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Determination of the Absolute Configuration of (+)-Pentahelicene.

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Determination of the Absolute Configuration of
( + )-Pentahelicene
By Hans Jiirgen Bestmann and Wolfgang Both[*'
Resolution of pentahelicene (4) was recently described['!
We have now succeeded in the first allocation of absolute
configuration to a helicene, namely to (+ )-(4).
[5] For nomenclature of helicenes cf. R. S. Cahn, Sir Ch. Ingold, and
V Prelog, Angew. Chem. 78,413 (1966); Angew. Chem. internat. Edit.
5, 385 (1966).
[6] D. D. Fitts and I . G. Kirkwood, J. Amer. Chem. SOC.77,4940(1955);
I. Tinoco and R. Woody, J. Chem. Phys. 40, 160 (1964); J . H . Brewster,
Topics Stereochem. 2,40 (1967).
[7] G. Wagniere in : Aromaticity, Pseudoaromaticity, Antiaromaticity.
The Jerusalem Symposium of Science, Jerusalem 1971.
[8] The sole exception is recorded by A. Moskowitz, Dissertation,
Harvard University 1957; Advan. Chem. Phys. 4,67 (1962).
[9] Cf. the agreement with thiaheterohelicenes: M . B. Groen, G. Stuten,
G. I . Visser, and H . Wynberg, J. Amer. Chem. SOC.92,7218 (1970).
Vinylogous Fulvalene: Synthesis and Electrocyclic
Reaction"] [**I
By Hubert Sauter and Horst Prinzbach"]
Angularly annelated indacenoid ring systems ( 5 ) have
been the subject of several quantum-mechanical calcula* ~ l have
.
tions['] and the aim of preparative s t ~ d i e s [ ~ We
sought the synthetic precursors (2) and (4) by u,o-cycloadditions to the fulvalenes (I) (m = 2, n = 1; m = 2, n =3 ;
m=2, n=5)[,' and by electrocyclic ring closure of the
vinylogous fulvalenes (3). We report here the first example
of the second of these synthetic routes[']: we have been
able to prepare compound (4a) (m=n=2) from (3a)
(m= n =2).
The fundamental investigations of Mislow et al.['], later
confirmed by X-ray structure analysis[31,have shown that
(-)-2,2'-bis(bromomethy1)-1,l'-binaphthyl (I) belongs to
the @)-series. The optically pure compound ( - ) - ( I ) with
triphenylphosphane in molar ratio 1:2 afford the ( + ) - ( S ) bisphosphonium salt (2) ([a]:;8 = 117.3", [a]:& =
+ 138.7 ', c= 5.48 mg/cm3 in chloroform; m. p. 274-277 "C
(dec.), m. p. (racemate) 289-290°C (dec.)[,]), which is
converted by NaIO, into the (S)-bisphosphoniumperiodate
(3). With lithium ethoxide at -5 to 0°C[41,(3) gives the
(+)-pentahelicene (4), which has absolute conformation
of the (P)-series['l because of its preparation from (-)-(S)(I); it has [~]:~~=2260",
[~]:~~=2730O,[a]:,,=682Oo,
c=2.90mg/cm3 in chloroform; [a]:$8= -1670°t'], [a]::,
= -4950"['] in isooctane ; m. p. 159
= - 2025 "['I,
to 161°C; the racemate has m.p. 176-177°C[41. Since we
started from optically pure (-)-@)-(I) and prepared (4)
at 0°C-a temperature at which no racemization of the
compound can be observed-we assume that the (+)-(P)pentahelicene formed is optically pure.
+
According to calculations partly based on CD measurements, ( +)-hexahelicene[6*71 and ( +)-heptahelicenef7]also
belong to the (P)-seriesL8].
We suspect that all (+)-helicenes
belong to the (P)- and all ( - )-helicenes to the (M)-conformation['].
1,2-Biscyclopentadienylideneethane(3a), the vinylog of
the extremely unstable fulvalene that has hitherto been
obtained only in highly dilute solution161,can be isolated
in pure form as violet-red needles from pentane. For this
purpose the deep red solution obtained when "glyoxal
sulfate" (6) reacts with cyclopentadienylsodium (equivalent amounts, THF, -20°C) in the presence of triethylamine, is chromatographed on A1'0, (neutral, deactivated
with 7% of H,O) at - 15°C with pentane as eluant; (3a)
crystallizes at -78°C from the pure fraction (yield 3 to
5 %)[71.
Received: December 13,1971 [Z 575 IE]
German version: Angew. Chem. 84,293 (1972)
[*IProf. Dr. H. J. Bestmann and Dip1.-Chem. W. Both
Institut fur Organische Chemie der Universitat Erlangen-Niirnberg
852 Erlangen, Henkestrasse 42 (Germany)
[l]Ch. Goedicke and H . Stegemeyer, Tetrahedron Lett. 1970, 937.
[2] D. D. Fitts, M . Siegel, and K . Mislow, J. Amer. Chem. SOC.80,480
(1958); K . Mislow and F. A . McGinn, ibid. 80, 6036 (1958); K . Mislow,
!I Prelog, and H . Scherrer, Helv. Chim. Acta 41, 1410 (1958); also the
review by K . Mistow, Angew. Chem. 70, 687 (1958).
[3] H . Akimoto, 7: Shioiri, Y. Jitaka, and S . Yamada, Tetrahedron
Lett. 1968, 97.
[4] H . J . Bestmann, R. Armsen, and H . Wagner, Chem. Ber. 102, 2259
(1969).
296
Crystalline (3a) is quite stable in an inert atmosphere at
-20°C but isomerizes rapidly and quantitatively in solution to tricyclo[7.3.0.02*6]dodeca-3,5,7,9,1I-pentaene (4a) ;
its half-lifetime, determined by W spectrometry (isosbestic
points at 281,362,399 nm) in pentane at 30°C amounts to
ca. 20min. Even under the influence of daylight the ring
closure (3a) + (4a) is reversible. On base catalysis (4a)
affords the known mixture of dihydro-as-indacenesobtain[*] Prof. Dr. H. Prinzbach and Dip1.-Chem. H. Sauter
Lehrstuhl fur Organische Chemie der Universitat
78 Freiburg, Albertstrasse 21 (Germany)
p*] This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.
Angew. Chem. internat. Edit. 1 Vol. I1 (1972) / N o . 4
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