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Synthesis and X‐Ray Analysis of a New[6]HeliceneFaouzi Aloui a , Riadh El Abed a , Taha Guerfel b & Béchir Ben Hassine aa Laboratoire de Synthèse Organique Asymétrique et Catalyse Homogène(O1UR1201), Faculté des Sciences , Monastir, Tunisieb Laboratoire de Chimie du Solide, Faculté des Sciences , Monastir, TunisiePublished online: 21 Aug 2006.

To cite this article: Faouzi Aloui , Riadh El Abed , Taha Guerfel & Béchir Ben Hassine (2006) Synthesisand X‐Ray Analysis of a New [6]Helicene, Synthetic Communications: An International Journal for RapidCommunication of Synthetic Organic Chemistry, 36:11, 1557-1567

To link to this article: http://dx.doi.org/10.1080/00397910600588934

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Synthesis and X-Ray Analysisof a New [6]Helicene

Faouzi Aloui and Riadh El Abed

Laboratoire de Synthese Organique Asymetrique et Catalyse Homogene

(O1UR1201), Faculte des Sciences, Monastir, Tunisie

Taha Guerfel

Laboratoire de Chimie du Solide, Faculte des Sciences,

Monastir, Tunisie

Bechir Ben Hassine

Laboratoire de Synthese Organique Asymetrique et Catalyse Homogene

(O1UR1201), Faculte des Sciences, Monastir, Tunisie

Abstract: A new disubstituted hexahelicene derivative 3 bearing methoxy functions at

positions 3 and 14 has been prepared in racemic form through a Heck reaction followed

by photocyclodehydrogenation. Suitable crystals of rac-3 were analyzed by X-ray

crystallography and showed similar geometry to the structure of hexahelicene itself.

Deprotection of 3 using boron tribromide led to 3,14-dihydroxyhexahelicene 4 in

quantitative yield. The complexation of transition metal atoms seemed to be quite

possible by these two bidentate hexahelicene derivatives.

Keywords: Heck reaction, hexahelicene, photocyclodehydrogenation, substituted

positions

Received in Poland October 31, 2005

Address correspondence to Bechir Ben Hassine, Laboratoire de Synthese

Organique Asymetrique et Catalyse Homogene (O1UR1201), Faculte des Sciences,

Avenue de l’environnement, 5019 Monastir, Tunisie. Tel.: 0021673500279;

Fax: 0021673500278; E-mail: bechir.benhassine@fsm.rnu.tn or bechirbenhassine@

yahoo.fr

Synthetic Communicationsw, 36: 1557–1567, 2006

Copyright # Taylor & Francis Group, LLC

ISSN 0039-7911 print/1532-2432 online

DOI: 10.1080/00397910600588934

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INTRODUCTION

Helicenes are benzologs of phenanthrene in which a regular cylindrical helix

is formed through an all-ortho annelation of the aromatic rings. Their helical

structure is a consequence of the repulsive steric interaction between terminal

aromatic rings. These polycyclic aromatic compounds exhibit a largely

specific rotation and a nonlinear optical property.[1] They are considered

potentially useful for asymmetric molecular recognition,[2] as an asymmetric

catalyst[3] and a liquid crystal molecule.[4]

Since the first study on hexahelicene and its optical resolution by

Newman and Lednicer in 1956,[5] this area has developed rapidly. However,

so far, many efforts have been devoted to the synthesis of helicenes through

several alternative strategies, including methods based on Diels–Alder cyclo-

addition reaction,[6] carbenoid insertion reactions,[7] and intramolecular

[2 þ 2 þ 2] cycloisomerization of aromatic triynes.[8] A synthetic sequence

relying on Heck-type and photocyclodehydrogenation reactions for the prep-

aration of functionalized helicenes has been developed in our laboratory.[9]

Although all the parent helicenes between [6]- and [14]helicene had been

prepared, the number of [6]helicenes described in the literature remains

relatively small and functionalized [6]helicenes bear OH and CO2H

functions with their congener.[10] These series usually have substituents in

positions 1,2 and 15,16 of a [6]helicene skeleton. Although Terfort et al.

reported the synthesis of 2,15-bis(diphenylphophino)hexahelicene 1 in

racemic form,[11] they did not report the antipode separation or the use in

catalysis. In 1997, Reetz and Sostmann prepared 1 in an enantiomerically

pure form and used it as a helical ligand for an enantioselective rhodium-

catalyzed allylic substitution.[12] In an independent study, Reetz and

coworkers described the synthesis of the first dihydroxyhexahelicene 2 in its

optically active form and its use as an enatioselective fluorescent sensor.[13]

The antipode separation of racemic 2 was achieved by HPLC using a chiral

stationary phase.

