Synthesis of helicene scaffolds via [2+2+2] cycloisomerization of aromatic triynes

Article abstract of DOI:10.1135/cccc20030917

Triynes R-C≡-C-CH₂CH₂-Ar-C≡C-Ar′- CH₂CH₂-C≡C-R were subjected to Co(I)- or Ni(0)-mediated intramolecular [2+2+2] cycloisomerization to provide corresponding tetrahydropentahelicene, tetrahydrohexahelicene, and t

Full text of DOI:10.1135/cccc20030917

Synthesis of Helicene Scaffolds
917
SYNTHESIS OF HELICENE SCAFFOLDS via [2+2+2]
CYCLOISOMERIZATION OF AROMATIC TRIYNES
1,
2,
3
Irena G. STARÁ *, Ivo STARÝ *, Adrian KOLLÁROVIČ, Filip TEPLÝ ,
4
David ŠAMAN and Pavel FIEDLER
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: stara@uochb.cas.cz,
2
3
4
stary@uochb.cas.cz, fteply@uochb.cas.cz, saman@uochb.cas.cz
Received Jan uary 8, 2003
Accepted March 5, 2003
Dedicated to the memory of Professor Otakar Červinka who deceased on May 24, 2002.
Triyn es R–C≡C–CH2CH2–Ar–C≡C–Ar′–CH2CH2–C≡C–R were subjected to Co(I)- or Ni(0)-
m ediated in tram olecular [2+2+2] cycloisom erization to provide correspon din g tetrah ydro-
pen tah elicen e, tetrah ydroh exah elicen e, an d tetrah ydroh eptah elicen e in good to m oderate
yields. Th e reaction tolerates various substituen ts at pen dan t acetylen e m oieties (R = H, CH3,
trim eth ylsilyl, n ot triisopropylsilyl) an d fluorin e in an arom atic part. By con trast, un der
Rh (I) or Pd(0) catalysis, triyn e with th e CH2OCH2 teth er lost pen dan t propargyl m oieties
an d an arom atic spiroketal was form ed preferen tially.
Keyw ord s: Alkyn es; Aren es; Cobalt catalysis; Helical ch irality; Helicen es; Nickel catalysis;
Palladium catalysis; Rh odium catalysis; Spiroketals; Tetrah ydroh elicen es; Triyn e cyclo-
trim erization .
Helically ch iral m aterials such as n ucleic acids, protein s, an d polysacch a-
rides play an essen tial role in n ature. By con trast, with in th e realm s of
sm all ch iral organ ic m olecules1, th e “screw-like” structural m otif2 is less fre-
quen t. Th us, m ost exam ples of sm all h elices can be taken from th e fam ilies
of artificial m olecules such as h elicates3 an d h elicen es4. Th e latter com -
poun ds h ave been applied or at least th eoretically studied in diverse areas
ran gin g from asym m etric catalysis5 to n on lin ear optics6 an d m olecular
wires7 to provide prom isin g results. In spite of th is, th ere h as still been a
n oticeable discrepan cy between th e n um ber of origin al papers devoted to
h elicen e syn th esis publish ed durin g th e last two decades an d th e n um ber of
h elicen e exploitation s. Th e reason for th is is obvious: th e curren t m eth ods
for h elicen e syn th esis h ave been laborious or suitable on ly for preparin g in -
dividual structure types. In addition , n o h elicen e derivative is so far com -
Collect. Czech. Chem. Commun. (Vol. 68) (2003)
doi:10.1135/cccc20030917

918
Stará et al.:
m ercially available in m ultigram quan tities. Hen ce, a gen eral, m odular, an d
really efficien t syn th etic m eth odology for preparin g h elicen es an d th eir
substituted derivatives in a n on -racem ic form is required.
Recen tly, Katz h as con tributed substan tially to th e solution of th e h elice-
n es availability problem 8. However, a n ich e in th is area of organ ic syn th esis
h as still rem ain ed9. In our prelim in ary com m un ication 10 we publish ed an
altern ative route to tetrah ydroh elicen e scaffolds, wh ich relied on in tra-
m olecular [2+2+2] cycloisom erization of arom atic triyn es un der Co(I) or
Ni(0) catalysis. Herein , we report th e cyclization procedure in full details
bein g accom pan ied by furth er exam ples. Th is approach relates to th e syn -
th esis of h elicen e-like m olecules from arom atic triyn es11 an d it is com ple-
m en tary to our n on ph otoch em ical syn th esis of paren t h elicen es from
arom atic cis,cis-dien etriyn es12.
RESULTS AND DISCUSSION
Synthesis of Aromatic Triynes
Th e syn th esis of arom atic triyn es 1, 4–6, an d 8–11 (Sch em e 2, Tables I–III)
h ave been publish ed elsewh ere13. Fluorin ated triyn e 7 was prepared from
dibrom ide13 21 by a on e-pot procedure (Sch em e 1). Treatm en t of 21 with
LiCH2C≡CTMS (gen erated in situ from CH3C≡CTMS an d butyllith ium ) fol-
lowed by th e TMS group rem oval with tetrabutylam m on ium fluoride led to
triyn e 7 in good overall yield. Diol14 2 an d its TMS derivative13 3 were pre-
pared earlier.
