5954
P. N. Collier et al. / Tetrahedron Letters 42 (2001) 5953–5954
NHBoc
NHBoc
CO2Me
CO2Me
NHBoc
CO2Me
c, d
a
e
meso-DAP 1
6
+
CO2Me
BocHN
b
BocHN
BocN
OH
O
8
9
7
Scheme 2. Reagents and conditions: (a) 250 psi H2, [(COD)Rh((S,S)-Et-DuPHOS)]OTf, toluene, 60°C (7, 79%; 8, 15%); (b) TFA,
MeOH, 0°C, 89%; (c) Jones’ reagent, acetone; (d) TMSCHN2, MeOH–toluene (72% from 8); (e) (i) 5 M HCl, 70°C; (ii) propylene
oxide, EtOH (96% from 9).
with D2O).8 We then carried out the Suzuki coupling
procedure between organoborane 3 and bromoenamide
5, using [1,1% - bis - (diphenylphosphino)ferrocene]di-
chloropalladium(II) [PdCl2(dppf)·CHCl3]7 as catalyst
which proved successful in our earlier studies.5 We were
delighted to obtain the desired dehydroamino acid
derivative 6 from this reaction in 76% yield {[h]D −4.6
(c 2.3, CHCl3)}. We have previously shown that alkene
stereochemistry is conserved in these types of Suzuki
coupling reactions with Z-vinylbromides.5
References
1. (a) Caplan, J. F.; Zheng, R.; Blanchard, J. S.; Vederas, J.
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Tetrahedron Lett. 1999, 40, 5263–5266; (b) For our
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41, 7115–7119; (c) See also: Sabat, M.; Johnson, C. R.
Org. Lett. 2000, 2, 1089–1092.
6. Campbell, A. D.; Raynham, T. M.; Taylor, R. J. K.
Synthesis 1998, 1707–1709.
7. Miyaura, N.; Ishiyama, T.; Sasaki, H.; Ishikawa, M.;
Satoh, M.; Suzuki, A. J. Am. Chem. Soc. 1989, 111,
314–321.
8. Compound 5, which is novel, was prepared using the
method of: Miossec, B.; Danion-Bougot, R.; Danion, D.
Synthesis 1994, 1171–1174.
9. All novel compounds were fully characterised by NMR
spectroscopy and HRMS.
Asymmetric hydrogenation of 6 was achieved using
Burk’s Rh(I)-(S,S)-Et-DuPHOS catalyst10 and gave the
desired S-amino acid product 7 in 79% yield as a single
diastereomer11 (Scheme 2). Alcohol 8 (15%) was also
obtained from this reaction. Oxazolidine 7 was con-
verted into 8 using trifluoroacetic acid at 0°C. Oxida-
tion of alcohol 8 was performed using Jones’ reagent
and the resulting acid was converted into meso-diester 9
using TMS–diazomethane. The symmetrical nature of
compound 9 was established by proton NMR spec-
troscopy thus confirming the expected stereoselectivity
of the (S,S)-ligand in the hydrogenation step.10 Hydro-
lysis of 9 with 5 M HCl at 70°C followed by treatment
with propylene oxide furnished meso-DAP 1, which
displayed spectroscopic properties consistent with those
published [lCꢀO 173.0 ppm, lit.12 lCꢀO 172.2 ppm; mp
>300°C (dec.); lit.12 mp >300°C (dec.)].
In principle the above method can be used for the
preparation of (R,R)- and (S,S)-DAP simply by vary-
ing the choice of starting amino acid (L- or D-serine)
.
used in the formation of alkene 2, and the choice of
rhodium catalyst [(R,R)- or (S,S)-Et-DuPHOS lig-
ands). This methodology also allows for the prepara-
tion of differentially protected DAP analogues for
incorporation into peptides. We are currently using this
procedure to prepare (R,R)- and (S,S)-DAP and novel
DAP analogues of biological interest.
10. (a) Burk, M. J.; Feaster, J. E.; Nugent, W. A.; Harlow,
R. L. J. Am. Chem. Soc. 1993, 115, 10125–10138; (b) See
also: Holcomb, R. C.; Schow, S.; Ayral-Kaloustian, S.;
Powell, D. Tetrahedron Lett. 1994, 35, 7005–7008.
11. Hydrogenation of 6 was also carried out under standard
H2–Pd/C conditions to produce a mixture of 7 and the
corresponding R-amino acid derivative (99%, 52:48). In
the asymmetric reaction, the R-diastereomer could not be
Acknowledgements
1
observed by 270 MHz H NMR spectroscopy.
The University of York and AstraZeneca are gratefully
acknowledged for financial assistance (P.N.C.). We also
thank Dr. Andrew D. Campbell for helpful discussions.
12. Arakawa, Y.; Goto, T.; Kawase, K.; Yoshifuji, S. Chem.
Pharm. Bull. 1998, 46, 674–680.