Tri-n-butylstannane1

n-Bu3SnH

[688-73-3]  · C12H28Sn  · Tri-n-butylstannane  · (MW 291.11)

(source of Bu3Sn&bdot; radical which produces carbon radicals by (a) abstraction of X from C-X derivatives1,2 and (b) addition to alkenes, alkynes,3,4 and carbonyl compounds; hydrogen donor (with concomitant generation of the chain carrier, Bu3Sn&bdot;) for radicals;2 hydrostannylation of alkenes, alkynes,5,6 and carbonyl compounds;7 desulfurative stannylation of propargylic or allylic sulfides;8 catalyst for SH2 reactions of allylic stannanes;9 radical ring expansion;10,11 selective reduction of acid chlorides to aldehydes;12 oxygenation13 and carbonylation14 of radicals; source of tributyltin anion15)

Alternate Name: tributyltin hydride; TBTH.

Physical Data: bp 80 °C/0.4 mmHg; d 1.082 g cm-3.

Solubility: freely sol organic solvents.

Form Supplied in: clear colorless liquid; 97% pure.

Analysis of Reagent Purity: major impurity is oxidation product (Bu3Sn)2O; purity ascertained by gas volumetric methods using dichloroacetic acid or by IR spectroscopy.1e

Handling, Storage, and Precautions: irritant. Even though this reagent appears to be relatively innocuous,1a tin compounds in general are toxic16 and should be handled with care in a fume hood. It should be kept in brown bottles away from light and air. Some pressure may develop upon long term storage. Workup procedures for the isolation of tin-free organic products from reactions involving TBTH have been published.1e,77b

Introduction.

Tributyltin hydride is the most commonly used source of Bu3Sn&bdot; which initiates a variety of radical chain reactions.1,2 Bu3Sn&bdot; may be generated either thermally or photochemically. Azobisisobutyronitrile (AIBN) is the most commonly used radical initiator. Three important classes of reactions have been recognized for R3Sn&bdot;: (a) atom or group abstraction, (b) addition to multiple bonds, and (c) homolytic substitution reactions. The primary products of (a) and (b) are themselves radicals and they undergo a variety of useful transformations such as atom abstraction reactions, rearrangements, fragmentation reactions, and intra- and intermolecular additions to carbon-carbon and other multiple bonds. Tributyltin hydride acts as a hydrogen atom source to trap the radical products, with concomitant generation of the radical chain carrier Bu3Sn&bdot;. These primary steps are illustrated for prototypical radical reactions in eqs 1-6.

The lifetimes of the radicals R&bdot; and R&bdot;, are determined by the chain transfer steps 3 and 4 and side reactions of steps 5 and 6. For the efficient formation of preparatively useful radical intermediates R and R&bdot;, the kinetics of each step must cooperate. For example, only reactions that are faster than the chain transfer step 3 can be executed in step 2. It should be understood that the overall rates of the chain transfer steps can be controlled to some extent by the concentration of the Bu3SnH reagent. Likewise, the rates of step 2 can be altered by electronic and steric characteristics of R&bdot; and of any reaction partners involved. Fortunately kinetics, thermodynamics,17 and substituent effects for the individual steps have been studied in some detail and it is possible to design useful synthetic strategies.18

Reactions Initiated by Bu3Sn Radical.

C-X Homolysis followed by H abstraction.

Dehalogenation.

Chemoselective replacement of halogens (except fluorine) with hydrogen is one of the major uses of TBTH in synthesis.1h The examples below show the versatility of the method and the range of substrates that can be used in this reaction. Replacement of bridgehead halogen (eq 7)19 and selective removal of one of the halogens from a geminal dihalocyclopropane (eq 8)20 are particularly noteworthy. TBTH can be generated catalytically for the dehalogenation reaction (eq 9).21

Applications in the b-lactam22 and carbohydrate (eqs 10 and 11)23,24 areas illustrate the compatibility of the radical intermediates with a wide range of functional groups and reaction conditions. It should be noted that unlike reactions involving polar intermediates, hydroxyl and amino groups need not be protected under conditions where radicals are generated. Bu3SnT is the reagent of choice for the regiochemical introduction of tritium into the steroid molecule (eq 12).25 Debromination with TBTH was a crucial step in a model study directed towards the synthesis of a highly labile thromboxane A2 analog (eq 13).26

Deoxygenation.

