Modified synthesis of heftahelicene and its resolution into single enantiomers

Article abstract of DOI:10.1135/cccc20061256

A practical synthesis of racemic heptahelicene has been develeped being based on key [2+2+2] cycloisomerization of bis[2-(but-3-yn-1-yl)-1-naphthyl] acetylene under CpCo(CO)₂/PPh₃ or CpCo(C₂H ₄)₂ catalysis. The application of the Ni(COd)₂ catalyst with (-)-(S_a)-(2′-methoxy-1,1′-binaphthalen-2- yl)diphenylphosphane resulted in enantioselective triyne cyclization to provide (+)-7,8,11,12-tetrahydroheptahelicene in 40% ee. Optically pure (-)-(M)- and optically highly enriched (+)-(P)-heptahelicene were obtained on a milligram scale by resolution of racemate by chiral HPLC on a semipreparative Whelk-O1 column.

Full text of DOI:10.1135/cccc20061256

1256
Alexandrová, Sehnal, Stará, Starý, Šaman, Urquhart, Otero:
MODIFIED SYNTHESIS OF HEPTAHELICENE AND ITS RESOLUTION
INTO SINGLE ENANTIOMERS
Zuzana ALEXANDROVÁa1, Petr SEHNALa2, Irena G. STARÁa3,*, Ivo STARÝa4,*,
b1
b
David ŠAMANa5, Stephen G. URQUHART and Edwige OTERO
a Center for Biomolecules and Complex Molecular Systems,
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic,
1
Flemingovo nám. 2, 166 10 Prague 6, Czech Republic; e-mail: alexa@uochb.cas.cz,
2
3
4
5
sehnal@uochb.cas.cz, stara@uochb.cas.cz, stary@uochb.cas.cz, saman@uochb.cas.cz
b
Department of Chemistry, University of Saskatchewan,
Saskatoon SK S7N 5C9, Canada; e-mail: stephen.urquhart@usask.ca
1
Received Jun e 2, 2006
Accepted July 18, 2006
Dedicated to Professor Antonín Holý on the occasion of his 70th birthday.
A practical syn th esis of racem ic h eptah elicen e h as been develeped bein g based on key
[2+2+2] cycloisom erization of bis[2-(but-3-yn -1-yl)-1-n aph th yl]acetylen e un der
CpCo(CO)2/PPh 3 or CpCo(C2H4)2 catalysis. Th e application of th e Ni(cod)2 catalyst with
(–)-(Sa)-(2′-m eth oxy-1,1′-bin aph th alen -2-yl)diph en ylph osph an e resulted in en an tioselective
triyn e cyclization to provide (+)-7,8,11,12-tetrah ydroh eptah elicen e in 40% ee. Optically pure
(–)-(M)- an d optically h igh ly en rich ed (+)-(P)-h eptah elicen e were obtain ed on a m illigram
scale by resolution of racem ate by ch iral HPLC on a sem ipreparative Wh elk-O1 colum n .
Keyw ord s: Helicen es; Heptah elicen e; Alkyn es; Triyn es; Cycliosom erization ; Cobalt catalysis;
Nickel catalysis; En an tioselective catalysis; Racem ate resolution ; Ch iral HPLC.
In an alogy to CD spectroscopy at UV/VIS/IR wavelen gth s, we propose to
m easure th e CD spectra of ch iral organ ic m olecules at X-ray wavelen gth s
(X-ray n atural CD, or XNCD), wh ere well defin ed carbon 1s→p* tran sition s
occur. Advan ces in syn ch rotron tech n ology, particularly th e developm en t
of elliptically polarized un dulators, m ake sen sitive X-ray CD m easurem en ts
possible. X-ray CD spectroscopy h as been exten sively studied for m agn etic
CD spectroscopy. Ab in itio calculation s predict th e X-ray CD an isotropy to
be sm all1 an d, to date, m easurem en ts h ave been elusive on sm all m olecules
such as m eth yloxiran e2 an d am in o acids3. We h ave ch osen carboh elicen es
as targets for th e XNCD experim en ts. Th ese species h ave a series of sh ape,
n arrow carbon 1s→p* tran sition s with a ch iral orbital ch aracter th at are
ideal for m easurin g th e XNCD effect.
Collect. Czech. Chem. Commun. 2006, Vol. 71, No. 8, pp. 1256–1264
© 2006 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc20061256

