Modern Organocopper Chemistry

68 2 Transmetalation Reactions Producing Organocopper Reagents

Transmetalation of this type has also been used to assist palladium(0)-catalyzed cross-coupling reactions in sterically congested substrates. Transmetalation of stannanes into alkenylcopper intermediates considerably accelerates subsequent palladium(0)-catalyzed cross-coupling with arylsulfonates (Scheme 2.50) [103].

Scheme 2.50. Copper(I) chloride as a promoter of Stille cross-coupling.

These transmetalations may be performed not only with copper(I) halides in DMF [104], but also by using Me2CuLi LiCN. This transmetalation has been used in the synthesis of prostaglandin derivatives (Scheme 2.51) [105].

Scheme 2.51. Prostaglandin synthesis using SnaCu transmetalation.

As well as alkenylstannanes [106–108], other classes such as a-heteroatom- substituted alkyltributylstannanes [109] and, more importantly, allylic stannanes [110, 111] also undergo these SnaCu transmetalations. Otherwise di cult to prepare, allylic copper reagents may, however, be obtained by treatment of allylic stannanes (produced in turn from organolithium, magnesium, or zinc organometallics) with Me2CuLi LiCN. They enter into cross-coupling reactions with alkyl bromides [110] or vinyl triflates (Scheme 2.52) [111].

Michael additions [112] and other reactions typical of organocopper species can also be performed with silylcopper reagents such as TBDMSCu, prepared by Sn/Cu exchange [113] between Me3SnSiMe2(tBu) and Bu(Th)CuLi LiCN (Th ¼ 2-thienyl) (Scheme 2.53) [113, 114].

Transmetalation of thioethers to organocopper compounds can also be performed in some special cases. Thus, treatment of the ester 119 with Me2CuLi LiCN provides the copper reagent 120, which can be treated successfully with several electrophiles such as allyl bromide or acid chlorides to a ord the expected products such as 121 (Scheme 2.54) [115, 116].

This reaction can be extended to cyanoketone dithioacetals [117]. Alkenyltellu-

2.5 Transmetalation of Organotin, Organosulfur, and Organotellurium Reagents 69

Scheme 2.52. Cross-coupling of allylic copper compounds.

Scheme 2.53. Preparation of silylcuprates by Sn/Cu-transmetalation.

Scheme 2.54. Sulfur/copper exchange reaction.

rium species also undergo exchange with Me2CuLi LiCN. The synthetic importance of this exchange is due to the easy availability of (Z)-alkenyltellurium species by reduction of alkynyl tellurides such as 122 (Scheme 2.55) [118].

Scheme 2.55. Te/Cu exchange reactions of (Z)-alkenyltellurium species.

702 Transmetalation Reactions Producing Organocopper Reagents

2.6

Transmetalation of Organotitanium and Organomanganese Reagents

Transmetalations with first row transition metal elements such as titanium or manganese have produced useful synthetic applications. Organotitanate species of type 123 show the advantage of high SN20 selectivity in the anti stereochemistry of the resulting copper(I) intermediates (Scheme 2.56) [119, 120].

Scheme 2.56. Copper(I)-catalyzed anti-SN20 substitution of allylic phosphates.

Organomanganese reagents are very useful organometallics, reacting with high chemoselectivity with acid chlorides [121] and several other classes of electrophiles [122]. The scope of organomanganese reagents can be greatly increased by use of copper(I) catalysis. Especially impressive is the performance of Michael additions [123–128]. Thus, the Michael addition between BuMnCl and pulegone 124, furnishing 125, proceeds in excellent yield in the presence of Li2CuCl4 (3 mol%) (Scheme 2.57) [128].

Scheme 2.57. Copper-catalyzed Michael addition reactions between organomanganese reagents and pulegone.

Acylation reactions can also be greatly improved in this way, with t-alkyl- or sec- alkyl-manganese reagents reacting with acid chlorides in excellent yields [123]. The related addition-elimination to 3-ethoxy-2-cyclohexenone is also improved, resulting after acidic aqueous workup in 3-methyl-2-cyclohexenone [125]. The perillaketone 126 was prepared in an improved yield using copper(I) catalysis (Scheme 2.58) [129].

2.7 Transmetalation of Organozirconium and Organosamarium Reagents 71

Scheme 2.58. Preparation of perilla-ketone using copper-catalyzed acylation.

