Tin(IV) Chloride

SnCl4

[7646-78-8]  · Cl4Sn  · Tin(IV) Chloride  · (MW 260.51)

(strong Lewis acid used to promote nucleophilic additions, pericyclic reactions, and cationic rearrangements; chlorination reagent)

Alternate Name: stannic chloride.

Physical Data: colorless liquid; mp -33 °C; bp 114.1 °C; d 2.226 g cm-3.

Solubility: reacts violently with water; sol cold H2O; dec hot H2O; sol alcohol, Et2O, CCl4, benzene, toluene, acetone.

Form Supplied in: colorless liquid; 1 M soln in CH2Cl2 or heptane; widely available.

Purification: reflux with mercury or P2O5 for several hours, then distill under reduced nitrogen pressure into receiver with P2O5. Redistill. Typical impurities: hydrates.

Handling, Storage, and Precautions: hygroscopic; should be stored in a glove box or over P2O5 to minimize exposure to moisture. Containers should be flushed with N2 or Ar and tightly sealed. Perform all manipulations under N2 or Ar. Solvating with H2O liberates much heat. Use in a fume hood.

Introduction.

SnCl4 is used extensively in organic synthesis as a Lewis acid for enhancing a variety of reactions. SnCl4 is classified as a strong Lewis acid according to HSAB theory, and therefore interacts preferentially with hard oxygen and nitrogen bases. Six-coordinate 1:2 species and 1:1 chelates are the most stable coordination complexes, although 1:1 five-coordinate species are also possible.1 SnCl4 can be used in stoichiometric amounts, in which case it is considered a promoter, or in substoichiometric amounts as a catalyst, depending upon the nature of the reaction. SnCl4 is an attractive alternative to boron, aluminum, and titanium Lewis acids because it is monomeric, highly soluble in organic solvents, and relatively easy to handle. SnCl4 and TiCl4 are among the most common Lewis acids employed in chelation control strategies for asymmetric induction. However, SnCl4 is not often the Lewis acid of choice for optimum selectivities and yields.

SnCl4 is also the principal source for alkyltin chlorides, RnSnCl4 - n.2 Allyltrialkyltin reagents react with SnCl4 to produce allyltrichlorotin species through an SE2 pathway (eq 1).3 Silyl enol ethers react with SnCl4 to give a-trichlorotin ketones (eq 2).4 Transmetalation or metathesis reactions of this type are competing pathways to nucleophilic addition reactions where SnCl4 is present as an external Lewis acid. As a consequence, four important experimental variables must be considered when using SnCl4 as a promoter: (1) the stoichiometry between the substrate and the Lewis acid; (2) the reaction temperature; (3) the nature of the Lewis base site(s) in the substrate; and (4) the order of addition. These variables influence the reaction pathway and product distribution.5

Nucleophilic Additions to Aldehydes.

SnCl4 is effective in promoting the addition of nucleophiles to simple aldehydes. Among the most synthetically useful additions are allylstannane and -silane additions. The product distribution in the stannane reactions can be influenced by the order of addition, stoichiometry, and reaction temperature. The anti geometry of the tin-aldehyde complex is favored due to steric interactions. Furthermore, the six-coordinate 2:1 complex is most likely the reactive intermediate in these systems. The use of crotylstannanes provides evidence for competing transmetalation reaction pathways (eq 3).6 Superior selectivities are provided by Titanium(IV) Chloride.

The presence of additional Lewis base sites within the molecule can result in the formation of chelates with SnCl4 or TiCl4, which can lead to 1,2- or 1,3-asymmetric induction with the appropriate substitution at the C-2 or C-3 centers. NMR studies have provided a basis for explaining the levels of diastereofacial selectivity observed in nucleophilic additions to Lewis acid chelates of b-alkoxy aldehydes with substitution at the C-2 or C-3 positions.7 These studies reveal that SnCl4 chelates are dynamically unstable when substrates are sterically crowded at the alkoxy center, thus enhancing the formation of 2:1 complexes and/or competing metathesis pathways. Furthermore, for b-siloxy aldehydes, the 2:1 SnCl4 complex is formed preferentially over the corresponding chelate.8

Mukaiyama Aldol Additions.

