A stereospecific, intermolecular biaryl-coupling approach to korupensamine A en route to the michellamines

Article abstract of DOI:10.1002/(SICI)1521-3773(19991203)38:23<3530::AID-ANIE3530>3.0.CO;2-W

By what means and how well can axial chirality be controlled in an intermolecular Suzuki biaryl cross-coupling reaction? The directionality of reductive elimination [Eq.(1)] is completely controlled by using a strategically positioned internal ligand L to afford a single biaryl atropisomer corresponding to the korupensamine A skeleton. TIPS = iPr₃Si, Ts = H₃CC₆H₄SO₂.

Full text of DOI:10.1002/(SICI)1521-3773(19991203)38:23<3530::AID-ANIE3530>3.0.CO;2-W

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[10] The workup was carried out chromatographically (SiO2, 40 ± 60 m;
dichloromethane/tert-butyl methyl ether 12/1). The mixture of the
isomers of rotaxane 1 showed the expected mass spectrum: m/z 3148.6
OH
HN
‡
‡
[M‡H] (FAB, 9-nitroanthracene (9-NA)) m/z 3167.4 [M‡Na]
OH
HO
R/M
(MALDI-TOF; calcd for C204H220N10O17S2: 3148.12). The compound
has been characterized by 1H and 13C NMR spectroscopy. M.p. 2788C;
elemental analysis calcd for C204H220N10O17S2 ´ 3CH2Cl2: C 73.06, H
6.69, N 4.12, S 1.88; found: C 72.98, H 6.77, N 4.13, S 2.31; the
dichloromethane is detected by NMR spectroscopy.
OMe
MeO
HO
HO
S/P
[11] a) E. Yashima, Y. Okamoto, Bull. Chem. Soc. Jpn. 1995, 68, 3289; b) Y.
Okamoto, E. Yashima, Angew. Chem. 1998, 110, 1072 ± 1095; Angew.
Chem. Int. Ed. 1998, 37, 1020 ± 1043; c) for polarimetric detection
during liquid chromatography, see A. Mannschreck, N. Pustet, F.
Brandl, GIT Spezial 1999, 1, 34 ± 36.
NH
HO
Michellamine B
[12] Each fraction was analyzed by mass spectroscopy (MALDI-TOF-
MS). Every stereoisomer had the same mass peak. Conditions of the
separation: Column Chiralpak AD (25 Â 0.46 cm inner diameter),
amylose tris[3,5-(dimethylphenyl)carbamate], eluent n-hexane/etha-
nol 81/19; flow rate 1.3 mLmin 1; pressure 23 bar; detector UV
(Millipore Waters, Lambda Max, Model 481, LC Spectrophotometer);
sample 10 mgmL 1 in chloroform/n-hexane (1/1).
[13] Optical rotation: JASCO Polarimeter P-1020, cell 100 Â 3.5 mm inner
diameter (CHCl3); circular dichroism spectra: JASCO Spectropo-
larimeter J-720, cell 0.1 mm (trifluoroethanol).
[14] S. Grimme, J. Harren, A. Sobanski, F. Vögtle, Eur. J. Org. Chem. 1998,
1499 ± 1509.
[15] a)D. A. Leigh, A. Murphy, Chem. Ind. 1999, 3, 178; b) K. Mislow,
Topics in Stereochemistry, Vol. 2 (Ed.: Scott E. Denmark), Wiley-
VCH, Weinheim, 1999.
HO
HO
OMe
HO
OMe
R/M
S/P
HO
HO
NH
NH
HO
Korupensamine A
Korupensamine B
Scheme 1. Retrosynthesis of michellamine B to its components korupens-
amines A and B.
The guiding principles upon which our route was developed
(Scheme 2) focus on the presence of a hydroxyl ªhandleº in
each of the components. These were anticipated to afford
considerable flexibility in fine-tuning the selectivity in the
BnO
OMe
A Stereospecific, Intermolecular Biaryl-
Coupling Approach to Korupensamine A
En Route to the Michellamines**
steric effect on
biaryl coupling?