Alkylation of organomanganese reagents with alkyl bromides can also be improved by addition of CuCl (3 mol%). The reactions proceed at room temperature and are complete within a few hours [123, 130]. The opening of epoxides is also improved under these conditions. The reaction also features good chemoselectivity, tolerating the presence of sensitive functions such as ketones (Scheme 2.59) [130].

Scheme 2.59. Copper-catalyzed alkylation of alkyl manganese reagents.

Benzylic organomanganese reagents prepared by direct insertion of activated manganese metal display the same behavior (Scheme 2.60) [131]. Excellent results are also obtained for 1,4-additions of organomanganese reagents to unsaturated esters in the presence of CuCl (3 mol%) [127].

Scheme 2.60. Copper-catalyzed acylation of benzylic manganese reagents.

2.7

Transmetalation of Organozirconium and Organosamarium Reagents

Transmetalation reactions of organozirconium reagents were pioneered by Schwartz [130–132], with improved procedures developed more recently by Lipshutz [133] and Wipf [134]. The hydrozirconation of 1-hexene with H(Cl)ZrCp2 at 25 C under sonication conditions produces the n-hexylzirconium complex 127, which adds to cyclohexenone in the presence of CuBr Me2S (10 mol%) to a ord the desired product 128 in 79% isolated yield (Scheme 2.61) [134].

72 2 Transmetalation Reactions Producing Organocopper Reagents

Scheme 2.61. Copper-catalyzed 1,4-addition of alkylzirconium derivatives.

Similarly, alkenylzirconium species prepared by the hydrozirconation of alkynes add in a conjugated fashion to enones. Formation of an intermediate zincate prior to transmetalation to the copper species facilitates the Michael addition (Scheme 2.62) [135]. This methodology has been applied to the preparation of protected misoprostol 129 (Scheme 2.63) [136, 137].

Scheme 2.62. ‘‘Michael addition of an alkenylzirconium compound’’, by successive transmetalation into zinc and copper intermediates.

Scheme 2.63. Synthesis of protected misoprostol 129.

The mechanism and the nature of the reaction intermediates have been carefully studied by Wipf, revealing an activation of the carbonyl group of the enone by the zirconium complex. Remarkably, a variety of primary and secondary alkylzirconium complexes can be added to enones in 1,4-fashion under mild conditions [134, 138]. Interestingly, treatment of zirconocyclopentadienes such as 130 with alkynes such as dimethyl acetylenedicarboxylate in the presence of CuCl gives benzene derivatives such as 131 [136, 137]. A transmetalation from Zr to Cu has been postulated in this reaction. Annelation reactions involving a similar transmetalation of 130 and cross-coupling with 1,2-diodobenzene proceeds in high yield to a ord 132 (Scheme 2.64) [139, 140].

2.7 Transmetalation of Organozirconium and Organosamarium Reagents 73

Scheme 2.64. Copper-catalyzed reactions of zirconocyclopentadienes.

Cross-coupling reactions between alkenylzirconocenes such as 133 and aryl or alkenyl iodides occur readily in the presence of CuCl and Pd(PPh3)4, producing tetrasubstituted olefins such as 134 in good yields (Scheme 2.65) [141, 142].

Scheme 2.65. Cross-coupling between alkenylzirconocene complexes and aryl iodides.

Carbocupration of alkynes by zirconacyclopentane derivatives can be performed according to the same procedure. Thus, the zirconocyclopentane 135, obtained by treatment of Bu2ZrCp2 with 1,6-heptadiene, reacts at room temperature with phenylacetylene to a ord the adduct 136 through a carbocupration-reductive elimination mechanism. Cross-coupling followed by intramolecular carbocupration takes place in the case of the reaction with 1-bromohexyne, producing 137 (Scheme 2.66) [143].

Scheme 2.66. Copper-catalyzed reactions of zirconacyclopentane derivatives.

74 2 Transmetalation Reactions Producing Organocopper Reagents

Finally, spiro-compounds such as 138 can be prepared by treatment of zirconacylopentadienes such as 139 with 3-iodo-2-cyclohexenone in the presence of CuCl (2 equiv.) (Scheme 2.67) [144].

Scheme 2.67. Spirometalation of zirconacyclopentadienes.

