π-allyl cation cyclisations initiated by silver(I)-promoted electrocyclic ring opening of ring-fused gem-dibromocyclopropanes possessing tethered nucleophiles: The influence of chiral auxiliaries on the diastereoselectivity of cyclisations involving meso-substrates

Article abstract of DOI:10.1039/b002525i

π-allyl cations derived from electrophilic ring-opening of meso-compounds were subjected to distereoselective ring-closure when the nucleophile had a chiral auxiliary associated with it. Synthesis of the epimeric forms of meso-substrates were also shown. The results suggested that π-stacking effects could be used to advantage in enhancing the selectivities associated with π-allyl cation cyclizations.

Full text of DOI:10.1039/b002525i

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ꢀ-Allyl cation cyclisations initiated by silver(I)-promoted
electrocyclic ring opening of ring-fused gem-dibromocyclopropanes
possessing tethered nucleophiles: the influence of chiral auxiliaries
on the diastereoselectivity of cyclisations involving meso-substrates
1
Martin Banwell,* Alison Edwards, Joanne Harvey, David Hockless and Anthony Willis
Research School of Chemistry, Institute of Advanced Studies, The Australian National
University, Canberra, ACT 0200, Australia
Received (in Cambridge, UK) 30th March 2000, Accepted 1st June 2000
Published on the Web 23rd June 2000
The epimeric pairs of ring-fused gem-dibromocyclo-
propanes 17/18 and 27/28, each of which possesses an
internal plane of symmetry and has a tethered carbamate
moiety with an associated chiral auxiliary, react with silver
perchlorate to give, in a diastereoselective manner, the
corresponding pairs of azabicyclic compounds, 19/20 and
29/30 respectively.
Ring-fused gem-dibromocyclopropanes are generally easy to
prepare (normally via addition of dibromocarbene to the corre-
sponding cycloalkene)1 and often engage in thermal- or
silver()-promoted electrocyclic ring-cleavage of the three-
membered ring2 to give π-allyl cations. These latter species can
be trapped, in an inter- or intra-molecular fashion,1 by various
carbon- and hetero-atom centred nucleophiles.3,4 The cyclis-
ation reactions associated with the intramolecular trapping
processes generate new polycyclic compounds that possess a
cycloalkenyl bromide moiety capable of participating in Pd(0)-
catalysed cross-coupling reactions. Thus, for example, treat-
ment of either epimer of gem-dibromocyclopropane 1 with
silver perchlorate results in the smooth and near quantitative
formation of the hexahydroindole 2,3g a compound that read-
ily engages in Suzuki cross-couplings5 with aryl boronic acids.
One of the products of such a coupling reaction has been
exploited3g,3h in a concise synthesis of the alkaloidal degrad-
ation product γ-lycorane. On the basis of this and a modest
number of additional examples,3a,3b,3f,4 the title reactions would
seem to offer considerable potential in chemical synthesis. At
present, however, this type of methodology is under-utilised,
perhaps because the basic scope and limitations of such chem-
istry remain largely undefined. Consequently, we now report
that π-allyl cations derived from electrocyclic ring-opening of
meso-compounds such as 6,6-dibromobicyclo[3.1.0]hexanes
possessing a carbamate unit tethered through C-3 (e.g. 3) are
subject to diastereoselective ring-closure when the nucleophile
has a chiral auxiliary associated with it.6 The reactions
described herein should prove valuable for the synthesis, in
enantiopure form, of various azabicyclic species7 related to
bioactive natural products such as epibatidine8 and anatoxin-a
(very fast death factor).3b,3f,9
Syntheses of the epimeric forms of meso-substrates of the
general type 3 (n = 1) are shown in Scheme 1. All start with
Scheme 1 Reagents and conditions (i) LiAlH4 (0.8 mol equiv.), THF, 66 ЊC, 6 h ; (ii) Ac2O (1.9 mol equiv.), pyridine, 18 ЊC, 16 h; (iii) CHBr3 (excess),
C6H6, 50% aq. NaOH, TEBAC, 0–18 ЊC, 42 h; (iv) K2CO3 (1.7 mol equiv.), CH3OH, 18 ЊC, 8 h; (v) Tf2O (2.1 mol equiv.), 2,6-lutidine (2.3 mol equiv.),
CH2Cl2, Ϫ60 ЊC, 0.66 h; (vi) TMGA (3.5 mol equiv.), CH2Cl2, Ϫ60–18 ЊC, 6.33 h; (vii) H2 (excess), 10% Pd on C, THF, 18 ЊC, 0.5 h; (viii)
(Ϫ)-menthyl-, (Ϫ)-8-phenylmenthyl- or (1R)-trans-2-phenylcyclohexyl chloroformate (3.33 mol equiv.), Et3N, THF, 0 ЊC, 7 h; (ix) AgClO4 (2 mol
equiv.), THF, 18 ЊC, 7 h; (x) NaOCH3 (28 mol equiv.), CH3OH, 80 ЊC (sealed tube), 48 h then HCl (gas, excess), 18 ЊC, 0.1 h then (Ϫ)-menthyl
chloroformate or (Ϫ)-8-phenylmenthyl chloroformate (3 .5 mol equiv.), pyridine, 18 ЊC, 24 h. TEBAC = triethylbenzylammonium chloride.
