A facile access to densely functionalized substituted cyclopentanes and spiro cyclopentanes. Carbocation stabilization directed bond migration in rearrangement of cyclobutanes

Full text of DOI:10.1021/jo970283m

J . Org. Chem. 1997, 62, 5211-5214
5211
A F a cile Access to Den sely F u n ction a lized
Su bstitu ted Cyclop en ta n es a n d Sp ir o
Cyclop en ta n es. Ca r boca tion Sta biliza tion
Dir ected Bon d Migr a tion in
Rea r r a n gem en t of Cyclobu ta n es
Azizul Haque,† Anjan Ghatak,† Subrata Ghosh,*,† and
Nanda Ghoshal‡
Department of Organic Chemistry, Indian Association for
the Cultivation of Science, J adavpur, Calcutta 700 032,
India, and Medicinal Chemistry Division, Indian Institute
of Chemical Biology, J adavpur, Calcutta 700 032, India
Received February 14, 1997
Substituted cyclopentanes are widely represented in
nature and have served as valuable precursors1 in the
total synthesis of natural products. The cyclopentanone
I having substituents at the 2, 3, and 5 positions as
present in the novel sesquiterpene herbadysidolide II2
and related compounds poses considerable synthetic
challenge. Although much effort has been spent on
annulation of cyclopentane rings3 onto preexisting rings,
little attention has been given to the synthesis of cyclo-
pentanones4 with multiple substitution as represented
by the structure I. Construction of cyclopentanones of
this type through stepwise addition of substituents to
prebuilt cyclopentanone is a lengthy process and is in
general attended with poor regio- and stereoselectivity.
An approach in which substituents are directly generated
during construction of the cyclopentane ring is likely to
have great synthetic importance in terms of selectivity
and step economy. Development of a general methodol-
ogy to construct cyclopentanones represented by the
general structure I, thus, became our primary objective.
Resu lts a n d Discu ssion
It appeared to us that an intramolecular [2 + 2]
photocycloaddition of the diene IV (R3,R4 ) O) would be
the most straightforward route to the cyclobutanes III
(R3,R4 ) O). Although the diallyl ether 1a undergoes
smooth copper(I) catalyzed photocycloaddition7 to produce
2a , relating to another synthetic program, we have noted8
that the allyl ester 1b failed to undergo photocycloaddi-
tion to produce 2b under sensitized or catalyzed condi-
tions. We believe that the failure of 1b to undergo
cycloaddition is possibly due to the inability of the diene
1b to adopt the requisite conformation for cycloaddition.
Thus, in the present investigation we decided to use the
cyclobutane derivatives III (R3 ) R4 ) H).
We anticipate that acid-induced rearrangement5 of the
cyclobutane ring in the tricyclic compound III (R3,R4 )
O) may produce the desired cyclopentanone I provided
1,5-bond migration takes place preferentially over the
1,7-bond. In light of the stereoelectronic requirement6
in the pinacol rearrangement, the 1,7-bond migration
may be competitive. Thus it is of considerable interest
to investigate the rearrangement of the cyclobutane
derivatives III leading to a general route for the direct
synthesis of cyclopentanones with functionalised substit-
uents.
The preparation of the dienes and their photocycload-
dition were carried out according to Scheme 1 and are
illustrated with the transformation of acetone 3a to the
photoadduct 6a . Reaction of acetone 3a with (dihydro-
furyl)lithium afforded the carbinol 4a in 71% yield.
Coupling of the carbinol 4a with allyl bromide afforded
the diene 5a in 80% yield. The diene 5a was easily
identified by its 1H NMR spectral data. Irradiation of
the diene 5a in a diethyl ether solution in the presence
of CuOTf as catalyst according to the procedure of
Salomon7 afforded the bis tetrahydrofuran cyclobutane
6a in 50% yield after chromatographic purification. The
disappearance of the olefinic protons of the starting diene
5a in the 1H NMR spectrum of the product is a clear
indication of the cycloaddition. Further, the presence of
two Me singlets at δ 1.13 and 1.18, two one-proton
quartets at δ 2.5 (J ) 6 Hz) and 2.85 (J ) 6 Hz) assigned
to the C7 and C5 methine protons, and the resonance at
δ 3.66 (1H, d, J ) 10 Hz), 3.84-4.03 (2H, m), and 4.20
(1H, t, J ) 7.5 Hz) for the oxomethylene protons indicated
the identity of the structure 6a . This structural assign-
ment is corroborated by 13C NMR (DEPT) spectral data
showing two Me’s at δ 20.1 and 20.6, two methine carbons
at δ 36.4 and 40.4, the OCH2 of the tetrahydrofuran at δ
† Indian Association for the Cultivation of Science.
