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Disaccharide Mimetics by Enzymatic Tandem Aldol Additions.

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Disaccharide Mimetics by Enzymatic Tandem
Aldol Additions**
Oliver Eyrisch and Wolf-Dieter Fessner"
+
I b
Dctlicurerl I O Profissor R. R. Schmidt
on rlw omision of'liis 60tli birthduj.
The increasing amount of detailed knowledge on the mode of
action of complex oligosaccharides in biological recognition
phenomena and the occurrence of longer chain sugars as the
bioactive principle of compounds like the tunicamycins have
placed the synthesis of higher homologous sugars containing
8 - 13 carbons and their derivatives in the focus of modern
carbohydrate
Of
H
o+
H
$
particular interest are C-disaccharidesr2' as nonhydrolyzing
HooH HO OH
antimetabolites. that is, potenTunicamine
tial inhibitors of glycosideprocessing enzymes. Classical
synthetic strategies --iterative chain elongation or convergent
block condensations on the basis of available pentoses or hexoses- require the free, unmasked carbonyl group of the sugars
and thus intensive protecting group manipulations. Limited
availability or reactivity of suitably configurated precursors
from the "chiral pool" are obvious further restrictions.
In the context of the developing enzymatic de novo syntheses
of simple sugarsr3]and extending their chains by a C, unit to
form C , and C', homoiogues by using a l d o l a ~ e s , ~we
~ . present
a new proccdure: twofold chain elongation ("tandem" aldolization["]) of simple, readily available dialdehydes afford dodeco2.1 1-diuloses of doubly furanoid or pyranoid nature[']-formally derived from ketohexoses by C-coupling tail to tail-in a
highly stereoselective fashion and with deliberately variable substitution patterns.
The enzyme-mediated aldol reaction of dihydroxyacetone
phosphate (DHAP) and 2- or 3-hydroxylated aldehydes leads to
ketofuranoses or ketopyranoses, respectively, while the absolute
and rciative configurations of all stereocenters in the products
can be controlled by the choice of the aldolase (C-3. C-4)[81and
by adopting a
or thermodynamically selective[9b]
reaction mode (C-5, C-6). In principle, such a process should
also be applicable to difunctional
A plausible candidate for initial tests was 2,5-dihydroxyhexanedial (2).Dial 2 is
smoothly formed by ozonolysis of racemic cyclohex-2-ene-l,4diol (1)["] followed by reductive workup and, according to an
N M R analysis. exists in solution as a mixture of several constitutional isomers (hydrates, hemiacetals; mostly 2a) (Scheme I ) .
Its FruA-caialyzed[81coupling with DHAP. generated in situ by
retroaldolization of fructose 1.6-bisphosphate (FBP), proceeded at a sntisfactory rate (about 10% rel. cmaX)with transient
formation of a monoadduct (3; TLC analysis). The secondary
addition to form 4a may be at a certain kinetic disadvantage.
because in the equilibrium the pyranose form 3a with a masked
aldehyde group is presumably more stable than 3b, a furanose
having ii free aldehyde (hydrate) group, but the intermediate is
[*I
l ' r d I>r. W - D . Fessncr. Dr. 0 . Eyrisch
lnsi~tuirut- Organischc C'hemie dcr Technischen Hochschule
I'i-(,fc\\[,r-f'irlrt-StTasbe 1 . D-52056 Aachen (Germany)
T d c l a x 1111. code + (241) X888 127
c-iiiiiil fes\tict- ( I rwth-aachcn.de
I**]En/qmc\
i n Organic Synthesis. Part 10. This work was supported by the
Dcut\che l~or\chunfs~emeinschaft
(Grant Fe 244:2-2) and thc Schwerpunkts-
pr(>gr;inim "Synthesc-Enzyme" from the state of Baden Wiirttemberg We
thank (~ht-i\iiii;iSchiitte lor experimental asistance. Part 9: (51
~
1
\b
Scheme I [8]. a ) 0,MeOH. - 7 X C: Me$. room temperature ( R T ) .h) 1.3 equh
FBP. FruA (125 U),triosephosphate isomerase (2.50 U). pll 7 1. 2 d q a . c) Acid
phosphatase (25 U),pH 5.8. 3 days: 37% 4h Itom i u - I .
practically completely consumed in the presence of a slight excess of DHAP (FBP). After enzymatic dephosphorylation of 4a
with acid phosphatase, chromatography on Ca' ion-exchange
resin, and crystallization from water - acetone. a bisadduct was
isolated in 37% overall yield (based on rw-1: 74% based on
(R,R)-1). which according to its spectra is unequivocally the
C,-symmetrical, all-trans-configurated 4b,[''I a 6.6'4inked
dimer of D-fructose. In the crude product mixture no significant
amounts of the diastereomer stemming from the (XS)antipode
of 2 were detectable ( 55 Y O )which
,
is expected to be significantly less stable owing to a twofold cis-vicinal substitution.