O R
9'
M
1
internal ligand for
PdII intermediate
Bruce H. Lipshutz* and John M. Keith
O
L
I
3
MeO
The michellamines make up a rather unusual group of
highly active antiviral natural products, with michellamine B
receiving most of the attention as a potent anti-HIV-1 and -2
agent.[1] The components of this alkaloid, korupensamines A
and B, are related as diastereomers with respect to their axial
chirality (Scheme 1).[2] All attempts to effect a direct, highly
stereocontrolled biaryl coupling between the naphthyl and
tetrahydroisoquinoline portions have thus far met with
limited success,[3] which has encouraged a number of groups
to devise clever, albeit indirect, alternatives.[4] We now
describe a novel solution to this problem that allows for
exclusive entry to the korpensamine A series through a Pd0-
mediated intermolecular biaryl cross-coupling reaction.
N
R'
MeO
2
Scheme 2. Possible influence of substituents on the hydroxymethyl
ªhandlesº.
biaryl-forming step. Such a residue at C-9' in 1 might facilitate
the introduction of a potentially influential steric component,
while an OH function on the methyl group at C-3 in 2 would
provide an opportunity for intramolecular chelation, thereby
directing biaryl formation from one face of a transient
polycyclic array.
Our route to nonracemic, protected hydroxymethyliodo-
tetrahydroisoquinoline
9 began with aryl chloride 3
[*] Prof. B. H. Lipshutz, Dr. J. M. Keith
Department of Chemistry
(Scheme 3).[5] A copper-catalyzed opening of commercially
available (S)-TBS-glycidol (TBS ˆ tert-butyldimethylsilyl)
with the Grignard reagent derived from 3 leads to alcohol 4,
which is suitable for a Mitsunobu inversion[6] with phthalimide
followed by hydrazinolysis to give nonracemic amine 5.
Acetamide formation sets the stage for a temperature-
sensitive[7] Bischler± Napieralski cyclization to imine 6. Care-
ful reduction with a 1:7 mixture of Me3Al (in heptane) and
University of California
Santa Barbara, CA 93106 (USA)
Fax: (‡001)805-893-8265
E-mail: lipshutz@chem.ucsb.edu
[**] We warmly thank the NIH (GM-40287) and the Universitywide Task
Force on AIDS Research for supporting our programs. We also
appreciate most valuable discussions with Prof. G. Bringmann
(University of Wurzburg).
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to use a boronate (12; Scheme 4), rather than the precursor
boronic acid, which has good hydrolytic stability due to the
presence of the OTIPS residue in this molecule. Initial model
couplings with a simple 1-naphthalene boronate, while
OTBS
MeO
Cl
MeO
A
B
OH
MeO
MeO
MeO
3
4
OTBS
OTBS
MeO
C
O
D
BnO OMe
BnO OMe
NH2
N
A-C
O
MeO
MeO
5
B
Br
OH
OTIPS
O
O
OTBS
F
OTBS
E
MeO
MeO
MeO
MeO
11
12
HN
N
Scheme 4. A) TIPS-OTf, Et3N, CH2Cl2, RT, 24 h, 82%; B) nBuLi, 788C,
B(OiPr)3, RT, overnight, then NH4Cl, 81%; C) HO(CH2)3OH, PhCH3, RT,
molecular sieves, 18 h, 98%.
O
OH
G
MeO
MeO
MeO
OTIPS
H
6
8
NH
NH
successful, did not translate to the ªrealº system, and revealed
a critical need for 2,6-di-tert-butyl-4-methylphenol (BHT;
0.7 equiv)[12] in the reaction medium (DMF). The coupling of
iodide 9 with 12 occurred efficiently under these modified
conditions (81%; Scheme 5), but the 5:4 ratio of S(P):R(M)
atropisomers obtained indicated that steric factors alone at
each hydroxymethyl site in 1 and 2 (R ˆ L ˆ TIPS) are not
sufficient for high diastereocontrol in the biaryl forming step.