‡ Indian Institute of Chemical Biology.
(1) For example see. (a) Stork, G.; Saccomano, N. A. Tetrahedron
Lett. 1987, 28, 2087. (b) Mehta, G.; Krishnamurthy, N.; Karra, S. R.
J . Am. Chem. Soc. 1991, 113, 5765. (c) Ho, T. L.; Liang, F. S. J . Chem.
Soc., Chem. Commun. 1996, 1887.
(2) Faulkner, D. J . Nat. Prod. Rep. 1995, 12, 223. (b) Schram, T.
J .; Cardellira, J . H., II. J . Org. Chem. 1985, 50, 4155.
(3) Ramaiah, M. Synthesis 1984, 529. (b) Paquette, L. A. Top. Curr.
Chem. 1984, 119, 1. (c) Little, R. D. In Comprehensive Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991;
Vol. 5, p 239. (d) Habermas, K. L.; Denmark, S. E. Org. React. 1994,
45, 1.
(4) Hatanaka, M.; Ishida, A.; Tanaka, Y.; Uedo, I. Tetrahedron Lett.
1996, 37, 401. (b) Patra, D.; Ghosh, S. J . Org. Chem. 1995, 60, 2526
and references cited therein.
(5) For pinacol type rearrangement of cyclobutane derivatives to
form cyclopentanones see: (a) Ikeda, M.; Takahashi, M.; Uchini, T.;
Ohno, K.; Tamura, Y.; Kido, M. J . Org. Chem. 1983, 48, 4241. (b)
Moriarty, K. J .; Shen, C. C.; Paquette, L. A. Synlett 1990, 263. (c)
J amart-Gregoire, B.; Brosse, N.; Ianelli, S.; Nardelli, M.; Caubere, P.
Tetrahedron Lett. 1991, 32, 3069. (d) Nath, A.; Venkateswaran, R. V.
J . Chem. Soc., Chem. Commun. 1993, 281.
(7) Ghosh, S.; Raychaudhuri, S. R.; Salomon, R. G. J . Org. Chem.
1987, 52, 83.
(6) Mundy, B. P.; Otzenbenger, R. D. J . Chem. Educ. 1971, 48, 431.
(8) Ghosh, S.; Sarkar, S. Unpublished result.
S0022-3263(97)00283-1 CCC: $14.00 © 1997 American Chemical Society

5212 J . Org. Chem., Vol. 62, No. 15, 1997
Notes
less sterically crowded Cu-complexed diene 137 in which
CH2CH2Ph group, bulkier than Me, occupied an exo
position. In accord with this stereochemical outcome in
photocycloaddition, the diene 5c in which the CH2CH2-
Ph group has been replaced by the more bulkier CH2Ph
group produced mainly the photoadduct 6c in 70% yield.
A second series of cyclobutane derivatives (9a -c) was
also prepared for this investigation. The ketones 3a -c
were transformed to the dienes 8a -c by reaction with
(dihydropyranyl)lithium followed by coupling of the
resulting carbinols 7a -c with allyl bromide. Photo-
cycloaddition of these dienes (8a -c) produced the cy-
cloadducts 9a -c with stereochemical outcome (Table 1)
more or less identical to that observed for cycloaddition
of the dienes 5a -c prepared from dihydrofuran.
Sch em e 1a
After the cyclobutane derivatives were successfully
prepared, they were subjected to acid induced rearrange-
ment. Rearrangement was found to take place very
efficiently when a solution of the cyclobutane derivatives
in trifluoroacetic acid (TFA) was heated with a catalytic
quantity of trifluoromethane sulfonic acid (TfOH) at 50-
55 °C for about an hour. The results are summarized in
Table 1. The cyclobutane derivative 6a produced a
mixture of only two cyclopentanone derivatives in 3:1
ratio. A 13C NMR spectrum of the major component in
the mixture revealed the presence of two Me’s at δ 18.2
and 24.5, two hydroxy methylene units at 61 and 63, two
methines at 46.3 and 47.6, a quaternary center at 47.2,
and a trisubstituted cyclopentanone at 225.5. J ones
oxidation of this product followed by diazomethane
treatment produced a mixture of the diesters in a 3:1
ratio as indicated by 1H NMR. NMR spectral data
combined with its transformation to the diesters indi-
cated the product to be either a mixture of the regioiso-
mers 10a and 11a or a mixture of the stereoisomers
resulting from epimerization of either of the regioisomers.