Very few transmetalations between organolanthanides and organocopper reagents have been reported. Organosamarium(III) reagents, prepared by treatment of SmI2 with alkyl halides in THF/HMPA, undergo easy conjugate addition to unsaturated ketones and nitriles in the presence of TMSCl, producing the corresponding Michael adducts. Functionalized alkyl bromides such as 140 react chemoselectively with cyclohexenone in the presence of TMSCl and CuBr Me2S (0.1 equiv.) to a ord the polyfunctional ketone 141 in 60% yield (Scheme 2.68) [145].

Scheme 2.68. Copper-catalyzed 1,4-addition of organosamarium reagents.

2.8

Conclusion

Transmetalations of various organometallic species with copper salts have been found to produce highly useful organocopper reagents of great synthetic interest. Many di erent organometallic precursors have proved valuable, depending on the functionality present in the copper reagent. The scope of organocopper chemistry has been greatly enhanced by these new transmetalation reactions and these reagents have found many applications in organic synthesis.

			References	75
References
1	B. H. Lipshutz, S. Sengupta, Org.	22	M. Abarbri, P. Knochel, Synlett
	React. 1992, 41, 135.		1999, 1577.
2	P. Wipf, Synthesis, 1993, 537.	23	F. Dehmel, M. Abarbri, P. Knochel,
3	E. Negishi, Organometallics in Organic		Synlett 2000, 345.
	Synthesis, Wiley: New York, 1980.	24	M. Abarbri, J. Thibonnet,
4	N. Krause, Angew. Chem. 1999, 111,		L. Be´rillon, F. Dehmel,
	83; Angew. Chem. Int. Ed. Engl. 1999,		J. Org. Chem. 2000, 65, 4618.
	38, 79.	25	M. Abarbri, F. Dehmel, P. Knochel,
5	G. Boche, F. Bosold, M. Marsch,		Tetrahedron Lett. 1999, 40, 7449.
	K. Harms, Angew. Chem. 1998, 110,	26	B. H. Lipshutz, D. A. Parker, S. L.
	1779; Angew. Chem. Int. Ed. Engl.		Nguyen, K. E. McCarthy, J. C.
	1998, 37, 1684.		Barton, S. E. Whitney, H. Kotsuki,
6	C. E. Tucker, T. N. Majid, P.		Tetrahedron 1986, 42, 2873.
	Knochel, J. Am. Chem. Soc. 1992,	27	G. Varchi, A. Ricci, G. Cahiez, P.
	114, 3983.		Knochel, Tetrahedron 2000, 56, 2727.
7	P. Knochel, M. C. P. Yeh, S. C.	28	G. Cahiez, C. Chaboche, M.
	Berk, J. Talbert, J. Org. Chem. 1988,		Je´ze´quel, Tetrahedron 2000, 56, 2733.
