16.5 Cyclic structures

16.3CyclicStructures 501
16.5Cyclicstructures
AIMS: To describethe bonding thot resultsin cyclic formsof
,ugors. To clqssifysimplesugorsos a pyronoseor o
furonoseond as a hemiocetitor o heiiiketot.
Focus
Pentosesand hexosesexist
mainly as ring structures.
until now; saccharideshave been depicted as straight-chain compounds.
The reality is somewhat different, however, for pen6ses and hexoses
exist
primarily in flve- and six-membered rings or cyclic forms. Examples
of two
such cyclic forms, those of o-glucose and l-fructose, are shown in Figure
t6.4.
Thefiue-membered sugar ring system is giuen the generar name furanose after the parent cyclic ether furan; the sii-memberid sugar ring system
is considered a deriuatiue of pyran and is called apyranose.
,'o\
L/
Furan
Pyran
To name a sugar in its cyclic form precisely,remove the -sefrom the sugar
rlme and addfuranose or pyranose accordingto whether the cyclic form
is
a five-membered or a six-membered ring. T[us the cyclic form of l-fructose is properly called o-fructofuranose and that of o-glucose is called
o-glucopyranose.
To understand how these cyclic structures are formed from the
straight-chain sugars, recall that alcohols can add to carbonyl groups of
aldehydes or ketones to form hemiacetals or hemiketals, and dd u"e d- X
membered rings are more stable than smaller rings. Hemiacetal formation,' '
discussedprwiously (sec. 13.6),is the addition oian alcohol to a carbonyl
$o-"q of an aldehyde. Hemiketals are formed in a similar way from ketones
and alcohols.
Sugarssuch as o-glucose and o-fructose contain a carbonyl group and
several hydroxyl groups in the same molecule. Figure 16.5 iliustrates the
4
OH
Cyclic form of
o-fructose
(o-fructofuranose)
Cyclic form of
o-glucose
(o-glucopyranose)
Figure16.4
Thecyclicformsof two sugarsdrawnasin theirtrueshapes.
Thefive-andsixmemberedringsresemble
thoseof cyclopentane
andcy.loh"*an",respectively.
502
l6 Carbohydrates
CHAPTER
H
o.
.H
\//
I
I
HO-C-H
I
H-C-OH
I
H-C-OH
:-
,C-oH6
H ,/l
H
l,/ H
^'/
c
.L.,
'i,/./ \\.
oH- H
r\
H6 \i
H-C-oH7
I
a/6tty
cH2oH
z
Cyclic
hemiacetals
(*hemiacetalcarbon)
o
c_c
II
HOH
L\
\
cH2oH
Open chain structures
Figute15.5
of a sugar,
and hydroxylSrouPs
reactionbetweenthe aldehyde
An intramolecular
hemiacetal'
of
a
cyclic
formation
resultsin
in thiscaseo-glucose,
possible internal hemiacetal or hemiketal formation. The internal hydroxyl
group that is selected for reaction is one that will give a flve- or six-membered ring depending on the saccharide. Smaller rings would be unstable
becauseof ring strain; larger rings are not formed becausethe more distant
ends of the molecule do not often collide in solution.
16.4Howorth projections
for the commonsimple
AIMS: Todrow Haworth Proiections
os on olpha or beto
sugor
o
simple
To
identify
sugors,
oiomer. To exploin the interconversionof closed'choin
formsof sugors.
The stereochemistry of the
cyclic forms of sugars is
depicted by Haworth
projections.
The stereochemistry of the cyclic forms of sugars is often represented by
their Haworth proiectio ns-standardized waysof depicting the positions of
hydroxyl groups in space.In viewing Haworth projections, envision the
plane of the ring as tilted perpendicular to the plane of the paper. The
attached groups.are above and below the plane of the ring. Figure 16.6
shows Haworth projections for n- glucopyranose and o-fructofuranose.
TWoforms of each of these sugarsare possible; these are designated alpha
and beta. TWo cyclic forms are possible because, in going from a straight
chain to a ring, a new asymmetric carbon is introduced at carbon 1 (the
hemiacetal carbon) of aldoses and carbon 2 (the hemiketal carbon) of
16.4 Haworth Projections
6
"o?l-"\o*t
z
? HA o
o-hvdroxr.l
group
OH\
B-o-Fructofuranose
A-o,ofi
l(o" )
\r
KnoH r)
Ho\-q-4loH
tt\
HO
6
,- B-hydrorryl
CH2OH /
giu,,p
o
CH,OH
L:o
cH2oH
)ry
a-o-Fructofuranose
t/''
Hol'
N
OH
HO
Figure15.6
Haworth projectionsof
o-glucopyranose
and
o-fructofuranose.
Two
differentstereoisomers,
labeleda and B, may be
formedon hemiacetal
formation.
