Stereoselective synthesis of the disaccharide unit of incednine

Article abstract of DOI:10.1021/ol303093z

A stereoselective synthesis of a fully protected version of the disaccharide unit (2) of incednine (1) is described. The synthesis of 2 proceeds in 4.7% overall yield from commercially available allyl α-D- galactopyranoside over the longest linear sequence.

Full text of DOI:10.1021/ol303093z

ORGANIC
LETTERS
2013
Vol. 15, No. 1
62–64
Stereoselective Synthesis of the
Disaccharide Unit of Incednine
Jason R. Abbott and William R. Roush*
Department of Chemistry, The Scripps Research Institute;Florida,
130 Scripps Way Jupiter, Florida 33458, United States
roush@scripps.edu
Received November 9, 2012
ABSTRACT
A stereoselective synthesis of a fully protected version of the disaccharide unit (2) of incednine (1) is described. The synthesis of 2 proceeds in
4.7% overall yield from commercially available allyl R-D-galactopyranoside over the longest linear sequence.
The structure determination of incednine (1), isolated
from Streptomyces sp. ML694-90F3 using a cell-based
chemical-genetic screen targeting inhibitors of the onco-
protein Bcl-xL, was reported in 2008.1 It was reported in
this initial publication that incednine induced apoptosis in
Bcl-xL-overexpressing cells, thereby sensitizing these other-
wise resistant cells to chemotherapeutic treatment. Unlike
most existing Bcl-xL inhibitors, which recognize surface
binding pockets on this protein, incednine neither dis-
rupted the binding of Bcl-xL to the pro-apoptotic Bcl-2
family of proteins nor decreased expression levels of this
oncoprotein.1,2 These data suggest that the mechanism of
action of incednine is distinct from that of existing Bcl-xL
inhibitors. Further studies are necessary to determine the
biological target(s) of this compound.3
The intriguing biological properties of incednine define
it to be an important target for chemical synthesis and
further biological studies. While a total synthesis of in-
cednine has not yet been reported, syntheses of the aglycon
and of the disaccharide unit have been described.4 We
describe here our synthesis of the incednine disaccharide 2
in fully protected form.
Our strategy for the synthesis of disaccharide 2 is sum-
marized in Figure 1. We envisaged that the diamine pre-
cursor 3 could be obtained from a tandem azide reduction
and alcohol deoxygenation sequence carried out on thio-
carbonate 4. Intermediate4 originatesfrom disaccharide5,
which is the product of a β-selective glycosylation reaction
between the glycosyl donor 6 and acceptor 7.
(1) Futamura, Y.; Sawa, R.; Umezawa, Y.; Igarashi, M.; Nakamura,
H.; Hasegawa, K.; Yamasaki, M.; Tashiro, E.; Takahashi, Y.; Akamatsu,
Y.; Imoto, M. J. Am. Chem. Soc. 2008, 130, 1822.
(2) (a) Reed, J. C. Curr. Opin. Oncol. 1999, 11, 68. (b) Adams, J. M.;
Cory, S. Oncogene 2007, 26, 1324. (c) Wang, J.-L.; Liu, D.; Zhang, Z.-J.;
Shan, S.; Han, X.; Srinivasula, S. M.; Croce, C. M.; Alnemri, E. S.;
Huang, Z. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 7124. (d) Oltersdorf,
T.; Elmore, S. W.; Shoemaker, A. R.; Armstrong, R. C.; Augeri, D. J.;
Belli, B. A.; Bruncko, M.; Deckwerth, T. L.; Dinges, J.; Hajduk, P. J.;
Joseph, M. K.; Kitada, S.; Korsmeyer, S. J.; Kunzer, A. R.; Letai, A.; Li,
C.; Mitten, M. J.; Nettesheim, D. G.; Ng, S.; Nimmer, P. M.; O’Connor,
J. M.; Oleksijew, A.; Petros, A. M.; Reed, J. C.; Shen, W.; Tahir, S. K.;
Thompson, C. B.; Tomaselli, K. J.; Wang, B.; Wendt, M. D.; Zhang, H.;
Fesik, S. W.; Rosenberg, S. H. Nature 2005, 435, 677. (e) Shoemaker,
A. R.; Oleksijew, A.; Bauch, J.; Belli, B. A.; Borre, T.; Bruncko, M.;
Deckwirth, T.; Frost, D. J.; Jarvis, K.; Joseph, M. K.; Marsh, K.;
McClellan, W.; Nellans, H.; Ng, S.; Nimmer, P.; O’Connor, J. M.;
Oltersdorf, T.; Qing, W.; Shen, W.; Stavropoulos, J.; Tahir, S. K.; Wang,
B.; Warner, R.; Zhang, H.; Fesik, S. W.; Rosenberg, S. H.; Elmore, S. W.
Cancer Res. 2006, 66, 8731.
The synthesis of thioglycoside donor 6 originated from
the known azido sugar 9, which was prepared in nine steps
(23% overall yield) from commercially available allyl
R-D-galactopyranoside (8) according to a literature proce-
dure (Scheme 1).5 Removal of the 1,2-propylidene acetal
was accomplished by treatment of 9 with aqueous trifluoro-
acetic acid. Subsequent treatment of the crude diol with
acetic anhydride in pyridine provided diacetate 10 in
90% yield for the two steps. Conversion of 10 to the
(4) (a) Ohtani, T.; Tsukamoto, S.; Kanda, H.; Misawa, K.; Urakawa,
Y.; Fujimaki, T.; Imoto, M.; Takahashi, Y.; Takahashi, D.; Toshima, K.
Org. Lett. 2010, 12, 5068. (b) Ohtani, T.; Sakai, S.; Takada, A.;
Takahashi, D.; Toshima, K. Org. Lett. 2011, 13, 6126.
(3) Kobayashi, H.; Harada, H.; Nakamura, M.; Futamura, Y.; Ito,
A.; Yoshida, M.; Iemura, S.-i.; Shin-ya, K.; Doi, T.; Takahashi, T.;
Natsume, T.; Imoto, M.; Sakakibara, Y. BMC Chem. Biol. 2012, 12, 2.
€
(5) Tietze, L. F.; Bohnke, N.; Brasche, G. ARKIVOC 2007, 12.
r
10.1021/ol303093z
Published on Web 12/18/2012
2012 American Chemical Society