	53, 2390.	29	W. Dohle, L. Be´rillon, M. Wimmer,
8	Y. Horiguchi, S. Matsuzawa,		P. Knochel, manuscript in
	E. Nakamura, I. Kuwajima,		preparation.
	Tetrahedron Lett. 1986, 27, 4025.	30	N. Miyaura, N. Sasaki, M. Itoh,
9	J. F. Cameron, J. M. J. Fre´chet,		A. Suzuki, Tetrahedron Lett. 1977,
	J. Am. Chem. Soc. 1991, 113, 4303.		18, 173.
10	C. Najera, M. Yus, Recent Res. Devel.	31	H. Yatagai, J. Org. Chem. 1980, 45,
	in Organic Chem. 1997, 1, 67.		1640.
11	C. Gomez, F. F. Huerta, M. Yus,	32	H. C. Brown, G. Molander, J. Org.
	Tetrahedron Lett. 1997, 38, 687.		Chem. 1981, 46, 645.
12	C. Gomez, F. F. Huerta, M. Yus,	33	J. Ichikawa, S. Hamada, T. Sonoda,
	Tetrahedron 1998, 6177.		H. Kobayashi, Tetrahedron Lett. 1992,
13	D. J. Ramon, M. Yus, Tetrahedron Lett.		33, 337.
	1993, 34, 7115.	34	P. Knochel, Synlett, 1995, 393.
14	C. Gomez, F. F. Huerta, M. Yus,	35	R. E. Ireland, P. Wipf, J. Org. Chem.
	Tetrahedron 1998, 54, 1853.		1990, 55, 1425.
15	F. Foubelo, A. Gutierrez, M. Yus,	36	P. Wipf, J. H. Smitrovich, C.-W.
	Tetrahedron Lett. 1997, 38, 4837.		Moon, J. Org. Chem. 1992, 57, 3178.
16	A. Guijarro, M. Yus, Tetrahedron	37	P. Wipf, L. Lim, Angew. Chem. 1993,
	1995, 51, 231.		105, 1095; Angew. Chem. Int. Ed. Engl.
17	R. D. Rieke, Science 1989, 246, 1260.		1993, 32, 1068.
18	T. P. Burns, R. D. Rieke, J. Org.	38	S. Sharma, A. C. Oehlschlager,
	Chem. 1987, 52, 3674.		J. Org. Chem. 1989, 54, 5064.
19	M. Rottla¨nder, L. Boymond, L.	39	E. Frankland, Liebigs Ann. 1849, 71,
	Be´rillon, A. Lepreˆtre, G. Varchi,		171.
	S. Avolio, H. Laaziri, G.	40	E. Negishi, Acc. Chem. Res. 1982, 15,
	Que´guiner, A. Ricci, G. Cahiez, P.		340.
	Knochel, Chem. Eur. J. 2000, 6, 767.	41	P. Knochel, R. Singer, Chem. Rev.
20	L. Boymond, M. Rottla¨nder, G.		1993, 93, 2117.
	Cahiez, P. Knochel, Angew. Chem.	42	P. Knochel, J. J. Almena Perea,
	1998, 110, 1801; Angew. Chem. Int. Ed.		P. Jones, Tetrahedron 1998, 54, 8275.
	Engl. 1998, 37, 1701.	43	C. E. Tucker, P. Knochel, J. Org.
21	L. Be´rillon, A. Lepreˆtre, A. Turck,		Chem. 1993, 58, 4781.
	N. Ple´, G. Que´guiner, G. Cahiez,	44	K. Soai, S. Niwa, Chem. Rev. 1992, 92,
	P. Knochel, Synlett 1998, 1359.		833.

