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Origins of the proton NMR chemical shift non-equivalence in the diastereotopic methylene protons of camphanamides
T&r&&u Vd. 42. No. 2. pp. 617 to 622.1966 Riaedinoratnrihin
ORIGINSOF
lltE PROTON NM CHWCAL
SHIFT
NON-EQUIVALRNCE IN
'IW DIASTFRECTOPIC lIEMyLENEPROTOM OF CAUPRANAIUDES D. PARKERI( and R.J. TAYLOR Departmentof Chemistry,Universityof Durham,South Road, Durham DHl 3LE C. FERGUSON Departmentof Chemistry,Universityof Guelph,Guelph,OntarioNlG 2Wl A. IQNGE Glaxo Group ResearchLtd., GreenfordRoad, GreenfordUD6 ORR. (Roceiredis UK23 October 1985)! of the diastereotopic methylene Abstract- The observedchemicalshift non-equivalence protons in primarycamphanamides is rationalizedin terms of chiralityindependent, differentialshieldingof IiSby the amide carbonyl,as deducedby nmr solution,X-ray structuralstudiesand molecularmechanicscalculations. In proton nmr spectroscopy, the methyleneprotonsat a prochiralcentre in a chiralmoleculeexist in magneticallynon-equivalent environmentsand may exhibitdifferentchsmicalshifts. It was proposedthat the observedchemicalshift non-equivalence, Adobs., is made up of an intrinsic term, A6c, dependentupon the relative diastereotopic shieldingterm, A6i and a confotuational fractionalpopulationand the corresponding individualchemicalshifts of low-energymolecular ' More recentlyit has been shown that the intrinsicdiastereotopic shieldingis conformations. has been insignificant in % nmr.2 The magnitudeof the observedchemicalshift non-equivalence observedto be a functionof the separationof the prochiraland chiral centres,of molecular rigidityand of temperatureand solventi.e. parameterswhich affect the molecularconformation term, A6c.3 In the II!n.m.r. spectraof a series of alkyl camphanamides, la-lf we have found that the --7 pro 2 hydrogenconsistently resonatesto higher frequencyof HR.4 This permitsthe determination of both the enantiomericpurity and the absoluteconfiguration of o-deuteriated primaryamines. The chemicalshifts of the diastereotopic methyleneprotonsin la-f are recordedin Table 1; the pro E and pro 2 protonresonancesare cleanlyseparatedat medium field in d6-benzenesolvent, with the chemicalshift differencebetweenHg and HR varyingbetween0.12 and 0.21 ppm. An example is shown in Figure 1, in which part of the 400 MI% *H n.m.r. spectrumof E
is illustrated. HS and
HR are clearlyseparated. Each proton is coupledto the amide NH and to the adjacentmethylene protonsto give the observedmultiplet.
a)
R - Me
d) R =CH2Et
b)
R -Et
elR=Ph
cl
R -CH2CH2Br
f 1 R = p-Brl’h
In order to probe the originsof the anisochronismof the geminalmethyleneprotonsin la-f, the para-bromobenzyl derivativerf has been examinedin detail. Rxaminationof molecularmodels suggeetsthat the observednon-equivalence of the diastereotopic ethylene protonsis due to the neighbouring&de
carbonylanisotropy.
617
D. PARKER etal,
618
At
=O-21ppm
400 MHz
I ,-.------
8
*---------I
315
3.05
2-95
2.05
Ppm
Fig. 1. 400 wlz Illn.m.r. spectrumof (lS,QR)-(-IN-propylcamphanamide (lJ),showingthe ciiastereotopic methyleneprotons. Table 1. NMR Data for Diastereotopic Methyleneprotonsin -_ la-lfa Compound
R A6RS'k
-la
ne
3.03
2.89
0.14
17.5
b
Et
3.12
2.91
0.21
14.3
-1c Ir!