Deoxygenation of secondary alcohols is best carried out by treatment of the corresponding thiocarbonyl derivatives with TBTH (eq 14).27,28 Many common functional groups such as amine, alcohol, amido, carbonyl, epoxide, and tosylate are stable to the reaction conditions.

Typical thiocarbonyl derivatives are xanthates (eq 15),29 thiocarbonyl imidazolides (R = imidazolyl in eq 14), and phenoxythionocarbonate (eq 16).30 Thioimidazolides, which are prepared under essentially neutral conditions from alcohols and 1,1-Thiocarbonyldiimidazole, are best suited for acid- or base-sensitive substrates (eq 17).31 Pentafluoro- or trichlorophenyl thionocarbonate is used for the deoxygenation of a primary alcohol under similar conditions.32 A new procedure for the deoxygenation of tertiary alcohols33 has also been reported.

Decarboxylation.

Photolysis or thermolysis of acyl derivatives of N-hydroxy-2-thiopyridone in the presence of TBTH (or, better, t-butyl thiol) gives the norhydrocarbon derived from the acyl moiety (eq 18). This method is applicable to a wide variety of aliphatic carboxylic acids (eq 19).34

Homolysis of C-N Bonds.

Isocyanides,35 isothiocyanates, and nitro compounds36 undergo homolysis of the C-N bond upon treatment with TBTH. Primary amino groups are converted into isocyanides via dehydration of the corresponding formamides. A highly selective reaction of 6b-isocyano-b-lactam (eq 20)37 proceeds in 63% yield; as expected, the convex a-face of the bicyclic skeleton is more accessible to the hydride reagent.38

Homolysis of C-S, C-Se, and C-Te Bonds.

TBTH, in the presence of AIBN, reduces primary and secondary thiols,39 sulfides with at least one radical stabilizing group (for example, a-carbonyl, a-thio, benzyl, t-Bu), and thiones to the corresponding hydrocarbons.40 Se-41 and Te-C42 bonds are also cleaved by tin hydrides. Since the Ph-S or Ph-Se bond is almost never cleaved, thiophenyl and selenophenyl derivatives are among the most widely used radical precursors for C-C bond forming reactions (see below). Selenophenyl esters provide aldehydes (80 °C or hn, rt) or the norhydrocarbon (164 °C), depending on the reaction conditions.41

Radical Rearrangements.

An alternative to the classical Wharton reaction uses the thioimidazolide intermediate for the radical generation and subsequent epoxide opening (eq 21).43

Ring opening of the cyclopropylmethyl radical is one of the most studied radical reactions, though synthetic applications in this area involving TBTH are limited.44,45 A recent example is shown in eq 22.46 Opening of N-acylaziridines by TBTH has also been described.47

1,2-Migrations to a radical center are rare and take place only at low TBTH concentrations.48 The example shown in eq 2349 is particularly noteworthy since it provides a ready access to 2-deoxy sugar derivatives.

An interesting protocol for the ring expansion of ketones relies on the ability of carbonyl groups to act as acceptors for radicals. The subsequent fragmentation of the resultant oxy radicals is controlled by strategically placed stabilizing groups (eq 24)10,11 or leaving groups.50

Intramolecular Additions to Unsaturated Centers.