Scheme 2. Synthesis of Glycosyl Acceptor 7
Figure 1. Retrosynthetic analysis of disaccharide 2.
thioglycoside 6 proceeded smoothly upon treatment with
phenylthiotrimethylsilane (PhSTMS) inthe presenceof tri-
methylsilyl trifluoromethanesulfonate (TMSOTf), which
provided 6 in 91% yield as an inconsequential 70:30 R:β
mixture of anomers.
77% yield over the two steps. Installation of the azide
function at C(2) by conversion of 15 to the corresponding
triflate and displacement using NaN3 in DMSO proceeded
smoothly to give the C(2)-equatorial azide 16 in 94% yield.
With the required functionalities in place, only removal
of the allyl group remained for completion of the synthesis
ofthetargetedglycosyl acceptor 7. Attemptstoachievethis
transformation by treatment of 16 with PdCl2, NaOAc, and
AcOH7 were low yielding, and treatment of 16 with sodium
borohydride and iodine led only to decomposition.8 Grat-
ifyingly, a two-step deallylation protocol involving, first,
isomerization of the allyl group to the propenyl ether us-
ing catalytic amounts of the iridium catalyst Ir(COD)-
[PMePh2]PF6, followed by hydrolysis of the propenyl ether
with mercury(II) chloride and mercury(II) oxide in aqueous
acetone, provided the glycosyl acceptor 7 in 97% yield.9,10
The coupling of the two protected monosaccharide units
is summarized in Scheme 3. Treatment of thioglycoside 6
with N-iodosuccinimide (1.2 equiv) in the presence of
trimethylsilyl trifluoromethanesulfonate (0.15 equiv) and
acceptor 7 (1.1 equiv) at ꢀ78 °C provided disaccharide 5 in
82% yield and with excellent β:R selectivity (97:3).
Scheme 1. Synthesis of Thioglycoside Donor 6
We focused next on the synthesis of glycosyl acceptor
7 (Scheme 2). Persilylation of commercially available
D-lyxose (11) in the presence of triethylamine produced 12
in essentially quantitative yield. Treatment of 12 first with
iodotrimethylsilane, followed by benzyl alcohol and 2,6-
di-tert-butylpyridine (2,6-DTBP), and finally with MeOH
provided the benzyl glycoside 13 in 55% yield as a single
anomer after chromatographic purification.6 Selective
protection of the cis 2,3-diol unit of 13 was best achieved
by using trichloromethyl chloroformate in pyridine. Sub-
sequent O-allylation of the remaining C(4)-hydroxyl group
with allyl bromide in the presence of silver(I) oxide furnished
carbonate 14 in 71% yield for the two operations. Removal
of the 2,3-O-carbonate by treatment of 14 with NaOMe in
MeOH followed by selective silylation of the equatorial
C(3)-hydroxyl group with triisopropylsilyl trifluormethane-
sulfonate (TIPS-OTf) at ꢀ78 °C provided alcohol 15 in
Two key transformations then remained to complete the
synthesis of the fully protected disaccharide 2: deoxygena-
tion of C(20) of the forosamine unit [deriving from 6] and
reduction of the azide functionalities present in both mono-
saccharide units. Previous experience with this sequence of
transformations suggested that these two operations could
(7) (a) Ogawa, T.; Horisaki, T. Carbohydr. Res. 1983, 123, C1. (b)
Smith, A. B., III; Rivero, R. A.; Hale, K. J.; Vaccaro, H. A. J. Am. Chem.
Soc. 1991, 113, 2092. (c) Lohman, G. J. S.; Seeberger, P. H. J. Org. Chem.
2004, 69, 4081.
(8) Thomas, R. M.; Mohan, G. H.; Iyengar, D. S. Tetrahedron Lett.
1997, 38, 4721.
(9) Gigg, R.; Warren, C. D. J. Chem. Soc. C 1968, 1903.
(10) Oltvoort, J. J.; Van Boeckel, C. A. A.; De Koning, J. H.; Van
Boom, J. H. Synthesis 1981, 305.
(6) This procedure is known to produce R-glycosides with high
selectivity: Uchiyama, T.; Hindsgaul, O. Synlett 1996, 499.
Org. Lett., Vol. 15, No. 1, 2013
63

