10,2-Camphorsultam1

(-)-D-(2R)

[94594-90-8]  · C10H17NO2S  · 10,2-Camphorsultam  · (MW 215.31) (+)-L-(2S)

[108448-77-7]

(versatile chiral auxiliary: N-enoyl derivatives undergo highly stereoselective [2 + 4] Diels-Alder2 and [2 + 3]3 cycloadditions, cyclopropanations,4 aziridinations,5 dihydroxylations,6 hydrogenations,7 azido-iodinations8 and conjugate hydride,9 Grignard,10 cuprate,11 allylsilane12 and thiolate13 additions; radical additions14 and SN2 reactions15 at the a-position also occur stereoselectively; enolates of N-acyl derivatives participate in highly stereoselective aldolizations,16 alkylations,17 halogenations, and aminations, the latter three types of reactivity being useful for a-amino acid preparation;18 free radicals generated at the a-position of N-acyl derivatives participate in stereoselective intra- and intermolecular addition reactions;19 the N-fluoro derivative functions as an enantioselective, electrophilic fluorinating reagent20)

Alternate Name: bornane-10,2-sultam.

Physical Data: mp 183-185 °C (EtOH). (-)-D-(2R) enantiomer: [a]D20 -31±1° (CHCl3, c 2.3). (+)-L-(2S) enantiomer: [a]D20 +34±1° (EtOH, c 1.00).

Form Supplied in: white crystalline solid; both enantiomers are commercially available (~same price) or may be readily prepared (3 steps, >70% overall yield) from 10-Camphorsulfonic Acid.21

Handling, Storage, and Precautions: stable indefinitely at ambient temperature in a sealed container; mild irritant.

Introduction.

Exploitation of chiral auxiliary controlled face discrimination in the reaction of a reactant with a prochiral molecule or functional group is a powerful strategy in asymmetric synthesis.22 Clearly the choice of auxiliary for a desired chemical transformation is crucial for optimal synthetic efficiency. Hence, the ease with which the auxiliary can be introduced, the extent of stereoselection it imparts to the desired transformation, and the ease of its nondestructive removal are of critical importance. The 10,2-camphorsultam not only meets these criteria for a range of transformations, but also generally imparts crystallinity to all derived intermediates, thereby facilitating purification and isolation of enantiomerically pure products. Indeed, 10,2-camphorsultam derivatization alone allows for facile crystallographic determination of absolute configuration.23

Preparation of Derivatives.

N-Acyl- and N-enoylsultam derivatives are routinely prepared in good yields using either sodium hydride-acid chloride16a or trimethylaluminum-methyl ester18g single-step protocols. A variant of the former method employing in situ stabilization of labile enoyl chlorides with CuCl/Cu has also been reported.3k A two-step procedure via the N-TMS derivative (1) is useful when a nonaqueous work-up is desirable and for synthesis of the N-acryloyl derivative.24 N-Enoyl derivatives may also be prepared via the phosphonate derivative (2) by means of an Horner-Wadsworth-Emmons reaction (eq 1).2c,2d

An N-acyl-b-keto derivative has been prepared by reaction with a diketene equivalent17b and the trans-N-cinnamoyl derivative by a Heck type coupling reaction.4 The N-fluoro derivative (3) is prepared by direct fluorination (eq 2).20

Reactions of N-Enoyl Derivatives.

[4 + 2] Diels-Alder Cycloadditions (Alkene -> Six-Membered Cycloadduct).2

N-Enoylsultam derivatives were originally devised as activated chiral dienophiles for stereoselective Diels-Alder reactions.1,2a

Thermal reactions of N-enoylsultams generally show only moderate endo and p-face selectivity, e.g. N-acryloyl- and N-crotonoyl-10,2-camphorsultams (4) and (6) with cyclopentadiene (eq 3, Table 1).2g The thermal hetero-Diels-Alder reaction of N-glyoxaloyl-10,2-camphorsultam with 1-methoxybuta-1,3-diene also proceeds with moderate exo and p-face selectivity (57% exo, 46% de).2h Thermal hetero-Diels-Alder reactions of N-acylnitroso-10,2-camphorsultam with cyclopentadiene and 1,3-cyclohexadiene, however, proceed with excellent selectivity (>98% ee, p-face selectivity not established).2i