Lewis acid-promoted additions of a chiral aldehyde to a silyl enol ether or silyl ketene acetal (the Mukaiyama9 aldol addition) occurs with good diastereofacial selectivity.10 The reaction has been investigated with nonheterosubstituted aldehydes, a- and b-alkoxy aldehydes,11 a- and b-amino aldehydes,12 and thio-substituted aldehydes.13 High diastereoselectivity is observed in the SnCl4- or TiCl4-promoted aldol addition of silyl enol ethers to a- and b-alkoxy aldehydes. Prior chelation of the aldehyde before addition of the enol silane is important because certain enol silanes interact with SnCl4 to produce a-trichlorostannyl ketones, which provide lower selectivity.14 Simple diastereoselectivity is independent of the geometry of the enol silane, and the reaction does not proceed through prior Si-Ti or Si-Sn exchange. Good anti selectivities (up to 98:2) are obtained in the SnCl4-promoted reactions of chiral a-thio-substituted aldehydes only with a-phenylthio-substituted aldehydes (eq 4). Stereorandom results are obtained with SnCl4 when other alkylthio-substituted aldehydes, such as a-isopropylthio-substituted aldehydes, are used. Boron Trifluoride Etherate catalysis gives better anti selectivities than SnCl4 for aldehydes with smaller alkylthio substituents. Excellent syn selectivities are obtained for a-thio-substituted aldehydes with TiCl4.

Additions to Nitriles.

SnCl4-promoted addition of malonates and bromomalonates to simple nitriles (not electron deficient) gives a,b-dehydro-b-amino acid derivatives (eq 5).15 SnCl4 is the Lewis acid of choice for the condensation of aroyl chlorides with sodium isocyanate, affording aroyl isocyanates in 70-85% yields.16 Nonaromatic acyl chlorides react under more variable reaction conditions.

Hydrochlorination of Allenic Ketones.

SnCl4 is also a source for generating chloride anions which form new carbon-chlorine bonds. This occurs through a ligand exchange pathway which has been exploited in the formation of b-chloro enones from conjugated allenic ketones (eq 6).17 Yields range from 36-82% with complete selectivity for the trans geometry. A variety of substituents (R1, R2) can be tolerated including aryl, rings, and alkoxymethyl groups (R1).

Glycosylations.

The reaction of glycofuranosides having a free hydroxyl group at C-2 with functionalized organosilanes, in the presence of SnCl4, provides C-glycosyl compounds in high stereoselectivity (eq 7).18 Organosilanes such as 4-(chlorodimethylsilyl)toluene, chlorodimethylvinylsilane, Allyltrimethylsilane, and allylchlorodimethylsilane are effective reagents. The presence of a leaving group on the silane is essential for good selectivity since the reaction proceeds intramolecularly through a 2-O-organosilyl glycoside. The availability of furanosides in the ribo, xylo, and arabino series make this reaction valuable for the stereoselective synthesis of C-furanosides. Regioselective glycosylation of nitrogen-containing heterocycles is also effectively promoted by SnCl4, and has been used in the synthesis of pentostatin-like nucleosides, such as (1).19

Selective De-O-benzylation.

Regioselective de-O-benzylation of polyols and perbenzylated sugars is achieved with organotin reagents or other Lewis acids.20,21 The equatorial O-benzyl group of 1,6-anhydro-2,3,4-tri-O-benzyl-b-D-mannopyranose is selectively cleaved by SnCl4 or TiCl4 (eq 8).2 The equatorial O-benzyl group is also selectively cleaved when one of the axial O-benzyl groups is replaced by an O-methyl group. The 2-O-benzyl group of 1,2,3-tris(benzyloxy)propane is selectively cleaved (eq 9), but no debenzylation is observed with 1,2-bis(benzyloxy)ethane.

Rearrangement of Allylic Acetals.

Lewis acid-promoted (SnCl4 or Diethylaluminum Chloride) rearrangements of allylic acetals provide substituted tetrahydrofurans.22 Upon addition of Lewis acid, (2) rearranges to the all-cis furan (3) (eq 10). No racemization is observed with optically active allylic acetals; however, addition of KOH completely epimerizes the furan-carbonyl bond, as does quenching at room temperature. Acetals successfully undergo similar rearrangement provided the alkene is substituted. Completely substituted tetrahydrofurans are synthesized stereoselectively (>97% ee) by the rearrangement of disubstituted allyl acetals (eq 11). This reaction is related to the acid-catalyzed rearrangements of 5-methyl-5-vinyloxazolidines to 3-acetylpyrrolidines, which involves an aza-Cope rearrangement and Mannich cyclization.23

The rearrangement is also useful for furan annulations, through enlargement of the starting carbocycle.24 Thus addition of SnCl4 to either diastereomer of the allylic acetal (4) produces the cis-fused cycloheptatetrahydrofuran (5) in 48-76% yield (eq 12). Acetals derived from trans-diols rearrange to the same cis-fused bicyclics in higher yield. The stereochemistry of a terminal alkene is transmitted to the C-3 carbon of the bicyclic products (eq 13). Rearrangements of acetals require substitution at the internal alkene carbon.

a-t-Alkylations.

