hydroxy acid 16 (quant), and the resulting 16 was condensed
with (S)-homoserine lactone hydrobromide (17) using EDC
with HOBt14 and Et3N to give the desired (2S,3′R)-1. The
1H and 13C NMR spectra of (2S,3′R)-(-)-1 were, as expected,
identical with those of the natural product, for which the
absolute configuration of both stereogenic centers had been
determined.6 Unfortunately, the optical rotation of the natural
product was not available, thus prohibiting the direct
comparison of the absolute configuration of the natural
product with the synthetic product. Because agreement of
spectroscopic data does not necessarily prove identical
stereochemistry,15 diastereomer (2R,3′R)-1 was synthesized
from 16 and (R)-homoserine lactone hydrochloride (18). The
1H and 13C NMR spectra of the product were clearly different
from those of synthetic (2S,3′R)-(-)-1 and the natural
product, showing that the diastereomers of 1 can be distin-
was employed. The Z-selective semihydrogenation of 19 with
Lindlar catalyst20 (Z:E ) >20:1) and Dess-Martin oxida-
tion21 gave 25 (87%). In this step, BER-Ni2B12 was not an
effective catalyst because a saturated alcohol, caused by
undesired overhydrogenation, was produced in 10-15% and
could not be separated from the product. According to the
modified conditions reported by Smith,22 Nagao aldol
reaction of 25 with (S)-26 gave a diastereomeric mixture (ca.
1
5:1 by H NMR analysis) which could be easily separated
by silica gel column chromatography. The relative stereo-
chemistry of isomers 27a and 27b were determined after their
conversion to 1. Major isomer 27a was treated with (S)-17
and (R)-18 in the presence of Et3N to give (2S,3′S)-(+)-1
and (2R,3′S)-(+)-1, respectively, in good yields, and no
signals from the diastereomer were observed. Minor isomer
27b could also be converted to the two isomers of 1 in the
same manner. Since both enantiomers of the Nagao ligand
are readily available, all the stereoisomers of 1 can be
synthesized using the above methodology.
With the four stereoisomers of 1 in hand, NMR studies
of the synthetic samples with TFAE (2) were conducted for
verification of the configuration of the asymmetric carbons.
1H NMR spectra of the four stereoisomers of 1 in the
presence of both enantiomers of TFAE (2) were recorded.
The correlation of the signal displacements of 1 induced by
(S)- or (R)-TFAE (2) with those of the corresponding protons
in the two reference compounds [(2S,3′R)- and (2S,3′S)-
HBHL (3)]10a induced by (S)-TFAE, yielded four graphs for
each stereoisomer of 1 (Figure 2). In each set of graphs, the
correct absolute configurations were indicated by the best
correlation between the data. In this way for all four
stereoisomers, the expected configurations were confirmed
in the same way as the absolute configuration of the natural
product was confirmed to be 2S,3′R.6
1
guished by their H and 13C NMR spectra.
Although the (3′S)-isomers of 1 could be prepared using
(R)-13 as the starting material, we investigated a more
versatile methodology for the synthesis of 3′-hydroxy HSLs
(Scheme 2). Diethyl malonate was alkylated with the iodide
11, and the product was heated with LiCl in wet DMSO16
Scheme 2. Synthesis of 1 via Nagao Aldol Reaction
Next, the biological activity of the four stereoisomers of
1 was compared by estimating the minimum amount required
to give an inhibition zone on plates with the sensitive
bacterium R. leguminosarum RBL552323 (Figure 3). Quanti-
ties of 1 varying from 10 µg to 5 ng were applied on filter
paper discs and tested on plates. Three experiments were
done with only slightly varying results due to not completely
equal culture conditions. Figure 3, representing one of the
experiments, gives an impression of the inhibition zones
obtained applying concentration series of the four stereo-
(13) Yoon, N. M.; Choi, J. Tetrahedron Lett. 1996, 37, 1057–1060.
(14) Downing, S. V.; Aguilar, E.; Meyers, A. I. J. Org. Chem. 1999,
64, 826–831.
(15) (a) Nicolaou, K. C.; Li, J.; Zenke, G. HelV. Chim. Acta 2000, 83,
1977–2006. (b) Mori, K.; Tashiro, T.; Akasaka, K.; Ohrui, H.; Fattorusso,
E. Tetrahedron Lett. 2002, 43, 3719–3722. (c) Tashiro, T.; Akasaka, K.;
Ohrui, H.; Fattorusso, E.; Mori, K. Eur. J. Org. Chem. 2002, 3659–3665.
(16) Krapcho, A. P. Synthesis 1982, 805–822.
(17) Ley, S. V.; Norman, J.; Griffith, W. P.; Marsden, S. P. Synthesis
1994, 639–666.
to give an ester. LAH reduction of the ester gave alcohol 19
(91%), which was oxidized under Ley conditions17 (50%)
to give aldehyde 20. Keck asymmetric aldol reaction18 of
20 with 21 gave (S)-22 (65%). The enantiomeric purity of
(S)-22 was determined by GC analysis of the corresponding
methyl ester to be 72% ee, and its enantiomeric purity could
not be improved by simple recrystallization. Thus, to prepare
stereochemically pure samples, the Nagao aldol reaction19
(18) Keck, G. E.; Krishnamurthy, D. J. Am. Chem. Soc. 1995, 117, 2363–
2364.
(19) (a) Nagao, Y.; Hagiwara, Y.; Kumagai, T.; Ochiai, M.; Inoue, T.;
Hashimoto, K.; Fujita, E. J. Org. Chem. 1986, 51, 2391–2393. (b) Nagao,
Y.; Dai, W.-M.; Ochiai, M.; Shiro, M. J. Org. Chem. 1989, 54, 5211–
5217.
(20) Yuasa, H.; Satoda, S.; Aoyama, M. Jpn. Kokai Tokkyo Koho
2003206254, 2003.
(21) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155–4156.
Org. Lett., Vol. 10, No. 10, 2008
2049