CR2cH2Br
3.00
2.88
0.12
13.7
n-R
3.11
2.95
0.16
13.6
-le
w
4.25
4.12
0.13
14.8
E
-p-Br-rJh
4.12
3.96
0.16
14.7
a Spectrawere recordedin d6-benzeneat 298 K; chemicalshifts are given in pp and couplingconstantsin Hz. X-Ray StructuralStudies The structureof rf has been detetined by single crystalX-ray diffraction.+ There are two independentmoleculesi and B in the asmtric
unit: these moleculesare linked into A-B pairs
by NH--O hydrogenbonds betweenthe NH group of one moleculeand the lactonecarbonylof another. A view of the two moleculesia shown in Figure 2 with the cry&allographicnumbering scheme. MoleculesA and 2 are conformersrelatedby rotationabout the N-C(ll)bond vith ' Crystalsof &
were grown from CRC13/CC14:hexane (1:l). Crystaldata:- Cl,R20BrN03,Mr =
366.3,monoclinic,space-groupC2, a = 25.454(6),
b = 6.507(J),c = 21.947(7)8,B = ll2.4',U =
3362(2)x,Z = 8, DC = 1.45 g CID-~, F(ooO) = 1504, ~1fn0-K~) = 24.3 cm-'. At convergenceR = 0.064 and Rn = 0.086 for the 1903 observedreflections. The atomic coordinatesfor this work are availableon requestfrom the Directorof the CambridgeCrystallographic Data Centre,University ChericalLaboratories,LensfieldRoad, CambridgeCB2 1EU. Any request should the full literaturecitationfor this comunication.
be accompaniedby
si8ultaneous
reorisatatfonof the pheql
anglesare 96.6,audShefor
torsion ring. The values of the C(1O)-N-G(11)-C(l2)
the A and B moleculesrespectively.Tbe bromopbmyl riny, are
ariestedabout the G(ll)412) bonda such that the N-C(li)-C(12)-4(13) torsiohau&xi are 31.1 and 325.T"forA and B respeotively:
Fig. 2. Molecularetructuresof the two iwlemdeut molecules,_3: (left)and B (right). The relativeorientationof the bicycllcriu~ is the same iu both casee. In molecule_ir, the pro S hydrogenis closer than HK to the magueticallyanisotropiccarbouyl group; in uolewle 2 the situationis reversed. Rirthexmore,HS ie closer to the aside carbonyl
SolutionW.a.r. Studies In the 'Iin.m.r. spectrum of Lf in d6-beuzeaeat 298 K, two doubletof doubletsMY be observed, ceutredat 64.11 and 3.95 pm, correflponding to Hs and HK respectively.The chemicalshift differencebetuem HS aud HK iucreaeeelinearlywith decreasiugtemperature,(Table21; Mar where h6 in 0.12 pgnnat 320 K aud r&see linearlyto 0.15 ppm at V#t 283 K. The chemicalshift differenceis indepoudeutof tide concentratiou, precludiuSauy effects
behaviouris observedwith 2
due to intemolecularassociation. Carefulexawinatiouof the temperaturedepeudenceof MI, SHR revealsthat as the taaperatureis lowered,it is Hs that shifts to h&beer frequencyrelativeto I$ and other resouauceeiu the molecule. Such MaHour is cousisteutwith HS spendingsore tti, on average,iu a regneticallydeshieldieKeuvironmmt i.e. proximateto the amide carbonyl. The observedanisochronisa~ is also ssnsitiveto the n.m.r. solventused. With lf, ad nSKKta (Table2). Sinilarbahaviourvasobserr~vfth~,vithrhich A8 at 308 K is 0.03 ppa in CDC13, 0.05 pp KS% in @X4, 0.06 pps in d6-acetoneand 0.15 ppm in c6h6' The observatiouthat Mobs, is a m&.mum in aromaticsolventssu&~wts that one of the aromaticsolventmoleculeslien cloee to the amide group. maximumwith non-polararomaticsolventsand is lower in polar, aliphaticeolwts,
Some form of ‘we&* *-rcCinteraotionbstueeathe aromaticv cloud and the carbonyldouble bond may be
620
D. Pm
et sl.
Table 2. Variationof Abobs with Terperatureand Solventa . TeiperatureK;
A6
ppm
Solventb; A6
%AR 320
0.137
C6D6
0.15
310
0.148
‘7D8
0.14
300 295
0.162 0.171
zN
Z'Z .
zgo
0.177
3 (CD3)2C0
285
0.185
GD30D
0.07 0.06
a%corded at 200 Hlz. ' At 308 K. involved,enhan*
the anisotropicenvironmentexperiencedby IIsa&l&.