F
F
Br
Br
a
21
7
(a) LiCH2C CTMS (2.1 equiv.), THF, –78 oC, 1 h, then n-Bu4NF (10.0 equiv.), rt, overnight, 76%
SCHEME 1
Syn th esis of triyn e 7
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Synthesis of Helicene Scaffolds
919
[2+2+2] Cycloisomerization of Aromatic Triynes
Th e results of our attem pts to pursue [2+2+2] cycloisom erization of aro-
m atic triyn es 1, 4–11 are sum m arized in Sch em e 2 an d Tables I–III. We
h ave already publish ed th at triyn e 1 un derwen t a sm ooth cyclization un der
th e cobalt(I) catalysis to afford 12 with a h elical scaffold11 (Sch em e 2). To
explore th e catalytic efficien cy of oth er tran sition m etal com plexes in th is
process, we paid atten tion to n ickel(0), rh odium (I), an d palladium (0) catal-
ysis. Displacin g Co(I) by Ni(0), th e startin g m aterial 1 was partially con -
sum ed but a com plex m ixture was form ed. Most aston ish in gly, th e
Wilkin son ’s catalyst15 prom oted th e form ation of an un expected spiro-
ketal16 13 in stead of givin g rise to th e h elicen e-like derivative 12. Th e yield
was low but spiroketal 13 was th e on ly product detected in th e reaction
m ixture. Sim ilarly, th e application of palladium on ch arcoal with ch loro-
(trim eth yl)silan e17 led to spiroketal 13, wh ich was produced, con trary to
O
O
a
O
O
b or c
1
12
OR
OR
O
O
d or e
13
2, R = H
3, R = TMS
(a) CpCo(CO)2 (40 mole %), PPh3 (80 mole %), decane, 140 oC, 0.5 h, irradiated
with a halogen lamp, 89% (ref.11);
(b) Rh(PPh3)3Cl (10 mole %), ethanol, 120 oC, 3 h, in a sealed tube, 22%;
(c) 10% Pd/C (5 mole %), TMSCl (300%), tetrahydrofuran, 120 oC, 3 h, in a sealed tube, 95%;
(d) 2, 10% Pd/C (5 mole %), TMSCl (300%), tetrahydrofuran, 120 oC, 10 h, in a sealed tube, 42%;
(e) 3, 10% Pd/C (5 mole %), TMSCl (300%), tetrahydrofuran, 120 oC, 10 h, in a sealed tube, 66%
SCHEME 2
Cyclization of 1, 2, an d 3
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

920
Stará et al.:
th e previous run , in h igh yield. Apparen tly, both propargyl m oieties in 1
are lost to afford spiroketal 13. We h ypoth esize a com petitive Rh - or Pd-
m ediated propargyl eth er-to-allen yl eth er isom erization operates at 1 in -
stead of en terin g th e desired [2+2+2] cycloisom erization catalytic cycle. Th e
allen yl eth er m oiety m igh t be cleaved with th e assistan ce of a tran sition
m etal Lewis acid or h ydrogen ch loride (gen erated in situ from TMSCl an d
rem n an t water in th e reaction m edium ) to produce diol 2 or its TMS deriva-
tive 3. Th en a double in tram olecular addition of oxygen n ucleoph iles
across th e triple bon d takes place. Th ere is a com m on sen se th e reaction is
catalyzed by tran sition m etals, particularly by palladium 18, via electroph ilic
activation of a m ultiple carbon –carbon bon d towards n ucleoph ilic addi-
tion . To validate th e occurren ce of an in ferred h ydroxy or silyloxy in term e-
diate, we subjected diol 2 an d silyl eth er 3 to th e reaction in th e presen ce of
Pd/C an d TMSCl. In deed, both 2 an d 3 were cyclized to spiroketal 13 in
m oderate an d good yield, respectively.
Triyn e 4 was cyclized un der Co(I) catalysis to tetrah ydro[5]h elicen e 14 in
good yield (Table I, en try 1). In com parison with triyn e 1 (Sch em e 2),
wh ich differs on ly in th e presen ce of th e oxygen atom in th e teth er be-
tween pen dan t acetylen e un its an d th e core diph en ylacetylen e m oiety,
triyn e 4 is m ore susceptible to [2+2+2] cycloisom erization . Th is differen ce
em erged profoun dly wh en decreasin g th e reaction tem perature. Wh ile
triyn e 1 required h eatin g at 140 °C to accom plish th e con version , triyn e 4
could be cyclized even at 50 °C with out an y irradiation to provide 14 in
still reason able yield (Table I, en try 2). However, th e presen ce of triph en yl-
ph osph in e was essen tial to keep a cobalt(I) catalyst active th rough out th e
lon g reaction period. In th e absen ce of triph en ylph osph in e, triyn e 4 fur-
n ish ed 14 in a very low yield (Table I, en try 3). As regards th e catalytic sys-
tem , n ickel(0) was foun d to be superior to cobalt(I) in term s of reactivity
an d m ildn ess of reaction con dition s used. In th e sh arp con trast to triyn e 1,
Ni(0)-catalyzed cyclization of 4 proceeded sm ooth ly to afford 14 in good
yield (Table I, en try 4). Despite th e kn own activity of TaCl5 in cyclo-
isom erization of alkyn es19, it was foun d to be in effective in th e case of 4
(Table I, en try 5).
Our n ext goal was to explore th e reaction s poten tial for th e syn th esis of
substituted tetrah ydro[5]h elicen es. In th e presen ce of a Ni(0) com plex,
triyn e 5 possessin g on ly n on -term in al acetylen e un its furn ish ed th e tetra-
h ydro[5]h elicen e derivative 15 in acceptable yield (Table II, en try 1). Triyn e
6 with m ore bulky TMS groups at th e pen dan t alkyn e un its provided un der
Co(I) or Ni(0) catalysis th e correspon din g derivative 16 in good yields20
(Table II, en tries 2 an d 3). In order to exam in e th e accessibility of h elicen es
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Synthesis of Helicene Scaffolds
921
with m odulated electron ic properties, we attem pted th e syn th esis of a
m odel fluoro derivative. Th e Co(I)-catalyzed cyclization of 7 led to fluoro-
tetrah ydro[5]h elicen e 17 in m oderate yield (Table II, en try 4).
As th e gen eral approach to tetrah ydro[5]h elicen es h as been proved to be
feasible, th e n ext ch allen ge was to syn th esize h igh er h om ologues. Triyn e 8
with th e un sym m etrical n aph th yl(ph en yl)acetylen e core provided un der
Co(I) catalysis tetrah ydro[6]h elicen e 18 in good yield (Table III, en try 1).