Synthesis of Heptahelicene
1257
For practical reason s, we h ave decided to deal with h eptah elicen e (6)
(Sch em e 1) wh ich is, alon g with its [5]h elicen e an d [6]h elicen e con gen ers,
th e icon of h elicen e ch em istry4. Notably, n on racem ic h eptah elicen e exh ibits
a good con figuration al stability wh ich m an ifests itself by th e 41.7 kcal/m ol
barrier to racem ization 5. Th is attribute is suggested to allow for th erm al de-
position of (+)-(P)- or (–)-(M)-6 on a solid ph ase with out a con figuration al
scram blin g.
I
TIPS
TIPS
+
2
1
(i)
TIPS
TIPS
(ii)
4
3
(iii) or (iv)
(v)
rac-5
rac-6
(i) 1 (1.2 equiv.), 2 (1.0 equiv.), Pd(PPh3)4 (5%), CuI (10%), i-Pr2NH, 80 oC, 1.5 h, 82%;
(ii) Bu4NF (2.4 equiv.), THF, r.t., 20 min, 65%; (iii) CpCo(CO)2 (20%), PPh3 (40%), decane,
halogen lamp, 140 oC, 2 h, 64%; (iv) CpCo(C2H4)2 (20%), THF, r.t., 20 min, 64%;
(v) Ph3CBF4 (3.4 equiv.), 1,2-dichloroethane, 80 oC, 20 h, 90%
SCHEME 1
Syn th esis of rac-h eptah elicen e 6
Collect. Czech. Chem. Commun. 2006, Vol. 71, No. 8, pp. 1256–1264

1258
Alexandrová, Sehnal, Stará, Starý, Šaman, Urquhart, Otero:
Sin ce its first syn th esis by Martin in 1967 usin g th e ph otodeh ydro-
cyclization m eth odology6, several oth er n on ph otoch em ical approach es
h ave appeared in th e literature7. However, all of th em provided h epta-
h elicen e 6 in racem ic form an d, th erefore, its asym m etric syn th esis h as re-
m ain ed an un solved problem 8. Th us, th e on ly m eth odologies leadin g to
n on racem ic 6 (ref.9) h ave been m ech an ical separation of en an tiom eric crys-
tals of a con glom erate10 or an HPLC resolution on a ch iral station ary
ph ase11. Herein , we report a m odified syn th esis of rac-6, an attem pt at ap-
plyin g en an tioselective catalysis to get a n on racem ic h elical backbon e, an d
resolution of rac-6 in to en an tiom ers on a sem ipreparative scale usin g HPLC
on a Wh elk O1 ch iral sem ipreparative colum n .
RESULTS AND DISCUSSION
We h ave recen tly proved th at h elically ch iral m olecules can be syn th esized
in a gen eral an d efficien t way usin g in tram olecular [2+2+2] cycloisom er-
ization of arom atic triyn es12. Earlier, takin g advan tage of th is m eth odology,
we prepared rac-6 by cyclizin g an appropriate cis,cis-dien etriyn e7a. In th is
case th e key [2+2+2] cycloisom erization led directly to th e fully arom atic
h elical backbon e. However, th ere m igh t be an altern ative wh ich does n ot
in volve th e above m en tion ed cis,cis-dien etriyn e th at exh ibited on ly m oder-
ate stability. Th erefore, we h ave turn ed our atten tion to tetrah ydroh epta-
h elicen e rac-5 represen tin g a direct precursor of rac-6 (Sch em e 1) th at we
prepared earlier12c,12d
.
To prepare triyn e 3 (refs12c,12d), we h ave developed a differen t syn th esis
sch em e circum ven tin g th e Son ogash ira couplin g of 2 with gaseous acety-
len e. Alth ough such an approach to 3 is th e m ost straigh tforward, th e cou-
plin g reaction m igh t n ot be clean an d th e preparative yield m igh t vary.
Accordin gly, we h ave ch osen a com plem en tary process startin g from
kn own diyn e 1 (ref.12b) an d iodide 2 (ref.13). Un der Pd0/CuI catalysis, a
clean reaction took place affordin g triyn e 3 in h igh yield. Th e followin g
TBAF-assisted rem oval of triisopropylsilyl groups provided key triyn e 4 in
good yield. We publish ed earlier th at [2+2+2] cycloisom erization of 4 can
be catalyzed by CpCo(CO)2 in th e presen ce of PPh 3 un der con com itan t irra-
diation with visible ligh t at 140 °C to afford tetrah ydroh eptah elicen e rac-5
in 64% yield12c. To fin d a m ore reactive catalyst en ablin g us to sh orten th e
reaction period, to decrease th e reaction tem perature an d to in crease th e
preparative yield, we decided to use th e Jon as catalyst CpCo(C2H4)2 (ref.14).
In deed, th is cobalt com plex sh owed superior reactivity wh en cyclizin g 4 to
rac-5 at room tem perature with in 20 m in but th e yield was th e sam e as
Collect. Czech. Chem. Commun. 2006, Vol. 71, No. 8, pp. 1256–1264

