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Contribution of an Intermolecular Charge-Transfer Band to the Optical Activity of Electron-DonorAcceptor Complexes.

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N-Cfo-amino acids are formed by the reaction of furfuryl
alcohol with the esters of a-isocyanofatty acids and subsequent saponification [l], or directly from amino acids
and furfuryl chloroformate, which is produced at -60 "C
in toluene from equivalent amounts of furfuryl alcohol,
phosgene, and triethylamine. The Cfo compounds are
isolated as their dicyclohexylammonium salts. Table 2
shows examples.
The Furfuryloxycarbonyl Group,
an N-Protecting Group for Peptide Syntheses
By Prof. Dr. G. Losse, Dr. H. Jeschkeit, and
Dip1.-Chem. E. Willenberg
Institut fur Organische Chemie
der Universitat Halle-Wittenberg (Germany)
Urethane groups have proved useful as N-protecting groups
in peptide syntheses: they can be removed under mild
conditions.
Systematic studies with new urethane groupings have shown
that the furfuryloxycarbonyl (carbofuroxy or Cfo) residue is
extremely acid-labile and is removed nearly quantitatively
from the amino group at room temperature after 3 min with
two equivalents of HBr in glacial acetic acid at a concentration
of 85 mg/cm3 (corresponding about to a 6 % solution); cf.
Table I. The fission also proceeds quickly and quantitatively
at 0°C.
Received, January 16th, 1964
[Z 658/486 IEI
German version: Angew. Chem. 76, 271 (1964)
[I] F. C. McKay and N . F. Albertson, J. Amer. chem. SOC.79,
4686 (1957): St. Goldschmidf and M. Wick, Liebigs Ann. Chem.
575, 217 (1952).
Contribution of an IntermolecularCharge-Transfer
Band to the Optical Activity of Electron-Donor/
Acceptor Complexes
By Prof. Dr. G. Briegleb and Dip1.-Chem. H. G. Kuball
Table 1. Acidolysis of Cfo-glycine benzyl ester ( J ) , glycine benzyl ester
( 2 ) , and Cbo-glycine ( 3 ) at 20 "C in HBr/AcOH.
Substance
equiv. of HBr
per equiv. of
i J ) ,( 2 ) . of (3)
2
HBr
2
Proportion [%I of ( I ) , ( 2 ) .
or (3) split after
2 1 3
450
200
L
I
85
85
50
50
85
85
90
87
79
65
21
15
<l
0
90
91
85
75
30
24
90
92
87
79
40
30
Cl
0
90
91
90
84
54
43
5 1
92
87
70
I
59
<I
2
92
90
83
73
91
85
77
<l
4
Under these conditions, N-Cbo- and C-benzyl ester groups
remain absolutely intact. In contrast, Cfo and Cbo residues
are removed with almost equal rate in neutral solution by
hydrogenolysis.
The high speed of acidolysis of the Cfo residue is due to the
formation of the strongly mesomerically stabilized furfuryl
cation. Being vinylogous acetals, furfuryl esters are just as
sensitive t o acids as acetals.
H
Table 2. Cfo-amino acids, dipeptides, and dipeptide benzyl esters
Cfo compound [a]
1 M.P I"C1
Cfo-glycine
Cfo-Dr-alanine
Cfo-DL-valine
Cfo-DL-leucine
CfO-DL-phenylahine
76-77
82
100- 101
95-96
100-102
Cfo-glycylglycine
Cf o-glycyl-m-alanine
Cfo-glycyl-DL-valine
Cfo-glycyl-DL-phenylalanine
12s
155-156
106-107
146-147
Cfo-DL-valylglycine benzyl ester
Cfo-DL-phenylalanylglycinehenzyl ester
Cfo-Dr-leucylglycine benzyl ester
132
118-120
79-80
Angew. Chem. internat. Edit. I Vol. 3 (1964) No. 4
Institut fur Physikalische Chemie
der Universitat Wurzburg (Germany)
If the electron donor or acceptor in electron-donor/acceptor
(EDA) complexes [l] is optically active, the following influences of EDA interaction on rotatory dispersion and circular dichroism may be expected:
1. Change in the partial rotational strength [Z] of a given
absorption band which is responsible for the optical activity
of the optically active component.
2. Rotatory dispersion and circular dichroism should be
detectable within the range of the charge-transfer (CT) band.
3. Any absorption band characteristic for the optically in-
active component should exhibit circular dichroism as a
result of loss of symmetry.