DOI: 10.1039/b002525i
J. Chem. Soc., Perkin Trans. 1, 2000, 2175–2178
This journal is © The Royal Society of Chemistry 2000
2175

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Table 1 Diastereomeric ratios of products obtained from the title cyclisation reactions
Ratio of product
diastereomers
Entry
Substrate(s)
Auxiliary (R*)
1
2
3
4
5
6
17a/18aa
17b/18ba
17c/18ca
27a/28aa
27b/28bb
27c/28cb
(Ϫ)-trans-2-Phenylcyclohexyl
(Ϫ)-Menthyl
(Ϫ)-8-Phenylmenthyl
(Ϫ)-trans-2-Phenylcyclohexyl
(Ϫ)-Menthyl
3:7 (19a/20a)
2:3 (19b/20b)
2:9 (19c/20c)
5:9 (29a/30a)
5:6 (29b/30b)
3:11 (29c/30c)
(Ϫ)-8-Phenylmenthyl
a Substrates were subjected to the title reaction either individually or as an epimeric mixture. The product ratios were the same in each case.
b Substrates were subjected to the title reaction as an epimeric mixture.
Scheme 2 Reagents and conditions (i) NaCN (21.5 mol equiv.), DMPU, 18 ЊC, 68 h; (ii) H2 (excess), PtO2ؒH2O, CHCl3, EtOH, 18 ЊC, 2 h; (iii) (Ϫ)-
menthyl-, (Ϫ)-8-phenylmenthyl- or (Ϫ)-(1R)-trans-2-phenylcyclohexyl chloroformate (ca. 2.0 mol equiv.), pyridine, 0–18 ЊC, 15 h; (iv) AgClO4 (10.0
mol equiv.), DME, 3 Å molecular sieves, 18 ЊC, 19 h.
conversion of the known acid 410 into the corresponding alco-
hol 511 (95%), the acetate derivative 6 (97%) of which readily
reacts with dibromocarbene generated under Makosza condi-
tions.1 The resulting ca. 2:1 mixture of adducts 7† and 8 (81%
combined yield) was then hydrolysed to the corresponding mix-
ture of alcohols 9 (mp = 46–48 ЊC) and 10 (95% combined
yield). The latter mixture was subjected to reaction with triflic
anhydride and the resulting triflates, 11 and 12, immediately
treated with tetramethylguanidinium azide (TMGA)12 so as to
form the corresponding azides, 13 and 14 (86% combined yield)
respectively. Reduction of the latter compounds under hydro-
genolysis conditions13 gave a ca. 2:1 mixture of amines, 15 and
16, which reacted with triethylamine and either (Ϫ)-menthyl
chloroformate (Aldrich), (Ϫ)-8-phenylmenthyl chloroform-
ate 14 or (Ϫ)-(1R)-trans-2-phenylcyclohexyl chloroformate15 to
give the corresponding pairs of epimeric carbamates. These
could be separated from one another by flash chromatography
and in this manner pure samples of compounds 17a {74% from
16, [α]D = Ϫ33 (c 1.8)‡}, 17b {68% from 16, mp = 117–118 ЊC,
[α]D = Ϫ27 (c 1.0)}, 17c {50% from 16, [α]D = Ϫ4 (c 1.7)}, 18a
{74% from 15, mp = 101–103 ЊC, [α]D = Ϫ35 (c 1.0)}, 18b {68%
from 15, mp = 90–91 ЊC, [α]D = Ϫ36 (c 1.4)} and 18c {50% from
(15), [α]D = Ϫ6 (c 1.3)} were obtained. The structure of carb-
amate 18b was determined by single-crystal X-ray analysis§
which served to establish the anti-relationship between the
cyclopropyl and side-chain moieties and, thereby, the structures
of all of the related compounds 17a–c, 18a and 18c.