Synthesis of Heptahelicene
1259
with CpCo(CO)2. Fin ally, th e syn th esis of rac-h eptah elicen e 6 was com -
pleted by sm ooth arom atization of rac-5 with trityl cation wh ich h as been
foun d earlier to be a reagen t of ch oice to arom atize tetrah ydroh elicen es12b
.
As far as n on racem ic h eptah elicen e 6 is con cern ed, we h ave proposed
first to explore en an tioselective [2+2+2] cycloisom erization to con trol th e
h elicity sen se of precursor 5. Stem m in g from successful catalytic experi-
m en ts in th e tetrah ydroh exah elicen e series, wh ere we obtain ed m oderate
en an tioselectivities12b,12d, we used Ni(cod)2 as a catalyst an d Hayash i’s
m on oden tate bin aph th yl-derived ph osph in e MOP 15 as a ch irality in ducer
in en an tioselective cyclization of 4 (Sch em e 2). Alth ough we m on itored
prom isin g 40% ee in favor of (+)-5, such an en an tioselectivity level
(i)
Ni(cod)2
(+)-5
PPh2
40% ee (41%)
OCH3
4
(–)-(S)-MOP
(i) Ni(cod)2 (20%), (–)-(S)-MOP (40%), THF, r.t., 1.5 h
SCHEME 2
Asym m etric syn th esis of (+)-7,8,11,12-tetrah ydroh eptah elicen e
ach ieved was n ot sufficien t to use th is m eth od for producin g optically pure
h eptah elicen e 6. Th us, we paid atten tion to th e h eptah elicen e racem ate res-
olution in to en an tiom ers by liquid ch rom atograph y on a ch iral station ary
ph ase (Sch em e 3). For th is purpose we used com m ercially available sem i-
preparative (R,R)-Wh elk-O1 colum n in com bin ation with an autom ated
preparative HPLC system . As th e resolution of sin gle en an tiom ers of rac-6
did n ot proceed en tirely down to th e base lin e an d, th erefore, a h igh er col-
um n loadin g was n ot possible, m ultiple in jection s of rac-6 were n ecessary
to get optically pure or optically h igh ly en rich ed (–)-(M)-6 (>99% op) an d
(+)-(P)-6 (92% op), respectively (Fig. 1).
Collect. Czech. Chem. Commun. 2006, Vol. 71, No. 8, pp. 1256–1264

1260
Alexandrová, Sehnal, Stará, Starý, Šaman, Urquhart, Otero:
(–)-(M)-6
(i)
rac-6
(+)-(M)-6
(i) HPCL separation on semipreparative (R,R)-Whelk-O1 column (250 x 10mm, 5 µm),
heptane, automated repeated 0.5–0.7 mg injections of rac-6, 43% of (–)-(M)-6 (>99% op),
40% of (+)-(M)-6 (92% op)
SCHEME 3
Resolution of rac-h eptah elicen e to sin gle en an tiom ers via ch iral HPLC
FIG. 1
Ch rom atograph ic an alyses (–)-(M)-6 an d (+)-(P)-6 after resolution . Resolution of rac-6 in to
(–)-(M)-6 an d (+)-(P)-6 en an tiom ers by ch iral HPLC on an (R,R)-Wh elk-O1 colum n (250 ×
4.6 m m , 5 µm , n-h eptan e, 1.0 m l/m in , 25 °C, UV detector)
Collect. Czech. Chem. Commun. 2006, Vol. 71, No. 8, pp. 1256–1264