Th e triyn e 8 was reactive en ough to un dergo cyclization at 50 °C (with out
irradiation ) but th e preparative yield was lower in com parison with th e
an alogous experim en t in a tetrah ydro[5]h elicen e series (cf. Table III, en try 2
an d Table I, en try 2). In th e presen ce of a Ni(0) com plex, triyn e 8 was
im m ediately cyclized to tetrah ydro[6]h elicen e 18 in good yield (Table III,
en try 3). Th e attem pt to apply a Ni(0) catalyst gen erated in situ from
n ickelocen e an d triph en ylph osph in e21 failed an d n o cyclized product 18
was observed (Table III, en try 4). Th e an alogous dim eth yl derivative 9 was
cyclized un der Co(I) catalysis to 19 in good yield (Table III, en try 5). Ex-
TABLE I
Cyclization of m odel substrate 4
Startin g
m aterial
MLn (m ole %)
Ligan d (m ole %)
Con dition sa
°C, h
Yieldb
%
En try
Product
CpCo(CO)2 (20)
PPh 3 (40)
1
140, 1c
72
14
4
CpCo(CO)2 (100)
PPh 3 (200)
2
3
4
5
4
4
4
4
50, 72
14
14
14
63
8d
66
CpCo(CO)2 (100) 50, 8 days
Ni(cod)2 (20)
r.t., 0.5
PPh 3 (40)
TaCl5 (100)
reflux, 24
n o reaction e
a
Th e reaction s were run in decan e (Co catalysis), tetrah ydrofuran (Ni catalysis), or ben zen e
b
(Ta catalysis) un til th e startin g m aterial disappeared or n o progress was m on itored. Iso-
lated. Th e reaction m ixture was irradiated by a 250 W h alogen lam p. Th e startin g m ate-
rial was recovered (58%). Th e startin g m aterial rem ain ed un ch an ged.
c
d
e
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922
Stará et al.:
ten din g th e triyn e core to a din aph th ylacetylen e as in 10, led to successful
cycloisom erization un der Co(I) catalysis givin g tetrah ydro[7]h elicen e 20
again in good yield (Table III, en try 6). In th e case of 10, th e Ni(0)-catalyzed
reaction produced 20 in low yield an d m ost of th e startin g m aterial poly-
m erized in con trast to th e form ation of lower h om ologues 14 an d 18 (cf.
Table III, en try 7 versus Table I, en try 4 an d Table II, en try 3). Not surpris-
in gly, th e attem pt at cyclization of triyn e 11 bearin g bulky TIPS groups was
un successful an d startin g m aterial rem ain ed (Table III, en try 8).
TABLE II
Cyclization of m odel substrates 5–7
Startin g
m aterial
MLn (m ole %)
Ligan d (m ole %)
Con dition sa
°C, h
Yieldb
%
En try
Product
Ni(cod)2 (100)
PPh 3 (200)
1
r.t., 20
42
5
15
TMS
TMS
CpCo(CO)2 (40)
PPh 3 (80)
TMS
TMS
2
3
4
140, 2.5c
71
70
33
6
6
16
16
Ni(cod)2 (100)
PPh 3 (200)
r.t., 20
F
F
CpCo(CO)2 (40)
PPh 3 (80)
140, 3c
7
17
a
Th e reaction s were run in decan e (Co catalysis) or tetrah ydrofuran (Ni catalysis) un til th e
b
c
startin g m aterial disappeared or n o progress was m on itored. Isolated. Th e reaction m ix-
ture was irradiated by a 250 W h alogen lam p.
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Synthesis of Helicene Scaffolds
923
TABLE III
Cyclization of m odel substrates 8–11
Startin g
m aterial
MLn (m ole %)
Ligan d (m ole %) °C, h
Con dition sa
Yieldb
%
En try
Product
CpCo(CO)2 (20)
PPh 3 (40)
1
140, 0.5c
64
8
18
18
CpCo(CO)2 (20)
PPh 3 (40)
2
3
4
8
8
8
50, 4 days
reflux, 0.1
50, 12
26
75
Ni(cod)2 (200)
18
Cp2Ni (120)
PPh 3 (120)
n o reaction d
CpCo(CO)2 (20)
PPh 3 (40)
5
140, 1.0c
68
19
9
CpCo(CO)2 (20)
PPh 3 (40)
6
7
8
140, 2.0c
reflux, 0.1
r.t., 2
64
24
20
20
10
10
Ni(cod)2 (200)
Ni(cod)2 (100)
PPh 3 (200)
n o reaction d
TIPS
TIPS
11
a
Th e reaction s were run in decan e (Co catalysis) or tetrah ydrofuran (Ni catalysis) un til th e
b
c
startin g m aterial disappeared or n o progress was m on itored. Isolated. Th e reaction m ix-
ture was irradiated by a 250 W h alogen lam p. Th e startin g m aterial rem ain ed un ch an ged.
d
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

924
Stará et al.:
CONCLUSION
Model arom atic triyn es 1 an d 4–10 h ave been sh own to un dergo Co(I)- or
Ni(0)-catalyzed in tram olecular [2+2+2] cycloisom erization affordin g tetra-
h ydroh elicen es. Th e reaction can be used to con struct pen ta-, h exa- or
h eptacyclic h elical scaffolds an d displays reason able toleran ce towards sub-
stituen ts an d con dition s. Th e use of oth er tran sition m etal catalysts is lim -
ited – Ta(V) is in active an d Rh (I) or Pd(0) can prom ote, in an in dividual
case, a con curren t reaction ch an n el to deliver a spiroketal product. Th e
study h as establish ed a n ovel m eth odology for preparin g h elicen e con ge-
n ers in a n on ph otoch em ical way an d illustrates clearly its efficien cy. Tran s-
form ation of tetrah ydroh elicen es to paren t h elicen es is un derway an d will
be publish ed separately.
EXPERIMENTAL
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), couplin g con stan ts J are given in Hz. HMBC experim en ts were setup for JCH = 5 Hz.
For correct assign m en t of both 1H an d 13C NMR spectra of key com poun ds, th e HCOSY an d
HMQC experim en ts were perform ed. For all th e oth er com poun ds, th e gen eral sem i-
em pirical equation s were applied to th e ch em ical sh ift assign m en ts. IR spectra (ν, cm –1) were
m easured in CHCl3. EI MS spectra were determ in ed at an ion izin g voltage of 70 eV, m/z val-
ues are given alon g with th eir relative in ten sities (%). FAB MS spectrum was m easured usin g
th e bis(2-h ydroxyeth yl) disulfide m atrix. HR MS spectra were obtain ed by th e EI tech n ique.