3,4-Dihydroxyhexanedial(6) was generated by oxidative ring
cleavage of doubly homoallylic diol mc-5[13i (Scheme 2) and
+
HO
OH
HO
OH
r
'(
0a R = PO
-:
bR=H
1
7
Scheme 2 [XI. a ) O,.'MeOH. -78 C: Me,S. RT. b) 1.0 e q u i FBP. FruA (500 U).
triosephosphate isomerase (500 U). pH7.2. 7 days: 40% conieriion. c ) Acid phosphatase ( 5 0 U ) . pH5.8, 16 h: 13"X 8h from roc-S.
shown by NMR spectroscopy to exist in aqucous solution as
mixed bicyclic anomers 6a ( m : x P : P P =7:9:2). with no indication of a prevalence of a linear species ( 5 2 %). Accordingly, the
primary aldolization step with DHAPIFruA proceeded considerably more sluggishly than the addition to 2. The intermediate
(presumably mainly pyranose 7) was not detectable, and the
only bisphosphate product was isolated after 7 days of reaction.
Dephosphorylation provided in 13 YO total yield (based on
ruc-5; 26% based on (R,R)-5; about 60-70% when corrected
for conversion) the crystalline bipyranoid diketose 8b. Its C,symmetry and the bisequatorial connection between the rings
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are conclusively corroborated by the simple N M R spectra"']
and by the typical occurrence of a pseudo-quartet for the axial
protons 5-H and 8-H with almost identical geminal and vicinal
couplings of approximately 12 Hz. Despite the use of an excess
of DHAP (as FBP) no adduct of the ( S , S )enantiomer of 6 had
been formed. Its constitution (demanding a diaxial linkage) thus
seems to be thermodynamically highly disadvantageous even
relative to the furanoid FBP.
The me.ro-dialdehyde 10 can stabilize itself only by single cyclization (lOa, Scheme 3) and therefore should display much
better substrate qualities. Indeed, relatively rapidly a bisadduct
(+) 6 a
Scheme 4 IS]. a ) 2 5 equiv. of DHAP. RhuA. pH 6.8 b) + FruA. c) 0.75 equiv. of
FBP. RhuA (150 U ) , FruA (150 U ) , triosephosphate isomerase (500 U). pH7.2.
20 h. d ) Acid phosphatase (50 U). pH5.8. 16 h: 4 6 % 15b from rm-6
I b
t
13
OAc
I
OH
'la b R == H
Scheme 3 [ S ] . a ) 0,;MeOH. -78 'C; Me,S. RT. b) 1.0 equiv FBP. FruA (250 U).
triosephosphate isomerase (500 U ) , pH7.2. 16 h. c) Acid phosphatase (50 U ) .
pH5.8, 1 6 h ; 7 1 % from 9. d ) I l b MeOH, Dowex 5OW-XX. reflux; 8 7 % 12
(corresponds to 13, where H replaces Ac). e) Ac,O/py, RT; 91 "/o 13.
+
accumulated directly from the enzymatic reaction with DHAP/
FruA; consequently a potential kinetic preference of the aldolase for the 3-(S)- or 3-(R)-hydroxyaldehyde moieties within 10
could not be uncovered. Neither phosphate ester l l a , nor the
free syrupy sugar l l b , obtained in 70% overall yield by conventional processing, allowed a satisfactory N M R analysis to be
carried out, because a complex equilibrium of anomers and/or
conformers was apparently established, further complicated by
the unequivocal participation of partially acyclic forms. Complete signal" and conformational analysis was possible for 13,
which was obtained by peracetylation of the sparingly soluble
dimethyl glycoside 12 (87 YO).This revealed that the pyranose
subunit stemming from the (S)-configurated part of the dialdehyde has to tolerate two axial 0-acyl substituents at C-3'/C4' in
order to obviate an unfavorable equatorial to axial ring connection.
An application of the RhuA, stereocomplementary to
FruA,[81tested with 6 as example, furnished two rather startling
results (Scheme 4): 1) Rapid consumption of DHAP abruptly
ceased with formation of a monoadduct 14, from which a bisadduct (15a) could only be obtained upon addition of the FruA
enzyme; 2) N M R analysis" of 15b unambiguously established
a cis-vicinaf substitution pattern in one of the two rings, rather
than the expected two t r a m forms.