Treatment of compound 18a with silver perchlorate in THF
at 18 ЊC resulted in its smooth conversion into a 3:7 mixture
(as determined by GLC analysis) of the diastereoisomeric 6-
azabicyclo[3.2.1]oct-3-enes 19a {26%, [α]D = 18 (c 1.6)} and 20a
{60%, mp = 87–88 ЊC, [α]D = Ϫ121 (c 1.7)} (Table 1). These
products could be separated from one another by flash chrom-
atography and the structure of compound 20a was established
by single-crystal X-ray analysis.§ Reaction of epimer 17a under
analogous conditions gave the same mixture of products in 63%
(combined) yield.¶ Related cyclisations were carried out using
substrates 17b and 18b each of which afforded a separable and
ca. 2:3 mixture of products 19b {[α]D = 20 (c 0.5)} and 20b
{[α]D = Ϫ136 (c 0.8)} in 67–73% combined yield. Similarly, each
of compounds 17c and 18c gave a separable and ca. 2:9 mixture
of 6-azabicyclo[3.2.1]oct-3-enes 19c {[α]D = 21 (c 0.2)} and 20c
{[α]D = Ϫ99 (c 1.9)} in 84% combined yield. The structures of
compounds 20b and 20c were established via their inde-
pendent synthesis from congener 20a. Thus, carbamate 20a was
hydrolysed to the corresponding amine which was immediately
treated with (Ϫ)-menthyl chloroformate or (Ϫ)-8-phenyl-
menthyl chloroformate to give authentic samples of com-
pounds 20b and 20c, respectively.
The next higher homologues of substrates 17a–c and 18a–c,
viz. compounds 27a–c and 28a–c, were prepared by the route
shown in Scheme 2. Thus, the mixture of mesylates 21 and
22 derived16 from the corresponding ca. 2:1 mixture of
alcohols 9 and 10 was converted into the nitriles, 23 and 24
(83% combined yield), by reaction with sodium cyanide in
N,NЈ-dimethylpropyleneurea (DMPU). Hydrogenation of the
latter compounds using PtO2 as catalyst and in the presence
of chloroform afforded the hydrochloride salts of amines 25
and 26 which were immediately treated with pyridine and
either (Ϫ)-menthyl chloroformate, (Ϫ)-8-phenylmenthyl chloro-
formate or (Ϫ)-(1R)-trans-2-phenylcyclohexyl chloroformate.
In this fashion inseparable and ca. 2:1 mixtures of carbamates
27b and 28b [56% combined yield from precursors 23 and 24]
and compounds 27c and 28c (71% combined yield) were
obtained. In contrast, congeners 27a {51% from (23), mp = 87–
89 ЊC, [α]D = Ϫ29 (c 0.8)} and 28a {51% from (24), mp = 108–
109 ЊC, [α]D = Ϫ32 (c 0.5)} could be separated from one another
by HPLC. Reaction of compounds 27a and 28a, either
independently or as a 2:1 mixture, with silver perchlorate
afforded, in 58% combined yield, a ca. 5:9 mixture of the 2-
azabicyclo[3.3.1]non-7-enes 29a {[α]D = 49 (c 0.5)} and 30a
{[α]D = Ϫ131 (c 0.6)} (Table 1) which were separated from one
another by HPLC. Substrates 27c and 28c behaved in a similar
manner and afforded, in 33% combined yield, a ca. 3:11 mix-
ture of products 29c {[α]D = 110 (c 0.9)} and 30c {[α]D = Ϫ108
(c 1.5)}. Reaction of compounds 27b and 28b under the same
conditions gave, in 54% combined yield, a ca. 5:6 mixture of
2-azabicyclo[3.3.1]non-7-enes (29b) {mp = 85–87 ЊC, [α]D = 102
(c 0.4)} and 30b {[α]D = Ϫ145 (c 0.2)}. Each of the above-
mentioned 2-azabicyclo[3.3.1]non-7-enes was accompanied by
as yet uncharacterised hydrolysis products. The structure of
compound 29b, the minor cyclisation product derived from
precursors 27b and 28b, was established by single-crystal X-ray
analysis.§ This observation, together with the fact that the
specific rotations for all of compounds 20a–c and 30a–c are of
the same (negative) sign, effectively implies that the absolute
stereochemistries of the azabicyclic ring systems associated
with the major cyclisation products are equivalent for the entire
series of reactions reported herein.