Reagen t grade m aterials purch ased from Sigm a–Aldrich , Fluka, an d Merck were used as re-
ceived. Th e syn th eses of triyn es 1, 4–6, an d 8–11 (ref.13), diol 2 (ref.14) an d its TMS deriva-
tive 3 (ref.13) were publish ed previously. A stock solution of Ni(cod)2 in tetrah ydrofuran (ca
0.06 m ol/l) was stored at –78 °C un der argon an d reused with in two weeks. Tetrah ydrofuran
was fresh ly distilled from sodium /ben zoph en on e un der n itrogen ; ben zen e was distilled from
calcium h ydride un der argon ; decan e an d UV grade eth an ol were degassed by th ree freeze-
pum p-th aw cycles before use. TLC was perform ed on Silica gel 60 F254-coated alum in ium
sh eets (Merck) an d spots were detected by th e solution of Ce(SO4)2·4H2O (1%) an d
H3P(Mo3O10
) (2%) in sulfuric acid (10%). Flash ch rom atograph y was perform ed on Silica
4
gel 60 (0.040–0.063 m m or <0.063 m m , Merck).
Gen eral Procedures for th e Cyclization of Triyn es 1, 4–11
Procedure A. Cobalt catalysis with halogen lamp irradiation. A m ixture of triyn e (0.100 m m ol)
an d decan e (3 m l) un der argon was stirred an d h eated at 100 °C by two 250 W h alogen
lam ps to dissolve th e startin g m aterial. A h ot solution of triph en ylph osph in e (10.5 m g,
0.040 m m ol, 40 m ole %) in decan e (1 m l) was added an d th e m ixture was furth er irradiated
to ach ieve 140 °C. CpCo(CO)2 (ca 2.7 µl, 0.020 m m ol, 20 m ole %) in decan e (0.1 m l) was
added via syrin ge an d th e resultin g oran ge-brown solution was irradiated un der stirrin g at
140 °C (m easured in side th e reaction vessel) un til th e startin g m aterial disappeared or n o re-
action progress was m on itored. Th e reaction m ixture was cooled to room tem perature,
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Synthesis of Helicene Scaffolds
925
poured on to a silica gel colum n , an d th e product was eluted. Stoich iom etric version or a re-
action with out h alogen lam p irradiation was perform ed an alogously.
Procedure B. Nickel catalysis. In a Sch len k flask, triph en ylph osph in e (10.5 m g, 0.040 m m ol,
40 m ole %) was dissolved in tetrah ydrofuran (1 m l) un der argon , a stock solution of
Ni(cod)2 (0.06 M in tetrah ydrofuran , 330 µl, 0.020 m m ol, 20 m ole %) was added an d th e
m ixture was stirred at room tem perature for 5 m in . Triyn e (0.100 m m ol) in tetrah ydrofuran
(2 m l) was added an d th e m ixture was stirred at room tem perature or un der reflux un til th e
startin g m aterial disappeared or n o reaction progress was m on itored. Th e solven t was evapo-
rated in vacuo an d th e residue was ch rom atograph ed on silica gel to obtain th e product.
Stoich iom etric version was perform ed an alogously.
2-But-3-yn -1-yl-1-[(2-but-3-yn -1-ylph en yl)eth yn yl]-4-fluoroben zen e (7)
Butyllith ium (1.6
M solution in h exan es, 480 µl, 0.768 m m ol, 2.1 equiv.) was added
dropwise over a 2-m in period to a solution of trim eth yl(prop-1-yn -1-yl)silan e (115 µl, 0.777
m m ol, 2.1 equiv.) in tetrah ydrofuran (2 m l) at –78 °C un der argon . After stirrin g at –78 °C
for 3 h , dibrom ide13 21 (140 m g, 0.366 m m ol) in tetrah ydrofuran (1 m l) was added
dropwise. Th e m ixture was stirred at –78 °C for 1 h an d th en warm ed to room tem perature.
Tetrabutylam m on ium fluoride (1.0 M solution in tetrah ydrofuran , 3.65 m l, 3.65 m m ol,
10.0 equiv.) was added an d th e m ixture was stirred at room tem perature overn igh t. Th e so-
lution was evaporated in vacuo to dryn ess an d th e residue was ch rom atograph ed on silica gel
(h exan e–eth er 97:3) to get triyn e 7 (83.1 m g, 76%) as an am orph ous solid. Th e product was
im m ediately subjected to th e Co-catalyzed cyclization . IR: 3308 s, 3071 w, 2960 s, 2928 vs,
2873 m , 2856 s, 2207 w, 2118 w, 1608 s, 1580 s, 1496 s, 1485 m (sh ), 1452 m , 1432 m ,
1416 w, 1340 w, 1304 m (sh ), 1278 s, 1263 s, 1184 m , 1161 m , 1137 m , 1104 m , 954 m ,
874 m , 822 m , 640 s, 542 w, 495 w, 453 w. 1H NMR (200 MHz): 1.99 (1 H, t, J = 2.4,
CH2CH2C≡CH); 2.00 (1 H, t, J = 2.4, CH2CH2C≡CH); 2.55–2.64 (4 H, m , CH2CH2C≡CH);
3.07 (2 H, t, J = 7.1, CH2CH2C≡CH); 3.11 (2 H, t, J = 7.1, CH2CH2C≡CH); 6.95–7.74 (7 H, m ,
arom .). EI MS: 300 (M• +, 2), 299 (7), 283 (10), 260 (99), 246 (19), 239 (100), 233 (24), 133
(30), 115 (17), 109 (21), 98 (10), 87 (8), 81 (10), 75 (13), 63 (19), 57 (27), 51 (24), 39 (57).
HR EI MS: calculated for C22H16F (M – 1) 299.1236; foun d 299.1240.
1,4-Dih ydro-3′H-spiro[2-ben zopyran -3,1′-isoben zofuran ] (13)
A sealed tube was ch arged with triyn e 1 (21 m g, 0.067 m m ol) an d Rh (PPh 3)3Cl (6.5 m g,
0.007 m m ol, 10 m ole %) an d flush ed with argon . Eth an ol (3 m l) was added, th e tube was
tigh tly closed an d h eated at 120 °C for 3 h . Th e reaction m ixture was evaporated to dryn ess
in vacuo an d th e residue was ch rom atograph ed on silica gel (petroleum eth er–eth er–aceton e
80:10:10) to obtain spiroketal16 13 (3.5 m g, 22%) as an oil.
Cyclization of 1 un der Pd catalysis was carried out an alogously: 1 (20 m g, 0.064 m m ol),
10% Pd on ch arcoal (3.4 m g, 0.003 m m ol, 5 m ole %), ch loro(trim eth yl)silan e (24 µl,
0.189 m m ol, 300 m ole %), tetrahydrofuran (3 m l), 120 °C, 3 h. Flash chrom atography afforded
spiroketal 13 (14.5 m g, 95%).