The possibilitythat the amide N-H may form an intramolecular hy&ogen bond with the lactone ether oxygen in non-polarsolvents,at the concentrations used in the-a.m.r.experiments,was considered. However an FT Lr. study in Ccl4 and C6D6 revealedthat at cemoemtrations of -If of -1 1o-2 Jland below, only a mingle sharp band at 3435 cm was observed. Ibis correspondsto the free N-H stretch,and no other bands were observed,in this region,over the concentrationrange -2 10-s to 10 w. At higher oencentrations uhiehgrew as a a broad band appearedat 338O,cm-';' functionof increasingcoaoe&rationand is assignedto an intermolecular hydrog&.bondedNH
.’
tion, hydrogenbondingbetweemthe tide KH of one stretch. In the crystalstructuredetermina moleculeand the lactoneoarbonylof anotherwas revealed. Such interactionsalso appear to occur in solutionat higher concentrations. MolecularMechanicsCalculations Using a model of the camphanamide constructedfrom fragmentstaken from the Cambridge 5 molecularmechanicsconformationenergy calculationswere performedon Crystallographic Database, -If. The variationin molecnlarpotentialenergy with changesin torsionangle was calculatedby the eumrationof the separatecomponentsfor non-bonded,torsionaland electrostatic energies, (furtherdetailsare in Kxperimental).In Figure 3 the variationin molecularpotentialenergy with changesin the C(lO)-N-C(ll)-C(12) and N-C(ll)-C(lZ)-C(l3) torsionangles is illustrated. The contourlines are in energyunits of kcal mol-'. Comparisonwith the observedtorsionangles for moleculesi and B, revealedby the crystalstructure,shows good agr-t.
MoleculeA is
the lower energy conformer,residingin the bottom left hand cornerof the energy plot, with B in the top right hand corner. Ibe calculatedenergy differencebetween1 and B is 1.3 kJ 1101-l. The local energy minima are calculatedto be within 4 kJ mol-' of the absoluteenergiesfor the observedcrystalconformations, that is, within the limit of the largestprobabledistortionof a 6 small moleculeconformer,in the crystallinestate, from the minimumenergy form. Furthermore, it is well establishedthat any conforrpation observedin the crystallinestate is likely to be importantin solution.6 Confolrerh is the lower energy state and in solutionas the temperature is lowered,vi11 be preferentially populatedover B. Conclusions The studies relatedabove confirmthat the observedchemicalshift nonequivalenceof the gcainalsethyleneprotonsin camphanamides, 1, is due to differentialshieldingof the diastereotopic methyleneprotonsby the anisotropicamide carbonylgroup. In the preferredconforsler 4, the pro S hydrogenis closer on averageto the amide carbonylgroup and hence resonatesto higher frequencyof I$. As the temperatureis loweredthis differentialshieldingis more pronouncedas conformer&is preferomtially populated. Given that HS consistentlyresonatesto higher frequency of liKin all studiedcaqhanamides,4'7the rationaleput forwardmay afford a generalexplanation for the originsof the observedgeminalproton anisochronism in camphanamides and relatedcompounds.
fig 3. Mofolecular potentialenergymap as a functionof the C(lO)-Nap C(l3) torsionangles. Contoursare iu I kcal mol" steps.
and N-C(lL)-C(lz)-
RXPRRIRRNTAL $i n.m.r. spectrawere xwcordedon either a RrukerWR360,a RrukerW200 or a Rruker WR@C
._
iusttaeot. Roleculwqecbanics coufomatioualehergycahxlat$onsuere Researchmolecularmodellingsysteqruuuiugon a VAX 11-750
performedontbeClax0
miuicomputercoupledto Rogatekand
Sigma displayteminsls. The force-fieldcalculationsused standardRuckingbasand single-texm 8 cosine potentialfunctions for the non-bondedand torsionalenergies,respectively, and the electrostatic euergy was calculatedfrom a Cqulombicpotentialusiug a distance-dependeut dfelectricters.9 The associatednon-bondedand torsionalforce-fieldparameterswere setlo to reproducein 'rigid-rotor' models (i.e.withoutfull relaxationof moleculargeometry), experireutaltorsionalbarriersin mall mole~ules~~and the observeddistributionof +, Gaugles iu peptideaand proteins. l2 A standardset of partialchargeswers placed on those atoms iuyolved in potentialhydrogen-bonding groups. The crystalstructurewas detemiued by the heavy atom method and refinedby full matrix lesst-squares calculation& All hydrogenatoss were located from the differencemaps and includedbut not refiuedin the calculations.Allowanceno8 made for the auomalouedispersionof brosine. Reflectionswere seaswed with a CAD4 diffractometer and all calculationswere made with the SDP system of programson a PDP-11/73computersystar. The cbiral anides -la-le were preparedaccordingto literatureprocsdures,4and E preparedas describedbelow.