The origins of the, thus far, modest diastereoselectivities
observed in the above-mentioned cyclisation reactions (see
Table 1) are the subject of ongoing studies. At this point it
2176
J. Chem. Soc., Perkin Trans. 1, 2000, 2175–2178

View Article Online
§ Crystal data for compound 18b: C18H29Br2NO2, M = 451.24, T =
193 K, orthorhombic, space group P212121 (#19), Z = 4, a = 7.080(3),
b = 9.637(1), c = 29.064(2) Å, U = 1982.9(9) Å3, µ(Cu-Kα) = 52.94 cmϪ1
1730 unique data (2Θmax = 119.9Њ), 1605 with I > 3σ(I); R = 0.033,
Rw = 0.041, S = 2.56.
seems reasonable to suggest that since the highest diastereo-
meric excesses are observed with the 8-phenylmenthyl-based
auxiliary,17 π-stacking effects could be used to advantage in
enhancing the selectivities associated with the title processes.
,
Crystal data for compound 20a: C20H24BrNO2, M = 390.32, T = 193
K, orthorhombic, space group P212121 (#19), Z = 4, a = 9.591(2),
b = 12.8947(9), c = 14.871(2) Å, U = 1839.2(4) Å3, µ(Cu-Kα) = 31.38
Experimental
Formation of compounds 19c and 20c
cmϪ1, 5862 reflections measured, 2738 unique (Rint = 0.037, 2Θmax
120.1Њ), 2682 with I > 2σ(I); R = 0.020, Rw = 0.026, S = 1.17.
=
A magnetically stirred solution of carbamate 18c (135 mg,
0.256 mmol) in THF (7.0 ml) maintained under a nitrogen
atmosphere at 18 ЊC was treated, in one portion, with silver
perchlorate (104 mg, 0.502 mmol). After 7 h the reaction mix-
ture was filtered through a plug of CeliteTM which was washed
with dichloromethane (20 ml). The combined filtrates were
concentrated under reduced pressure (CAUTION—use a blast
shield and do not heat) to give a brown oil which was subjected
to flash chromatography (silica gel, 1:7 v/v ethyl acetate–hexane
elution). In this manner two fractions, A and B, were obtained.
Concentration of fraction A (Rf 0.4) afforded the 6-aza-
bicyclo[3.2.1]oct-3-ene 19c (18 mg, 16%) as a clear colourless oil
Crystal data for compound 29b: C19H30BrNO2,
M = 384.36,
T = 200(1) K, monoclinic, space group P21, Z = 2, a = 8.4008(5),
b = 9.9077(6), c = 11.8739(8) Å, β = 97.278(4)Њ, U = 980.3(5) Å3,
µ(Mo-Kα) = 21.1 cmϪ1
, 9089 reflections measured, 3263 unique
(Rint = 0.048, 2Θmax = 25.0Њ), 2453 with I > 3σ(I); R = 0.042, Rw = 0.045,
S = 1.18.
The structures of 18b and 20a were refined by full-matrix least
squares analysis on F using the teXsan structure analysis software of
Molecular Structure Corporation.18 Data for compound 29b were
extracted using the DENZO package.19 Structure solution was by direct
methods (SIR92)20 and refinement was by full matrix least-squares on F
using the maXus program package.21 CCDC reference number 207/442.
See http://www.rsc.org/suppdata/p1/b0/b002525i/ for crystallographic
files in .cif format.
¶ These observations indicate that the cyclisation process is insensitive
to the stereochemical relationship between the tethered nucleophile and
cyclopropyl moiety. In our view, such observations also suggest the
intermediacy of a discrete and common π-allyl cation.
(Found Mϩ., 447.1597. C24H3281BrNO2 requires Mϩ , 447.1596).
ؒ
1H NMR (300 MHz, CDCl3) δ 7.30–7.19 (complex m, 4H),
7.09 (m, 1H), 5.88 (s) and 5.79 (s) (rotamers,|| 1H), 4.82 (m,
1H), 4.50 (d, J = 4.8 Hz) and 4.42 (d, J = 4.8 Hz) (rotamers,
1H), 3.17 (d, J = 10.7 Hz) and 2.62 (d, J = 10.7 Hz) (rotamers,
1H), 2.42–2.31 (complex m, 2H), 2.23 (m, 1H), 2.08–1.42
(complex m, 8H), 1.33 (s, 3H), 1.38–1.08 (complex m, 2H),
1.18 (s, 3H), 1.01–0.83 (partially concealed m, 1H), 0.87 (d,
J = 6.5 Hz, 3H); νmax (KBr) 2953, 2923, 1697, 1407, 1324,
1234, 1104, 1030, 763 and 700 cmϪ1; m/z 447 (1%) and 445
|| For an up-to-date discussion regarding the barriers to rotations in
carbamates see C. Cox and T. Lectka, J. Org. Chem., 1998, 63, 2426.