Here we report the synthesis of two new bidentate hexahelicene deriva-

tives 3 and 4, substituted at positions 3,14, through a Heck reaction

followed by photocyclodehydrogenation.

RESULTS AND DISCUSSION

The Heck reaction[14] of 1,4-dibromobenzene 6 with the 3-methoxystyrene 5

in the presence of sodium acetate and Hermann’s catalyst in N,N-dimethyl-

acetamide affords the diarylethylene 7 in 81% yield. Compound 7

was dissolved in toluene and irradiated with a 150-W pressure mercury

lamp to give 3-bromo-7-methoxyphenanthrene 8 in 67% yield (Scheme 1).

A 3-bromo-5-methoxy-phenanthrene isomer was isolated as a minor product

in the reaction mixture (23% yield). The use of 3-hydroxystyrene instead of

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3-methoxystyrene 5 leads to the corresponding diene in 33% yield. However,

the photocyclization step is less efficient.

The selective Heck coupling[14] of the phenanthrene derivative 8 with the

6-methoxy-2-vinylnaphthalene, using 1% of Hermann’s catalyst, allowed

the coupling product 9, which is assumed to have an E-stereochemistry at

the double bond. The photolysis of 9 (Heraeus high-pressure mercury lamp,

150 W) was performed in toluene, for about 120 min, on a 150-mg scale in

the presence of a stoechiometric amount of iodine and an excess of

propylene oxide[15] as a hydrogen iodide scavenger and afforded 3,14-

dimethoxy[6]helicene 3. The deprotection of 3 using boron tribromide leads

to the new 3,14-dihydroxy[6]helicene 4 (Scheme 1).

Because the ring closure is not completely regioselective, an isomer 10

was obtained (Figure 1). Although the ratio of 3 in the mixture was relatively

high, the isolated yield decreased to around 35% because of the difficult sep-

aration of 3 from 10 by column chromatography. However, as deprotection

using boron tribromide is an easier purification procedure, it allowed us to

obtain deprotected 3 in 51% yield. The 1H NMR spectrum of compound 10

displays characteristic signals at a low field for both 7-H and 8-H

(8.97 ppm; 9.39 ppm).[2b,16]

Scheme 1. Structure of compound 10.

Synthesis and X-Ray Analysis of a New [6]Helicene 1559

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Compound 3 forms pale yellow crystals from chloroform. The substance

crystallizes in the form of a racemate. It is stable in air and light. The X-ray

analysis of the [6]helicene 3 was carried out on a single crystal obtained

from racemic 3 as shown in Figure 2. The two oxygen atoms span a

Figure 1. Structure of compound 10.

Figure 2. ORTEP drawing of (+) 2 3.

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distance of 5.876 A, which seems appropriate to the complexation of large

transition-metal atoms.[11,12a] The inner pitch elevation (C1 . . . C16

distance) is 3.041 A. The ORTEP drawing of 3 reveals that the dihedral

angles (C1-C16e-C16d-C16c), (C16-C16a-C16b-C16c), (C16e-C16d-C16c-

C16b), and (C16a-C16b-C16c-C16d) are respectively 15.28, 13.68, 27.18,and 25.88. The deformation of the dihedral angles of the interior side in 3 is

mainly attributed to the methoxy groups in comparison with those of

[6]helicene[17] (see Fig. 3) (C1-C16e-C16d-C16c: 15.28; C16-C16a-C16b-

C16c: 11.28; C16e-C16d-C16c-C16b: 30.38; C16a-C16b-C16c-C16d: 30.08).3,14-Dihydroxy[6]helicene 4 is a light yellow solid, sensitive to light. It

shows a violet fluorescence when dissolved and decomposes without

melting. It is characterized by 1H and 13C NMR spectroscopy. Figure 4

shows the two NMR spectra.