was
622
D. PtuKKu 8l ai.
para-Brosobensylauinarine hydrochloride(227 ug, 1 nmol) was suspwded in dichlorcsethane (7 cn3) at O'C, and trietJ#anine (150 mg, 1.5 wl)
and (-)-casphanoyl chloride(241 sg, 1.1 roll added
in sequaice. The sixturewas stirredfor 3 h at O'C, then poured in sodim hydroxidesdlution (0.1 H, 50 cr3) and the aqueousphase extractedvith dicbloraatbane(3 x 10 cu3). The organic phase was washed with dilutehydrochloricacid (0.1 U, 2 x 20 cu31, water (2 x 25
cm3),
dried over
anhydroueuagne&us sulphate,filtered,and solventraao~eduuder reducedpressureto give a colourlesssolid (220 sg, 60$) which may be recrystallissd fros chloroform-carbon tetraclrloride (1:l)and tmxane,up 126-127°C.'Found: C 55.6, Ii5.28, N 3.61. C17R20BrN03requiresC 55.7, H 5.46, N 3.82. $
(C6D6)6.77 (lR, br, NHCO), 4.12 (18, dd, I$,>,3.96 (lE, dd, RR, J 14.71, %% 2.30 (Hi, u), 1.56 (lH, a), 1.33 (LH,R), 1.25 (Hi, n), 0.83,,0.82(3R + 3H, s,+ s, )b2C), 0.69
(3% s, C&f. s/t 367/365CR+,, 186/184,149, 136, 109, 83. .v(KBr)3380 (H-bondedR@, 2960, 2920, 2860 (III), 1780 (rs), lactoneCO), 1675 (vs, RRCO), 1530, 1175. h Ltd. for n CASE studentshipand - We thank SFXC and Glaxo Group Researc Acknovledgeuent Dr. I. Sadler (vniversityof Kdinburgh)for high-fidldn.u.r. spectra. GF thanksNSPX! (Canada) for continuingsupportvia operatinggrants.
1li.S. Gutowsky,J. C&u. fhys. g, 2196 (1962). 2P.J. Stiles,Chea. Pays. Lett. a, 23 (1976). "R.D. Norris and G. Binsch,J. Aaer. Chem. Sot. 9& J. Aser. Cban. Sot. s,
182 (1973); G.R. Fronraaand 0. Binsch,
175 (1973);.Y.B. Jennings,Cha. Rev. 7&‘307 (1975).
%. Parker,J. Cher. Sot. PerkinTrans; II, 83 (1983)and referencestherein. %.A. AlIen, S. Bellard,I4.C.Brice,B.A. Carturigbt,A. Doubleday,H. Riggs, T. -link, 8.0. lhmseUnk-Peters,0. Keamard,W.D.S. Motherwell,J.R. Rodgersand D.G. Watson, Act8 CrybtaIIogr,Sect. B B35, 2331 (1979). 6P. Rurray-Rustin 'Molecular Structureand BiologicdlActivity',eds. J.F. Griffinand Y.L. Duax, Eleevier(19823,p. 118. 7D. Gani, P.B. Hitchcockand D.Y. Young, J. Chew Sot, Chm. Comun. 898 (1983). *D.N.J.White and M.J. Bovill,J. C&I. Sot. Perkin Trans. II, 1610 (1977). 9A.J. Hopfinger,%onforsatiouaIPropertiesof 14acrouoIecules1, Academic Press,
New York <1979),
P. 59. Tonge, unpublishedresults.
l’A.P.
'1V. Sackwildand U.G. Richards,J. Molec. Struct.3,
269 (1982).