1 M. G. Banwell and M. E. Reum, in Advances in Strain in Organic
Chemistry, ed. B. Halton, JAI Press, Greenwich, Connecticut, 1991,
vol. 1, pp. 19–64 and references cited therein.
2 For an excellent discussion of such processes see E. N. Marvell,
Thermal Electrocyclic Reactions, Academic Press, New York, 1980,
pp. 23–53.
(1) [Mϩ ], 234 (56) and 232 (58), 119 (100) (C9H11ϩ), 118 (75)
ؒ
ϩ
[C9H10 ], 91 (67) (C7H7ϩ).
ؒ
Concentration of fraction B (Rf 0.2) afforded the 6-aza-
bicyclo[3.2.1]oct-3-ene 20c (78 mg, 68%) as a clear colourless
3 For examples involving heteroatom-centred nucleophiles see
(a) R. L. Danheiser, J. M. Morin Jr., M. Yu and A. Basak, Tetra-
hedron Lett., 1981, 22, 4205; (b) R. L. Danheiser, J. M. Morin Jr.
and E. J. Salaski, J. Am. Chem. Soc., 1985, 107, 8066; (c) T. H.
Hemmingsen, J. S. Svendsen and L. K. Sydnes, Acta Chem.
Scand., Ser. B, 1988, 42, 651; (d) M. G. Banwell, C. J. Cowden
and M. F. Mackay, J. Chem. Soc., Chem. Commun, 1994, 61;
(e) M. G. Banwell, C. J. Cowden and R. W. Gable, J. Chem. Soc.,
Perkin Trans. 1, 1994, 3515; ( f ) B. M. Trost and J. D. Oslob, J. Am.
Chem. Soc., 1999, 121, 3057; (g) M. G. Banwell and A. W. Wu,
J. Chem. Soc., Perkin Trans. 1, 1994, 2671; (h) M. G. Banwell,
J. E. Harvey, D. C. R. Hockless and A. W. Wu., J. Org. Chem., 2000,
65, 4241. For a related example involving a gem-dichlorocyclo-
propane see J. L. Castro, L. Castedo and R. Riguera, J. Org. Chem.,
1987, 52, 3579.
4 For examples involving carbon-centred nucleophiles see P. G.
Gassman, L. Tan and T. R. Hoye, Tetrahedron Lett., 1996, 37, 439.
5 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
6 For a related enantioselective (and catalytic) process involving
epoxide ring-cleavage by tethered alcohols see M. H. Wu, K. B
Hansen and E. N. Jacobsen, Angew Chem., Int. Ed., 1999, 38, 2012.
7 For leading references on the preparation, synthetic utility and bio-
logical activities of 6-azabicyclo[3.2.1]octanes see (a) H. Hiemstra,
W. J. Klaver and W. N. Speckamp, Recl. Trav. Chim. Pays-Bas, 1986,
105, 299; (b) D. J. Callis, N. F. Thomas, D. P. J. Pearson and B. V. L.
Potter, J. Org. Chem., 1996, 61, 4634; (c) J. Quirante, C. Escolano
and J. Bonjoch, Synlett., 1997, 179; (d) J. H. Rigby and J. H. Meyer,
Synlett., 1999, 860; for related papers on 2-azabicyclo[3.3.1]-
nonanes (morphans) see (e) M. C. Desai and S. L. Lefkowitz, Tetra-
hedron Lett., 1994, 35, 4701; ( f ) N. Yamazaki, H. Suzuki and
C. Kibayashi, J. Org. Chem., 1997, 62, 8280; (g) J. Quirante,
C. Escolano, F. Diaba and J. Bonjoch, Heterocycles, 1999, 50, 731;
(h) J. Quirante, C. Escolano, F. Diaba and J. Bonjoch, J. Chem. Soc.,
Perkin Trans. 1, 1999, 1157.