In summary, we have prepared in racemic form and characterized two

new bidentate hexahelicene derivatives substituted at positions 3,14. The

optical resolution of 4 is under investigation in our laboratory. The X-ray

diffraction analyses confirm that the functionalization at positions 3,14 of

the hexahelicene framework makes possible the chelation of larger

transition-metal atoms. Thus, nonracemic [6]helicenes 3 and 4 can serve as

chiral auxiliaries or chiral ligands in the asymmetric synthesis.

Figure 3. Dihedral angles (in 8) and bond distances (in A) of 3 and hexahelicene.

Synthesis and X-Ray Analysis of a New [6]Helicene 1561

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EXPERIMENTAL

All reactions were perfomed under an argon atmosphere and were monitored by

TLC. Melting points were measured in open capillary tubes and are uncorrected.

NMR spectra were recorded on a Bruker AC-300 spectrometer at 300 MHz (1H)

and 75 MHz (13C). Photocyclizations were performed in a water-cooled Quartz

photoreactor equipped with a high-pressure mercury immersion lamp (Heraeus

TQ 150). Silica gel 60 mesh was used for column chromatography.

General Procedure A for the Heck Reactions

A solution of haloarene (6 mmol) and dry sodium acetate (541 mg, 6.1 mmol) in

N,N-dimethylacetamide (12 mL) was placed in a double-necked flask fitted with

a septum and was repeatedly degassed and purged with argon. The styrene

derivative (12 mmol) was added, and the mixture was heated to 1008C. When

this temperature was reached, a solution of trans-di(m-acetato)bis[o-(di-o-tolyl-

phosphanyl)benzyl]dipalladium (Herrmann catalyst, 56 mg, 1%) in N,N-

dimethylacetamide (6 mL) was added, and the reaction mixture was heated to

1408C. Heating was maintained for about 48 h. The reaction was quenched

by an addition of 5% HCl solution stirred for 30 min at room temperature and

then extracted with CH2Cl2. The combined layers were dried (MgSO4) and

evaporated to dryness. The final product was purified as indicated.

trans-1-Bromo-4-(3-methoxystyryl)benzene (7): Compound 7 was obtained

in 81% yield according to procedure A. It was purified by column

Figure 4. (a) 1H NMR; (b) 13C NMR spectra of 4 in d6-acetone.

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chromatography with cyclohexane/ethyl acetate (98:02) as eluent (Rf ¼ 0.39).

Colorless oil. 1H NMR (300 MHz, CDCl3): d ¼ 3.67 (s, 3H, OCH3), 6.49

(d, J ¼ 12.3 Hz, 1H, H vinyl), 6.59 (d, J ¼ 12.3 Hz, 1H, Hvinyl), 6.75 (2

H), 6.80 (d, J ¼ 7.5 Hz, 1H, H-3), 7.11 (d, J ¼ 8.7 Hz, 2H), 7.16

(d, J ¼ 7.5 Hz, 1H), 7.33 (d, J ¼ 8.4 Hz, 2H) ppm; 13C NMR (75 MHz,

CDCl3): d ¼ 55.49 (OCH3), 113.7 (C–H), 114.3 (C–H), 121.4 (C–Br),

121.7 (C–H), 129.6 (C–H), 129.8 (C–H), 131.0 (2C–H), 131.3 (C–H),

131.7 (2C–H), 136.5 (C), 138.6 (C), 159.9 (C–OCH3) ppm; m/z (EI) 290

(78%, Mþ.), 288 (81, Mþ.), 273 (1), 258 (3), 245 (3), 207 (24), 208 (31),

194 (52), 193 (10), 178 (76), 164 (50), 165 (100), 152 (5), 139 (11).

General Procedure B for the Photocyclization Reactions

Iodine (1.1 equiv.) was added to a solution of the olefin in toluene. The

solution was degassed for 15–30 min, and propylene oxide (50 equiv.) was

added. Irradiation was performed using a falling-film photoreactor and a

high-pressure Hg-vapor lamp (150 W, Heraeus). The argon flow was main-

tained throughout the irradiation. The reaction was monitored by TLC.

Then the reaction mixture was concentrated in vacuo, and the crude product

was purified by column chromatography on silica gel.