76 2 Transmetalation Reactions Producing Organocopper Reagents

45	S. C. Berk, M. C. P. Yeh, N. Jeong,	65	L. Micouin, P. Knochel, Synlett
	P. Knochel, Organometallics 1990, 9,		1997, 327.
	3053.	66	R. Duddu, M. Eckhardt,
46	H. P. Knoess, M. T. Furlong, M. J.		M. Furlong, H. P. Knoess, S.
	Rozema, P. Knochel, J. Org. Chem.		Berger, P. Knochel, Tetrahedron
	1991, 56, 5974.		1994, 50, 2415.
47	C. Janikaram, P. Knochel,	67	L. Schwink, P. Knochel, Tetrahedron
	Tetrahedron 1993, 49, 29.		Lett. 1994, 35, 9007.
48	T. N. Majid, P. Knochel, Tetrahedron	68	A. Devasagayaraj, L. Schwink,
	Lett. 1990, 31, 4413.		P. Knochel, J. Org. Chem. 1995, 60,
49	T. M. Stevenson, A. S. B. Prasad,		3311.
	J. R. Citineni, P. Knochel,	69	A. Longeau, F. Langer, P.
	Tetrahedron Lett. 1996, 37, 8375.		Knochel, Tetrahedron Lett. 1996, 37,
50	L. Zhu, R. M. Weymeyer, R. D.		2209.
	Rieke, J. Org. Chem. 1991, 56, 1445.	70	F. Langer, L. Schwink,
51	R. D. Rieke, M. V. Hanson, J. D.		A. Devasagayaraj, P.-Y. Chavant,
	Brown, Q. J. Niu, J. Org. Chem. 1996,		P. Knochel, J. Org. Chem. 1996, 61,
	61, 2726.		8229.
52	M. V. Hanson, R. D. Rieke, J. Am.	71	C. Darcel, F. Flachsmann,
	Chem. Soc. 1995, 117, 10775.		P. Knochel, Chem. Commun. 1998,
53	H. Stadtmu¨ller, R. Lentz, C. E.		205.
	Tucker, T. Stu¨demann, W. Do¨rner,	72	A. Boudier, F. Flachsmann,
	P. Knochel, J. Am. Chem. Soc. 1993,		P. Knochel, Synlett 1998, 1438.
	115, 7027.	73	A. Boudier, E. Hupe, P. Knochel,
54	H. Stadtmu¨ller, C. E. Tucker,		Angew. Chem. 2000, 112, 2396; Angew.
	A. Vaupel, P. Knochel, Tetrahedron		Chem. Int. Ed. Engl. 2000, 39, 2294.
	Lett. 1993, 34, 7911.	74	S. Berger, F. Langer, C. Lutz,
55	A. Vaupel, P. Knochel, Tetrahedron		P. Knochel, T. A. Mobley, C. K.
	Lett. 1994, 35, 8349.		Reddy, Angew. Chem. 1997, 109, 1603;
56	H. Stadtmu¨ller, A. Vaupel, C. E.		Angew. Chem. Int. Ed. Engl. 1997, 36,
	Tucker, T. Stu¨demann, P. Knochel,		1496.
	Chem. Eur. J. 1996, 2, 1204.	75	C. Lutz, P. Knochel, J. Org. Chem.
57	A. Vaupel, P. Knochel, Tetrahedron		1997, 62, 7895.
	Lett. 1995, 36, 231.	76	P. Jones, P. Knochel, J. Chem. Soc.
58	H. Stadtmu¨ller, P. Knochel, Synlett		Perkin Trans. 1 1997, 3117.
	1995, 463.	77	H. G. Chen, J. L. Gage, S. D.
59	R. Giovannini, T. Stu¨demann,		Barrett, P. Knochel, Tetrahedron
	A. Devasagayaraj, G. Dussin,		Lett. 1990, 31, 1829.
	P. Knochel, J. Org. Chem. 1999, 64,	78	F. Du¨bner, P. Knochel, Tetrahedron
	3544.		Lett. 2000, 41, 9233.
60	P. Jones, P. Knochel, J. Org. Chem.	79	M. J. Dunn, R. F. W. Jackson, J.
	1999, 64, 186.		Chem. Soc. Chem. Commun. 1992, 319.
61	N. Millot, P. Knochel, Tetrahedron	80	B. H. Lipshutz, K. Woo, T. Gross,
	Lett. 1999, 40, 7779.		D. J. Buzard, R. Tirado, Synlett 1997,
62	M. J. Rozema, A. Sidduri,		477.
	P. Knochel, J. Org. Chem. 1992, 57,	81	W. Blankenfeldt, J.-W. Liao, L.-C.
	1956.		Lo, M. C. P. Yeh, Tetrahedron Lett.
63	M. J. Rozema, C. Eisenberg,		1996, 37, 7361.
	H. Lu¨tjens, R. Ostwald, K. Belyk,	82	J. H. Rigby, M. Kirova-Snover,
	P. Knochel, Tetrahedron Lett. 1993,		Tetrahedron Lett. 1997, 38, 8153.
	34, 3115.	83	M. C. P. Yeh, P. Knochel,
64	A. B. Charette, A. Beauchemin,		Tetrahedron Lett. 1989, 30, 4799.
	J. F. Marcoux, J. Am. Chem. Soc.	84	H. So¨rensen, A. E. Greene,
	1998, 120, 5114.		Tetrahedron Lett. 1990, 31, 7597.