Synthesis of Heptahelicene
1261
Sum m arizin g, we h ave developed a practical syn th esis of racem ic h epta-
h elicen e wh ich is based on key [2+2+2] cycloisom erization of arom atic
triyn e un der CoI catalysis. Th e application of th e Ni0 catalyst bearin g ch iral
m on oph osph in e ligan ds resulted in en an tioselective triyn e cyclization to
provide tetrah ydroh eptah elicen e (+)-5 in m oderate en an tiom eric excess.
Optically pure h eptah elicen e (–)-(M)-6 an d optically h igh ly en rich ed
(+)-(P)-6 were obtain ed on a m illigram scale by resolution of rac-6 by sem i-
preparative HPLC on a ch iral colum n . Th e XNCD experim en ts with n on -
racem ic h eptah elicen e are curren tly in progress.
EXPERIMENTAL
Gen eral
1H NMR spectra were m easured at 200.0, 499.8 or 500.13 MHz, 13C NMR spectra at 125.7 MHz,
in CDCl3 with TMS as an in tern al stan dard. Ch em ical sh ifts are given in ppm (δ-scale), cou-
plin g con stan ts (J) in Hz. Com poun ds 1 (ref.12b), 2 (ref.13), an d (–)-(S)-MOP (ref.15) were pre-
pared accordin g to th e literature procedures. Com m ercially available reagen t grade m aterials
were used as received. Solid Ni(cod)2 (Sigm a–Aldrich ) an d Jon as catalyst, CpCo(C2H4)2
(ref.14), were h an dled in a glovebox. Decan e an d diisopropylam in e were degassed by th ree
freeze-pum p-th aw cycles before use; 1,2-dich loroeth an e was distilled from calcium h ydride
un der argon ; THF was fresh ly distilled from sodium /ben zoph en on e un der n itrogen . TLC was
perform ed on Silica gel 60 F254-coated alum in ium sh eets (Merck) an d spots were detected
with a solution of Ce(SO4)2·4H2O (1%) an d H3P(Mo3O10
) (2%) in sulfuric acid (10%). Flash
4
ch rom atograph y was perform ed on Silica gel 60 (0.040–0.063 m m or <0.063 m m , Merck) or
on Biotage KP-Sil Silica cartridges (0.040–0.063 m m ) used in Horizon or Sp1 HPFC sys-
tem (Biotage, In c.). HPLC separation s on a ch iral sem ipreparative (R,R)-Wh elk-O1 colum n
(250 × 10 m m , 5 µm , Merck) were perform ed with th e Agilen t 1100 system equipped with
a UV detector, autosam pler an d fraction collector. En an tiom eric excess of (+)-5 was deter-
m in ed by ch iral HPLC on a (R,R)-Wh elk-O1 colum n (250 × 4.6 m m , 5 µm , n-h eptan e,
1.0 m l/m in , 25 °C) wh ile optical purity of (–)-(M)-6 an d (+)-(P)-6 was determ in ed on
a
Ch iralcel OD-H colum n (250 × 4.6 m m , n-h eptan e, 0.8 m l/m in , 25 °C) usin g UV an d
polarim etric detectors.
Bis{2-[4-(triisopropylsilyl)but-3-yn -1-yl]-1-n aph th yl}acetylen e (3)
A Sch len k flask was ch arged with n aph th yl diyn e 1 (1.12 g, 3.11 m m ol), n aph th yl iodide 2
(1.58 g, 3.42 m m ol, 1.1 equiv.), Pd(PPh 3)4 (180 m g, 0.156 m m ol, 5 m ole %), CuI (59 m g,
0.310 m m ol, 10 m ole %), an d flush ed with argon . Diisopropylam in e (50 m l) was added an d
th e reaction m ixture was stirred at 80 °C for 2 h . Th e solven t was rem oved in vacuo an d th e
residue was ch rom atograph ed on silica gel (petroleum eth er–eth er, 100:0 to 98:2) to obtain
3 (1.78 g, 82%) as an oil. 1H NMR (200 MHz, CDCl3) spectrum was in accord with th e litera-
ture data13
.
Collect. Czech. Chem. Commun. 2006, Vol. 71, No. 8, pp. 1256–1264