Since the charge transfer is a permissible electrical dipole
pm
transfer, the dissymmetry factor g = - - = CL - [*] is very
E
k
small. On the other hand, since pe is relatively large, the
intensity of absorption of the CT band will be correspondingly large. It is therefore necessary for measurements within
the range of the CT band to select the product [concentration]
%[layer thickness] relatively small, so that the effects measured (difference in optical activity between the components
and the EDA complex [3]), are extremely small. The measuring apparatus required has to be particularly sensitive.
Furthermore, EDA complexes with as low CT band intensities as possible have to be used.
In practice, a high transmission polarimeter with an accuracy
of i: 2x 10-3 degrees was assembled according to an electronic
measuring principle described by Wenking 141. For the
measurement of circular dichroism, a method based on the
principle of Senarmont [ 5 ] was developed. The ellipticity was
measured with an accuracy of i 2x 1 0 - 5 ~ 2x.
Seven EDA complexes which have all relatively small chargetransfer moments within the range of the CT band, were
examined.
With the first complex, (+)-camphor tetracyanoethylene ( I ) ,
in heptane, a negative Cotton curve and negative circular
dichroism were found for the EDA portion, within the frequency range of the CT band. By way of contrast, the complex (+)-fenchone tetracyanoethylene (2) yielded a positive
Cotton curve and positive circular dichroism in heptane. The
rotational strengths were calculated to be R1 = - 1 . 2 ~10-40
[cgs] and RZ= l . l x 10-40 [cgs], respectively. The dissymmetry
factors gl = -3.7% 10-4 and g2 = 4.4%10-4 are of the magnitude expected for the charge-transfer moments of EDA
complexes [l]. Also, the circular dichroism calculated from
the amplitude of the Cotton curve according to an expression
given by Kuhn [6] is in agreement with the experimental
values.
307
It has thus becn proved for the first timc with the complexes
( 1 ) and (2) that the EDA-CT band in EDA complexes with
an optically active component can develop circular dichroism and contribute to optical activity.
As stated above, the optically active part of a charge transfer is extremely small. Accordingly, this effect could not be
detected with another EDA complex, (+)-methylcyclohexanone tetracyanoethylene (3), in heptane.
Plain dispersion curves [2] with a superimposed positive
Cotton curve were obtained for the E D A complexes of
methyl (-)-d~-[2,4,5,7-tetranitrofluorenylideneaminooxy]propionate as optically active acceptor with 2-methylnaphthalene
(4) or stilbene (5) in benzene and carbazole (6) in dioxan.
Methyl (+)- and (-)-~-[2,4,5,7-tetranitrofluorenylideneaminooxylpropionates with anthracene (7) in benzene yielded plain
dispersion curves with a superimposed positive or negative
Cotton curve, respectively. Circular dichroism could not be
observed, within the limits of accuracy, for the E D A complexes (4)-(7). The effect referred to under 1 . above, viz.
change in the absorption band of the optically active component responsible for the optical activity, comes into play
in the case of complexes (4)-(6). This leads to a change
in the rotatory dispersion in the appropriate frequency range
of the CT band.
As regards complex (7), it is a moot point whether its positive
Cotton curve is a result of the effect referred to under 1. above
or is due to a partial contribution of the CT band.
While effect 1 . is clearly in operation in the case of complexes
(4)-(7), it is not noticeable, within the limits of experimental
accuracy, for (1)-(3). This may however be explained by the
octant rule [2], since tetracyanoethylene lies in one of the
planes of symmetry of the carbonyl orbital and should therefore have only a minor effect on the optical activity of the
carbonyl band.
Received, January 17th, 1964
[Z 647/481 IE]
German version: Angew. Chem. 76, 228 (1964)
2,3,5,6-tetrahydro-7-aryliniidazo[2,1-b]t
chloridcs,
which are convertible into other salts, r.g. ( I ) , Ar = CsH5,
X = C1 : m.p. 96 -97 "C [*I; ( J ) , Ar = CoH5, X = HSOd:
m.p. 1 6 8 - 1 6 9 ' C ; ( I ) , A r = C ~ H ~ , X - C 1 0 4 :m.p. 145- 146°C.
The structure of ( I ) is supported by the absence of non-ionic
chlorine, by C=N absorption in the infrared, and by its degradation reactions. The salts ( I ) react with a small excess
of alkali on contact with air to yield the thiol (2) and, as a
by-product, the disulfide (3). e . g . Ar = C6H5: m. p. 157 ^C.
Under nitrogen, however, the only product is I-(@-mercapto-
ethyl)-3-arylimidazolid-2-one(2), e.g. Ar = C6H5 : m. p.
