57.3, 61.1,
66.8, 70.6, 72.2, 128.0, 128.1, 128.4, 136.3, 156.0, 171.8,172.4; HRMS
(+TOF) calcd for C27H46N2O7Si (M + H)+: 538.3074; obsd: 538.3065.
Lactone (2S,4S)-17. A mixture of trifluoroacetic acid (100 µL)
and CH2Cl2 (2 mL) was added to compound 24 (43 mg, 0.10 mmol)
at 0 °C under N2. The mixture was gradually warmed to rt overnight,
concentrated, and the product isolated by flash chromatography eluting
Acknowledgment. We thank the Department of Chemistry at
LSU for financial support. We also thank Dr. Frank Fronczek,
Department of Chemistry, LSU, for the determination of the crystal
structure of (2S,4S)-26.
Supporting Information Available: Procedures for the
preparation of compounds 4, (()-10, (+)-10 and (-)-11, and 13.
1H and 13C NMR spectra for pertinent compounds and crystal-
lographic data for compound (2S,4S)-26. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO900459F
4136 J. Org. Chem. Vol. 74, No. 11, 2009