Lewis acid-mediated reactions of N-enoylsultams, on the other hand, occur under very mild conditions and with high levels of endo and p-face selectivity (eq 3, Table 1).2b,2g Dicoordinate TiCl4, EtAlCl2, and Me2AlCl are particularly effective and their role in the stereodifferentiating process, which results in almost exclusive C(a)-re face dienophile attack, has been rationalized.2g Both inter- and intramolecular reactions proceed well even on a preparative scale (e.g. >100 g), often requiring just a single recrystallization to furnish isomerically pure products, valuable as synthetic intermediates (eq 4, the key step in a synthesis of (-)-pulo'upone).2d The hetero-Diels-Alder reaction of N-glyoxaloyl-10,2-camphorsultam with 1-methoxybuta-1,3-diene also proceeds efficiently and with high endo and p-face selectivity in the presence of 2% Eu(fod)3 (90% endo, 88% de).2h These levels of asymmetric induction compare very favorably with those obtained using alternative auxiliaries (see Related Reagents below) for most substrates.

[3 + 2] Cycloadditions (Alkene -> Five-Membered Cycloadduct).3

The levels of selectivity found for 1,3-dipolar cycloaddition reactions are not as high as those obtained for Lewis acid-catalyzed Diels-Alder reactions. However, the 10,2-camphorsultam auxiliary can achieve synthetically useful levels of induction in these reactions, and this has been attributed to efficient enoyl conformational control by the sultam moiety leading to preferred C(a)-re face attack even in the absence of metal complexation.1d

The reactions of N-enoyl-10,2-camphorsultams with various nitrile oxides to give isoxazolines have been well studied.3a-c Indeed, the high regioselectivity and high p-face selectivity (62-90% de)3a observed in reactions with the N-acryloyl compound (4) have been exploited in synthesis (eq 5, the key step in a synthesis of (+)-hepialone3b), although related toluene-2,a-sultam auxiliaries provide still higher selectivity (see a-Methyltoluene-2,a-sultam). Isoxazolines may also be obtained by regioselective and similarly p-face selective cycloadditions of silyl nitronates followed by acid catalyzed elimination of TMS alcohol3d-f (eq 6, the key step in a synthesis of (+)-methylnonactate).3f A cyclic, photochemically generated azomethine ylide also participates in exo and p-face selective 1,3-dipolar cycloaddition with (ent-4), a reaction for which alternative auxiliaries were significantly less effective (eq 7, the key step in a synthesis of (-)-quinocarcin).3g-i

Nickel catalyzed [3 + 2] cycloadditions of methylenecyclopropane and 2,2-dimethylmethylenecyclopropane with (4) afford 3-methylenecyclopentane derivatives with extremely high p-face selectivities (91% and 98% de respectively); five alternative auxiliaries were found to be less effective.3j,3k Palladium catalyzed [3 + 2] cycloaddition of 2-(TMS-methyl)-3-acetoxy-1-propene with an N-enoylsultam, however, proceeds with disappointing selectivity (4-26% de).3l A norephedrine derived auxiliary ((4R,5S)-4-methyl-5-phenyl-2-oxazolidinone) was similarly ineffective in this instance.3l

Cyclopropanation and Aziridination (Alkene -> Three-Membered Cycloadduct).4,5

Cyclopropanation of various trans-N-enoyl derivatives using diazomethane with Pd(OAc)2 as catalyst affords cyclopropyl products with good C(a)-re p-facial control (eq 8).4 Similarly, aziridination with N-aminophthalimide-lead tetraacetate affords N-phthalimidoaziridines with variable but generally good p-face selectivity (33-95% de).5