71.5 "C, which is oxidized by air or iodine quantitatively to
(3). The disulfide (3) is reducible under nitrogen to the thiol
(2).
N
I
R-NCS
+
3N(CH2CH2C1)2
4*
Formation and Behavior of 2,3,5,6-Tetrahydro7-arylimidazo~2,1-blthiazolium
Salts [l]
By Dr. H. Dorn
lnstitut fur Organische Chemie der Deutschen Akademie der
Wissenschaften zu Berlin, Berlin-Adlershof (Germany)
The bicyclic system imidazo[2,1-b]thiazole [2] and 6,7-dihydro-5H-imidazo[2,l-b]thiazoliumsalts [3] are accessible
from 2-mercaptoimidazole or iniidazoline-2-thione and M halogeno ketones. Aryl isothiocyanates and bis-(9-chloroethy1)amine react i n benzene at room temperature to give
308
" f Y N3
1'5J
I~-N-C-N(CI12CI12CI)2
I I1
€1 s
f
[ I ] Summary on EDA interaction, cf. G . Briegleb: ElektronenDonator-Acceptor-Komplexe. Springer, Berlin-Gottingen-Heidelberg 1961.
[2] Definitions and nomenclature, cf. F. S. Muson, Molecular
Physics 5, 343 (1962); W . Moffitit and A . Mosrowitz, J . Chem.
Physics 30, 648 (1959); C. Djerassi; Optical Rotatory Dispersion.
McCraw-Hill, New York 1960.
[MI is the molecular rotation. The circular dichroism is the
difference in the molecular absorption coefficients of left and
right circularly polarized light, respectively.
[*] i = decadic extinction coefficient.
p m = magnetic dipole-transition moment.
IJ.~
= electric dipole-transition moment.
[3] The contribution made to rotation by the EDA band is the
difference A[M] =z [ME~A]-[Mopt. act.], where [MEDA] and
] are the [MI-values of the EDA complex and of the
pure, optlcally active component, respectively, within the range
of the EDA band. The strength (R) of rotation is the imaginary
part of the product of electrical and magnetic charge-transition
moments.
[4] H. Wenking, Z. Instrumentenkunde 66, 1 (1958).
[5! H. G. Jerrard, J. opt. SOC.America 38, 35 (1948).
[6] W. Kirhn et al., Annual Rev. physic. Chem. 9, 417 (1958).
+-
41
The claim [41 that thioureas (4) arise from isothiocyanates
and bis-(p-chloroethy1)amine must be treated with reserve.
We succeeded in isolating an intermediate of the bicyclic
viz. the 2-iniinothiazolidine derivative ( 5 ) ,
product (Z),
R = 2,3,4,6-tetra-O-acetyI-l-~-i~-glucosyl,
and its salts.
Received, January 20th, 1964
IZ 673/497 IE]
German version: Angew. Chem. 76, 301 (1964)
[ I ] Potential Cytostatics, Part 2. - Part 1 : H . Dort7, Mber. dtsch.
Akad. Wiss.. Berlin 3, 683 (1961).
[2] E. Ochiai, Ber. dtsch. chem. Ges. 6Y, 1650 (1936); H. Andersug
and K. Wrstphol, Ber. dtsch. chem. Ges. 70, 2044 (1937).
[3] W. Wilson andR. Wuodger, J.chem.Soc.(London) 1955,2943.
[*I i l ) , Ar = C6H5, X -= CI, crystallizes with 1 H2O. On heating
to 75-77 'C, water is lost and three modifications melting at
96-97 'C, 124-125 "C and 138-139 "C are obtained.
[4] F. D. Popp and H. Swnrz, J. org. Chemistry 26. 4764 (1961).
Cleavage of the f-Arnide Bond in Cyanocobalarnin
Analogues by Propionibacterium shermanii
By Doz. Dr. W. Friedrich and Dr. E. Konigk
in collaboration with Waltraud Sandeclc
Physiologisch-Chemisches lnstitut
der Universitit Hamburg (Germany)
ln the biosynthesis of vitamin B12 analogues [l] by Propiotiibucterium sherniaizii from cobinamide analogues, whose
alkanolamine residues are fluorinated, often practically all
the starting material is converted into vitamin B12 if cobaltfree media are used. This is especially noticeable when the
alkanolamine residue is I-amino-2-hydroxy-2-(o-fluorophenyl)ethane, l-amino-2-hydroxy-2-(p-fluorophenyl)ethane,
or
l-aniino-2-hydroxy-3,3,4,4,5,5,5-heptafluoropentane.
Aitgew. Cliem. intertiat. Edit./ Vol. 3 (1964) Nu. 4
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