Kew York 12G.E.SchuItaand R.H. Schiruer,%%.nciples of ProteinStructure',Springer-VerIag, (19781,p. 21.
ORIGINSOF
lltE PROTON NM CHWCAL
SHIFT
NON-EQUIVALRNCE IN
'IW DIASTFRECTOPIC lIEMyLENEPROTOM OF CAUPRANAIUDES D. PARKERI( and R.J. TAYLOR Departmentof Chemistry,Universityof Durham,South Road, Durham DHl 3LE C. FERGUSON Departmentof Chemistry,Universityof Guelph,Guelph,OntarioNlG 2Wl A. IQNGE Glaxo Group ResearchLtd., GreenfordRoad, GreenfordUD6 ORR. (Roceiredis UK23 October 1985)! of the diastereotopic methylene Abstract- The observedchemicalshift non-equivalence protons in primarycamphanamides is rationalizedin terms of chiralityindependent, differentialshieldingof IiSby the amide carbonyl,as deducedby nmr solution,X-ray structuralstudiesand molecularmechanicscalculations. In proton nmr spectroscopy, the methyleneprotonsat a prochiralcentre in a chiralmoleculeexist in magneticallynon-equivalent environmentsand may exhibitdifferentchsmicalshifts. It was proposedthat the observedchemicalshift non-equivalence, Adobs., is made up of an intrinsic term, A6c, dependentupon the relative diastereotopic shieldingterm, A6i and a confotuational fractionalpopulationand the corresponding individualchemicalshifts of low-energymolecular ' More recentlyit has been shown that the intrinsicdiastereotopic shieldingis conformations. has been insignificant in % nmr.2 The magnitudeof the observedchemicalshift non-equivalence observedto be a functionof the separationof the prochiraland chiral centres,of molecular rigidityand of temperatureand solventi.e. parameterswhich affect the molecularconformation term, A6c.3 In the II!n.m.r. spectraof a series of alkyl camphanamides, la-lf we have found that the --7 pro 2 hydrogenconsistently resonatesto higher frequencyof HR.4 This permitsthe determination of both the enantiomericpurity and the absoluteconfiguration of o-deuteriated primaryamines. The chemicalshifts of the diastereotopic methyleneprotonsin la-f are recordedin Table 1; the pro E and pro 2 protonresonancesare cleanlyseparatedat medium field in d6-benzenesolvent, with the chemicalshift differencebetweenHg and HR varyingbetween0.12 and 0.21 ppm. An example is shown in Figure 1, in which part of the 400 MI% *H n.m.r. spectrumof E
is illustrated. HS and
HR are clearlyseparated. Each proton is coupledto the amide NH and to the adjacentmethylene protonsto give the observedmultiplet.
a)
R - Me
d) R =CH2Et
b)
R -Et
elR=Ph
cl
R -CH2CH2Br
f 1 R = p-Brl’h
In order to probe the originsof the anisochronismof the geminalmethyleneprotonsin la-f, the para-bromobenzyl derivativerf has been examinedin detail. Rxaminationof molecularmodels suggeetsthat the observednon-equivalence of the diastereotopic ethylene protonsis due to the neighbouring&de
carbonylanisotropy.
617
D. PARKER etal,
618
At
=O-21ppm
400 MHz
I ,-.------
8
*---------I
315
3.05
2-95
2.05
Ppm
Fig. 1. 400 wlz Illn.m.r. spectrumof (lS,QR)-(-IN-propylcamphanamide (lJ),showingthe ciiastereotopic methyleneprotons. Table 1. NMR Data for Diastereotopic Methyleneprotonsin -_ la-lfa Compound
R A6RS'k
-la
ne
3.03
2.89
0.14
17.5
b
Et
3.12
2.91
0.21
14.3
-1c Ir!