The hex-5-enyl radical cyclization is one of the most useful radical reactions and this subject has been extensively reviewed.2,51,57 The examples shown (eqs 25-29)52-56 illustrate the range of tolerance to various functional groups and the choice of radical precursors possible for this versatile reaction. The stereochemistry of hex-5-enyl radical cyclization has been the subject of a recent review.57

Appropriately placed haloalkyl side chains, which are readily prepared from allylic or propargylic alcohols, can be used for the stereocontrolled introduction of the hydroxymethyl group (eq 30)58,60 or lactone annulation.59 The silacyclopentane in eq 30 produces a 1,3-diol upon Tamao oxidation with Hydrogen Peroxide and Potassium Fluoride60 or it can be completely desilylated by treatment with Potassium t-Butoxide in DMSO.61

Several examples of the use of vinyl2d,3,63,64 and allyl (eq 31)65 radicals in synthesis have been described. Curran has described the use of a catalytic amount of TBTH for atom transfer reactions.66

Exo-hept-6-enyl radical cyclization is about 30 times slower than the corresponding hex-5-enyl cyclization. Nonetheless, by increasing the rate of cyclization (for example, in eq 3259 with an electron-withdrawing group on the alkene), synthetically useful reactions to make six-membered rings can be accomplished.67-69

Acyl radicals generated from selenoesters undergo facile cyclization or intermolecular addition to electron-deficient alkenes faster than H-atom abstraction to give five- and six-membered ketones.69

TBTH is useful for the formation of macrolides via radical cyclization from iodoacrylates, if the concentration of both the reagent and substrate are kept low (eq 33). This method is not applicable for medium sized rings, but proceeds well for compounds with 11 or more atoms in the ring. Endo cyclization mode is favored when n = 16 or higher.70

Intermolecular Additions.

With reactive acceptors in high concentrations, intermolecular addition of radicals proceed with considerable ease and several synthetically useful reactions have been discovered (eqs 34 and 35).71,72 The example of the C-glycoside synthesis (eq 35) is particularly noteworthy; only the a-glycoside is obtained. Stereochemical control in these reactions has been the subject of a recent review.73 A recent example illustrates the power of this method for rapid assembly of prostaglandin analogs (eq 36).74

Radicals generated at the end of an intramolecular cyclization can be trapped if the acceptors are sufficiently reactive (eq 37).62 Such radicals have also been trapped by t-Butyl Isocyanide to provide the corresponding nitrile.75 One-carbon elongation can also be achieved by addition of an electrophilic radical to an enamine (eq 38).76 Clever use of available kinetic and thermodynamic data can lead to the development of new annulation reactions, as shown in eq 39.77

Fragmentation and Homolytic Substitution Reactions.

Allyltributyltin derivatives,78,79 which have found wide applicability, may be synthesized by intermolecular SH2 reaction of an appropriately substituted allylic compound (eq 40).8,80 Intramolecular9,81 SH2 reactions have been used for cyclizations.

Tributyltin hydride has ben used in catalytic amounts to effect carbocyclic ring expansions of b-stannyl ketones.50 As shown in eq 41, the fragmentation reaction can be coupled to a radical cyclization reaction to produce ring expanded products from relatively simple substrates.

Hydrostannylation of Alkenes, Alkynes, and Carbonyl Compounds.

Hydrostannylation of alkynes and alkenes is a well known reaction1,6 which may be carried out under either thermal (eqs 42 and 43)21,4 or sonochemical82 initiation conditions. The intermediate radical formed upon the addition of R3Sn&bdot; to alkynes and alkenes can be trapped by an appropriately placed double bond to give carbocyclic compounds.63,64,83 TBTH adds to the strained single bond of [1.1.1]propellane (eq 44).84

Addition of TBTH to carbonyl compounds requires catalysis by Pd0 and a promoter Lewis acid (eq 45),7 or a proton source.85 Mechanistic studies suggest that TBTH is a hydride donor. Even though the radical additions to simple ketones are slow, a,b-enones readily generate O-stannyl ketyls which participate in hex-5-enyl radical cyclization reactions.86

Applications in Organometallic Methodology.