NMR spectral data were inadequate to differentiate
between these two possibilities. The mass spectrum of
the product mixture showed the base peak at m/e 142.
This peak can be attributed to loss of CH3CHO through
McLafferty fragmentation from the regioisomer 10a only.
Thus, the product obtained from rearrangement of the
cyclobutane derivative 6a was a mixture of the two
diastereoisomers 10a resulting from epimerization at C5
and rearrangement of the cyclobutane derivative 6a has
involved migration of 1,5-bond.
Structural assignment to the rearrangement product
gets additional support from rearrangement of the cy-
clobutane derivative 6c to produce, instead of the diol
10c, a single compound, devoid of any carbonyl group (IR
and 13C NMR). J ones oxidation of this product followed
by diazomethane treatment to produce a monoester
instead of the expected diester corresponding to the diol
10c, combined with 13C NMR spectral data (two oxo-
methylene units at δ 64.0 and 69.4 and two quaternary
olefinic carbons at δ 104.3 and 146.5 in addition to the
aromatic carbons), dictated the cyclic enol ether struc-
tures 15 and 16 for the rearrangement product and the
monoester, respectively. The stereochemical assignment
to the rearrangement product 15 was based on the carbon
chemical shift of the Me group. In general, in vicinally
substituted cyclopentane derivatives, the Me group syn
to the vicinal substituent is shielded by ∼5 ppm over the
one having with an anti Me9 due to steric interaction of
a
Reagents: i, 2,3-dihydrofuran, THF, t-BuLi, -70 °C. ii, NaH,
THF-HMPA, allyl bromide, reflux. iii, hν, CuOTf, Et2O. iv,
TfOH-TFA, 50-55 °C, 1 h, then NaOH-H2O, reflux. v, 3,4-
dihydropyran, THF, t-BuLi, -70 °C.
Ta ble 1. Syn th esis of Cyclop en ta n es
dienes
photoadducts
(% yield)
cyclopentanes
(% yield)b
entry ketones (% yield)
1
2
3
4
5
6
7
8
9
3a
3b
3c
3a
3b
3c
3d
3e
3d
3e
5a (57)
5b (63)
5c (55)
8a (57)
8b (66)
8c (56)
5d (48)
5e (62)
8d (50)
8e (55)
6a (50)
10a (55)
10b (58)
15 (60)
12a (58)
12b (67)
12c (57)
10d (50)
10e (58)
12d (50)
12e (67)
6b (59) 5.3:1a
6c (70) >99.1a
9a (46)
9b (45) 7.6:1a
9c (40) >99:1a
6d (45)
6e (60)
9d (58)
10
9e (57)
a
b
Represents ratio of the two diastereoisomers. Cyclopen-
tanones obtained in each case were mixture of the diastereoisomers
epimeric at C5 in about 2.5:1 ratio.
68.3 and 70.5, and the quaternary carbons of the tet-
rahydrofuran at δ 78.4 and 96.4 in addition to cyclo-
butane methylene at δ 25.9 and the C8-methylene at δ
31.7. Following the above sequence, the unsymmetrical
ketones 3b and 3c were transformed to the cyclobutanes
6b and 6c through the dienes 5b and 5c, respectively.
The results are summarized in Table 1. As expected, the
diene 5b produced a mixture of the photoadduct 6b along
with its C2-diasteroisomer in a 4:1 ratio. The gross
structures of the photoadducts were evident from their
1H and 13C NMR spectra, which are closely comparable
to those of the cyclobutane derivative 6a . The stereo-
chemical assignment to the major component as 6b
followed from its formation through cycloaddition of the
(9) Funk, R. L.; Vollhardt, K. P. C. J . Am. Chem. Soc. 1980, 102,
5253.