Dihydroxylation, Azido-Iodination and Hydrogenation (Alkene -> a,b-Addition Product).6-8

syn-Dihydroxylation of b-substituted N-enoylsultams using N-methylmorpholine with a catalytic amount of OsO4 affords vicinal diol products with good C(a)-re p-facial selectivity (80-90% de) (eq 9, the key step in a synthesis of (+)-LLP 880b).6a Similar levels of selectivity but lower chemical yields are obtained using KMnO4 and N-dienoylsultams.6b Regioselective but poorly stereoselective trans addition of iodine azide to N-crotonoyl- and N-cinnamoyl-10,2-camphorsultams has also been reported (34% and 47% de, respectively). The sense of addition corresponds to iodonium ion formation from the C(a)-re face followed by SN2 attack of azide at the b-position.8 Heterogeneous syn hydrogenation of b,b-disubstituted enoylsultams over Pd/C using gaseous hydrogen (100 psi) affords reduced products, again with excellent C(a)-re topicity (90-96% de).7

1,4-Hydride, Grignard, Cuprate, Allylsilane, and Thiolate Addition (Alkene -> b- or a,b-Functionalized Product).9-13

b,b-Disubstituted enoylsultams undergo efficient reduction with L-Selectride®.9a The syn hydrogenated products obtained result from conjugate hydride delivery (and protonation) on the opposite p-face [i.e. C(a)-si] to that from hydrogenation (90-94% de) (eq 10).9b Similarly, simple alkylmagnesium chlorides also undergo 1,4-addition-protonation with trans-b-substituted enoylsultams from this face (72-89% de).10 Use of a-substituted N-enoyl substrates,9a or trapping of the intermediate aluminum or magnesium enolates with other electrophiles, allows creation of two asymmetric centers in one synthetic operation.9a,10 The observed topicity is that of syn addition from the C(a)-si face. As PBu3 stabilized alkylcopper reagents,11a,11b Grignard reagents (in the presence of copper salts),11c and cuprates (Gilman reagents)11d participate in analogous reactions but show reversed p-face selectivity, an appropriate 1,4-addition-trapping protocol can be devised to generate products with any desired configuration at both the a- and b-positions (eq 11).11c This complementarity has been rationalized.11 Phosphine stabilized alkyl- and alkenylcopper reagents also add to N-(b-silylenoyl)sultams (giving aldols after C-Si oxidative bond cleavage). In this case, either p-face selectivity can be achieved, depending on the promoting Lewis acid employed.11b Similar Lewis acid dependent selectivity is observed for addition of allyltrimethylsilane to N-enoylsultams.12

Stereoselective anti addition of thiophenol to N-[b-(n-butyl)methacryloyl]-10,2-camphorsultam [the key step in a synthesis of (+)-trans whiskey lactone] has been explained by a sulfur-induced, stereoelectronically directed protonation following C(b)-re face conjugate addition.13

Radical Addition and SN2 Displacement (Alkene -> a-Functionalized Product).14,15

Stereoselective radical additions to N-enoylsultams occur at the a-position, while additions to the b-position are essentially nonselective.1d,14 The SN2 displacement of g-bromo-N-enoylsultams with higher order cyanocuprates occurs with good p-face selectivity (90-96% de).15

Reactions of N-Acyl Derivatives.

Aldolization (Acyl Species -> b-Hydroxyacyl Product).16

Chiral oxazolidin-2-ones and 10,2-camphorsultams presently represent state of the art aldol reaction mediators. Both auxiliaries have similarly high p-facial preferences (totally overwhelming any modest facial preference of most chiral aldehydes), allowing the predictable formation of essentially one (of four possible) diastereomeric aldol type products by judicious choice of auxiliary antipode and reaction conditions.16a Although sultam mediated aldolizations generally require a 2-3 fold excess of aldehyde to go to completion (cf. 1-2 equiv when oxazolidin-2-one mediated), which is clearly wasteful when employing a valuable aldehyde, the superior crystallinity and cleavage properties of the sultam adducts makes the choice of auxiliary for a given aldolization dependent on the specific substrate.