Cyclization of 2 un der Pd catalysis was carried out an alogously: 2 (27.1 m g, 0.114 m m ol),
10% Pd on ch arcoal (6.3 m g, 0.006 m m ol, 5 m ole %), ch loro(trim eth yl)silan e (43 µl,
0.339 m m ol, 300 m ole %), tetrah ydrofuran (2 m l), 120 °C, 10 h . Flash ch rom atograph y af-
forded spiroketal 13 (11.4 m g, 42%).
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

926
Stará et al.:
Cyclization of 3 un der Pd catalysis was carried out an alogously: 3 (24 m g, 0.063 m m ol),
10% Pd on ch arcoal (3.3 m g, 0.003 m m ol, 5 m ole %), ch loro(trim eth yl)silan e (24 µl,
0.189 m m ol, 300 m ole %), tetrah ydrofuran (2 m l), 120 °C, 10 h . Flash ch rom atograph y af-
forded spiroketal 13 (9.9 m g, 66%). IR: 2948 w, 2867 w, 1602 w, 1590 w, 1496 w, 1484 w,
1459 w, 1375 m , 1197 w, 1114 w, 1098 w, 1062 s, 1037 w, 1008 s, 939 w, 886 m , 865 vw,
591 vw. 1H NMR (200 MHz): 3.08 (1 H, d, J = 16.2, Ph -CH2-C); 3.58 (1 H, d, J = 16.2,
Ph -CH2-C); 4.87 (1 H, d, J = 14.5, Ph -CH2-O); 5.07 (1 H, d, J = 12.4, Ph -CH2-O); 5.17 (1 H, d,
J = 14.5, Ph -CH2-O); 5.26 (1 H, d, J = 12.4, Ph -CH2-O); 7.04–7.40 (8 H, m , arom .). FAB MS:
239 ((M + H)+), 238, 221, 179, 149, 147, 95, 81, 73, 69, 55.
5,6,9,10-Tetrah ydropen tah elicen e (14)
Procedure A – Catalytic with h alogen lam p irradiation . Triyn e 4 (72 m g, 0.255 m m ol),
CpCo(CO)2 (7 µl, 0.053 m m ol, 20 m ole %), triph en ylph osph in e (26.7 m g, 0.102 m m ol,
40 m ole %), decan e (3 m l), 140 °C, 1 h . Flash ch rom atograph y on silica gel (petroleum
eth er) afforded 14 (52 m g, 72%) as an am orph ous solid. Procedure A – Stoich iom etric with -
out h alogen lam p irradiation . Triyn e 4 (49 m g, 0.174 m m ol), CpCo(CO)2 (23 µl, 0.174 m m ol,
100 m ole %), triph en ylph osph in e (91 m g, 0.347 m m ol, 200 m ole %), decan e (3 m l), 50 °C,
72 h . Flash ch rom atograph y afforded 14 (30.7 m g, 63%). Procedure A – Stoich iom etric with -
out h alogen lam p irradiation an d with out triph en ylph osph in e. Triyn e 4 (50 m g, 0.178 m m ol),
CpCo(CO)2 (24 µl, 0.181 m m ol, 100 m ole %), decan e (2 m l), 50 °C, 8 days. Flash ch rom a-
tograph y afforded 14 (4 m g, 8%) an d recovered 4 (29 m g, 58%). Procedure B – Catalytic.
Triyn e 4 (40 m g, 0.142 m m ol), Ni(cod)2 (0.06 M in tetrah ydrofuran , 470 µl, 0.028 m m ol,
20 m ole %), triph en ylph osph in e (14.8 m g, 0.056 m m ol, 40 m ole %), tetrah ydrofuran (5 m l),
room tem perature, 30 m in . Flash ch rom atograph y afforded 14 (26.2 m g, 66%). IR: 3100,
3064, 2942, 2901, 2841, 1601, 1570, 1489, 1436, 1427, 1410, 824. 1H NMR (500 MHz):
2.62–3.00 (8 H, m , H-5, 6, 9, 10); 6.91 (2 H, dt, J = 7.7, 7.7, 1.5, H-2, 13); 7.10 (2 H, s, H-7,
8); 7.10 (2 H, dt, J = 7.5, 7.5, 1.3, H-3, 12); 7.22 (2 H, dd, J = 7.9, 1.5, H-1, 14); 7.25 (2 H,
dd, J = 7.5, 1.3, H-4, 11). 13C NMR: 29.8 (t, C-5, 10), 30.2 (t, C-6, 9), 125.4 (d, C-2, 13),
126.6 (d, C-7, 8), 126.7 (d, C-3, 12), 127.3 (d, C-4, 11), 130.2 (d, C-1, 14), 132.7 (s, C-14b,
14c), 134.8 (s, C-14a, 14d), 138.9 (s, C-4a, 10a), 139.2 (s, C-6a, 8a). EI MS: 282 (M• +, 100),
265 (15), 254 (9), 239 (8), 138 (5), 126 (10), 69 (9). HR EI MS: calculated for C22H18
282.1409; foun d 282.1470.