MeO
7
I
OTIPS
OTIPS
MeO
MeO
MeO
I
J
N
N
N
N
Ts
Ts
MeO
MeO
9
I
I
OH
K
O-L
MeO
Ts
Ts
BnO OMe
MeO
MeO
10
2
OTIPS
BnO OMe
Scheme 3. A) 1) Mg, THF, BrCH2CH2Br D, 10 h; 2) (S)-TBS-glycidol,
10% CuBr´ SMe2, 208C, 4.5 h, 92%; B) phthalimide, PPh3, DEAD, THF,
08C to RT, 20 h, 88%; C) H2NNH2, EtOH, D, 7 h, 89%; D) Ac2O, Et3N,
CH2Cl2, RT, overnight, 99%; E) POCl3, CH3CN, 2,4,6-collidine, 858C, 3 h,
97%; F) LAH, Me3Al, THF/hexanes, 788C to RT, overnight, 97% (3:1
trans:cis); G) NaH, TIPS-OTf, THF, 788C, 85%; H) TsCl, Et3N, CH2Cl2,
RT, 93%; I) PhI(O2CCF3)2, I2, CH2Cl2, 10 to 08C, 89%; J) TBAF, THF,
RT, 2 h, 99%; K) NaH, THF, ClPPh2, 08C to RT, 21 h, then BH3 ´ THF, RT,
40 min, 83%, or (2-diphenylphosphanyl)benzoic acid, DCC, CH2Cl2, RT,
14 h, 99%. DCC ˆ dicyclohexylcarbodiimide; DEAD ˆ diethyl azodicarb-
oxylate; TBAF ˆ tetrabutylammonium fluoride; Ts ˆ p-H3CC6H4SO2.
B
O
O
OTIPS
OTIPS
A
MeO
12
N
I
Ts
MeO
MeO
OTIPS
13
N
Ts
81 %; S(P) : R(M) = 5 : 4
MeO
9
Scheme 5. A) cat. [Pd(PPh3)4], K3PO4, BHT, DMF, 968C.
lithiumaluminum hydride (LAH; solid)[8] gives 7 in a 3:1 ratio
of trans:cis products[9] in excellent yield, albeit with concom-
itant loss of the TBS residue. To maximize the size of the L
group in 2, the triisopropylsilyl (TIPS) moiety was installed in
8 by conversion of 7 into its corresponding alkoxide followed
by the low-temperature addition of TIPS-OTf (Tf ˆ F3CSO2).
N-Sulfonylation was accomplished without incident, and
iodination[10] proceeds regiospecifically at 10 to 08C to give
the coupling partner 9.
The halonaphthol precursor to 1, bromide 11, was prepared,
with minor modifications, according to Bringmann,[3a] thereby
providing the hydroxymethyl ªhandleº required to test the
effects of a bulky methyl group at this site. The nine-step
sequence gave the desired derivative 11 in good overall yield
(24%).
The attachment of an internal chelating phosphane through
the free hydroxyl group in 10 was far more rewarding.[13] After
conversion of 10 into the diphenylphosphane derivative 14
(Scheme 6), its Pd0-mediated coupling with 12 afforded a
modest (36%) yield of cross-coupling product; however, a
single diastereomer was produced. A switch to the ortho-
diphenylphosphanylbenzoic acid ester derivative 15[14] was
made because of the less than satisfactory level of efficiency
and the tendency of 16 (L ˆ PPh2) to autoxidize to the
corresponding phosphane oxide upon workup. The coupling
of ester 15 with 12, which was found to be better catalyzed by
[Pd(dppf)Cl2], also afforded a single diastereomer, now in
71% yield (dppf ˆ 1,1'-bis(diphenylphosphanyl)ferrocenyl).
Complete conversion into 16 could be accomplished by
increasing the amount of catalyst and raising the temperature,
thereby boosting the yield of the isolated product to 81%.
The sole formation of the S(P) atropisomer can be
rationalized as viewed in Scheme 7. The PPh2-substituted
benzoate residue is best accommodated with the phosphorus
Numerous experiments designed to ascertain the most
effective organometallic compound (M in 1) for the inter-
molecular Pd0-catalyzed coupling led us to focus on Suzuki-
based methodology, a conclusion also reached by Hoye and
Chen for this purpose.[11] However, it was essential in this case
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opportunities exist for directing the orientation of the two aryl
units so as to encourage formation of the R(M) atropisomer
associated with korupensamine B. These and related applica-
tions to tetrahydroisoquinoline-based biaryl alkaloids, as well
as to other challenging naturally occurring biaryls (for
example, vancomycin) are currently under study.
BnO OMe
OTIPS
BnO OMe
B
O
O
OTIPS
O-L
A
MeO
12
N
O-L
Ts
I
Ts
MeO
Received: March 23, 1999 [Z13207IE]
German version: Angew. Chem. 1999, 111, 3743 ± 3746
MeO
N
16
MeO
2
Keywords: atropisomerism ´ biaryls ´ korupensamines ´
michellamines ´ tetrahydroisoquinolines
yield of 16
L in 2:
PPh2
(14)
(15)
36 %
Ph2P
81 %
C
O
[1] a) K. P. Manfredi, J. W. Blunt, J. H. Cardellina II, J. B. McMahon, L. L.