syn-Aldols with (R) configuration at the a-position are obtained from boryl enolates of (-)-10,2-camphorsultam derivatives (8) on condensation with aldehydes.16b The observed topicity is consistent with C(a)-si/C=O-re interaction of the nonchelated (Z)-enolate and the aldehyde.16b syn-Aldols with (S) configuration at the a-position are obtained from lithium (BuLi-THF) or better tin(IV) enolates of the same derivatives (8), and this outcome is consistent with C(a)-re/C=O-si interaction of the chelated (Z)-enolate and the aldehyde.16b anti-Aldols with (S) configuration at the a-position are obtained from in situ prepared O-silyl-N,O-ketene acetals of sultams (8) on condensation with aldehydes in the presence of TiCl416c (Mukaiyama aldolization). This topicity arises from C(a)-re/C=O-re interaction of the (Z)-N,O-ketene acetal and the Lewis acid coordinated aldehyde (eq 12).16c These same anti-aldols can also be obtained from sultams (8) with similarly excellent stereocontrol using boryl enolates in the presence of TiCl4, and this unique procedure is the method of choice when using crotonaldehyde or methacrolein.16d anti-Aldols with (R) configuration at the a-position should be obtained from sultams (ent-8) using the above Mukaiyama conditions. Enantiocontrolled synthesis of a-unsubstituted b-hydroxy carbonyl compounds from the N-acetyl derivative is best accomplished using the Mukaiyama conditions (58-93% de).16e The synthesis of beetle sex pheromone (-)-serricorole serves to highlight the power of the above methods.16f,16g

Alkylation (Acyl Species -> a-Alkylated Acyl Product).17

An efficient procedure for the C(a)-re alkylation of lithium and sodium enolates of N-acylsultams with various (even nonactivated) primary halides in the presence of HMPA has been developed (88.7-99% de).17a Alkylation with ClCH2NMeCO2Bn enables a two-step b-lactam synthesis.1c,17a Michael-type alkylation of a b-keto derivative with arylidenemalononitriles in toluene containing piperidine has been reported to give 4H-pyrans (60-70% de).17b

a-Amino Acid Preparation.18

Three distinct strategies for the asymmetric preparation of a-amino acids using the 10,2-camphorsultam auxiliary have been developed. The first is a glycine anion strategy18a centered on alkylation, with excellent C(a)-si p-face stereocontrol, of lithium enolates of sultam derivative (9a) [mp 107-109 °C (EtOH)]18b,18c to give adducts (10) (eq 13). a-Amino acids are obtained simply by Schiff base hydrolysis (0.5N HCl, rt) and auxiliary cleavage (LiOH, aq THF). Compound (9b) has also been reported to participate in analogous chemistry,18d but it is not crystalline and its derivatives require more vigorous hydrolysis. Commercially available (9a) is thus the preferred reagent, comparing favorably with other glycine anion synthetic equivalents (see 4-t-Butoxycarbonyl-5,6-diphenyl-2,3,5,6-tetrahydro-4H-oxazin-2-one). Promising preliminary results of deprotonation-alkylation of (9a) under phase transfer catalysis have also been disclosed.1c,18c,18j

The second strategy involves a bromination-azide displacement-hydrogenolysis protocol. Treatment of boryl enolates of N-acylsultams (8) with NBS provides the key a-bromo derivatives (11) with good C(a)-re topicity (eq 14).1c,18b Stereospecific substitution with tetramethylguanidinium azide [(Me2N)2C=NH2+N3-], hydrogenolysis (H2-Pd/C), and auxiliary cleavage provides a-amino acids in good overall yield.1c,18b As with the previous strategy, given that an appropriate derivative is crystallized to enantiomeric homogeneity, the enantiomeric purity of the product will reflect the extent of racemization during auxiliary hydrolysis (e.g. phenylglycine: 90.3% ee, isoleucine: >99% ee).1c This problem can be circumvented by the use of nonbasic Ti(O-i-Pr)4 assisted transesterification with allyl alcohol then rhodium-catalyzed deprotection. This allows for the preparation of either free or N-Fmoc a-amino acids of excellent enantiomeric purity.18e