7,8-Dim eth yl-5,6,9,10-tetrah ydropen tah elicen e (15)
Procedure B – Stoich iom etric. Triyn e 5 (77 m g, 0.248 m m ol), Ni(cod)2 (0.06 M in tetra-
h ydrofuran , 4.0 m l, 0.240 m m ol, 100 m ole %), triph en ylph osph in e (130 m g, 0.496 m m ol,
200 m ole %), tetrah ydrofuran (4 m l), room tem perature, 20 h . Flash ch rom atograph y on sil-
ica gel (petroleum eth er) afforded 13 (32.5 m g, 42%) as an am orph ous solid. IR: 3098 w,
3073 m , 3059 m , 3031 w, 3018 w, 3003 w, 2940 m , 2896 m , 2840 m , 1602 w, 1586 w,
1558 vw, 1489 m , 1480 m , 1456 w (sh ), 1435 s, 1420 w (sh ), 1378 vw, 1281 w, 1182 w,
1156 w, 1111 w, 1089 w, 1070 w, 1028 m , 1000 w, 973 w, 720 w, 617 w, 431 w. 1H NMR
(500 MHz): 2.31 (6 H, brs, 2 × CH3); 2.86–2.91 (8 H, m , 2 × CH2CH2); 6.86 (2 H, dt, J = 7.6,
7.6, 1.4, H-2, 13); 7.06 (2 H, dt, J = 7.3, 7.3, 1.3, H-3, 12); 7.08 (2 H, brdd, J = 7.1, 1.3, H-1,
14); 7.24 (2 H, ddq, J = 7.3, 1.4, 0.8, 0.8, 0.8, H-4, 11). 13C NMR: 16.3 (q, 2 × CH3), 27.0 (t,
C-6, 9), 29.8 (t, C-5, 10), 125.3 (d, C-2, 13), 126.1 (d, C-3, 12), 126.8 (d, C-4, 11), 130.7 (d,
C-1, 14), 131.0 (s, C-14b, 14c), 132.8 (s, C-7, 8), 135.7 (s, C-14a, 14d), 137.3 (s, C-4a, 10a),
Collect. Czech. Chem. Commun. (Vol. 68) (2003)

Synthesis of Helicene Scaffolds
927
138.1 (s, C-6a, 8a). EI MS: 310 (M• +, 21), 262 (100), 183 (54), 152 (7), 108 (26), 69 (8), 55
(12), 43 (13). HR EI MS: calculated for C24H22 310.1722; foun d 310.1723.
7,8-Bis(trim eth ylsilyl)-5,6,9,10-tetrah ydropen tah elicen e (16)
Procedure A – Catalytic with h alogen lam p irradiation . Triyn e 6 (102 m g, 0.239 m m ol),
CpCo(CO)2 (13 µl, 0.098 m m ol, 40 m ole %), triph en ylph osph in e (50 m g, 0.191 m m ol,
80 m ole %), decan e (4 m l), 140 °C, 2.5 h . Flash ch rom atograph y on silica gel (petroleum
eth er–eth er 100:0 to 98:2) afforded 16 (72.4 m g, 71%) as a solid an d a m ore polar un stable
cobalt com plex as an oil. Procedure B – Stoich iom etric. Triyn e 6 (72 m g, 0.169 m m ol),
Ni(cod)2 (0.06 M in tetrah ydrofuran , 2.8 m l, 0.168 m m ol, 100 m ole %), triph en ylph osph in e
(89 m g, 0.339 m m ol, 200 m ole %), tetrah ydrofuran (4 m l), room tem perature, 20 h . Flash
ch rom atograph y afforded 14 (50.7 m g, 70%), m .p. 191–193 °C (ch loroform ). IR: 3093 vw,
3068 w, 2951 m , 2899 m , 2839 w, 1601 vw, 1520 vw, 1516 vw, 1486 w, 1471 w, 1252 s,
1160 vw, 1113 w, 1040 w, 947 w, 852 vs, 628 m , 585 w, 496 w, 424 w. 1H NMR (500 MHz):
0.41 (18 H, s, 2 × (CH3)3Si); 2.47 (2 H, dd, J = 15.4, 13.9, H-5, 10); 2.79 (2 H, brd, J = 13.0,
H-6, 9); 2.93 (2 H, dd, J = 15.2, 13.0, H-6, 9); 3.29 (2 H, brd, J = 15.7, H-5, 10); 6.86 (2 H, dt,
J = 7.7, 7.7, 1.4, H-2, 13); 7.00 (2 H, dd, J = 7.8, 1.3, H-1, 14); 7.07 (2 H, dt, J = 7.4, 7.4, 1.3,
H-3, 12); 7.24 (2 H, brdd, J = 7.5, 1.1, H-4, 11). 13C NMR: 4.0 (q, 2 × (CH3)3Si), 29.9 (t, C-5,
10), 30.5 (t, C-6, 9), 125.3 (d, C-2, 13), 126.5 (d, C-3, 12), 126.9 (d, C-4, 11), 131.0 (d, C-1,
14), 131.5 (s, C-14b, 14c), 135.1 (s, C-14a, 14d), 139.3 (s, C-4a, 10a), 144.2 (s, C-6a, 8a),
147.1 (s, C-7, 8). EI MS: 426 (M• +, 78), 411 (11), 395 (9), 354 (98), 338 (12), 279 (75), 265
(10), 131 (6), 73 (100), 59 (29), 45 (12). HR EI MS: calculated for C28H34Si2 426.2199; foun d
426.2159.
3-Fluoro-5,6,9,10-tetrah ydropen tah elicen e (17)
Procedure A – Catalytic with h alogen lam p irradiation . Triyn e 7 (70 m g, 0.233 m m ol),
CpCo(CO)2 (13 µl, 0.098 m m ol, 40 m ole %), triph en ylph osph in e (49 m g, 0.187 m m ol,
80 m ole %), decan e (4 m l), 140 °C, 3 h . Flash ch rom atograph y on silica gel (h exan e) af-
forded 17 (22.7 m g, 33%) as an am orph ous solid. IR: 3100 vw, 3064 w, 3037 w, 2943 s,
2903 m , 2843 m , 1625 w (sh ), 1610 m , 1595 m , 1584 m , 1571 m , 1495 vs, 1485 m (sh ),
1467 w, 1436 vs, 1419 m , 1404 w, 1348 w, 1318 vw, 1294 m , 1279 m , 1258 w, 1235 w, 1188 m ,
1164 s, 1130 w (sh ), 1121 m , 1110 w, 1029 w, 948 w, 886 s, 824 vw, 812 w (sh ), 703 m ,
695 m , 628 m , 587 w, 542 s, 529 w, 508 w, 485 w, 451 w, 432 w. 1H NMR (500 MHz):
2.60–2.96 (8 H, m , 2 × CH2CH2); 6.80 (1 H, dt, J = 8.4, 8.4, 2.6, H-2); 6.90 (1 H, dd, J = 10.8,
2.6, H-4); 6.95 (1 H, dt, J = 7.6, 7.6, 1.4, H-13); 7.11 (1 H, d, J = 7.5, H-8); 7.13 (1 H, d, J =
7.5, H-7); 7.13 (1 H, dt, J = 7.5, 7.5, 1.4, H-12); 7.20 (1 H, dd, J = 8.3, 5.6, H-1); 7.20 (1 H,
dd, J = 7.5, 1.4, H-14); 7.27 (1 H, brdd, J = 7.7, 1.3, H-11). 13C NMR: 29.1 (t, C-5), 29.8 (t,
C-10), 30.1 (t, C-6), 30.3 (t, C-9), 113.4 (dd, JCF = 21.5, C-2), 116.6 (dd, JCF = 22.9, C-4),
125.6 (d, C-13), 126.6 (d, C-12), 127.1 (d, C-7, 8), 127.5 (d, C-11), 128.4 (dd, JCF = 8.3, C-1),
130.1 (d, C-14), 132.1 (s, C-14c), 132.2 (d, JCF = 9.7, C-4a), 133.0 (s, C-14b), 134.1 (s, C-14a),
134.4 (d, JCF = 1.0, C-14d), 139.0 (s, C-10a), 139.1 (s, C-8a), 139.3 (s, C-6a), 160.8 (d, JCF
=
241.2, C-3). EI MS: 300 (M• +, 100), 283 (14), 270 (9), 257 (6), 201 (5), 183 (4), 135 (10), 77
(12), 71 (13), 57 (22), 51 (10), 43 (19). HR EI MS: calculated for C22H17F 300.1314; foun d
300.1322.