Pannell, G. M. Cragg, J. Med. Chem. 1991, 34, 3402 ± 3405; b) M. R.
Boyd, Y. F. Hallock, J. H. Cardellina II, K. P. Manfredi, J. W. Blunt,
J. B. McMahon, R. W. Buckheit, G. Bringmann, M. Schaffer, G. M.
Cragg, D. W. Thomas, J. G. Jato, J. Med. Chem. 1994, 37, 1740 ± 1745;
c) Y. F. Hallock, K. P. Manfredi, J. R. Dai, J. H. Cardellina II, R. J.
Gulakowski, J. B. McMahon, M. Schaffer, M. Stahl, K. P. Gulden, G.
Bringmann, G. Francois, M. R. Boyd, J. Nat. Prod. 1997, 60, 677 ± 683;
d) G. Bringmann, F. Pokorny in The Alkaloids, Vol. 46 (Ed.: G. A.
Cordell), Academic Press, New York, 1995, p 127 ± 271.
Scheme 6. A) for 12 ‡ 14: 10 mol% [Pd(PPh3)4], K3PO4, BHT, DMF,
998C; for 12 ‡ 15: 20 mol% [Pd(dppf)Cl2], K3PO4, BHT, DMF, 1178C.
OBn
MeO
Ferrocene
Ph
P
reductive
TIPSO
MeO
16
Ph
Pd
Ph
[2] Y. F. Hallock, K. P. Manfredi, J. W. Blunt, J. H. Cardellina II, M.
Schäffer, K. P. Gulden, G. Bringmann, A. Y. Lee, J. Clardy, G.
François, M. R. Boyd, J. Org. Chem. 1994, 59, 6349 ± 6355.
elimination
P
O
Ph
N
[3] a) G. Bringmann, R. Götz, P. A. Keller, R. Walter, P. Henschel, M.
Schäffer, M. Stäblein, T. R. Kelly, M. R. Boyd, Heterocycles 1994, 39,
503 ± 508; b) P. D. Hobbs, V. Upender, J. Liu, D. J. Pollart, D. W.
Thomas, M. Dawson, Chem. Commun. 1996, 923 ± 924; P. D. Hobbs, V.
Upender, M. I. Dawson, Synlett 1997, 965 ± 967; c) T. R. Hoye, M.
Chen, Tetrahedron Lett. 1996, 37, 3099 ± 3100; T. R. Hoye, M. Chen, L.
Mi, O. P. Priest, Tetrahedron Lett. 1994, 35, 8747 ± 8750; d) T. R. Hoye,
L. Mi, J. Org. Chem. 1997, 62, 8586 ± 8588.
[4] a) G. Bringmann, M. Breuning, S. Tasler, Synthesis 1999, 525 ± 558; G.
Bringmann, M. Ochse, Synlett 1998, 1294 ± 1296; G. Bringmann, R.
Gotz, P. A. Keller, R. Walter, M. R. Boyd, F. Lang, A. Garcia, J. J.
Walsh, I. Tellitu, V. Bhaskar, T. R. Kelly, J. Org. Chem. 1998, 63, 1090 ±
1097; G. Bringmann, D. Vitt, J. Org. Chem. 1995, 60, 7674 ± 7681; b) T.
Watanabe, M. Uemura, Chem. Commun. 1998, 871 ± 872; c) A. V.
Rama Rao, M. K. Gurjar, V. Ramana, A. K. Chheda, Heterocycles
1996, 43, 1 ± 6. Other representative alternative routes to atropselec-
tive biaryl couplings include T. Hayashi, S. Niizuma, T. Kamikawa, N.
Suzuki, Y. Uozumi, J. Am. Chem. Soc. 1995, 117, 9101 ± 9102; A. P.
Degnan, A. I. Meyers, J. Am. Chem. Soc. 1999, 121, 2762 ± 2769.
[5] For a related route to the required 1,3-dimethylated tetrahydroiso-
quinolines, see G. Bringmann, R. Weirich, H. Reuscher, J. R. Jansen,
L. Kinzinger, T. Ortmann, Liebigs Ann. Chem. 1993, 877 ± 888.
[6] O. Mitsunobu, Synthesis 1981, 1 ± 28; D. Hughes, Org. React. (N.Y.)