The third strategy involves electrophilic amination of sodium enolates of N-acylsultams (8) using 1-Chloro-1-nitrosocyclohexane as an [NH2+] equivalent.18f-i The reaction proceeds via nitrone intermediates which are routinely hydrolyzed without isolation to give the key a-N-hydroxyamino derivatives (13) with outstanding C(a)-re p-facial control (eq 15). Nitrogen-oxygen bond hydrogenolysis (Zn, aq HCl, AcOH), then auxiliary cleavage, affords a-amino acids.18f,18g Omission of the hydrogenolysis step allows access to N-hydroxy-a-amino acids, which are extremely difficult to prepare by alternative means.18f The scope of the reaction has been extended to encompass the use of 1-chloro-1-nitrosocyclohexane as an electrophilic partner in conjugate addition-trapping reactions [allowing an expedient preparation of (2S,3S)-isoleucine],18f,18g N-alkyl-a-amino acid preparation,18h and homochiral a-substituted cyclic nitrone formation [giving a concise preparation of the piperidine alkaloid (-)-pinidine].18i

a-Radical Addition (Acyl Species -> a-Functionalized Acyl Product).19

Radicals derived from a-iodo-N-acylsultams give high levels of asymmetric induction in intramolecular addition reactions with allyltributylstannanes (85->94% de)19a and 5-exo-dig type cyclizations and annulations (eq 16).19a In addition, zipper type manganese(III) promoted oxidative radical cyclization of N-(trans-4-methyl-4,9-nonadienoyl)-10,2-camphorsultam gives a cis-fused hydrindane derivative with modest (50% de) selectivity at the a-center.19b All these reactions proceed at or above room temperature, making the levels of induction remarkable. Furthermore, effective alternative auxiliaries are scarce.1d,14

Nondestructive Auxiliary Cleavage.

One feature which makes the sultam chiral auxiliary, and to an even greater extent the related toluene-2,a-sultam auxiliaries (see a-Methyltoluene-2,a-sultam), so versatile is the ease with which N-acyl bond fission occurs in derivatives. A great variety of extremely mild, bimolecular and intramolecular nondestructive cleavage protocols have been developed which tolerate a wide array of molecular functionality, simple extraction and crystallization usually providing almost quantitative auxiliary recovery without loss of enantiomeric purity.

Saponification with LiOH6a or H2O2-LiOH16b in aqueous THF is routinely employed for conversion of N-acylsultams to enantiomerically pure carboxylic acids. A variant conducted in aprotic media with phase transfer catalysis has also been reported.18d If base sensitive functionality is present, then the corresponding esters can be prepared by nonbasic titanium mediated alcoholysis. This can be accomplished with ethyl,11b benzyl,4 or allyl18e alcohols, and in the latter two instances the carboxylic acids can be subsequently liberated by neutral hydrogenolysis or RhCl(PPh3)3 catalyzed hydrolysis,18e respectively. Lactones and esters can also be formed by intra-18j and intermolecular2d sultam cleavage with lithium alkoxides and bromomagnesium alkoxides.11d b-Lactams can be prepared by intramolecular ring closure of metallated b-aminomethyl derivatives1b,17a and an aluminum thiobenzyloxy ate complex has been used to obtain thioester derivatives.13 Reductive cleavage of N-acylsultams using lithium aluminum hydride2f or L-Selectride®3b,3f in THF gives rise to the corresponding primary alcohols.

Auxiliary cleavage with concomitant carbon-carbon bond formation is a particularly attractive option, which has been demonstrated in a bimolecular sense using the dianion of methyl sulfone (giving a methyl ketone),16f and in an intramolecular sense using a Claisen-type condensation of a b-acetoxy enolate (giving a d-lactone).25 An interesting halolactonization procedure has also been devised; for certain a-aryl-bis-(g-unsaturated)-N-acyl derivatives this allows for highly efficient auxiliary cleavage and asymmetric formation of two stereocenters, one of which is quaternary (eq 17), the key step in a synthesis of (-)-mesembrine).26

Enantioselective, Electrophilic Fluorination.

(-)-N-Fluoro-10,2-camphorsultam (3) [mp 112-114 °C (CH2Cl2-pentane)] is an enantioselective, electrophilic fluorinating agent.20 Fluorination of stabilized enolates occurs with highly variable yield (5-63%) and stereoselectivity (10-70% de).