The failure of the RhuA enzyme in the second addition step
has to be assigned to the anionic charge of the phosphate ester
moiety in the intermediate 14a (likewise, glyceraldehyde 3-phosphate is not accepted['41). According to molecular models this
charge can be oriented spatially close to the aldehyde group.
Since on the other hand the FruA preferentially converts anionic substrates[41but accepts 6 only reluctantly, the two biocatalysts complement each other almost perfectly for usage in a
one-pot synthesis in which 15a is indeed generated by simulta-
neous action of both aldolases on 6 and on FBP with high
selectivity ( 290 % content of 15a in the crude product; 46 %
total yield of 15b). N M R analysis of the adduct and spectral
comparison with 8b substantiated the diequatorial ring connection, an all-equatorially substituted pyranose, and an axially
oriented 3-OH group in the second ring (J3,4
= 9.4, J3.,4.
=
3.2Hz). Because of the employment of rac-6 and "enantiogenic" aldolases, the cis-configuration could, in principle, have
been created by either of the two enzymes (epi-4 OH by RhuA,
epi-3 OH by FruA; 15 or mnt-15, respectively), but may more
plausibly be rationalized merely as the result of an incorrect
binding of the aldehyde by the RhuA, since mechanistic arguments suggest enantiospecificity of all DHAP-dependent aldolases for C-3,[l5I whereas a limited diastereoselectivity of the
RhuA at C-4 for certain types of substrates has been documented."61 Equilibration at longer reaction times probably allows
the erroneously connected cis-adduct to accumulate.
The yields of the tandem reactions, some of which need improvement, on one hand reflect the inferior substrate qualities of
aldehydes masked as hemiacetals and presumably having high
apparent K, values, and on the other hand the as yet nonoptimized reaction conditions, which, particularly due to the choice
of fructose 1,6-bisphosphate as the source for DHAP, may have
potential weaknesses in the slow attainment of equilibria of
unknown nature; oxidative generation of D H A P in sit^["^
seems to be a reasonable alternative. However, the fact that
enantiomerically pure complex sugar skeletons of high structural variability can be made from readily accessible, racemic
starting materials in a single synthetic step with creation of four
new asymmetric centers and differentiation of two more (one of
which is independent), makes enzymatic tandem aldolizations
highly attractive.
Received- March 23, 1'195 [Z78241E]
German version: Afigew. Clirm. 1995, 1fJ7. 1738 1740
Keywords: aldolases . carbohydrates
tandem reactions
*
disaccharide mimetics
*
111 a ) S. J. Danishefsky. M. P. DeNinno, Angi'tr. CIwm. 1987. 99. 15-23; A n g r ~
Clie~n.11ir. Ed. Engl. 1987, 26, I 5 23: b) C.-H. Wong. G. M Whitesides.
Efiqwie.s iii S j n r h e t i c Organic Cherni.srrj., Pergamon, Oxford. 1994; C:H.
Wong, R . L. Halcoinb, Y. Ichikawa. T. KaJimoto, Arige~v.C k m . 1995, 107.
453 474: A f z ~ e i i Chrtii. h i t . E d Engl. 1995, 34. 412-432; 1995. 107. 569 593
and 1995. 34. 511 546.
[4 R . M Paton, K. J. Penman. 17rruherlrori Lrrr. 1994. 35. 3163-3166; R. W.
Armstrong, D. P. Sutherlin. hid. 1994.35, 7743~-7746:L. Lay. F. Nicotra, C.
Pangrario, L Panza. G Russo, J C h m . Snc. Pcrkiii T,.on.s. 1. 1994, 333-338.
and references therein.
[3] W:D. Fessner. C. Walter. Anget!. C'/iet?T. 1992. 1114, 643--645; Atigeiv. Chrm.
/ t i t . Ed. Digl. 1992. 31. 614-616.
[4] M. D. Bednarski, H. J. Waldmann, G . M. Whitesides. Tc,?.truhrdronLerr 1986,
27. 5807 -5X10.
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~~~~~-[B,H,PPh,{Fe(CO),)(Ir(CO),PPh,)l:
[5] 0 Eqrisch. %I.
Keller. W - D Fessner. Tt./riih~droiiLrrr. 1994,33. 9013 -9016.
[6] L E Tietfe. U Beifuss. Angebt. Chon. 1993.1115. 137- 170; A n g e u . Cheni. / n [ .
Ed. Eiig/. 1993,32, 131 163.
171 Thc diketows presented here (4.8.11- 12.15)are neither carba-sugars nor Cglycosides. Their structural relationship to disaccharides suggests biological
acli\:ities. \b hich are currently under investigation.