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Stará et al.:
5,6,9,10-Tetrah ydroh exah elicen e (18)
Procedure A – Catalytic with h alogen lam p irradiation . Triyn e 8 (67 m g, 0.202 m m ol),
CpCo(CO)2 (5 µl, 0.038 m m ol, 20 m ole %), triph en ylph osph in e (21 m g, 0.080 m m ol,
40 m ole %), decan e (3 m l), 140 °C, 30 m in . Flash ch rom atograph y on silica gel (petroleum
eth er–eth er 100:0 to 98:2) afforded 18 (42.9 m g, 64%) as an am orph ous solid. Procedure A –
Catalytic with out h alogen lam p irradiation . Triyn e 8 (91 m g, 0.274 m m ol), CpCo(CO)2
(7 µl, 0.053 m m ol, 20 m ole %), triph en ylph osph in e (29 m g, 0.111 m m ol, 40 m ole %), dec-
an e (3 m l), 50 °C, 4 days. Flash ch rom atograph y afforded 18 (23.7 m g, 26%). Procedure B –
Stoich iom etric. Triyn e 8 (33 m g, 0.099 m m ol), Ni(cod)2 (0.06 M in tetrah ydrofuran , 3.30 m l,
0.198 m m ol, 200 m ole %), tetrah ydrofuran (4 m l), refluxed, 5 m in . Flash ch rom atograph y
afforded 18 (24.8 m g, 75%). IR: 3098 w, 3055 m , 2942 s, 2837 m , 1619 w, 1603 w, 1595 w,
1587 w, 1570 w, 1560 w, 1509 m , 1489 m , 1436 s, 1425 m , 1416 w, 1250 w, 1159 w, 1113 w,
1037 w, 1027 w, 946 w, 865 w, 846 vs, 819 s, 589 w, 505 w. 1H NMR (500 MHz): 2.66 (1 H,
ddt, J = 15.0, 4.3, 1.4, 1.4, H-10); 2.69 (1 H, ddt, J = 15.2, 4.4, 1.4, 1.4, H-9); 2.75 (1 H, ddt,
J = 15.2, 4.4, 1.4, 1.4, H-10); 2.86 (1 H, ddd, J = 14.8, 4.3, 2.2, H-5); 2.88 (1 H, ddt, J = 14.1,
4.1, 2.0, 2.0, H-6); 2.91 (1 H, ddd, J = 15.2, 4.5, 2.1, H-9); 3.01 (1 H, brddd, J = 14.7, 10.1,
4.2, H-5); 3.04 (1 H, brddd, J = 14.2, 10.0, 4.4, H-6); 6.34 (1 H, ddt, J = 7.8, 6.8, 1.2, 1.2,
H-2); 6.36 (1 H, dd, J = 7.8, 1.9, H-1); 6.81 (1 H, ddd, J = 7.4, 6.8, 1.9, H-3); 6.84 (1 H, ddd,
J = 8.3, 6.8, 1.3, H-15); 7.10 (1 H, ddd, J = 8.0, 6.8, 1.2, H-14); 7.15 (1 H, dq, J = 7.3, 0.8, 0.8,
0.8, H-4); 7.20 (1 H, dd, J = 7.4, 1.0, H-7); 7.25 (1 H, dd, J = 7.4, 1.0, H-8); 7.47 (1 H, d, J =
8.0, H-11); 7.47 (1 H, dq, J = 8.5, 1.0, 1.0, 1.0, H-16); 7.64 (1 H, brd, J = 8.1, H-13); 7.70
(1 H, brd, J = 8.2, H-12). 13C NMR: 29.9 (t, C-6), 30.0 (t, C-5), 30.6 (t, C-9), 31.1 (t, C-10),
124.2 (d, C-14), 124.7 (d, C-15), 125.1 (d, C-2), 126.0 (d, C-11), 126.1 (d, C-8, 16), 126.2 (d,
C-3), 126.3 (d, C-7), 127.0 (d, C-4), 127.51 (d, C-12), 127.53 (d, C-13), 128.1 (d, C-1), 129.4
(s, C-16a), 131.0 (s, C-16c), 132.1 (s, C-16b), 133.0 (s, C-12a), 134.3 (s, C-16d), 135.4 (s,
C-16e), 137.8 (s, C-4a), 138.0 (s, C-6a, 10a), 140.6 (s, C-8a). EI MS: 332 (M• +, 100), 315 (11),
256 (12), 197 (9), 150 (8), 83 (6), 69 (9), 57 (8). HR EI MS: calculated for C26H20 332.1565;
foun d 332.1492.