CR2cH2Br
3.00
2.88
0.12
13.7
n-R
3.11
2.95
0.16
13.6
-le
w
4.25
4.12
0.13
14.8
E
-p-Br-rJh
4.12
3.96
0.16
14.7
a Spectrawere recordedin d6-benzeneat 298 K; chemicalshifts are given in pp and couplingconstantsin Hz. X-Ray StructuralStudies The structureof rf has been detetined by single crystalX-ray diffraction.+ There are two independentmoleculesi and B in the asmtric
unit: these moleculesare linked into A-B pairs
by NH--O hydrogenbonds betweenthe NH group of one moleculeand the lactonecarbonylof another. A view of the two moleculesia shown in Figure 2 with the cry&allographicnumbering scheme. MoleculesA and 2 are conformersrelatedby rotationabout the N-C(ll)bond vith ' Crystalsof &
were grown from CRC13/CC14:hexane (1:l). Crystaldata:- Cl,R20BrN03,Mr =
366.3,monoclinic,space-groupC2, a = 25.454(6),
b = 6.507(J),c = 21.947(7)8,B = ll2.4',U =
3362(2)x,Z = 8, DC = 1.45 g CID-~, F(ooO) = 1504, ~1fn0-K~) = 24.3 cm-'. At convergenceR = 0.064 and Rn = 0.086 for the 1903 observedreflections. The atomic coordinatesfor this work are availableon requestfrom the Directorof the CambridgeCrystallographic Data Centre,University ChericalLaboratories,LensfieldRoad, CambridgeCB2 1EU. Any request should the full literaturecitationfor this comunication.
be accompaniedby
si8ultaneous
reorisatatfonof the pheql
anglesare 96.6,audShefor
torsion ring. The values of the C(1O)-N-G(11)-C(l2)
the A and B moleculesrespectively.Tbe bromopbmyl riny, are
ariestedabout the G(ll)412) bonda such that the N-C(li)-C(12)-4(13) torsiohau&xi are 31.1 and 325.T"forA and B respeotively:
Fig. 2. Molecularetructuresof the two iwlemdeut molecules,_3: (left)and B (right). The relativeorientationof the bicycllcriu~ is the same iu both casee. In molecule_ir, the pro S hydrogenis closer than HK to the magueticallyanisotropiccarbouyl group; in uolewle 2 the situationis reversed. Rirthexmore,HS ie closer to the aside carbonyl
SolutionW.a.r. Studies In the 'Iin.m.r. spectrum of Lf in d6-beuzeaeat 298 K, two doubletof doubletsMY be observed, ceutredat 64.11 and 3.95 pm, correflponding to Hs and HK respectively.The chemicalshift differencebetuem HS aud HK iucreaeeelinearlywith decreasiugtemperature,(Table21; Mar where h6 in 0.12 pgnnat 320 K aud r&see linearlyto 0.15 ppm at V#t 283 K. The chemicalshift differenceis indepoudeutof tide concentratiou, precludiuSauy effects
behaviouris observedwith 2
due to intemolecularassociation. Carefulexawinatiouof the temperaturedepeudenceof MI, SHR revealsthat as the taaperatureis lowered,it is Hs that shifts to h&beer frequencyrelativeto I$ and other resouauceeiu the molecule. Such MaHour is cousisteutwith HS spendingsore tti, on average,iu a regneticallydeshieldieKeuvironmmt i.e. proximateto the amide carbonyl. The observedanisochronisa~ is also ssnsitiveto the n.m.r. solventused. With lf, ad nSKKta (Table2). Sinilarbahaviourvasobserr~vfth~,vithrhich A8 at 308 K is 0.03 ppa in CDC13, 0.05 pp KS% in @X4, 0.06 pps in d6-acetoneand 0.15 ppm in c6h6' The observatiouthat Mobs, is a m&.mum in aromaticsolventssu&~wts that one of the aromaticsolventmoleculeslien cloee to the amide group. maximumwith non-polararomaticsolventsand is lower in polar, aliphaticeolwts,
Some form of ‘we&* *-rcCinteraotionbstueeathe aromaticv cloud and the carbonyldouble bond may be
620
D. Pm
et sl.
Table 2. Variationof Abobs with Terperatureand Solventa . TeiperatureK;
A6
ppm
Solventb; A6
%AR 320
0.137
C6D6
0.15
310
0.148
‘7D8
0.14
300 295
0.162 0.171
zN
Z'Z .
zgo
0.177
3 (CD3)2C0
285
0.185
GD30D
0.07 0.06
a%corded at 200 Hlz. ' At 308 K. involved,enhan*
the anisotropicenvironmentexperiencedby IIsa&l&.