SnCl4-promoted a-t-alkylations of alkenyl b-dicarbonyl compounds is a particularly useful cyclization reaction.25 Cyclization occurs through initial formation of a stannyl enol ether, followed by protonation of the alkene to form a carbocation which undergoes subsequent closure (eq 14). The analogous a-s-alkylation reactions are best catalyzed by other Lewis acids.

This reaction is useful for cyclizations involving 6-endo-trigonal (eq 14) and allowed 7-endo trigonal processes (eq 15), but not for those involving 5-endo trigonal processes (eq 16). These observations are consistent with the Baldwin rules.

Reactions involving 4- and 6-exo trigonal cyclizations result in poor yields or undesired products, while those involving 5-exo trigonal cyclizations produce higher yields (eq 17). This synthetic strategy can also be used to form bicyclic and spiro compounds (eqs 18 and 19).

Alkene Cyclizations.

Cationic cyclizations of polyenes, containing initiating groups such as cyclic acetals, are promoted by SnCl4 and have been utilized in the synthesis of cis- and trans-decalins, cis- and trans-octalins, and tri- and tetracyclic terpenoids and steroids.26 In most instances, all-trans-alkenes yield products with trans,anti,trans stereochemistry (eq 20), while cis-alkenes lead to syn stereochemistry at the newly formed ring junctions. The stereoselectivity of polyene cyclizations are often greatly diminished when the terminating alkene is a vinyl group rather than an isopropenyl group. Acyclic compounds which contain terminal acyclic acetals and alkenes or vinylsilanes can be cyclized in a similar fashion to yield eight- and nine-membered cyclic ethers (eq 21).27

The analogous cyclization of chiral imines occurs in high yields (75-85%) with good asymmetric induction (36-65% ee).28 For example, the cyclization of aldimine (6), derived from methyl citronellal, using SnCl4 affords only the trans-substituted aminocyclohexane (7) in high yield (eq 22). Exo products are formed exclusively or preferentially over the thermodynamically favored endo products.

SnCl4-induced cyclizations between alkenes and enol acetates result in cycloalkanes or bicycloalkanes in high yield (eq 23). It is interesting to note that the TMSOTf-catalyzed reaction can yield fused products rather than bicyclo products. Alkenic carboxylic esters, allylic alcohols, sulfones, and sulfonate esters are also cyclized in the presence of SnCl4; however, alkenic oxiranes often cyclize in poor yield.26a

SnCl4 is also effective in the opening of cyclopropane rings to produce cationic intermediates useful in cyclization reactions. For example, the cyclization of aryl cyclopropyl ketones to form aryl tetralones, precursors of aryl lignan lactones and aryl naphthalene lignans, is mediated by SnCl4 (eq 24).29 The reaction is successful in nitromethane, but not in benzene or methylene chloride. Analogous cyclizations with epoxides result in very low yields (2-5%).

Polymerization.

Cationic polymerizations are catalyzed by SnCl4 and other Lewis acids (eq 25). Propagation is based upon the formation of a cationic species upon complexation with SnCl4.30 Radical pathways are also possible for polymer propagation.31

Diels-Alder Reactions.

Diels-Alder reactions are enhanced through the complexation of dienophiles or dienes by Lewis acids.32 Furthermore, Lewis acids have been successfully employed in asymmetric Diels-Alder additions.33 Although SnCl4 is a useful Lewis acid in Diels-Alder reactions, in most instances titanium or aluminum Lewis acids provide higher yields and/or selectivities. The stereoselectivity in Lewis acid-promoted Diels-Alder reactions between chiral a,b-unsaturated N-acyloxazolidinones shows unexpected selectivities as a function of the Lewis acid (eq 26).34 Optimum selectivity is expected for chelated intermediates, yet both SnCl4 and TiCl4 perform poorly relative to Et2AlCl (1.4 equiv). The formation of the SnCl4-N-acyloxazolidinone chelate has been confirmed by solution NMR studies.35 These data suggest that other factors such as the steric bulk associated with complexes may contribute to stereoselectivity.