Jonathan Bould, Nigam P. Rath, and Lawrence
[XI W - 0 . Fessner in Microhid Reagen/.c in Organic S).n/he.si:, (Ed. : S. Servi).
Barton
Kluwer Academic Publishers. Dordrecht, 1992,pp. 43-55. FruA = fructose
1.6-hisphosphate aldolase [EC 4.1.2.131, commercial enzyme from rabbit musrzido-Metallahexaboranes, that is species in which a metal
cle (Sipma I'ype IV), RhuA = rhamnulose I-phosphate aldolase [EC 4.1.2.191.
recombinant enzyme from E. co/i(purified according to ref. [16]). Amounts of
moiety replaces a BH group in nic[pB,H o, are convenient startcnryme acti\'ities correspond to respective D H A P equivalent.
ing materials for reactions of hexaboranes since they are easier
[ Y ] d l W-D Fcssner, J. Badia. 0. Eyriach. A. Schneider. G. Sinerius. Te~rahedrori
to
than B6H10 itself."] Two metallahexaboranes can be
LOII1992.33. 5231 5234: W.-D Fessner. A. Schneider, 0. Eyrisch. G. Sineri u \ . .I Badia. E,/rohei/roii
i m i t v r ! 1993,4.I 183 1192: b) J. R . Durrwachter.
easily prepared in high yield: nido-[B,H,.(Os(CO)(PPh,),)1"
c' H Wimp. J Orx. Chcm. 1988.53. 4175-4181: M. D. Bednarski. E. S. Siand nido-[B,H,{Ir(CO)(PPh,),J1 ( 1 ) , [ 2 , 3 1 While Some reactions
inoii. N U~schofberger.W.-D. Fessner. M.-J. Kim. W. Lees. T. Saito. H. Wdldwith osmahexaborane have been reported,"] other than recent
m;irin. G M . Whitesides. J. .4m. Cliiwi. Sot. 1989. ill, 627 635: W. J. Lees.
and our own preliminary report of the preparaCi. M.Whiteaidea. .L Or,?. Ciirm 1993.58. 1887-1894.
tion of [B,H,(Ir(CO)(PPh3)2}(p-H){PtCI(PMe2Ph))],[61
[lo] Although wveral attempts of additions to %.a-dialdehydes (glyoxal. glutaric
nothdialdchqdcl h;i\:e been reported i n the literature, in no case has a product been
ing has been published about the reactivity of the iridahexaboch.iractcrizcd 11x1. Our extensive studies corroborate that enzymatic assays
rane cluster since its synthesis was first reported in 1979.[31This
consumption of DHAP. but no defined products result (TLC. NMR
contrasts with. for example, meta~hdecaboraneswhich have
Anothcr problem is the cross-linking by aliphatic dialdehydes. which
ii-rcveriihl~dehtroys enzymatic activity. We attribute the success of the experibeen extensively studied and which show great similarities to the
menth presented here to the formation of stable hemiacetals by hydroxylated
parent deCaboraneS,171
di,ildehqde\. rslative rates of enzyme deactivation. however. have not been
dctcrrnined
[II ] J - t . Bhck\all. S E Bystrhm. R . E. Nordberz. J. Org. Chrnr
1984.4Y. 4619-4631.
H
H
[I21 4h M p I64 C, [XI;'
f 1 1 . 7 (< =0.3. H'O). ' H N M K
I
I
(400 MH7. D,O. B.p manomer) 0 = 4.08 (d. 3-.10-H), 4.02 (t.
4-.9-11). 3 77 (m, S-.X-H). 3 59 (d. la-.12a-H). 3.52 (d. lb-.12bH i . 3 82-1 7X (111. 6.7-H); '.'C NMR (100.6 MHz. DZO, p.p
:iiiomer) h 101 6. 80.2. 78.4. 75.9. 63.3. 29.9. 8b (monohydratej:
in.p X7 C'. [r]:' -59.7' (c, =1.1, MeOH). 'H N M R (400 MHz.
D,O) ii = 4 0 0 (d. 6-.7-Hj. j.97 (ddd, 4-.9-H). 3.70 (d. la-.12aH - B d H ' -A h
'
PPh3 H - B H H H /'
The First Structurally Characterized
closo-HeterobimetallaheptaboraneSystem **
~
*
4\T0 &bB
-
H ) . 3 5 5 (d. 1h-.12b-H), 3.42 (d 3-.10-H), 1.97 (ddd, Se-.Xe-Hj.
1 6.3 (q. 5;i-.Xa-H): " C N M R (100.6 MHz. D 2 0 ) d = 98.4. 72.5.