Notes
J . Org. Chem., Vol. 62, No. 15, 1997 5213
Sch em e 2
1,7-bond would be preferred over that of the 1,5-bond if
the reaction course is governed by a stereoelectronic
factor. This unusual selectivity in 1,5-bond migration is
attributed to be the result of the relative stabilities of
the cations 18 and 20 formed by migration of the 1,5-
and 1,7-bond, respectively, from the protonated species
17. The cation 18 obtained by migration of the 1,5-bond
can be stabilized through resonance with the structure
19. On the other hand, the cation 20, formed through
migration of the 1,7-bond, cannot be stabilized by reso-
nance with the structure 21 as its formation requires
generation of a bridgehead double bond. Molecular
mechanics calculation reveals that the cation 18a , having
an energy of 20.07 kcal mol-1, is indeed stabilized by 10
kcal mol-1 over cation 20a , having an energy of 30.26
kcal mol-1. Similarly the cation 18b (energy 24.76 kcal
mol-1) is stabilized by ∼12 kcal mol-1 over cation 20b
(energy 36.15 kcal mol-1). Thus in the present case
stabilization of the intermediate carbocation overrides the
stereoelectronic effect, resulting in exclusive migration
of a stereoelectronically disfavored bond. Loss of a proton
from the carbocation 19 then produces the cyclic enol
ether 22. The latter during aqueous workup undergoes
hydrolysis to produce the cyclopentanones 10 and 12. The
isolation of the enol ether 15 from rearrangement of the
cyclobutane derivative 6c is in agreement with this
rationalization. The failure of the enol ether 15 to
undergo hydrolysis to produce the corresponding cyclo-
pentanone 10c is possibly due to destabilization of the
latter arising from 1,3-eclipsing interaction between the
bulkier C2-benzyl and C5-alkyl substituents. The strain
arising through 1,3-eclipsing interaction is significantly
relieved through enolization which is further facilitated
by ring closure to form the very stable 5-membered ring.
In conclusion, this investigation has offered an excel-
lent route for direct synthesis of substituted cyclopen-
tanones and spiro cyclopentanones in which the substit-
uents are functionalized for further elaboration. The
synthesis had involved rearrangement of appropriately
constructed cyclobutane derivatives. The stability of the
carbonium ion formed after cyclobutane bond migration
dictated the reaction course during rearrangement.
the syn vicinal substituents. Thus, in the cyclopentanone
10a , the two Me groups absorbed at δ 18.0 (syn Me) and
24.5 (anti Me). The chemical shift (15.4 ppm) of the Me
carbon observed in the 13C NMR of cyclopentane 15 is
comparable to that of the syn Me of 10a . This confirms
that Me and CH2OH are syn to each other in cyclopen-
tane 15.
The cyclobutane derivatives 6b and 9a -c, under the
above condition, produced a mixture of cyclopentanones
in each case (Table 1). The structural assignments of
the cyclopentanones are based on a spectral data com-
parison with the cyclopentanone 6a . The syn orientation
of the Me and CH2OH in each of them was ensured from
the chemical shifts of the Me carbons (17.8-18.5 ppm).
The cyclopentanone 9a contains all of the requisite
substituents and functionalities for elaboration to
herbadysidolide.
The present methodology offers an excellent route for
the synthesis of spiro cyclopentanones.10 Thus in a
similar fashion cyclopentanone 3d and cyclohexanone 3e
provided the spiro cyclopentanones 10d ,e and 12d ,e.
The most remarkable feature in the rearrangement of
the cyclobutane derivatives 6 and 9 was the selectivity
observed in the migration of the 1,5-bond over the 1,7-
bond. In light of the stereoelectronic requirement6 in a
pinacol type rearrangement, antiperiplanarity of the
migrating bond with the bond to be broken is necessary.
Molecular mechanics calculated11 energy minimized con-
formation of the oxonium ions 17, formed through pro-
tonation of the bis tetrahydrofuran derivatives 6 and 9
(Scheme 2) having energies of 58.1 and 61.0 kcal mol-1
for 17a and 17b, respectively, reveals that it is the 1,7-
bond that meets this criterion. Hence, migration of the
Exp er im en ta l Section
Boiling points reported are uncorrected. The organic extracts
were dried over anhydrous Na2SO4 unless otherwise stated.