Related Reagents.

10-Dicyclohexylsulfonamidoisoborneol; 2-Hydroxy-1,2,2-triphenylethyl Acetate; (4S,5S)-4-Methoxymethyl-2-methyl-5-phenyl-2-oxazoline; a-Methyltoluene-2,a-sultam; (S)-4-Benzyl-2-oxazolidinone.


1. (a) Oppolzer, W. T 1987, 43, 1969. (b) Oppolzer, W. PAC 1988, 60, 39. (c) Oppolzer, W. PAC 1990, 62, 1241. (d) Kim, B. H.; Curran, D. P. T 1993, 49, 293.
2. (a) Oppolzer, W. AG(E) 1984, 23, 876. (b) Oppolzer, W.; Chapuis, C.; Bernardinelli, G. HCA 1984, 67, 1397. (c) Oppolzer, W.; Dupuis, D. TL 1985, 26, 5437. (d) Oppolzer, W.; Dupuis, D.; Poli, G.; Raynham, T. M.; Bernardinelli, G. TL 1988, 29, 5885. (e) Smith III, A. B.; Hale, J. K.; Laahso, L. M.; Chen, K.; Riera, A. TL 1989, 30, 6963. (f) Vandewalle, M.; Van der Eycken, J.; Oppolzer, W.; Vullioud, C. T 1986, 42, 4035. (g) Oppolzer, W.; Rodriguez, I.; Blagg, J.; Bernardinelli, G. HCA 1989, 72, 123. (h) Bauer, T.; Chapuis, C.; Kozac, J.; Jurczak, J. HCA 1989, 72, 482. (i) Gouverneur, V.; Dive, G.; Ghosez, L. TA 1991, 2, 1173.
3. (a) Curran, D. P.; Kim, B. H.; Daugherty, H.; Heffner, T. A. TL 1988, 29, 3555. (b) Curran, D. P.; Heffner, T. A. JOC 1990, 55, 4585. (c) Kim, K. S.; Kim, B. H.; Park, W. M.; Cho, S. J.; Mhin, B. J. JACS 1993, 115, 7472. (d) Kim, B. H.; Lee, J. Y.; Kim, K.; Whang, D. TA 1991, 2, 27. (e) Kim, B. H.; Lee, J. Y. TA 1991, 2, 1359. (f) Kim, B. H.; Lee, J. Y. TL 1992, 33, 2557. (g) Garner, P.; Ho, W. B. JOC 1990, 55, 3973. (h) Garner, P.; Ho, W. B.; Grandhee, S. K.; Youngs, W. J.; Kennedy, V. O. JOC 1991, 56, 5893. (i) Garner, P.; Ho, W. B.; Shin, H. JACS 1992, 114, 2767. (j) Binger, P.; Schafer, B. TL 1988, 29, 529. (k) Binger, P.; Brinkmann, A.; Roefke, P.; Schafer, B. LA 1989, 739. (l) Trost, B. M.; Yang, B.; Miller, M. L. JACS 1989, 111, 6482.
4. Vallgarda, J.; Hacksell, U. TL 1991, 32, 5625, and corrigendum ibid. TL 1991, 32, 7136.
5. Kapron, J. T.; Santarsiero, B. D.; Vederas, J. C. CC 1993, 1074.
6. (a) Oppolzer, W.; Barras, J.-P. HCA 1987, 70, 1666. (b) Walba, D. M.; Przybyla, C. A.; Walker, Jr, C. B. JACS 1990, 112, 5624.
7. Oppolzer, W.; Mills, R. J.; Reglier M. TL 1986, 27, 183.
8. Lee, P.-C.; Wu, C.-C.; Cheng. M.-C.; Wang, Y.; Wu, M.-J. J. Chinese Chem. Soc. 1992, 39, 87.
9. (a) Oppolzer, W.; Poli, G. TL 1986, 27, 4717. (b) Oppolzer, W.; Poli, G.; Starkemann, C.; Bernardinelli, G. TL 1988, 29, 3559.
10. Oppolzer, W.; Poli, G.; Kingma, A. J.; Starkemann, C.; Bernardinelli, G. HCA 1987, 70, 2201.
