edited bv JOHNJ. ALEXANDER
exam queftion exchange A Quizon Stereochemistry Abhlk Ghosh Jadavpur University Calcuna 700 032, India' The following problems on organic stereochemistry are in the form of an open-book quiz meant for senior undergraduates and graduate students in chemistry. Questions 1. A molecule of D-glucitol has two -CH20H (hydroxymethyl) groups, each of which may be oxidized t o a -CHO group leaving the other -CHzOH intact. Thus D-glucitol may be oxidized to D-glucoseas well as to the rare sugar L-gulose. This is nothing but a consequence of the diastereotopicity of the -CH20H groups in D-glucitol. Similar logic holds for Lglucitol. Thus the topicity of the -CH20H groups in alditols has some synthetic i m p o r t a n ~ e .Unlike ~ D-glucitol, many alditols cannot be oxidized to a pair of diastereomeric aldoses. In fact there happens to he just one other pair of enantiomeric aldohexitols which resembles D- or L-nlucitol in having diastereoptic -CH20H groups. Identify thispair of aldohexitols. Look UP any organic text if vou do not remember the stereostruct&e df the aldohexosei. 2. One of the first trulv catalvtic and efficient enantioselertiwnlkylariunas mpdiated hy n chiral phnir transfer mtalvst rPTC) was verformrd on the indunone svsttm I (Fir. 1 J. Here the OH- ion (which actually generates the indanone enolate anion) was transferred from the aqueous phase to a nonpolar phase like toluene by the chiral PTC I1 (Fig. 2). The benzylcinshoninium cation of I1 sat down preferentially on one of the two enantiotopic faces of the indanone enolate anion to give the species 111 (Fig. 3), leaving the other face open for enantioselective alkylation. Considering the charge transfer interactions can easily occur between the two overlapping benzene rings in I11 (see Fig. 3), a Hammett plot of ee/eeo versus the substituent constant o values of the R
University of Cincinnati Cincinnati. OH 45221
group (see Fig. 4) was made to determine the electronic effects of R. Here eeo stands for the enantiomeric excess in the product when R = H. Can you say which of the Hammett plots shown in Figure 4 is the most plausible for this system?"t might help if you think what kind of R ought to stabilize the species 111.
Figure 1. An ensntioselectiveaikylatian mediated by a chlral PTC.
Figure 2. A benzylcinchoninium bromide system that is now a commercially available (Aldrich)chiral PTC.
P\
-- - -
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Address all correspondence to the author to 5912A Pratapaditya Road, Alaka Flat B 44, Calcuna 700 026. India. Hanessian. S. Total Synthesis of Natural Products: The "Chiron" Approach; Pergamon: London. 1983; pp 15-16. Dolling, U-H.; Davis, P.; Grabowski. E. J. J. J. Am. Chem. Soc. 1984, 106,446.
Figure 3. ton pairing between the benzylcinchoninium cation and the indanone anion.
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Figure 5. A chird mown ether based on me (~,-1,l'-bi-2-naphthoIsystem.
Top-side a t t a c k with 0-(-1- t a r t r a t e s
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Figure 4. Possible Hammen plots. 3. Five molecules A, B, C, D, and E belong to point groups C3, C3", 0 3 , D3h, and Dad, respectively. Of course none of these molecules is "asymmetric" in the sense of being totally devoid of svmmetrv: thev all oossess a threefold nroner . . axis of symmetIy. Can& identify which (if any a t all) among these five molecules islare chiral? 4. How efficientlv a crown either solvates an alkali metal carbon cation depends on how closely the latter fits into the electron-rich cavitv of the crown svstem. For examole. an 18crown-6 system whose cavity diameter is about 280-320 pm very effectively solvates cations whose ionic diameters (twice the ionic radii) also fall in this range. Hence, which of the following four substances in aqueous solution is more likely to be resolved by the chiral crown either shown in Figure 5?'
l i o t t o m side attack with L-(+)-tartrates Figure 6. The shucture of geraniol
altritol and D-talitol are one and the same compound, just like Dglucitol and L-gulitol. 2. The nlot . (dl . is the most ~lausiblesince onlv this has a o ~ i t i v e shpe cc,rrcsponding to > U. This n p w s u,it h our exp~rtnrionthat incrpas~nglyrlrctn,n.u~irhdra~r ing R groupsshould rerulr in greatrr stal,il~zatmnot the rperles I l l by increasing fhe attra~ti\.echorgetransfer interactions between the overlapping benzene rings and thereby give greater ee's. The experimental value of p is 0.21 f 0.02. 3. The necessarycondition for chiralityis that a molecule lack an S, axis. Only A and C lack a (83,and ~(SI) or an S, (n > 2) and are therefore chiral. 4. Here it may appear at a first glance that I(+)-CHB-CHOHCOO-IKf is the most likely to be resolved, since crown-6 systems solvate Kt ions really very effectively. The student might easily overlook the fact that chirality in this ease is restricted to the anion,
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5. Prominent among all stereodiscriminatory reactions developed within the last decade is the Sharpless epoxidation of an allyic alcohol by a mixture of t-butylhydroperoxide, titanium tetraisopropoxide, a solvent like CH2CI2,and either (+)- or (-)-diethy1 tartrate. Which particular prochiral face of the double bond inthe allvlic alcohol will he epoxidized preferentially can be predicted from a knowledge of the chiralilty of the tartrate ester used, and very often the nature of the substituents present on the alylic alcohol system need not be considered. The rule for predicting this stereoselection is that when the double bond of the allylic alcohol is in the plane of the drawing (Fig. 6) with the -CHsOH substituent a t the lower right, the use of (-)-diethyl tartrate leads to addition of the epoxide "0"from the top, while the use of (+)-diethy1 tartrate leads to epoxidation from the bottom. Use this rule to determine the absolute configuration (in the R S svstem of nomenclature) of the major 2,3-epox~alcoholproduct obtained by sharpleis epoxidation of geraniol (Fig. 7) when the tartrate ester used is Acceptable Solutions I . The ~na~niumerir pair UI.-nltrirolresembles vL.glurirol in hmin:: diasrerrotupic -CHIOH groups. Thus 1,-nltritol is ohtnined by S n H H , reduc~imot either I)., +I-altrosen-I+,-talme. So 1)Stoddart, J. F. h g . Macrocyclic Chem. 1981, 2,173-250.
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whjch is not solvated by the crown ether. Only [(*IPhNH3CHCOOMelCI- has achiral cation and is therefore the most likely to be resolved by the ehiral crown ether. Crown-6 systems are well known as masking agents for potassium ions. An ordinary NH4+ion has about the same crystallographic radius as a Ki ion and so Kt and NHli ions may be used interrhonpnbg in man) chemical sttuntions. Herr, howewr, we are ronwrnrd nith a st~h%r~tuted ammonium ion of the type I1KH t. Smce R i~obvic,uslsrw, lnrae. i t i s m h rhc .NH