Highly selective reduction of acid chlorides (eq 46)87 and vinyl triflates6 may be achieved by the use of Bu3SnH under Pd catalysis. TBTH/Pd0 is also an effective system for the reduction of allylic substrates under mild conditions. This reaction is remarkably tolerant of other common functional groups like hydroxy, epoxide, aldehyde, nitrile, and lactone. Radical scavengers improve the yield of the reaction.88 Pd0 catalyzes carbonylation of aliphatic, vinyl, and aromatic halides under Carbon Monoxide in the presence of TBTH.6 A synthesis of alkyl hydroperoxide uses the homolytic cleavage of an Hg-C bond by tin hydride.89 Tributyltin hydride has been used to generate a low-valent niobium reagent which is useful for coupling of imines and aldehydes (eq 47).90

Carbonylation and Oxygenation of Radicals.

TBTH mediates the oxygenation (eq 48)13 and carbonylation (eq 49)14 of radicals.

TBTH as a Source of Bu3Sn Anion.

Tributyltin hydride is the best source of tributyltin Grignard or the corresponding lithium reagent, both of which have been used extensively in synthesis.91 Eqs 50 and 51 are illustrative.15,92,93

Related Reagents.

See also Trimethylstannane, Triphenylstannane, polymer-supported organotin hydride,94 Tris(trimethylsilyl)silane.95