Column chromatography was performed on silica gel (60-120
mesh). Petroleum refers to the fraction of petroleum ether
boiling in the range 60-80 °C. FT IR spectra were recorded
neat. 1H NMR spectra were recorded at 60 MHz in CCl4 and at
200 MHz in a CDCl3 solution. 13C NMR spectra were recorded
at 50 MHz in a CDCl3 solution. Mass spectra were recorded at
70 eV. Elemental analyses12 were performed in the microana-
lytical laboratory of this institute.
The general experimental procedure involved in the synthesis
of cyclopentanones from carbonyl compounds is illustrated by
the synthesis of the cyclopentanone 10a from acetone.
Syn th esis of th e Dien e. 5-(1′-(Allyloxy)-1′-m eth yleth -
yl)2,3-d ih yd r ofu r a n (5a ). To a magnetically stirred solution
of 2,3-dihydrofuran (3.4 mL, 45 mmol) in anhydrous THF (20
mL) under an argon atmosphere was added t-BuLi (20 mL, 30
mmol, 1.7 M in pentane) dropwise. After complete addition, the
bath temperature was slowly raised to -10 °C (45 min) and the
solution was stirred for 1 h at that temperature. The reaction
mixture was again cooled to -78 °C. A solution of the ketone
(10) For a comprehensive account of synthetic approaches to spiro
cyclopentanones, see: (i) Heathcock, C. H.; Graham, S. L.; Pirrung,
M. C.; Plavac, F.; White, C. T. The Total Synthesis of Sesquiterpenes;
ApSimon, J ., Ed.; Wiley: New York, 1983; Vol. 5, pp 264-313. (ii)
Vandewalle, M.; Clercq, P. D. Tetrahedron 1985, 41, 1767. (b) For
recent approaches, see: Patra, D.; Ghosh, S. J . Chem. Soc., Perkin
Trans. 1 1995, 2635 and references cited therein.
(11) The minimum energy conformations of the cations 17a ,b were
modeled using HyperChem Release 3 for Windows, a product of
Autodesk Inc. The energy optimization of the structures were carried
out using simulated annealing. Molecular mechanics calculations
using MM+ force field were used for energy minimization.
(12) Satisfactory microanalytical data (C, H) were obtained for all
compounds except the carbinols 4 and 7 and the dienes 5 and 8 which
on attempted purification through chromatography or distillation
underwent extensive decomposition.

5214 J . Org. Chem., Vol. 62, No. 15, 1997
Notes
3a (2.25 mL, 49 mmol) in THF (10 mL) was then added dropwise.
After complete addition, the bath temperature was slowly raised
to rt and stirring was continued for 1 h. The reaction mixture
was then cooled to -20 °C and quenched by addition of a
saturated aqueous NH4Cl solution (10 mL). The reaction
mixture was then extracted with ether (3 × 20 mL). The ether
extract after drying (K2CO3) was concentrated. The residual oil
was distilled to afford 4a (2.7g, 71%): bp 60-65 °C (20 mm); 1H
NMR (60 MHz) δ 1.3 (6H, s), 2.15-2.81 (3H, m), 4.28 (2H, t, J
) 9 Hz), 4.63 (1H, t, J ) 3 Hz). The carbinol obtained as such
was immediately transformed to the diene 5a using the following
procedure.
To a magnetically stirred suspension of NaH (670 mg, 11.2
mmol, 40% suspension in oil) in THF (35 mL) at rt under an N2
atmosphere was added a solution of the carbinol 4a (1g, 7.8
mmol) in THF (10 mL). After complete addition, the reaction
mixture was heated under reflux for 2 h and then cooled to 50
°C. HMPA (9 mL) was then added followed by addition of allyl
bromide (944 mg, 7.8 mmol) at a rate to maintain a mild reflux
of THF. The reaction mixture was then refluxed for 2 h, cooled
to rt and quenched by addition of water (20 mL). Finally, it
was extracted with ether (3 × 30 mL). The ether extract after
drying (K2CO3) was concentrated. The residual liquid, stabilized
by addition of 1% NEt3, was distilled to afford the diene 5a (1.07
g, 80%): bp 70-72 °C (2 mm); 1H NMR (60 MHz) δ 1.35 (6H, s),
2.61 (2H, dt, J ) 3 and 9 Hz), 3.81 (2H, dt, J ) 1 and 5 Hz),
4.33 (2H, t, J ) 10 Hz), 4.73 (1H, t, J ) 3 Hz), 4.86-6.20 (3H,
m).