In Lewis acid-promoted Diels-Alder reactions of cyclopentadiene with the acrylate of (S)-ethyl lactate, good diastereofacial and endo/exo selectivity are obtained with SnCl4 (84:16; endo/exo = 18:1) and TiCl4 (85:15; endo/exo = 16:1).36 It is interesting to note that boron, aluminum, and zirconium Lewis acids give the opposite diastereofacial selectivity (33:67 to 48:52). Competing polymerization of the diene is observed in methylene chloride, particularly with TiCl4, but not in solvent mixtures containing n-hexane.

Cycloalkenones generally perform poorly as dienophiles in Diels-Alder reactions but their reactivity can be enhanced by Lewis acids.37 SnCl4 is effective in promoting the Diels-Alder reaction between simple 1,3-butadienes, such as isoprene and piperylene, and cyclopentenone esters. For example, the SnCl4-promoted cycloaddition between (8) and isoprene is completely regioselective, providing the substituted indene in 86% yield (eq 27).38 However, cycloaddition does not occur in the presence of SnCl4 when the diene contains an oxygen-bearing substituent such as an alkoxy or siloxy group. In these cases, as is generally true for the Diels-Alder reactions of cycloalkenones, other Lewis acids are more effective. For example, SnCl4-promotion of the cycloaddition between (8) and 3-methyl-2-(t-butyldimethylsiloxy)butadiene yields 37% of the desired product, while Zinc Chloride provides a 90% yield. When furan or 2-methyl-1-alkylsiloxybutadiene are utilized as dienes, only decomposition of the starting material is observed with SnCl4.

The Lewis acid-promoted Diels-Alder reaction has been employed in the assembly of steroid skeletons.39 The cycloaddition reaction between a substituted bicyclic diene and 2,6-dimethylbenzoquinone produces two stereoisomers in a 1:5 ratio with a yield of 83% when SnCl4 is used in acetonitrile. TiCl4 provides slightly higher selectivities (1:8) but lower yield (70%) (eq 28).

When the dienophile N-a-methylbenzylmaleimide (9) is reacted with 2-t-butyl-1,3-butadiene in the presence of Lewis acids, cycloadducts (10) and (11) are formed (eq 29).40 While SnCl4 provides (10) and (11) in a 5:1 ratio, TiCl4 and EtAlCl2 both provide a 15:1 ratio. Polymerization of the diene competes with adduct formation under all conditions.

[4 + 3] Cycloadditions.

Oxyallyl cations,41 which react as C3 rather than C2 components in cyclization reactions, are generated by the addition of SnCl4 to substrates which contain silyl enol ethers which are conjugated with a carbonyl moiety. Thus 2-(trimethylsiloxy)propenal undergoes cyclization with cyclopentadiene or furan (eq 30).42 Substituted 1,1-dimethoxyacetones also form these intermediates and undergo subsequent cyclizations (eq 31).43 This method complements the usual synthesis of oxyallyl cations involving reductive elimination of halogens from halogenated ketones or electronically equivalent structures.44

[3 + 2] Cycloadditions.

Lewis acid-mediated [3 + 2] cycloadditions of oxazoles and aldehydes or diethyl ketomalonate have been observed using organoaluminum and SnIV Lewis acids.45 The reactions are highly regioselective, with stereoselectivity extremely dependent upon Lewis acid (eq 32). For example, the (BINOL)AlMe-promoted reaction between benzaldehyde and the oxazole (12) provides the oxazoline with a cis/trans ratio of 98:2. The selectivity is reversed with SnCl4 which provides a cis/trans ratio of 15:85. trans-5-Substituted 4-alkoxycarbonyl-2-oxazolines are synthesized under thermodynamic conditions in the aldol reaction of isocyanoacetates with aldehydes.46

[2 + 2] Cycloadditions.

The regioselectivity in the cycloaddition reactions of 2-alkoxy-5-allyl-1,4-benzoquinones with styrenes is controlled by the choice of TiIV or SnCl4 Lewis acids (eq 33).47 The use of an excess of TiCl4 or mixtures of TiCl4 and Ti(O-i-Pr)4 produces cyclobutane (13) as the major or exclusive product, while SnCl4 promotion with one equivalent of Lewis acid results in the formation of (14) only. These reactions represent a classic example of the mechanistic variability often associated with seemingly modest changes in Lewis acid.