1. (a) Neumann, W. P. The Organic Chemistry of Tin; Wiley: New York, 1970. (b) Kuivila, H. G. S 1970, 499. (c) Walling, C. T 1985, 41, 3887. (d) Pereyre, M.; Quintard, J. P.; Rahm, A. Tin in Organic Synthesis; Butterworth: London, 1987. (e) Neumann, W. P. S 1987, 665. (f) Harrison, P. G. Chemistry of Tin; Chapman and Hall: New York, 1989. (g) Giese, B. Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds; Pergamon: New York, 1986. (h) Metzger, J. O. MOC 1989, 19a.
2. (a) Beckwith, A. L. J. T 1981, 37, 3073. (b) Hart, D. J. Science 1984, 223, 883. (c) Curran, D. P. S 1988, 417, 489. (d) Stork, G. In Selectivity-A Goal for Synthetic Efficiency; Bartmann, W.; Trost, B. M., Ed.; Verlag Chemie: Basel, 1984; p 281. (e) Giese, B. AG(E) 1985, 24, 553. (f) Giese, B. AG(E) 1989, 28, 969. (g) Ramaiah, M. T 1987, 43, 3541.
3. Stork, G.; Mook, R., Jr. JACS 1987, 109, 2829.
4. (a) Nozaki, K.; Oshima, K.; Utimoto, K. T 1989, 45, 923. For a recent application see: (b) Satoh, S.; Sodeoka, M.; Sasai, H.; Shibasaki, M. JOC 1991, 56, 2278.
5. Negishi, E. Organometallics in Organic Synthesis; Wiley: New York, 1980.
6. Stille, J. K. AG(E) 1986, 25, 508.
7. (a) Four, P.; Guibe, F. TL 1982, 23, 1825. (b) For the use of a tin triflate catalyst, see: Yang, T. S.; Four, P.; Guibe, F.; Balavoine, G. NJC 1984, 8, 611.
8. Ueno, Y.; Okawara, M. JACS 1979, 101, 1893.
9. Baldwin, J. E.; Adlington, R. M.; Mitchell, M. B.; Robertson, J. CC 1990, 1574.
10. Beckwith, A. L. J.; O'Shea, D. M.; Gerba, S.; Westwood, S. W. CC 1987, 666.
11. (a) Dowd, P.; Choi, S. JACS 1987, 109, 6548. (b) See also: Tsang, R.; Dickson, J. K., Jr.; Pak, H.; Walton, R.; Fraser-Reid, B. JACS 1987, 109, 3484.
12. Four, P.; Guibe, F. JOC 1981, 46, 4439.
13. Nakamura, E.; Inubushi, T.; Aoki, S.; Machii, D. JACS 1991, 113, 8980.
14. Ryu, I.; Kusano, K.; Ogawa, A.; Kambe, N.; Sonoda, N. JACS 1990, 112, 1295.
15. Still, W. C. JACS 1978, 100, 1482.
16. Selwyn, M. J. In Chemistry of Tin; Harrison, P. G., Ed.; Chapman and Hall, New York, 1989; p. 359.
17. For a listing of important kinetic data, see: (a) Asmus, K. D.; Bonifacic, M.; Ingold, K. U.; Roberts, B. P. In Landolt-Bornstein, Radical Reaction Rates in Liquids; Fisher, H., Ed. Springer: Heidelberg, 1983; Vol. II 13 b, c. (b) Johnston, L. J.; Lusztyk, J.; Wayner, D. D. M.; Abeywickreyma, A. N.; Beckwith, A. L. J.; Scaiano, J. C.; Ingold, K. U. JACS 1985, 107, 4594.
18. For a lucid account of how available kinetic and thermodynamic data can be used in synthetic planning, see Refs. 1c, 2c, and 2e.
19. McDonald, I. A.; Dreiding, A. S.; Hutmacher, H.; Musso, H. HCA 1973, 56, 1385.
20. Seyferth, D.; Yamazaki, H.; Alleston, D. L. JOC 1963, 28, 703.
21. Corey, E. J.; Suggs, W. JOC 1975, 40, 2554.
22. Aimetti, J. A.; Hamanaka, E. S.; Johnson, D. A.; Kellog, M. S. TL 1979, 20, 4631.
23. Knapp, S.; Patel, D. V. JACS 1983, 105, 6985.
24. Cardillo, G.; Orena, M.; Sandri, S.; Tomasini, C. JOC 1984, 49, 3951.
25. Parnes, H.; Pease, J. JOC 1979, 44, 151.
26. Bhagwat, S. S.; Hamann, P. R.; Still, W. C. TL 1985, 26, 1955.
27. Barton, D. H. R.; McCombie, S. W. JCS(P1) 1975, 1574.
28. Hartwig, W. T 1983, 39, 2609.