7,8-Dim eth yl-5,6,9,10-tetrah ydroh exah elicen e (19)
Procedure A – Catalytic with h alogen lam p irradiation . Triyn e 9 (80 m g, 0.241 m m ol),
CpCo(CO)2 (6 µl, 0.045 m m ol, 20 m ole %), triph en ylph osph in e (25 m g, 0.095 m m ol,
40 m ole %), decan e (5 m l), 140 °C, 1 h . Flash ch rom atograph y on silica gel (petroleum
eth er–eth er 100:0 to 98:2) afforded 19 (54 m g, 68%) as an am orph ous solid. IR: 3056 m ,
3007 vs, 1620 w, 1603 w, 1595 w, 1586 w, 1572 w (sh ), 1560 w, 1509 m , 1488 m , 1480 m
(sh ), 1436 vs, 1376 m , 1260 w, 1142 w, 1135 vw, 1090 w, 1034 m , 1028 m , 999 w, 974 w,
866 w, 815 vs, 700 s, 610 m , 572 w, 539 s, 459 m , 430 m . 1H NMR (500 MHz): 2.39 (3 H, s,
CH3); 2.40 (3 H, s, CH3); 2.40–2.52 (2 H, m , H-9, 10); 2.91–3.03 (4 H, m , H-5, 6, 9, 10); 3.15
(1 H, ddd, J = 14.9, 3.9, 2.7, H-5); 3.21 (1 H, ddd, J = 15.5, 4.6, 2.6, H-6); 6.31–6.36 (2 H, m ,
H-1, 2); 6.77 (1 H, m , H-3); 6.86 (1 H, ddd, J = 8.6, 6.7, 1.4, H-15); 7.08 (1 H, ddd, J = 8.0,
6.7, 1.2, H-14); 7.12 (1 H, brdq, J = 7.2, 0.9, 0.9, 0.9, H-4); 7.47 (1 H, d, J = 8.1, H-11); 7.51
(1 H, ddt, J = 8.6, 1.2, 0.9, 0.9, H-16); 7.61 (1 H, ddt, J = 8.0, 1.4, 0.8, 0.8, H-13); 7.68 (1 H,
brd, J = 8.1, H-12). 13C NMR: 16.4 (q, CH3), 16.7 (q, CH3), 26.7 (t, C-6), 29.9 (t, C-5), 27.1 (t,
C-9), 31.0 (t, C-10), 124.0 (d, C-14), 124.7 (d, C-15), 125.0 (d, C-2), 125.7 (d, C-11), 125.8
(d, C-3), 125.9 (d, C-16), 126.5 (d, C-4), 127.1 (d, C-12), 127.5 (d, C-13), 128.3 (d, C-1),
129.0 (s, C-16c), 129.7 (s, C-16a), 132.7 (s, C-16b), 132.4 (s, C-7), 132.7 (s, C-16d), 132.90 (s,
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Synthesis of Helicene Scaffolds
929
C-8), 132.94 (s, C-12a), 135.7 (s, C-6a), 136.2 (s, C-16e), 137.21 (s, C-10a), 137.24 (s, C-4a),
138.7 (s, C-8a). EI MS: 360 (M• +, 15), 262 (100), 183 (71), 152 (9), 108 (37), 77 (15), 51 (26).
HR EI MS: calculated for C28H24 360.1878; foun d 360.1874.
7,8,11,12-Tetrah ydroh eptah elicen e (20)
Procedure A – Catalytic with h alogen lam p irradiation . Triyn e 10 (80 m g, 0.209 m m ol),
CpCo(CO)2 (5 µl, 0.038 m m ol, 20 m ole %), triph en ylph osph in e (22 m g, 0.084 m m ol,
40 m ole %), decan e (5 m l), 140 °C, 2 h . Flash ch rom atograph y on silica gel (petroleum
eth er-eth er 100:0 to 96:4) afforded 20 (51 m g, 64%) as an am orph ous solid. Procedure B –
Stoich iom etric. Triyn e 10 (28 m g, 0.073 m m ol), Ni(cod)2 (0.06
M in tetrah ydrofuran ,
2.40 m l, 0.144 m m ol, 200 m ole %), tetrah ydrofuran (4 m l), refluxed, 5 m in . Flash ch rom a-
tograph y afforded 20 (6.7 m g, 24%). IR: 3054 w, 3009 m , 2929 vs, 2856 m , 1620 vw, 1595 w,
1580 vw, 1566 vw, 1512 w, 1467 w, 1436 w, 1422 vw (sh ), 1377 w, 1329 vw, 1309 vw,
1261 w, 1244 w, 1159 w, 1024 w, 866 w, 823 m , 811 m , 683 w, 504 w, 454 w. 1H NMR
(500 MHz): 2.76 (2 H, dddd, J = 15.8, 14.9, 4.1, 1.3, H-7, 12); 2.96 (2 H, ddd, J = 14.8, 4.1,
2.0, H-8, 11); 2.99 (2 H, ddd, J = 15.8, 4.2, 2.0, H-7, 12); 3.10 (2 H, ddd, J = 14.9, 14.8, 4.2,
H-8, 11); 6.53 (2 H, ddd, J = 8.6, 6.8, 1.3, H-2, 17); 6.82 (2 H, ddd, J = 8.1, 6.8, 1.2, H-3, 16);
6.96 (2 H, dddd, J = 8.6, 1.3, 1.2, 0.7, H-1, 18); 7.20 (2 H, ddd, J = 8.1, 1.3, 0.7, H-4, 15);
7.33 (2 H, dd, J = 8.2, 1.0, H-5, 14); 7.34 (2 H, s, H-9, 10); 7.34 (2 H, d, J = 8.2, H-6, 13).
13C NMR: 30.4 (t, C-7, 12), 31.0 (t, C-8, 11), 123.5 (d, C-6, 13), 123.8 (d, C-1, 18), 123.9 (d,
C-2, 17), 125.8 (d, C-4, 15), 125.9 (d, C-3, 16), 126.6 (d, C-5, 14), 127.2 (d, C-9, 10), 128.9
(s, C-18a, 18f), 132.2 (s, C-18c, 18d), 132.7 (s, C-18b, 18e), 132.8 (s, C-4a, 14a), 137.5 (s,
C-6a, 12a), 139.6 (s, C-8a, 10a). EI MS: 382 (M• +, 100), 365 (4), 354 (7), 276 (3), 239 (4), 175
(4), 97 (6), 83 (9), 71 (14), 69 (17), 57 (36), 55 (28), 43 (43). HR EI MS: calculated for C30H22
382.1722; foun d 382.1710.
The financial support by the Grant Agency of the Czech Republic (Reg. No. 203/02/0248) and the
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic (Research
Project Z4 055 905) is gratefully acknowledged. We are very indebted to Dr J. Závada of this Institute
for valuable and stimulating discussions.
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