1992, 42, 335 ± 395.
O
MeO
Ts
Me
Scheme 7. The stereochemical course of the reductive elimination.
atom ligated to the palladium atom from the bottom of the
molecule relative to the plane of the tetrahydroisoquinoline
moiety. Both the aryl ring of the ester, as well as the remaining
two phenyl groups on phosphorus, combine to further block
the back and underside. Since a dppf ligand occupies the
fourth coordination site on the metal the bulky naphthylene
moiety is forced to protrude forward over the aryl portion of
the tetrahydroisoquinoline ring. Reductive elimination from
this cis disposition about the palladium atom would give the
observed isomer.
The assignment of the stereochemistry in 16 (L ˆ o-PPh2-
C6H4CO) as S(P), which is associated with the korupens-
amine A series, could be readily made on the basis of a
comparison of its CD spectrum, as well as that of its hydrolysis
product 16 (L ˆ H; NaOH, MeOH, RT, 85%), with those of
korupensamines A and B reported by Bringmann.[1c, 2] Both
derivatives of 16 (L ˆ H, o-PPh2-C6H4CO) display an initial
positive Cotton effect in the 230 ± 240 nm region characteristic
of korupensamine A, which is strongly indicative of the
chirality of the biaryl group being responsible for the
observed spectra.[15]
[7] Reactions run at 858C generally gave good results, but heating to
>908C afforded low yields. See also ref. [5].
[8] Solubilized LAH gave lower selectivities for the trans isomer.
[9] T. R. Hoye, M. Chen, L. Mi, O. P. Priest, Tetrahedron Lett. 1994, 35,
8747 ± 8750.
[10] E. B. Merkushev, N. D. Simakhina, G. M. Koveshinkova, Synthesis
1980, 486 ± 487.
[11] T. R. Hoye, M. Chen, J. Org. Chem. 1996, 61, 7940 ± 7942.
[12] C. Coudret, V. Manzenc, Tetrahedron Lett. 1997, 38, 5293 ± 5296;
F. Liu, E. Negishi, J. Org. Chem. 1997, 62, 8591 ± 8594.
[13] K. Bouruneau, A. C. Gaumont, J. M. Dennis, J. Organomet. Chem.
1997, 529, 205 ± 213.
[14] a) B. Breit, Liebigs Ann. Chem. 1997, 1841 ± 1851; B. Breit, Angew.
Chem. 1998, 110, 535 ± 538; Angew. Chem. Int. Ed. 1998, 37, 525 ± 527;
b) B. M. Trost, C. Marschner, Bull. Soc. Chim. Fr. 1997, 134, 263 ± 274;
B. M. Trost, D. L. van Vranken, C. Bingel, J. Am. Chem. Soc. 1992,
114, 9327 ± 9343; for examples of related directing effects by an
internal phosphane, see C. N. Farthing, P. Kocovsky, J. Am. Chem. Soc.
In summary, by virtue of a judisciously placed internal
phosphane group that acts as a coordinating ligand for a PdII
intermediate formed during a Suzuki biaryl coupling, the
incipient axial chirality resulting from an ensuing reductive
elimination can be fully controlled; in this case it leads to the
korupensamine A skeleton. With this discovery, a stereo-
specific route to half of the michellamine B molecule is a
reasonable expectation. Moreover, by altering the residue
attached to the hydroxymethyl ªhandlesº (as in 1 and 2),
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1998, 120, 6661 ± 6672; M. T. Didiuk, J. P. Morken, A. H. Hoveyda,
Tetrahedron, 1998, 54, 1117 ± 1130, and references therein.
O
O
(2)
+
SiMe3
R
SiMe3
[15] From seminal work by Bringmann[1d] it is appreciated that structural
changes with such tetrahydroisoquinolines can alter the observed CD
spectra, thereby leading to erroneous assignments. In the case of
derivatives 16, it is apparent that alterations at the C-3 methyl group
do not impact, let alone overshadow, the significant bias imposed in
each by the axially chiral, nonracemic, biaryl nuclei.