B-hydroxyl
group
6^/
cH2oH
HOCH2/,o
505
Hov_3/
tl
-'-
o-h-vdroxl'l
a-o-Glucopyranose
group
OH
B-o-Glucopyranose
p
t.,r.F1)
1I ".+
91"'l;
ketoses.In the Haworth projections of o-glucopyranose,for example,the
hydroxyl group at carbon 1, which is formed from the aldehyde functional
group of the straight-chain sugar,may end up below (alpha or Greek a) or
above (beta or Greek F) the plane of the pyranose ring.
Anomers are sugarsthat dffir in stereochemistryonly at the hemiacetal
or hemiketalcarbon.Thealpha and beta anomersof the cyclicforms of sugars have different melting points and different abilities to rotate planepolarized light. Alpha-o-glucosemelts at l4G'C, for example,but beta-lglucosemelts at 150"C. Thesedifferenceshelp to demonstrateagain how
small changesin molecular shape or structure dramatically affect the physical properties of molecules.
How to draw Haworth
proiections
Conversion of a straight-chain Fischer projectio4 to a ring in a Haworth
projection is easily accomplished for D sugars.Hydroxyl groups that point
to the left in a Fischer projection of a o sugar point up in the Haworth projection. Hydroxyl groups that point to the right in a Fischer projection point
down in the Haworth proj ection.
E X A M P I EI 6 . 5
Drawinga Haworthprojection
Draw the Haworth projection of a-o-glucopltanose.
SOLUTION
The a-r-glucopyranose
molecule is the cyclic form of o-glucose.Draw a
\
l''
fi,
504
16 Carbohydrates
CHAPTER
projection of u-glucose in the straight-chain form:
I- C H O
tl
H-C-OH
?l
I
HO-:C-H
I
'Ll
H-C-OH
I
- l
n-9c-oH
e-CH.OH
l
o-Glucose
Draw a six-membered pyranose ring in its abbreviated form as sholvn:
,l
. .i
t:-tri
l;iT
H$i
| ,*l
l,s
l'$
h$
t
c-o
/5
\
c4
\sz/
lc
)
C-C
Pyranosering
Abbreviated
pyranose ring
Put in the -CHrO'H group of carbon 6 of the hexose.tn a o sugar,the
qarbon is always above the plane of the ring as shor,rrn:
Fill in the -OH grcups-on carbons 2, 3, and 4. Notice that the o)rygen
carbon 5 of the chain form is now in the ring and need not concern us.
Hydroryl groups to the right in the Fischer projection go below the plane
the ring. Those to the left are above the plane. Ring hydrogens are usually
omitted for clarity.
ucHroH
,l=-O
t/''
\
KoH ')
Ho\_t_4/
t
tl
OH
Finally, write the anomeric -OH
(belowthe plane of the ring).
ucnon
ffi
LO
(;)
HoY-2roH
OH
c.n-Glucopyranose
group at carbon l-in
this case alpha
16.5Glycosides 505
PRACTICE
EXERCISE
T6.7
t,,. acetal carbon.
,'
t::
PRACTICE
EXERCTSE
t6.3
Drarthe,Haworth projection of a-o-ribofuranose.
Identify the hemiacetalcarbon.
PRACTICE
EXERGISE
I6.9
'
the Haworth projection of a_o_fructofuranose.
Identi$r the
Pruy
. hemiketal
carbon.
Interconversion
of straight-ch4tn
and rlng foms of sugars
The straight-chain sugarforms are in equilibrium
with the ring forms. The
ring forms are usually quite predominant. For example,
irsteie3crremically
pure a-o-glucopyranose is dissolved in an
acidic iolution, the ring wilr
open and close repe-atedly.In reclosing, some
B-l-glucopyranose is
formed' The finar equilibrium mixture
of about
-,i"y
63% B-o-glucopyranose' about 3T% a-o-grucopyranose,
"o.rsirt,
and o"ry a
uri'or.rt of the
btraight-chain aldehyde. r'-to- itt" p"r.rrtug", of products
formed, we can
say that B-o-glgcopylangse is-only slightry more preferred
than a-o-grucopyranose and that Oottr
anomers are much _";;;;_
9_Stygopyranose
ferred than the straight-chai" Aaet yae fo.- of
"_gl;;;;;."'*"
CHO
I
I
H-C-OH
HO-C-H
I
H-C-OH
OH (c)
OH
a-o-Glucopyranose
(about37%)
oioH (9)
H-C-OH
I
cHroH
o-Glucose
_.
gessthan
l%)
-p_o_Glucopyranose
(about63%)
16,5Glycosides
AIM: To describethe formation of grycosidicboinds
ond the
productsof their hydrolysis.
Focus
Alcohols react with closedchain forms of sugars to form
glycosidic bonds.
The closed-chain hemiacetal or hemiketal forms
of sugars may react with
alcohols to formQc-et4s-dfkEtri[stseesec. rs. 6). The
acitars orietars of sugars are called$ycosideC. - 'Glycosidic
bonds
The coualentether link betweenthe sugar hydroxyr
and the arcohor is a grycosidic bond. A simpre alcohor such ai *"ittu.roi
u. *_o_
""J;r;;;l,r"t