Ene Reactions.

The Lewis acid-catalyzed ene reaction is synthetically useful methodology for forming new carbon-carbon bonds.48 Ene reactions utilizing reactive enophiles such as formaldehyde and chloral can be promoted by SnCl4. SnCl4 also enhances intramolecular ene reactions, such as the cyclization of (15) which produces the a-hydroxy d-lactone in 85% yield (eq 34).49 The ene cyclization of citronellal to give isopulegol has also been reported.50 Proton scavenging aluminum Lewis acids such as RAlCl2 are most often used in ene reactions to eliminate proton-induced side reactions.

Related Reagents.

Tin(IV) Chloride-Zinc Chloride.


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5. (a) Keck, G. E.; Castellino, S.; Andrus, M. B. In Selectivities in Lewis Acid Promoted Reactions; Schinzer, D., Ed.; Kluwer: Dordrecht, 1989; pp 73-105. (b) Keck, G. E.; Andrus, M. B.; Castellino, S. JACS 1989, 111, 8136. (c) Denmark, S. E.; Wilson, T.; Wilson, T. M. JACS 1988, 110, 984. (d) Boaretto, A.; Marton, D.; Tagliavini, G.; Ganis, P. JOM 1987, 321, 199. (e) Yamamoto, T.; Maeda, N.; Maruyama, K. CC 1983, 742. (f) Quintard, J. P.; Elissondo, B.; Pereyre, M. JOC 1983, 48, 1559.
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7. (a) Keck, G. E.; Castellino, S. JACS 1986, 108, 3847. (b) Keck, G. E.; Castellino, S.; Wiley, M. R. JOC 1986, 51, 5478.
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9. Mukaiyama, T.; Banno, K.; Narasaka, K. JACS 1974, 96, 7503.
10. Review of Mukaiyama aldol reaction: Gennan, C. COS 1991, Vol. 2.
11. Reetz, M. T.; Kesseler, K.; Jung, A. T 1984, 40, 4327.
12. (a) Reetz, M. T. AG(E) 1984, 23, 556. (b) ref 11.
13. (a) Annunziata, R.; Cinquini, M.; Cozzi, F.; Cozzi, P. G.; Consolandi, E. JOC 1992, 57, 456. (b) Annunziata, R.; Cinquini, M.; Cozzi, F.; Cozzi, P. G. TL 1990, 31, 6733.
14. Nakamura, E.; Kawajima, I. TL 1983, 24, 3343.
15. Scavo, F.; Helquist, P. TL 1985, 26, 2603.
16. Deng, M. Z.; Caubere, P.; Senet, J. P.; Lecolier, S. T 1988, 44, 6079.
17. Gras, J. L.; Galledou, B. S. BSF(2) 1982, 89.
18. Martin, O. R.; Rao, S. P.; Kurz, K. G.; El-Shenawy, H. A. JACS 1988, 110, 8698.
19. Showalter, H. D. H.; Putt, S. R. TL 1981, 22, 3155.
20. Wagner, D.; Verheyden, J. P. H.; Moffat, J. G. JOC 1974, 39, 24.
21. Hori, H.; Nishida, Y.; Ohrui, H.; Meguro, H. JOC 1989, 54, 1346.
22. Hopkins, M. H.; Overman, L. E. JACS 1987, 109, 4748.
23. Overman, L. E.; Kakimoto, M. E.; Okazaki, M. E.; Meier, G. P. JACS 1983, 105, 6622.
24. Herrington, P. M.; Hopkins, M. H.; Mishra, P.; Brown, M. J.; Overman, L. E. JOC 1987, 52, 3711.
25. Review of a-alkylations to carbonyl compounds: Reetz, M. T. AG(E) 1982, 21, 96.
26. (a) Review of asymmetric alkene cyclization: Bartlett, P. A. Asymmetric Synthesis; Morrison, J. D., Ed.; Academic: New York, 1984; Vol. 3, Part B, pp 341-409. (b) Review of thermal cycloadditions: Fallis, A. G.; Lu, Y.-F. Advances in Cycloaddition; Curran, D. P., Ed.; JAI: Greenwich, CT, 1993; Vol. 3, pp 1-66.