Irradiation of trans-1-Bromo-4-(3-methoxystyryl)benzene (7)

The photocyclization of 600 mg (2 mmol) of stilbene 7 in 1 L of toluene

yielded 550 mg (92%) of a colorless solid after 1 h 30 min of irradiation

(general procedure B). Column chromatography on silica gel (eluent: cyclo-

hexane/ethyl acetate 98:02) leads to 400 mg of pure 3-bromo-7-methoxy-

phenanthrene 8 (67%) and 137 mg of 3-bromo-5-methoxyphenanthene (23%).

3-Bromo-7-methoxyphenanthrene (8): Compound 8 was obtained as a

colorless solid, Mp ¼ 69–708C, Rf ¼ 0.41 (cyclohexane/ethyl acetate

98:02). 1H NMR (300 MHz, CDCl3): d ¼ 3.85 (s, 3H, OCH3), 7.11

(d, J ¼ 2.4 Hz, H-8), 7.16 (dd, J1 ¼ 2.4 Hz, J2 ¼ 8.7 Hz, H-6), 7.49 (dd,

J1 ¼ 1.5 Hz, J2 ¼ 8.4 Hz, H-2), 7.54 (s, 2H, H-9/H-10), 7.58 (d, J ¼ 8.4 Hz,

H-1), 8.34 (d, J ¼ 9 Hz, H-5), 8.57 (d, J ¼ 1.5 Hz, H-4) ppm; 13C NMR

(75 MHz, CDCl3): d ¼ 55.8 (OCH3), 108.9 (C-8), 117.8 (C-6), 121.3 (C),

123.9 (C), 124.7 (C-5), 125.4 (C-4), 127.3 (C-9/C-10), 128.4 (C-2), 129.9

(C), 130.5 (C-1), 132.4 (C), 134.1 (C), 159.1 (C-7) ppm; m/z (EI) 288

(98%, Mþ.), 286 (100, Mþ.), 273 (5), 256 (2), 245 (56), 244 (9), 243 (58),

207 (2), 192 (4), 176 (10), 164 (33), 163 (57), 150 (3), 137 (3).

3-Bromo-5-methoxyphenanthrene: Colorless solid, mp ¼ 86–878C,Rf ¼ 0.3

(cyclohexane/ethyl acetate 98:02). 1H NMR (300 MHz, CDCl3): d ¼ 4.16 (s, 3H,

Synthesis and X-Ray Analysis of a New [6]Helicene 1563

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OCH3), 7.18 (dd, J1 ¼ 1.5 Hz, J2 ¼ 7.5 Hz, 1H, H-6), 7.53 (dd, J1 ¼ 1.5 Hz,

J2 ¼ 7.8 Hz, 1H, H-8), 7.58 (t, J ¼ 7.5 Hz, 1H, H-7), 7.69 (d, J ¼ 8.4 Hz, 1H),

7.70 (d, J ¼ 8.4 Hz, 1H), 7.73 (d, J ¼ 8.7 Hz, 1H), 7.75 (d, J ¼ 8.4 Hz, 1H),

9.87 (d, J ¼ 1.5 Hz, 1H, H-4) ppm; 13C NMR (75 MHz, CDCl3): d ¼ 56.1

(OCH3), 108.7 (C-6), 120.1 (C), 120.9 (2C), 121.9 (C-6), 127.6 (C–H), 127.7

(C–H), 128.0 (C–H), 129.4 (C–H), 129.9 (C–H), 131.6 (C–H), 131.9 (C),

135.1 (C-4), 159.1 (C-5) ppm; m/z (EI) 288 (40%), 286 (43), 243 (2), 205 (3),

193 (16), 192 (100), 176 (6), 164 (14), 163 (32), 150 (2), 137 (2).