Scheme 3. Synthesis of Disaccharide 2
1
be performed concomitantly, and indeed, this proved to be
the case.11
the H and 13C NMR spectra. Poor resolution was ob-
served in H and 13C NMR experiments performed at
1
Saponification of the C(20)ꢀOAc unit of 5 by treatment
with sodium methoxide in methanol proceeded smoothly
to give alcohol 17 (Scheme 3). The very minor amounts of
undesired R-disaccharide formed in the glycosylation reac-
tion were removed at this stage by silica gel chromatography.
Alcohol 17 was then acylated with phenyl chlorothionofor-
mate to furnish 4 (99% yield), the substrate for the tandem
deoxygenationꢀazide reduction step. Treatment of azido-
thiocarbonate 4 with excess tributyltin hydride and catalytic
amounts of AIBN in toluene at 85 °C effected reduction of
the thiocarbonate and azide functionalities.12 After filtration
of the reaction mixture through a short pad of silica gel,
diamine 3was obtained contaminated with small amounts of
tin-containing byproducts. This mixture was used directly in
the final transformations without additional purification.
Thus, treatment of the diamine intermediate with TeocOBt
and Et3N and subsequent bis-N-methylation of the two
carbamates by treatment with NaH and MeI in DMF
provided the targeted disaccharide 2 in 34% yield for the
final three operations. At this point it should be noted
that, although HRMS and IR confirmed the structure of
2, the presence of carbamate bond rotamers complicated
room temperature in a variety of solvents (chloroform-d,
benzene-d6, DMSO-d6), but acceptable NMR data were
obtained when samples of 2 were heated in DMSO-d6 at
80 °C. As further structural proof, a small sample of 2 was
treated with TBAF and the deprotected disaccharide
benzyl glycoside was characterized completely (see Sup-
porting Information).
In summary, our synthesis of the fully protected di-
saccharide unit (2) of incednine proceeded in 4.7%
overall yield from commercially available allyl R-D-galac-
topyranoside (18 steps, longest linear sequence). Our
efforts toward the synthesis of the incednine aglycon,
and the synthesis of incednine itself, will be reported in
due course.
Acknowledgment. Financial support provided by the
National Institutes of Health (GM038436) is gratefully
acknowledged. J.R.A. also thanks Eli Lilly for a graduate
fellowship.
Supporting Information Available. Experimental pro-
cedures and spectroscopic data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(11) Mergott, D. J.; Frank, S. A.; Roush, W. R. Proc. Natl. Acad. Sci.
U.S.A. 2004, 101, 11955.
(12) Barton, D. H. R.; McCombie, S. W. J. Chem. Soc., Perkin Trans.
1 1975, 1574–1585.
The authors declare no competing financial interest.
64
Org. Lett., Vol. 15, No. 1, 2013