1262
Alexandrová, Sehnal, Stará, Starý, Šaman, Urquhart, Otero:
Bis[2-(but-3-yn -1-yl)-1-n aph th yl]acetylen e (4)
Followin g th e literature procedure13, triyn e 3 (690 m g, 1.80 m m ol) in THF (15 m l) was
desilylated with tetrabutylam m on ium fluoride (1.0 M solution in THF, 2.40 m l, 2.40 m m ol,
2.4 equiv.) at room tem perature with in 20 m in . Flash ch rom atograph y on silica gel (petro-
leum eth er–eth er, 100:0 to 98:2) gave triyn e 4 (247 m g, 65%). 1H NMR (200 MHz, CDCl3)
spectrum was in accord with th e literature data13
.
7,8,11,12-Tetrah ydroh eptah elicen e (5)
A flask with triyn e 4 (162 m g) was flush ed with argon . Th e m aterial was dissolved in THF
(5 m l) an d CpCo(C2H4)2 (15 m g, 0.085 m m ol, 20 m ole %) in THF (2 m l) was added. Th e re-
action m ixture was stirred at room tem perature for 40 m in an d evaporated to dryn ess in
vacuo. Flash ch rom atograph y on silica gel (petroleum eth er–eth er, 100:0 to 98:2) provided
tetrah ydroh eptah elicen e 5 (104 m g, 64%) as an am orph ous solid. 1H NMR (200 MHz,
CDCl3) spectrum was in accord with th e literature data12c
.
(+)-7,8,11,12-Tetrah ydroh eptah elicen e (5)
A Sch len k flask was ch arged with (–)-(S)-MOP (9.3 m g, 0.020 m m ol, 40 m ole %), flush ed
with argon an d th e ligan d was dissolved in THF (2 m l) at room tem perature. Th en a solu-
tion of Ni(cod)2 (160 µl, 0.061 M in THF, 20 m ole %) was added an d th e m ixture was stirred
for 5 m in . Triyn e 4 (19 m g, 0.050 m m ol) in THF (1 m l) was added an d th e m ixture was
stirred at room tem perature for 1.5 h . Volatiles were rem oved in vacuo an d th e residue was
ch rom atograph ed on silica gel (petroleum eth er–eth er, 100:0 to 96:4) to obtain (+)-5
(7.7 m g, 41%, 40% ee) as an am orph ous solid.
Heptah elicen e (6)
A Sch len k flask was ch arged with tetrah ydroh eptah elicen e 5 (100 m g, 0.261 m m ol) an d
trityl tetrafluoroborate (290 m g, 0.878 m m ol, 3.4 equiv.) an d flush ed with argon . Th e m ate-
rials were dissolved in 1,2-dich loroeth an e (13 m l) an d th e reaction m ixture was stirred at
80 °C for 20 h . Th e m ixture was diluted with 1,2-dich loroeth an e an d wash ed with aqueous
KHCO3. Th e solution was dried over an h ydrous Na2SO4 an d evaporated to dryn ess. Flash
ch rom atograph y on silica gel (petroleum eth er–eth er, 100:0 to 98:2) afforded h eptah elicen e
6 (89 m g, 90%) as an am orph ous solid. 1H NMR (200 MHz, deuteroch loroform ) spectrum
was in accord with th e literature data7a
.
Resolution of rac-Heptah elicen e in to En an tiom ers
Usin g a sem ipreparative (R,R)-Wh elk-O1 colum n , m ultiple in jection s (0.5–0.7 m g portion s in
h eptan e) of racem ic h eptah elicen e rac-6 (35 m g) an d rein jection s of th e n ot fully resolved
fraction s led to optically pure (–)-(M)-6 (>99% op, 15 m g) an d optically h igh ly en rich ed
25
25
(+)-(P)-6 (92% op, 14 m g). (–)-(M)-6: [α]D –2276 (c 0.063, CHCl3)16. (+)-(P)-6: [α]D +2231
(c 0.084, CHCl3).
We are very grateful to Prof. K. Jonas, MPI Mülheim, Germany, for his generous gift of the
CpCo(C2H4)2 complex. This research was supported by the Ministry of Education, Youth and Sports of
Collect. Czech. Chem. Commun. 2006, Vol. 71, No. 8, pp. 1256–1264

Synthesis of Heptahelicene
1263
the Czech Republic (Barrande project 2005-06-041-1 and Center for Biomolecules and Complex
Molecular Systems, project LC512), Pico-Inside (6th Framework EU Integrated Project FP6-015847)
and the Institute of Organic Chemistry and Biochemistry (the work is a part of research project
Z4 055 0506).
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15. Uozumi Y., Tanahashi A., Lee S.-Y., Hayashi T.: J. Org. Chem. 1993, 58, 1945.
25
16. Optical rotation of (–)-(M)-6 published by Martin (ref.10, [α]579 –5900
200, c 0.06,
CHCl3) is substantially higher than the value measured by us, despite the fact that we
repeatedly checked the purity of (–)-(M)-6 (purified by HPLC or recrystallized from
heptane) by 1H NMR and 13C NMR and enantiopurity by HPLC on a chiral column. The
CD spectrum was in full agreement with the literature data.
Collect. Czech. Chem. Commun. 2006, Vol. 71, No. 8, pp. 1256–1264