			References	77
	E. J. Corey, C. J. Helal, Tetrahedron
85		105	J. R. Behling, K. A. Babiak, J. S. Ng,
	Lett. 1997, 38, 7511.		A. L. Campbell, R. Moretti,
86	S. Marquais, G. Cahiez,		M. Koerner, B. H. Lipshutz, J. Am.
	P. Knochel, Synlett 1994, 849.		Chem. Soc. 1988, 110, 2641.
87	A. Sidduri, N. Budries, R. M. Laine,	106	E. Piers, E. J. McEachern, P. A.
	P. Knochel, Tetrahedron Lett. 1992,		Burns, J. Org. Chem. 1995, 60, 2322.
	33, 7515.	107	E. Piers, J. M. Chong, Can. J. Chem.
88	H. G. Chen, C. Hoechstetter,		1988, 66, 1425.
	P. Knochel, Tetrahedron Lett. 1989,	108	E. Piers, H. E. Morton, J. M.
	30, 4795.		Chong, Can. J. Chem. 1987, 65, 78.
89	M. C. P. Yeh, P. Knochel, W. M.	109	J. R. Falck, R. K. Bhatt, J. Ye, J. Am.
	Butler, S. C. Berk, Tetrahedron Lett.		Chem. Soc. 1995, 117, 5973.
	1988, 29, 6693.	110	B. H. Lipshutz, R. Crow, S. H.
90	H. Tsujiyama, N. Ono, T. Yoshino,		Dimock, E. L. Ellsworth, R. A. J.
	S. Okamoto, F. Sato, Tetrahedron Lett.		Smith, J. R. Behling, J. Am. Chem.
	1990, 31, 4481.		Soc. 1990, 112, 4063.
91	T. Yoshino, S. Okamoto, F. Sato,	111	B. H. Lipshutz, T. R. Elworthy,
	J. Org. Chem. 1991, 56, 3205.		J. Org. Chem. 1990, 55, 1695.
92	K. Miyaji, Y. Ohara, Y. Miyauchi,	112	B. H. Lipshutz, J. I. Lee, Tetrahedron
	T. Tsuruda, K. Arai, Tetrahedron Lett.		Lett. 1991, 32, 7211.
	1993, 34, 5597.	113	B. H. Lipshutz, D. C. Reuter, E. L.
93	M. Kitamura, T. Miki, K. Nakano,		Ellsworth, J. Org. Chem. 1989, 54,
	R. Noyori, Tetrahedron Lett. 1996, 37,		4975.
	5141.	114	B. H. Lipshutz, S. Sharma, D. C.
94	G. Cahiez, P. Venegas, C. E.		Reuter, Tetrahedron Lett. 1990, 31,
	Tucker, T. N. Majid, P. Knochel,		7253.
	J. Chem. Soc., Chem. Commun. 1992,	115	M. Hojo, S. Tanimoto, J. Chem. Soc.
	1406.		Chem. Commun. 1990, 1284.
95	C. E. Tucker, P. Knochel, Synthesis	116	M. Hojo, H. Harada, A. Hosomi,
	1993, 530.		Chem. Lett. 1994, 437.
96	C. Jubert, P. Knochel, J. Org. Chem.	117	M. Hojo, H. Harada, C. Watanabe,
	1992, 57, 5431.		A. Hosomi, Bull. Chem. Soc. Jpn.
97	M. C. P. Yeh, P. Knochel, L. E. Santa,		1994, 67, 1495.
	Tetrahedron Lett. 1988, 29, 3887.	118	J. V. Comasseto, J. N. Berriel, Synth.
98	S. A. Rao, P. Knochel, J. Am. Chem.		Commun. 1990, 20, 1681.
	Soc. 1991, 113, 5735.	119	M. Arai, E. Nakamura, B. H.
99	P. Knochel, M. J. Rozema, C. E.		Lipshutz, J. Org. Chem. 1991, 56,
	Tucker, in Organocopper Reagents, A		5489.
	Practical Approach, R. J. K. Taylor	120	M. Arai, B. H. Lipshutz,
	(Ed.), Oxford University Press,		E. Nakamura, Tetrahedron 1992,
	Oxford, 1994, pp. 85–104.		48, 5709.
100	P. Knochel, P. Jones (Eds.),	121	G. Friour, G. Cahiez, J. F.
	Organozinc Reagents: A Practical		Normant, Synthesis 1984, 37.
	Approach; Oxford University Press,	122	G. Cahiez, An. Quim. 1995, 91, 561.
	Oxford, 1999.	123	G. Cahiez, S. Marquais, Pure Appl.
101	V. Farina, S. Kapdia, B. Krishnan,		Chem. 1996, 68, 53.
	C. Wang, L. S. Liebeskind, J. Org.	124	G. Cahiez, M. Alami, Tetrahedron
	Chem. 1994, 59, 5905.		1989, 45, 4163.
102	E. Piers, E. J. McEachern, P. A.	125	G. Cahiez, M. Alami, Tetrahedron
	Burns, Tetrahedron 2000, 56, 2753.		Lett. 1989, 30, 3541.
103	X. Han, B. M. Stoltz, E. J. Corey,	126	G. Cahiez, M. Alami, Tetrahedron
	J. Am. Chem. Soc. 1999, 121, 7600.		Lett. 1989, 30, 7365.
104	E. Piers, T. Wong, J. Org. Chem.	127	G. Cahiez, M. Alami, Tetrahedron
	1993, 58, 3609.		Lett. 1990, 31, 7423.