2-Methoxy-6-(6-methoxy-2-vinylnaphthyl)phenanthrene (9): Compound 9

was obtained as a colorless solid from 8 in 65% yield. It was purified by

column chromatography with cyclohexane/ethyl acetate (90:10) as eluent

(Rf ¼ 0.35), showing a violet fluorescence when dissolved. Mp ¼ 221–

2238C. 1H NMR (300 MHz, d6-DMSO): d ¼ 3.86 (s, 3H, OCH3), 3.94

(s, 3H, OCH3), 7.19 (dd, J1 ¼ 2.1 Hz, J2 ¼ 8.2 Hz, 1H), 7.36 (m, 2H), 7.47

(d, J ¼ 2.4 Hz, 1H), 7.60 (d, J ¼ 16.5 Hz, 1H, Hvinyl), 7.69 (d, J ¼ 16.5 Hz,

1H, Hvinyl), 7.75–7.97 (m, 7H, Ar), 8.02 (s, 1H), 8.83 (d, J ¼ 9.3 Hz, 1H),

8.91 (s, 1H) ppm; 13C NMR (75 MHz, d6-DMSO): d ¼ 56.4 (OCH3), 56.5

(OCH3), 107.3 (C–H), 109.9 (C–H), 118.3 (C–H), 120.1 (C–H), 122.1

(C–H), 124.6 (C–H), 125.1 (C–H), 125.3 (C–H), 125.8 (C), 127.5

(2C–H), 128.2 (C–H), 128.4 (C–H), 129.4 (C–H), 129.8 (C–H), 130.1

(C), 130.3 (C), 130.6 (C), 131.2 (C), 131.4 (C), 133.8 (C), 134.6 (C), 135.1

(C), 137.0 (C), 158.9 (C–O), 159.3 (C–O) ppm.

Irradiation of 2-Methoxy-6-(6-methoxy-2-vinylnaphthyl)phenanthrene (9)

The photocyclization reaction (general procedure B) of 150 mg (0.38 mmol) of

compound 9 in 1 L of toluene yielded 130 mg (87%) of a yellow solid, contain-

ing 3,14-dimethoxy[6]helicene 3 and the side compound 10. Purification by

column chromatography with cyclohexane/ethyl acetate (95:05) as the eluent

and crystallization from chloroform gave 53 mg of 3 as light yellow crystals.

3,14-Dimethoxyhexahelicene (3): Yield: 35%, light yellow crystals,

mp ¼ 245–2478C, Rf ¼ 0.23 (cyclohexane/ethyl acetate 95:05). 1H NMR

(300 MHz, CDCl3): d ¼ 3.75 (s, 6H, OCH3), 6.29 (dd, J1 ¼ 3 Hz,

J2 ¼ 9.3 Hz, 2H, H-2/H-15), 7.70 (d, J ¼ 3 Hz, 2H, H-4/H-13), 7.45

(d, J ¼ 9.3 Hz, 2H, H-1/H-16), 7.73 (d, J ¼ 8.4 Hz, 2H, H-5/H-12), 7.79

(d, J ¼ 8.4 Hz, 2H, H-7/H-10), 7.80 (d, J ¼ 7.5 Hz, 2H, H-6/H-11), 7.83

(d, J ¼ 8.7 Hz, 2H, H-8/H-9) ppm; 13C NMR (75 MHz, CDCl3): d ¼ 55.5

(2 OCH3), 107.2 (C-4/C-13), 116.2 (C-2/C-15), 123.8 (C), 125.0 (2C),

126.3. (C-7/C-10), 127.1 (C-6/C-11), 127.6 (C-5/C-8/C-9/C-12), 128.7

(2C), 129.73 (C-1/C-16), 130.4 (2C), 133.6 (C), 133.7 (2C), 157.5 (C-3/C-

14) ppm; anal. calcd. for C28H20O2: C, 86.57; H, 5.19. Found: C, 83.20; H, 5.03.

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Compound (10): Yield: 33%, fluffy yellow solid, mp ¼ 180–1828C,

Rf ¼ 0.21. 1H NMR (300 MHz, CDCl3): d ¼ 3.87 (s, 3H, OCH3), 3.91

(s, 3H, OCH3), 7.16–7.25 (2H), 7.28–7.32 (2H), 7.56 (d, J ¼ 8.7 Hz, 1H),

7.67 (d, J ¼ 8.7 Hz, 1H), 7.71–7.78 (2H), 7.83 (d, J ¼ 9 Hz, 1H), 7.89

(d, J ¼ 8.7 Hz, 1H), 8.66 (d, J ¼ 9 Hz, 1H), 8.97 (s, 1H), 9.09

(d, J ¼ 10.2 Hz, 1H), 9.39 (s, 1H) ppm; 13C NMR (75 MHz, CDCl3):