27. (a) Overman, L. E.; Blumenkopf, T. A.; Castaneda, A.; Thompson, A. S. JACS 1986, 108, 3516. (b) Overman, L. E.; Castaneda, A.; Blumenkopf, T. A. JACS 1986, 108, 1303.
28. Demailly, G.; Solladie, G. JOC 1981, 46, 3102.
29. Murphy, W. S.; Waltanansin, S. JCS(P1) 1982, 1029.
30. (a) Kamigaito, M.; Madea, Y.; Sawamota, M.; Higashimura, T. Macromolecules 1993, 26, 1643. (b) Takahashi, T.; Yokozawa, T.; Endo, T. Makromol. Chem. 1991, 192, 1207. (c) Ran, R. C.; Mao, G. P. J. Macromol. Sci. Chem. 1990, A27, 125. (d) Kurita, K.; Inoue, S.; Yamamura, K.; Yoshino, H.; Ishii, S.; Nishimura, S. I. Macromolecules 1992, 25, 3791. (e) Yokozawa, T.; Hayashi, R.; Endo, T. Macromolecules 1993, 26, 3313.
31. (a) Tanaka, H.; Kato, H.; Sakai, I.; Sato, T.; Ota, T. Makromol. Chem. Rapid Commun. 1987, 8, 223. (b) Yuan, Y.; Song, H.; Xu, G. Polym. Int. 1993, 31, 397.
32. Birney, D. M.; Houk, K. N. JACS 1990, 112, 4127.
33. For leading references on asymmetric Diels-Alder reactions, see: (a) Paquette, L. A. In Asymmetric Synthesis; Morrison, J. D., Ed.; Academic: New York, 1984; Vol. 3, pp 455-483. (b) Oppolzer, W. AG(E) 1984, 23, 876. (c) Carruthers, W. Cycloaddition Reactions in Organic Synthesis; Pergamon: New York, 1990; pp 61-72.
34. Evans, D. A.; Chapman, K. T.; Bisaha, J. JACS 1988, 110, 1238.
35. Castellino, S. JOC 1990, 55, 5197.
36. Poll, T.; Helmchen, G.; Bauer, B. TL 1984, 25, 2191.
37. (a) Fringuelli, F.; Pizzo, F.; Taticchi, A.; Wenkert, E. JOC 1983, 48, 2802. (b) Fringuelli, F.; Pizzo, F.; Taticchi, A.; Halls, T. D. J.; Wenkert, E. JOC 1982, 47, 5056.
38. Liu, H. J.; Ulibarri, G.; Browne, E. N. C. CJC 1992, 70, 1545.
39. Arseniyadis, A.; Rodriguez, R.; Spanevello, J. C.; Thompson, A.; Guittet, E.; Ourisson, G. T 1992, 48, 1255.
40. Baldwin, S. W.; Greenspan, P.; Alaimo, C.; McPhail, A. T. TL 1991, 42, 5877.
41. For a recent review of oxyallyl cations, see: Mann, J. T 1986, 42, 4611.
42. Masatomi, O.; Kohki, M.; Tatsuya, H.; Shoji, E. JOC 1990, 55, 6086.
43. Murray, D. H.; Albizati, K. F. TL 1990, 31, 4109.
44. Hoffman, H. M. R. AG(E) 1973, 12, 819; 1984, 23, 1.
45. Suga, H.; Shi, X.; Fujieda, H.; Ibata, T. TL 1991, 32, 6911.
46. For examples of enantioselective synthesis of trans-4-alkoxy-2-oxazolines, see: Ito. Y; Sawamura, M.; Shirakawa, E.; Hayashizaki, K.; Hayashi, T. TL 1988, 29, 235; T 1988, 44, 5253.
47. Engler, T. A.; Wei, D.; Latavic, M. A. TL 1993, 34, 1429.
48. Reviews of ene reactions: (a) Hoffman, H. M. R. AG(E) 1969, 8, 556. (b) Oppolzer, W.; Sniekus, V. AG(E) 1978, 17, 476. (c) Snyder, B. B. ACR 1980, 13, 426.
49. Lindner, D. L.; Doherty, J. B.; Shoham, G.; Woodward, R. B. TL 1982, 23, 5111.
50. Nakatani, Y.; Kawashima, K. S 1978, 147.

Stephen Castellino

Rhône-Poulenc Ag. Co., Research Triangle Park, NC, USA

David E. Volk

North Dakota State University, Fargo, ND, USA



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