11. (a) Oppolzer, W.; Mills, R. J.; Pachinger, W.; Stevenson, T. HCA 1986, 69, 1542. (b) Oppolzer, W.; Schneider, P. HCA 1986, 69, 1817. (c) Oppolzer, W.; Kingma, A. J. HCA 1989, 72, 1337. (d) Oppolzer, W.; Kingma, A. J.; Poli, G. T 1989, 45, 479.
12. Wu, M.-J.; Wu, C.-C.; Lee, P.-C. TL 1992, 33, 2547.
13. Miyata, O.; Shinada, T.; Kawakami, N.; Taji, K.; Ninomiya, I.; Naito, T.; Date, T.; Okamura, K. CPB 1992, 40, 2579.
14. Porter, N. A.; Giese, B.; Curran, D. P. ACR 1991, 24, 296.
15. Girard, C.; Mandville, G.; Bloch, R. TA 1993, 4, 613.
16. (a) Heathcock, C. H. In Modern Synthetic Methods, VCH-VHCA: Basel, 1982, 1. (b) Oppolzer, W.; Blagg, J.; Rodriguez, I.; Walther, E. JACS 1990, 112, 2767. (c) Oppolzer, W.; Starkemann, C.; Rodriguez, I.; Bernardinelli, G. TL 1991, 32, 61. (d) Oppolzer, W.; Lienard, P. TL 1993, 34, 4321. (e) Oppolzer, W.; Starkemann, C. TL 1992, 33, 2439. (f) Oppolzer, W.; Rodriguez, I. HCA 1993, 76, 1275. (g) Oppolzer, W.; Rodriguez, I. HCA 1993, 76, 1282.
17. (a) Oppolzer, W.; Moretti, R.; Thomi, S. TL 1989, 30, 5603. (b) Martin, N.; Martinez-Grau, A.; Seoane, C.; Marco, J. L. TL 1993, 34, 5627.
18. (a) Williams, R. M. Synthesis of Optically Active a-Amino Acids, Pergamon: Oxford, 1989. (b) Oppolzer, W. AP 1990, 190. (c) Oppolzer, W.; Moretti, R.; Thomi, S. TL 1989, 30, 6009. (d) Josien, H.; Martin, A.; Chassaing, G. TL 1991, 32, 6547. (e) Oppolzer, W.; Lienard, P. HCA 1992, 75, 2572. (f) Oppolzer, W.; Tamura, O. TL 1990, 31, 991. (g) Oppolzer, W.; Tamura, O.; Deerberg, J. HCA 1992, 75, 1965. (h) Oppolzer, W.; Cintas-Moreno, P.; Tamura, O. HCA 1993, 76, 187. (i) Oppolzer, W.; Merifield, E. HCA 1993, 76, 957. (j) Oppolzer, W.; Bienayme, H.; Genevois-Borella, A. JACS 1991, 113, 9660.
19. (a) Curran, D. P.; Shen, W.; Zhang, Z.; Heffner, T. A. JACS 1990, 112, 6738. (b) Zoretic, P. A.; Weng, X.; Biggers, C. K.; Biggers, M. S.; Caspar, M. L. TL 1992, 33, 2637.
20. Differding, E.; Lang, R. W. TL 1988, 29, 6087.
21. Weismiller, M. C.; Towson, J. C.; Davis, F. A. OS 1990, 69, 154.
22. Davies, S. G. Chem. Br. 1989, 25, 268.
23. Harada, N.; Soutome, T.; Nehira, T.; Uda, H. JACS 1993, 115, 7547.
24. Thom, C.; Kocienski, P. S 1992, 582.
25. Brandange, S.; Leijonmarck, H. TL 1992, 33, 3025.
26. (a) Yokomatsu, T.; Iwasawa, H.; Shibuya, S. CC 1992, 728. (b) Yokomatsu, T.; Iwasawa, H.; Shibuya, S. TL 1992, 33, 6999.

Alan C. Spivey

University of Cambridge, UK



Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.