d ¼ 54.3 (2OCH3), 106.7 (C–H), 108.3 (C–H), 115.4 (C–H), 116.5

(C–H), 120.0 (C–H), 123.2 (C–H), 123.5 (C–H), 124.2 (C), 125.6 (C–H),

125.7 (C–H), 125.8 (C–H), 126.0 (C–H), 126.3 (C), 126.4 (C), 126.5

(C–H), 127.0 (C), 127.2 (C–H), 127.5 (C), 128.1 (C–H), 128.3 (C), 128.6

(C), 131.3 (C), 132.4 (C), 134.0 (C), 156.3 (C–O), 157.6 (C–O) ppm; anal.

calcd. for C28H20O2: C, 86.57; H, 5.19. Found: C, 86.63; H, 4.93.

3,14-Dihydroxyhexahelicene (4): A 190-mg solution (0.49 mmol) of 3,14-

dimethoxyhexahelicene 3 in 15 mL of anhydrous methylene chloride is

treated dropwise with 1.47 mL (1.47 mmol) of 1 M solution of BBr3 in

methylene chloride at 08C.Thte mixture is stirred for 45 min at 08C and

afterward allowed to warm up to room temperature. After stirring for an

additional 12 h, the mixture is hydrolyzed using a small amount of water, and

the organic layer is separated. Then the aqueous phase is extracted three

times with 50 mL of ethyl acetate, and the extract is dried over MgSO4. The

evaporation of the solvent under vaccum and the purification by column chrom-

atography with chloroform/ethyl acetate (70:30) as the eluent (Rf ¼ 0.38)

yields 166 mg of the 3,14-dihydroxyhexahelicene 4 (94%) as light yellow

solid, mp . 3008C (decomposed). 1H NMR (300 MHz, d6-acetone): d ¼ 6.23

(dd, J1 ¼ 2.7 Hz, J2 ¼ 9.3 Hz, 2H, H-2/H-15), 7.11 (d, J ¼ 2.4 Hz, 2H, H-4/H-13), 7.35 (d, J ¼ 9.3 Hz, 2H, H-1/H-16), 7.68 (d, J ¼ 8.7 Hz, 2H, H-5/H-

12), 7.80 (d, J ¼ 8.4 Hz, 2H, H-6/H-11), 7.82 (d, J ¼ 8.1 Hz, H-7/H-

10),7.85 (d, J ¼ 8.1 Hz, 2H, H-8/H-9), 8.47 (s, 2H, OH) ppm; 13C NMR

(75 MHz, d6-acetone): d ¼ 111.1 (C-4/C-13), 116.7 (C-2/C-15), 124.3 (C),

125.1 (2C), 126.9 (C-7/C-10), 127.9 (C-6/C-11), 128.2 (C-5/C-12), 128.5

(C-8/C-9), 129.6 (2C), 130.4 (C-1/C-16), 131.2 (2C), 134.7 (C), 135.1 (2C),

156.6 (C-3/C-14) ppm.

X-Ray Crystal Structure Determination of 3,14-

Dimethoxyhexahelicene (3)

Single crystals of 3 were obtained by slow evaporation of chloroform at

ambient temperature. X-ray data were recorded on a Nonius MACH3/CAD4

diffractometer. Chemical formula C28H20O2, M ¼ 388.44, crystal dimensions

0.40 � 0.30 � 0.20 mm, orthorhombic, space group Pbca. At 208C:

a ¼ 10.140(4) A, b ¼ 17.144(7) A, c ¼ 23.491(9) A, volume 4084(3) A3,

Z ¼ 8, rcalcd ¼ 1.264 g/cm3, X-ray source MoKa, l ¼ 0.71073 A, measured

reflections 3941, independent reflections 3520, reflections used 3520,

Synthesis and X-Ray Analysis of a New [6]Helicene 1565

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refinement type Fmls, parameters refined 289, R1 ¼ 0.0537, wR2 ¼ 0.1349.

Further details of the crystal structure determination are available on request

from the Cambridge Crystallographic Data Centre on quoting the deposition

number CCDC 292055.

REFERENCES

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Synthesis and X-Ray Analysis of a New [6]Helicene 1567

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