29. Iacono, S.; Rasmussen, J. R. OS 1986, 64, 57.
30. Robins, M. J.; Wilson, J. S.; Hansske, F. JACS 1983, 105, 4059.
31. (a) Carney, R. E.; McAlpine, J. B.; Jackson, M.; Stanaszek, R. S.; Washburn, W. H.; Cirovic, M.; Mueller, S. L. J. Antibiot. 1978, 31, 441. (b) See also: Rasmussen, J. R. JOC 1980, 45, 2725.
32. Barton, D. H. R.; Blundell, P.; Dorchak, J.; Jang, D. O.; Jaszberenyi, J. C. T 1991, 47, 8969.
33. Barton, D. H. R.; Crich, D. CC 1984, 774.
34. Barton, D. H. R.; Crich, D.; Motherwell, W. B. T 1985, 41, 3901.
35. Barton, D. H. R.; Bringmann, G.; Motherwell, W. B. JCS(P1) 1980, 2665.
36. Ono, N.; Kaji, A. S 1986, 693.
37. John, D. I.; Tyrrell, N. D.; Thomas, E. J. T 1983, 39, 2477.
38. For a more extensive list, see Ref. 2g.
39. Vedejs, E.; Powell, D. W. JACS 1982, 104, 2046.
40. Gutierrez, C. G.; Summerhays, L. R. JOC 1984, 49, 5206.
41. Pfenninger, J.; Heuberger, C.; Graf, W. HCA 1980, 63, 2328.
42. Clive, D. L. J.; Chittattu, G. J.; Farina, V.; Kiel, W. A.; Menchen, S. M.; Russell, C. G.; Singh, A.; Wong, C. K.; Curtis, N. J. JACS 1980, 102, 4438.
43. Barton, D. H. R.; Motherwell, R. S. H.; Motherwell, W. B. JCS(P1) 1981, 2363.
44. Beckwith, A. L. J.; Ingold, K. U. In Rearrangements in Ground and Excited States; de Mayo, P., Ed.; Academic: New York, 1980; Vol. 1, p 161.
45. Harling, J. D.; Motherwell, W. B. CC 1988, 1380.
46. Clive, D. L. J.; Daigneault, S. CC 1989, 332.
47. Werry, J.; Stamm, H.; Lin, P.; Falkenstein, R.; Gries, S.; Irgartinger, H. T 1989, 45, 5015.
48. (a) Tada, M.; Akinaga, S.; Okabe, M, BCJ 1982, 55, 3939. (b) Wollowitz, S.; Halpern, J. JACS 1984, 106, 8319. (c) Barbier, M.; Barton, D. H. R.; Devys, M.; Topgi, R. S. CC 1984, 743.
49. (a) Giese, B.; Groninger, K. S. OS 1990, 69, 66. (b) See also: Giese, B.; Gilges, S.; Groninger, K. S.; Lamberth, C.; Witzel, T. LA 1988, 615.
50. Baldwin, J. E.; Adlington, R. M.; Robertson, J. T 1989, 45, 909.
51. Julia, M. ACR 1971, 4, 386.
52. Choi, J.; Ha, D.; Hart, D. J.; Lee, C.; Ramesh, S.; Wu, S. JOC 1989, 54, 279.
53. Curran, D. P.; Rakiewicz, D. M. T 1985, 41, 3943.
54. (a) Wilcox, C. S.; Gaudino, J. J. JACS 1986, 108, 3102. (b) See also: Gaudino, J. J.; Wilcox, C. S. JACS 1990, 112, 4374.
55. (a) RajanBabu, T. V. JOC 1988, 53, 4522. (b) See also: RajanBabu, T. V.; Fukunaga, T.; Reddy, G. S. JACS 1989, 111, 1759.
56. (a) Tsang, R.; Fraser-Reid, B. JACS 1986, 108, 2116. (b) See also: Alonso, R. A.; Vite, G. D.; McDevitt, R. E.; Fraser-Reid, B. JOC 1992, 57, 573.
57. RajanBabu, T. V. ACR 1991, 24, 139.
58. Stork, G.; Kahn, M. JACS 1985, 107, 500.
59. Stork, G.; Mook, R., Jr.; Biller, S. A.; Rychnovsky, S. C. JACS 1983, 105, 3741.
60. Nishiyama, H.; Kitajima, T.; Matsumoto, M.; Itoh, K. JOC 1984, 49, 2298.
61. Stork, G.; Sofia, M. J. JACS 1986, 108, 6826.
62. Stork, G.; Sher, P. M.; Chen, H. JACS 1986, 108, 6384.
63. Stork, G.; Mook, R., Jr. TL 1986, 27, 4529.
64. Beckwith, A. L. J.; O'Shea, D. M. TL 1986, 27, 4525.
65. Stork, G.; Reynolds, M. JACS 1988, 110, 6911.
66. Curran, D. P.; Chen, M.; Kim, D. JACS 1989, 111, 6265.
67. Munt, S. P.; Thomas, E. J. CC 1989, 480.
