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16.5 Cyclic Structures

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16.5 Cyclic Structures 16.3Cyclic Structures 501 16.5Cyclic structures AIMS: To describethe bonding thot resultsin cyclic formsof ,ugors. To clqssifysimple sugors os a pyronoseor o furonoseond as a hemiocetitor o heiiiketot. Focus until now; saccharideshave been depicted as straight-chain compounds. The reality is somewhat different, however, for pen6ses and hexosesexist Pentosesand hexosesexist primarily in flve- and six-membered rings or cyclic forms. Examples of two mainly as ring structures. such cyclic forms, those of o-glucose and l-fructose, are shown in Figure t6.4. The fiue-membered sugar ring system is giuen the generar name fura- nose 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-fruc- tose 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 Cyclic form of o-fructose o-glucose (o-fructofuranose) (o-glucopyranose) Figure16.4 Thecyclic forms of two sugarsdrawn as in theirtrue shapes. The five- and six- memberedrings resemble those of cyclopentaneand cy.loh"*an", respectively. 502 CHAPTERl6 Carbohydrates H o. .H a/6tty \// cH2oH z I ,C-oH6 L\ H-C-OH H ,/l H I l,/ H ^'/ Cyclic HO-C-H :- c .L., hemiacetals I r\ oH- H'i,/ ./ \\. (*hemiacetalcarbon) H-C-OH H6\i o I c_c H-C-oH7 II \ I HOH cH2oH Open chain structures Figute15.5 An intramolecularreaction between the aldehydeand hydroxyl SrouPs of a sugar, in thiscase o-glucose, results in formationof a cyclichemiacetal' possible internal hemiacetal or hemiketal formation. The internal hydroxyl group that is selected for reaction is one that will give a flve- or six-mem- bered 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 AIMS: Todrow Haworth Proiectionsfor the commonsimple sugors,To identify o simple sugor os on olpha or beto oiomer. To exploin the interconversionof closed'choin formsof sugors. The stereochemistry of the cyclic forms of sugars is often represented by their Haworth proiectio ns-standardized ways of depicting the positions of The stereochemistry of the hydroxyl groups in space.In viewing Haworth projections, envision the cyclic forms of sugars is plane of the ring as tilted perpendicular to the plane of the paper. The depicted by Haworth attached groups.are above and below the plane of the ring. Figure 16.6 projections. 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 505 B-hydroxyl group 6 6^/ HOCH2/,o cH2oH z "o?l-"\o*t ?Ho N Hol' A OH cH2oH )ry HO o-hvdroxr.l HO a-o-Fructofuranose group B-o-Fructofuranose o 6 ,- B-hydrorryl CH,OH CH2OH giu,,p Figure15.6 / Haworth projectionsof L:o t/'' \r A-o,ofi o-glucopyranoseand KnoH r) l(o" ) o-fructofuranose.Two Ho\-q-4loH Hov_3/ differentstereoisomers, tt\ tl OH\ -'- OH labeleda and B, may be o-h-vdroxl'l formedon hemiacetal a-o-Glucopyranose group B-o-Glucopyranose formation. p t.,r.F1) 1"+ I .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.The alpha and beta anomersof the cyclicforms of sug- ars have different melting points and different abilities to rotate plane- polarized light. Alpha-o-glucosemelts at l4G'C, for example,but beta-l- glucosemelts at 150"C. Thesedifferences help to demonstrateagain how small changesin molecular shape or structure dramatically affect the phys- ical propertiesof 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 pro- jection. Hydroxyl groups that point to the right in a Fischer projection point down in the Haworth proj ection. EXAMPIEI6.5 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'' 504 CHAPTER16 Carbohydrates fi, projection of u-glucose in the straight-chain form: -CHOI tl H-C-OH I ?l HO-:C-H I 'Ll H-C-OH I -l n-9c-oH el-CH.OH o-Glucose pyranose ring in its abbreviated form as sholvn: Draw a six-membered t ,l c-o ..i /5 \ t:-tri c4 lc ) l;iT \sz/ H$i C-C | ,*l l,s Pyranosering Abbreviated l'$ pyranose ring Put in the -CHrO'H group of carbon 6 of the hexose.tn a o sugar,the h$ 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/'' \ t KoH ') Ho\_t_4/ tl OH Finally, write the anomeric -OH group at carbon l-in this case alpha (belowthe plane of the ring). ucnon LO (;) ffi HoY-2roH OH c.n-Glucopyranose 16.5Glycosides 505 PRACTICEEXERCISE T6.7 t,,. acetal carbon. ,' PRACTICEEXERCTSE t6.3 Drarthe,Haworth projection of a-o-ribofuranose. Identify the hemi- acetalcarbon. t:: PRACTICEEXERGISE I6.9 ' the Haworth projection Pruy of a_o_fructofuranose. Identi$r the . hemiketal carbon. Interconversion of straight-ch4tn and rlng foms of sugars The straight-chain sugarforms are in equilibrium with the ring forms. The ring forms are usually quite predominant. For example, irsteie3crremically pure a-o-glucopyranose is dissolved in an acidic iolution, the ring wilr open and close repe-atedly. In reclosing, some B-l-glucopyranose is formed' The finar equilibrium mixture of about-,i"y 63% B-o-glucopy- ranose' about 3T% a-o-grucopyranose,"o.rsirt, and o"ry a uri'or.rt of the btraight-chain aldehyde. r'-to- itt" p"r.rrtug", of products formed, we can say that B-o-glgcopylangse is-only slightry more preferred than a-o-gru- copyranose and that Oottr 9_Stygopyranoseanomers are much _";;;;_ ferred than the straight-chai" Aaet yae fo.- of "_gl;;;;;."'*" CHO I H-C-OH I HO-C-H I oioH H-C-OH (9) OH (c) H-C-OH I OH cHroH a-o-Glucopyranose o-Glucose p_o_Glucopyranose (about37%) _. - gessthan l%) (about63%) 16,5Glycosides AIM: To describethe formation of grycosidic- boinds ond the productsof their hydrolysis. The closed-chain hemiacetal Focus or hemiketal forms of sugars may react with alcohols to formQc-et4s-dfkEtri[stsee sec. rs. 6). Theacitars orietars of sug- Alcohols react with closed- ars are called$ycosideC. - '- chain forms of sugars to form glycosidic bonds. Glycosidic bonds The coualentether link betweenthe sugar hydroxyr and the arcohor is a gry- cosidic bond. A simpre alcohor such ai *"ittu.roi u. *_o_ ""J;r;;;l,r"t .
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