70 7 , 6X.Y. 64 1. 34.9 12: M.p 228 230-C (decamp.). [a];'
- l S . X j ( = (I 6. H,O). I3C N M R (100.6 MHz. D,O) 0 = 101.9.
100 R. 73.4. 71 0.68.7.68.3. 66 9. 65 8, 61 2. 58.2. 48.5.48.2. 34.2.
2X -3 13 M p 189 C, [a];' -25 7 (c = 0.3, MeOH). 'H NMR
(400 M H / . CDCI,) ,5 = 5.28 (ddd, 4-H). 5 16 (d, 3-H), 4.95 (d.
10-HI. 3 YO (q. Y-Hj. 4.29. 4.16 ( 2 d. la-,l?a-H), 4.07. 4 04 (2 d. Ib-.lZb-H).
3.X5 (ddd. 7-HI. 3.72 (ddd. 6-H). 3.31. 3.26 ( 2 s. 2 OCH,). 2.44 (ddd. 5e-H).
2 1 I , 2 OY. 2 OR. 2.08. 2.03. 2.02 (6 s. 6 Ac). 1.95 (dni. Xe-H). 1.76 (ddd, 8a-H).
1.57 iq. 5ti-11) 1Sh. syrup. [ x ] i * -15.6 (L. = 2.4. MeOH). ' H N M R
(400 MHr. D l 0 j 0 = 4.17 (ddd. 9-H), 3.99 (d, 6.7-H). 3.97 (m. 4-Hj. 3.76 (d.
10-H). j.74. 3.71 (2 d. la-,lZa-H). 3.54. 3.53 ( 2 d. lb-,l?b-H). 3.45 (d, 3-H).
1 9X (ddd. 5c-F[). 1.80 (q. 8a-H). I . 6 X (ddd, Xe-H). 1.63 (4. 5a-H); 13C NMR
(100.6 MII/. D10)0 = 98.8. 98 4. 72.4, 70.9, 70.6. 68.9. 68.2, 66.2. 64.8. 64 2.
[I31 -74
H X.Suciiiunc.
29.5.
M. Hiruka, Chwi Phurni. Buil. 1989,37, 1379-1381
[I41 ( i Sineriu,. Di~sertrition.University of Freiburg. 1994.
1151 R A. Pcriaiia. K.Motiu-DeGrood, Y. Chiang. D. J. Hupe, J. Am. Chrni. Soc.
1980, 103. -3923 3927.
1161 W-D. tcsmcr. G Sinerius. A. Schneider. M. Dreyer, G. E. Schulz. J. Badia. J
hpuil'ir. , 4 r i w i 1 . Chcwi. 1991.103. 596-599: Angel!,. Chern. I n / . Ed ErigI. 1991.
.I/). 555 55s.
[I71 W-II tcs\iiei-. G. Sinerius, Arigeii C / i ~ n1994,106. 217 220; Angcw. C/ieni.
i l l ! . b,"/.En::/. 1994.33, 209 212
Ilx] 1. t,ffellherper,A , S t r a u b ,7 u , ~ r r ~ i P r ~ ~ , ~1987,18,1641
,r~r~l.
-1644: H , p, Brock.
~nn]i.M . R Kula, ibid. 1990.31, 7123-7126.
-B-
H
/
\ H
Bc/
H
\H/
~ B - B .
L P P h ,
\ H
f
H
H
2
1
H
I
\d
H-B-
'B-
\c
H H W-B-H
/ 'B
LH/
q
//H
co~~\co
co
3
The B,H,, cluster 2 contains a basal B-B bond with a high
degree of basic character.[*] This is a feature that has been
exploited in the formation of a number of compounds with
iron,i9B-ti1
rhodium,i9h1and platinum,iYr1in which
this bond donates two electrons to the metal. Thus for iron the
reaction of B,H,, with [Fe,(CO),] affords [B,H io[p-Fe(CO),}]
(3). It was therefore of interest to see if the iridahexaborane
analogue 1, which has a similar basal B-B bond as determined
by single-crystal X-ray diffraction (B-B distance 1.65 A vs.
1.63 8, in B6H1,),[3.'01would undergo an analogous reaction.
[*I
[**I
Prof. L. Barton. Dr. J. Bould. Dr N. P. Rath
Department of Chemistry
University of Missouri-St. Louis
St. Louis, MO 63121 ( U S A )
T e l e f u Int code + (314)816-5342
6 ~
e-mail: ~ 4 6 2 slvoxa.umsl.edu
W k thank the National Science Foundation, the Missouri Keseiii-ch Board. and
the University of Missouri-St Louis for financial suppoi-Iof this work and the
Johnson Mathey Company for a loan of I r C I , . 3 H 2 0 .
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