The possibilitythat the amide N-H may form an intramolecular hy&ogen bond with the lactone ether oxygen in non-polarsolvents,at the concentrations used in the-a.m.r.experiments,was considered. However an FT Lr. study in Ccl4 and C6D6 revealedthat at cemoemtrations of -If of -1 1o-2 Jland below, only a mingle sharp band at 3435 cm was observed. Ibis correspondsto the free N-H stretch,and no other bands were observed,in this region,over the concentrationrange -2 10-s to 10 w. At higher oencentrations uhiehgrew as a a broad band appearedat 338O,cm-';' functionof increasingcoaoe&rationand is assignedto an intermolecular hydrog&.bondedNH
.’
tion, hydrogenbondingbetweemthe tide KH of one stretch. In the crystalstructuredetermina moleculeand the lactoneoarbonylof anotherwas revealed. Such interactionsalso appear to occur in solutionat higher concentrations. MolecularMechanicsCalculations Using a model of the camphanamide constructedfrom fragmentstaken from the Cambridge 5 molecularmechanicsconformationenergy calculationswere performedon Crystallographic Database, -If. The variationin molecnlarpotentialenergy with changesin torsionangle was calculatedby the eumrationof the separatecomponentsfor non-bonded,torsionaland electrostatic energies, (furtherdetailsare in Kxperimental).In Figure 3 the variationin molecularpotentialenergy with changesin the C(lO)-N-C(ll)-C(12) and N-C(ll)-C(lZ)-C(l3) torsionangles is illustrated. The contourlines are in energyunits of kcal mol-'. Comparisonwith the observedtorsionangles for moleculesi and B, revealedby the crystalstructure,shows good agr-t.
MoleculeA is
the lower energy conformer,residingin the bottom left hand cornerof the energy plot, with B in the top right hand corner. Ibe calculatedenergy differencebetween1 and B is 1.3 kJ 1101-l. The local energy minima are calculatedto be within 4 kJ mol-' of the absoluteenergiesfor the observedcrystalconformations, that is, within the limit of the largestprobabledistortionof a 6 small moleculeconformer,in the crystallinestate, from the minimumenergy form. Furthermore, it is well establishedthat any conforrpation observedin the crystallinestate is likely to be importantin solution.6 Confolrerh is the lower energy state and in solutionas the temperature is lowered,vi11 be preferentially populatedover B. Conclusions The studies relatedabove confirmthat the observedchemicalshift nonequivalenceof the gcainalsethyleneprotonsin camphanamides, 1, is due to differentialshieldingof the diastereotopic methyleneprotonsby the anisotropicamide carbonylgroup. In the preferredconforsler 4, the pro S hydrogenis closer on averageto the amide carbonylgroup and hence resonatesto higher frequencyof I$. As the temperatureis loweredthis differentialshieldingis more pronouncedas conformer&is preferomtially populated. Given that HS consistentlyresonatesto higher frequency of liKin all studiedcaqhanamides,4'7the rationaleput forwardmay afford a generalexplanation for the originsof the observedgeminalproton anisochronism in camphanamides and relatedcompounds.
fig 3. Mofolecular potentialenergymap as a functionof the C(lO)-Nap C(l3) torsionangles. Contoursare iu I kcal mol" steps.
and N-C(lL)-C(lz)-
RXPRRIRRNTAL $i n.m.r. spectrawere xwcordedon either a RrukerWR360,a RrukerW200 or a Rruker WR@C
._
iusttaeot. Roleculwqecbanics coufomatioualehergycahxlat$onsuere Researchmolecularmodellingsysteqruuuiugon a VAX 11-750
performedontbeClax0
miuicomputercoupledto Rogatekand
Sigma displayteminsls. The force-fieldcalculationsused standardRuckingbasand single-texm 8 cosine potentialfunctions for the non-bondedand torsionalenergies,respectively, and the electrostatic euergy was calculatedfrom a Cqulombicpotentialusiug a distance-dependeut dfelectricters.9 The associatednon-bondedand torsionalforce-fieldparameterswere setlo to reproducein 'rigid-rotor' models (i.e.withoutfull relaxationof moleculargeometry), experireutaltorsionalbarriersin mall mole~ules~~and the observeddistributionof +, Gaugles iu peptideaand proteins. l2 A standardset of partialchargeswers placed on those atoms iuyolved in potentialhydrogen-bonding groups. The crystalstructurewas detemiued by the heavy atom method and refinedby full matrix lesst-squares calculation& All hydrogenatoss were located from the differencemaps and includedbut not refiuedin the calculations.Allowanceno8 made for the auomalouedispersionof brosine. Reflectionswere seaswed with a CAD4 diffractometer and all calculationswere made with the SDP system of programson a PDP-11/73computersystar. The cbiral anides -la-le were preparedaccordingto literatureprocsdures,4and E preparedas describedbelow.