68. See for example: (a) Crich, D.; Eustace, K. A.; Fortt, S. M.; Ritchie, T. J. T 1990, 46, 2135. (b) Marco-Contelles, J.; Pozuelo, C.; Jimeno, M. L.; Martinez, L.; Martinez-Grau, A. JOC 1992, 57, 2625. (c) Chuang, C.-P.; Gallucci, J. C.; Hart, D. J.; Hoffmann, C. JOC 1988, 53, 3218. (d) Gukumoto, K.; Taniguchi, N.; Yasui, K.; Ihara, M. JCSP(1) 1990, 1469. (e) Batty, D.; Crich, D.; Fortt, S. M. JCSP(1) 1990, 2875. (f) Bachi, M. D.; Frolow, F.; Hoornaert, C. JOC 1983, 48, 1841.
69. Boger, D. L.; Mathvink, R. J. JACS 1990, 112, 4003. See also Ref. 68(a).
70. (a) Porter, N. A.; Chang, V. H.-T. JACS 1987, 109, 4976. (b) Porter, N. A.; Magnin, D. R.; Wright, B. T. JACS 1986, 108, 278. (c) For an application see: Hitchcock, S. A.; Pattenden, G. TL 1990, 31, 3641. (d) See also Baldwin, J. E.; Adlington, R. M.; Mitchell, M. B.; Robertson, J. CC 1990, 1574.
71. Giese, B.; Gonzalez-Gomez, J.; Witzel, T. AG(E) 1984, 23, 69.
72. Giese, B.; Dupuis, J.; Nix, M. OS 1987, 65, 236.
73. Porter, N. A.; Giese, B.; Curran, D. P. ACR 1991, 24, 296.
74. Ono, N.; Yoshida, Y.; Tani, K.; Okamoto, S.; Sato, F. TL 1993, 34, 6427.
75. Stork, G.; Sher, P. M. JACS 1986, 108, 303.
76. (a) Shubert, S.; Renaud, P.; Carrupt, P.; Schenk, K. HCA 1993, 76, 2473. (b) See also: Ref. 2e. For another example of a one-carbon elongation that relies on a tin reagent, see: Hart, D. J.; Seely, F. L. JACS 1988, 110, 1631.
77. (a) Curran, D. P.; Chen, M.; Spletzer, E.; Seong, C. M.; Chang, C. JACS 1989, 111, 8872. (b) Curran, D. P.; Chang, C.-T. JOC 1989, 54, 3140.
78. Keck, G. E.; Enholm, E. J.; Yates, J. B.; Wiley, M. R. T 1985, 41, 4079.
79. Baldwin, J. E.; Kelly, D. R. CC 1985, 682.
80. Ueno, Y.; Sano, H.; Okawara, M. TL 1980, 21, 1767.
81. Danishefsky, S. J.; Panek, J. S. JACS 1987, 109, 917.
82. Nakamura, E.; Machii, D.; Inubushi, T. JACS 1989, 111, 6849.
83. Hanessian, S.; Leger, R. JACS 1992, 114, 3315.
84. (a) Toops, D.; Barbachyn, M. R. JOC 1993, 58, 6505. (b) See also: Belzner, J.; Szeimies, G. TL 1987, 28, 3099.
85. Keinan, E.; Gleize, P. A. TL 1982, 23, 477.
86. Enholm, E. J.; Kinter, K. S. JACS 1991, 113, 7784.
87. Four, P.; Guibe, F. JOC 1982, 46, 4439.
88. Keinan, E.; Greenspoon, N. TL 1982, 23, 241.
89. Bloodworth, A. J.; Khan, J. A.; Loveitt, M. E. JCS(P1) 1981, 621.
90. Pedersen, S. F.; Roskamp, E. J. JACS 1987, 109, 6551.
91. For a review see ref. 1f, p. 343. see also: (a) Nemoto, H.; Wu, X. M.; Kurobe, H; Ihara, M.; Fukumoto, K. TL 1983, 24, 4257. (b) Nakatani, K.; Isobe, S. TL 1985, 26, 2209. (c) Shibasaki, M.; Susuki, H.; Torisawa, Y.; Ikegami, S. CL 1983, 1303. (d) Fleming, I.; Urch, C. J. JOM 1985, 285, 173. (e) Kadow, J. F.; Johnson, C. R. TL 1984, 25, 5255.
92. Meyer, N.; Seebach, D. CB 1980, 113, 1290.
93. Chenard, B. L.; Laganis, E. D.; Davidson, F.; RajanBabu, T. V. JOC 1985, 50, 3666.
94. Gerigk, U.; Gerlach, M.; Neumann, W. P.; Vieler, R.; Weintritt, V. S 1990, 448.
95. Ballestri, M.; Chatgilialoglu, C.; Clark, K. B.; Griller, D.; Giese, B.; Kopping, B. JOC 1991, 56, 678.

T. (Babu) V. RajanBabu

The Ohio State University, Columbus, OH, USA



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