R
H
Me5
+
Me5
catalyst resting states
O
Co
Co
+
SiMe3
R
H
CO
SiMe3
R
1'
Me3Si
1
SiMe3
with the second row rhodium analogues should exhibit higher
energy barriers than the cobalt complexes; thus rhodium
analogues may exhibit more extensive reversibility and have
the potential to isomerize aldehydes prior to ketone forma-
tion.[14, 15] Indeed, we report here that rhodium(i) olefin
complexes of the type [C5Me5Rh(C2H3R)2] catalyze the
interconversion of linear and branched aldehydes as well as
transfer formylation reactions.[16]
Isomerization of Aldehydes Catalyzed by
Rhodium(i) Olefin Complexes**
Christian P. Lenges and Maurice Brookhart*
Reduction of [(C5Me5RhCl2)2] with zinc in the presence of
propene results in the formation of [C5Me5Rh(C2H3Me)2] (2),
which was isolated as an analytically pure yellow solid.[17, 18]
Characterization of 2 by NMR spectroscopy indicates a
mixture of isomers. Recrystallization from acetone generates
the single isomer 2a, which was characterized by X-ray
crystallographic analysis (Figure 1). When complex 2a is
dissolved in [D6]acetone a mixture of isomers 2a, 2b, and
2c[19] is generated in minutes. These correspond to isomers
generated by rotation around the Rh olefin bond. Eventually
a fourth isomer 2d is observed, which is generated by olefin
dissociation and rebinding (Scheme 1).
One of the earliest reported and most thoroughly inves-
tigated processes catalyzed by transition metals is the hydro-
formylation of olefins [Eq. (1)].[1±8] The oxidative addition of
O
H
O
H2 / CO
cat.
+
(1)
H
R
R
R
n
i
H2 to a transition metal center is combined with reversible
olefin and CO insertion processes to yield an acyl hydride
complex. The final step in this catalytic process is generally
accepted as the irreversible reductive elimination of the acyl
hydride to generate the aldehyde products.[9±11] In most
applications the linear product is desired, although branched
products are becoming increasingly important as indicated by
recent efforts directed at the asymmetric hydroformylation of
olefins.[7b, c] An ongoing effort in this area of research is
dedicated to altering the linear:branched isomer ratio by
varying the ligand and catalyst structure.
We have recently reported that [C5Me5Co(olefin)2] com-
plexes can be used to carry out intermolecular hydroacylation
reactions of bulky olefins such as vinyltrimethylsilane. The
resting state of the catalyst was established as a mixture of the
CoI bis-olefin complex and the CoIII dialkyl carbonyl complex
whose ratio is dependent on the substrate structures and
concentrations [Eq. (2)].[12, 13]
When n-butyraldehyde was used as the substrate traces of
isobutyraldehyde were evident near complete conversion of
aldehyde, which indicated some reversibility prior to the
reductive elimination of ketone.[12] Reductive elimination
[*] Prof. Dr. M. Brookhart, Dipl. Chem. C. P. Lenges
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, North Carolina 27599-3290 (USA)
Fax: (‡1)919-962-2476
Figure 1. ORTEP diagram of complex 2a; ellipsoids are drawn with 50%
probability. Selected bond distances [Š] and angles [8]: Rh C1 2.129(4),
Rh C2 2.129(5), Rh C4 2.133(5), Rh C5 2.134(4), C1 C2 1.404(6), C2 C3
1.493(7), C4 C5 1.416(6), C5 C6 1.517; C1-Rh-C2 38.51(18); C1-Rh-C4
108.61(19); C1-Rh-C5 87.38(18); C2-Rh-C4 85.15(20); C2-Rh-C5 87.00(20);
C4-Rh-C5 38.77(16); C1-C2-C3 124.5(5); C4-C5-C6 120.4(4); C5-Rh-C1-
C2 88.5(5); C4-Rh-C2-C1 128.4(6); C1-Rh-C4-C5 60.2(4); C2-Rh-C5-C4
86.0(5); C1-Rh-C2-C3 119.7(6); C2-Rh-C4-C5 91.2(5); C4-Rh-C5-C6
115.0(5); Rh-C1-C2-C3 110.1(7).
E-mail: brook@net.chem.unc.edu
[**] We thank the National Institutes of Health (GM 28938) for financial
support. We are also thankful to Dr. P. S. White (Dept. of Chemistry,
Univ. of North Carolina at Chapel Hill) for the X-ray structure
determination of 2. C.P.L. thanks the Fonds der Chemischen Industrie
Â
(Germany) for a Kekule fellowship.
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