96.45 (C2 or C1). Anal. Calcd. for C10H16O2: C, 71.39 H, 9.58.
Found: C, 70.92 H, 9.56.
Rea r r a n gem en t of th e Cyclobu ta n e Der iva tive 6a . Syn -
th esis of 2,2-Dim eth yl-3-(h yd r oxym eth yl)-5-(â-h yd r oxy-
eth yl)cyclop en ta n on e (10a ). A solution of the cyclobutane
derivative 6a (0.25 g, 1.49 mmol) in TFA (1 mL) and TfOH (50
µL) was heated in an oil bath (50-55 °C) for 1 h. The reaction
mixture was cooled, made alkaline by dropwise addition of
aqueous NaOH (10%), and extracted with dichloromethane (3
× 10 mL). The organic extract was dried and concentrated. The
residue was dissolved in MeOH (1 mL), and to it was added
aqueous NaOH (1 mL, 20%). The mixture was refluxed for 45
min and on cooling to rt was extracted with ethyl acetate (2 ×
15 mL). The organic extract was washed with brine, dried, and
concentrated. The residual mass was chromatographed (ethyl
acetate) to afford the cyclopentanone 10a (0.155 g, 55%) as a
yellow viscous liquid as an inseprable mixture in ca. 3:1 ratio
(1H and 13C NMR) of the diastereoisomers epimeric at C5; IR
3407, 1739 cm-1 1H NMR (200 MHz) δ 0.84 and 1.10 (two s,
;
Me’s for the major diastereoisomer), δ 0.95 and 1.04 (two s, Me’s
for the minor), 1.58 (1H, m), 1.83-2.06 (3H, m), 2.27-2.56 (2H,
m), 3.0 (br s), 3.53-3.79 (4H, m); 13C NMR (for the major) δ 18.2
(Me), 24.5 (Me), 30 (CH2), 33.3 (CH2), 46.3 (CH), 47.6 (CH), 61
(CH2), 63 (CH2), 47.2 and 225.5; (for the minor ketone), δ 18.6,
24.9, 29, 34.6, 43.3, 46.7, 60.8, 63 and 226; EIMS m/z 186 (M+)
169, 142 (100), 127, 109, 86, 55. Anal. Calcd for C10H18O3: C,
64.48 H, 9.70. Found: C, 64.10 H, 9.71.
Ir r a d ia tion of th e Dien e 5a . Syn th esis of 2,2-Dim eth yl-
3,10-d ioxa tr icyclo[5.3.01,5.01,7]d eca n e (6a ). A magnetically
stirred solution of the diene 5a (1.3 g, 7.8 mmol) in diethyl ether
(250 mL) containing CuOTf (0.2 g) under an Ar atmosphere was
irradiated with a 450 W medium pressure mercury vapor lamp
(Hanovia) through a double-walled water-cooled quartz immer-
sion well for 6-7 h. The reaction mixture was then washed
successively with ice-cold aqueous NH4OH (2 × 20 mL) and
water (2 × 20 mL) and dried. Removal of solvent followed by
column chromatography [ether-petroleum (1:9)] of the residual
oil afforded the cyclobutane derivative 6a (0.65 g, 50%) as a
colorless liquid: 1H NMR (200 MHz) δ 1.13 (3H, s), 1.18 (3H, s),
1.57-1.83 (4H, m), 2.5 (1H, q, J ) 6 Hz), 2.85 (1H, q, J ) 6 Hz),
3.66 (1H, d, J ) 10 Hz), 3.84-4.03 (2H, m), and 4.20 (1H, J )
7.5 Hz); 13C NMR δ 20.1 (Me), 20.6 (Me), 25.9 (CH2), 31.7 (CH2),
36.4 (CH), 40.4 (CH), 68.3 (CH2), 70.5 (CH2), 78.4 (C1 or C2) and
Ack n ow led gm en t. Financial support from the De-
partment of Science and Technology, Government of
India, New Delhi, is gratefully acknowledged. A.G.
thanks CSIR, New Delhi, for a Senior Research Fellow-
ship.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra of
5, 6, 8, 9, 10a ,b,d ,e, 12, 14, 15, and 16, 13C NMR spectra of 6,
9, 10a ,b,d ,e, 12, and 15, and mass spectra of 10a and 12a (54
pages). This material is contained in libraries on microfiche,
immediately follows the article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
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J O970283M