oil (Found Mϩ , 447.1596. C24H3281BrNO2 requires Mϩ
,
ؒ
ؒ
1
447.1596). H NMR (300 MHz, CDCl3) δ 7.33–7.23 (complex
m, 4H), 7.11 (m, 1H), 5.86 (broad s) and 5.72 (broad s) (rotam-
ers, 1H), 4.78 (dt, J = 10.6 and 4.3 Hz) and 4.71 (dt, J = 10.7
and 4.3 Hz) (rotamers, 1H), 4.44 (d, J = 4.4 Hz) and 2.94 (d,
J = 4.4 Hz) (rotamers, 1H), 3.43 (ddd, J = 10.7, 5.9 and 1.9 Hz,
1H), 3.09 (d, J = 11.0 Hz, 1H), 2.48 (m, 1H), 2.35 (dm, J = 18.1
Hz, 1H), 2.18–1.79 (complex m, 6H), 1.70–1.57 (complex m,
2H), 1.47 (broad m, 1H), 1.36 (s) and 1.33 (s) (rotamers, 3H),
1.18 (s, 3H), 1.14 (m, 1H), 0.94 (m, 1H), 0.88 (d, J = 6.5 Hz,
3H); 13C NMR (75 MHz, CDCl3) δ 153.0 and 152.9 (rotamers,
C), 127.8 (CH), 127.1 (CH), 125.2 (CH), 124.4 (CH), 123.2 and
123.0 (rotamers, C), 75.0 and 74.5 (rotamers, CH), 59.9 and
58.8 (rotamers, CH), 52.5 and 50.8 (rotamers, CH2), 50.8 and
50.6 (rotamers, CH), 42.6 and 41.9 (rotamers, C), 39.4 (CH2),
36.2 (CH2), 35.8 and 35.6 (rotamers, CH2), 35.0 and 34.7
(rotamers, CH2), 31.7 and 31.3 (rotamers, CH or CH3), 30.8
(CH or CH3), 29.7 (CH or CH3), 26.4 (CH2), 23.1 (CH or CH3),
21.9 (CH3), one signal due to a low field quaternary carbon not
observed; νmax (KBr) 2954, 2924, 1693, 1415, 1329, 1274, 1108,
788 and 763 cmϪ1; m/z 447 (3%) and 445 (3) [Mϩ ], 234 (70) and
ؒ
ؒ
232 (72), 119 (100) (C9H11ϩ), 118 (87) [C9H10 ], 105 (52), 91
ϩ
(69) (C7H7ϩ).
Acknowledgements
We are grateful to the Institute of Advanced Studies for
financial support including the provision of a PhD Scholarship
to J. H.
8 See, for example, J. R. Malpass and C. D. Cox, Tetrahedron Lett.,
1999, 40, 1419 and references cited therein.
9 P. J. Parsons, N. P. Camp, N. Edwards and L. R. Sumoreeah, Tetra-
hedron, 2000, 56, 309 and references cited therein.
Notes and references
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13 A. K. Ghosh, S. P. McKee, T. T. Duong and W. J. Thompson,
J. Chem. Soc., Chem Commun., 1992, 1308.
† All new and stable compounds had spectroscopic data [IR, UV,
NMR, mass spectrum] consistent with the assigned structure. Satisfac-
tory combustion and/or high-resolution mass spectral analytical data
were obtained for new compounds and/or suitable derivatives.
‡ All optical rotations were determined in chloroform solution at 20 ЊC
and are given in 10Ϫ1 deg cm2 gϪ1. Concentrations (c) are given in g 100
mlϪ1
.
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14 D. L. Comins, S. P. Joseph and R. R. Goehring, J. Am. Chem. Soc.,
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16 R. K. Crossland and K. L. Servis, J. Org. Chem., 1970, 35, 3195.
17 For a useful discussion regarding 8-phenylmenthyl and related
auxiliaries see G. Sarakinos and E. J. Corey, Org. Lett., 1999, 1,
1741.
18 teXsan: Single Crystal Structure Analysis Software, Version 1.8,
Molecular Structure Software Corporation, 3200 Reseach Forest
Drive, The Woodlands, TX 77381, USA (1997).
Data Collected in Oscillation Mode”, in Methods in Enzymology,
Volume 276: Macromolecular Crystallography, Part A, ed. C. W.
Carter Jr. and R. M. Sweet, Academic Press, 1997, pp. 307–326.
20 A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C.
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21 S. Mackay, C. J. Gilmore, C. Edwards, M. Tremayne, N. Stuart
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University of Glasgow, Scotland, Nonius BV, Delft, The
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19 Z. Otwinowski and W. Minor, “Processing of X-Ray Diffraction
2178
J. Chem. Soc., Perkin Trans. 1, 2000, 2175–2178