was
622
D. PtuKKu 8l ai.
para-Brosobensylauinarine hydrochloride(227 ug, 1 nmol) was suspwded in dichlorcsethane (7 cn3) at O'C, and trietJ#anine (150 mg, 1.5 wl)
and (-)-casphanoyl chloride(241 sg, 1.1 roll added
in sequaice. The sixturewas stirredfor 3 h at O'C, then poured in sodim hydroxidesdlution (0.1 H, 50 cr3) and the aqueousphase extractedvith dicbloraatbane(3 x 10 cu3). The organic phase was washed with dilutehydrochloricacid (0.1 U, 2 x 20 cu31, water (2 x 25
cm3),
dried over
anhydroueuagne&us sulphate,filtered,and solventraao~eduuder reducedpressureto give a colourlesssolid (220 sg, 60$) which may be recrystallissd fros chloroform-carbon tetraclrloride (1:l)and tmxane,up 126-127°C.'Found: C 55.6, Ii5.28, N 3.61. C17R20BrN03requiresC 55.7, H 5.46, N 3.82. $
(C6D6)6.77 (lR, br, NHCO), 4.12 (18, dd, I$,>,3.96 (lE, dd, RR, J 14.71, %% 2.30 (Hi, u), 1.56 (lH, a), 1.33 (LH,R), 1.25 (Hi, n), 0.83,,0.82(3R + 3H, s,+ s, )b2C), 0.69
(3% s, C&f. s/t 367/365CR+,, 186/184,149, 136, 109, 83. .v(KBr)3380 (H-bondedR@, 2960, 2920, 2860 (III), 1780 (rs), lactoneCO), 1675 (vs, RRCO), 1530, 1175. h Ltd. for n CASE studentshipand - We thank SFXC and Glaxo Group Researc Acknovledgeuent Dr. I. Sadler (vniversityof Kdinburgh)for high-fidldn.u.r. spectra. GF thanksNSPX! (Canada) for continuingsupportvia operatinggrants.
1li.S. Gutowsky,J. C&u. fhys. g, 2196 (1962). 2P.J. Stiles,Chea. Pays. Lett. a, 23 (1976). "R.D. Norris and G. Binsch,J. Aaer. Chem. Sot. 9& J. Aser. Cban. Sot. s,
182 (1973); G.R. Fronraaand 0. Binsch,
175 (1973);.Y.B. Jennings,Cha. Rev. 7&‘307 (1975).
%. Parker,J. Cher. Sot. PerkinTrans; II, 83 (1983)and referencestherein. %.A. AlIen, S. Bellard,I4.C.Brice,B.A. Carturigbt,A. Doubleday,H. Riggs, T. -link, 8.0. lhmseUnk-Peters,0. Keamard,W.D.S. Motherwell,J.R. Rodgersand D.G. Watson, Act8 CrybtaIIogr,Sect. B B35, 2331 (1979). 6P. Rurray-Rustin 'Molecular Structureand BiologicdlActivity',eds. J.F. Griffinand Y.L. Duax, Eleevier(19823,p. 118. 7D. Gani, P.B. Hitchcockand D.Y. Young, J. Chew Sot, Chm. Comun. 898 (1983). *D.N.J.White and M.J. Bovill,J. C&I. Sot. Perkin Trans. II, 1610 (1977). 9A.J. Hopfinger,%onforsatiouaIPropertiesof 14acrouoIecules1, Academic Press,
New York <1979),
P. 59. Tonge, unpublishedresults.
l’A.P.
'1V. Sackwildand U.G. Richards,J. Molec. Struct.3,
269 (1982).
Kew York 12G.E.SchuItaand R.H. Schiruer,%%.nciples of ProteinStructure',Springer-VerIag, (19781,p. 21.