J Pharm Innov
within the dendritic structures were connected via amide link-
ages formed by utilizing L-glutamic acid amino groups.
The synthesized dendritic molecules carried benzyl ester
of reflux and in the presence of protic solvent like methanol,
NaBH alone acts as sufficiently strong reducing agent [21].
4
Compound 5 was extracted in DCM and solid residue con-
taining inorganic salts was separated by filtration.
(3), carboxylic acid (4) and alcohol (5) functional groups at
dendron termini. Amongst these, carboxylic ester and acid
termini account for nonpolar neutral and polar charged periph-
eries, respectively. In order to synthesize dendritic lipopeptide
oligomers with polar neutral peripheries, current work includ-
ed alcohol termini analogues. Furthermore, the reaction con-
ditions adopted in the present work for synthesizing carbox-
ylic ester and acid termini were not only simplified in terms of
types of reagents employed, but also rapid. Synthesis was
conducted as described in Scheme 1. Synthesis of compound
For synthesis of compound 1, BOC-L-glutamic acid was
used as a di-carboxylic acid and L-glutamic acid dibenzyl
ester p-toluenesulfonate was used as an amine. Acid, amine,
EDC.HCl and 4-DMAP were employed in the equivalent ratio
of 1:2:2.5:6.125, respectively. The acid was activated by ini-
tial stirring with 4-DMAP. Addition of EDC.HCl at 0 °C
slows down the side reaction, which otherwise results into
formation of nonreactive N-acylurea by-product. Formation
of the same is also prevented due to the presence of 4-
DMAP, as this nucleophile activator prevents rearrangement
of O-acylisourea and maintains its reactivity. Addition of del-
iquescent EDC.HCl under nitrogen atmosphere is required to
prevent its inactivation. On reaction completion, 4-DMAP
was separated using saturated aqueous citric acid solution.
Cold n-pentane wash was required for solidification of oily
product. The obtained off-white solid product demonstrated
the presence of nonpolar impurity in TLC analysis. Hence, the
obtained crude product was purified using flash chromatogra-
phy. The initial mobile phase gradient of 100% DCM resulted
into separation of impurity, whereas increase in polarity there-
after resulted into elution of compound after 9 min.
1
was performed using carbodiimide coupling agent
EDC.HCl. It is often a carbodiimide of choice because its
nonreactive N-acylurea by-product is water soluble, and can
be eliminated easily by aqueous washes during work up [14].
Both DCM and DMF are the most widespread solvents used
in coupling reactions, the former is neutral aprotic and latter is
basic aprotic [15, 16]. We preferred DMF as a solvent since it
maintains non-acidic reaction conditions by neutralising HCl
separated from coupling agent. Also, instead of 1-
hydroxybenzotriazole (HOBt), we used 4-DMAP as a stron-
ger carbodiimide nucleophile activator [17, 18]. Nucleophile
activators are coupling additives which form corresponding
active esters, preventing conversion of O-acylisourea to non-
reactive N-acylurea [19]. Due to its basic nature, 4-DMAP
also helps in activation of acid through proton abstraction.
Hence, the presence of DMF and 4-DMAP eliminated need
of additional base such as TEA.
Although synthesis of compound 1 as well as compound 3
involved amide coupling, reaction conditions used for former
were not suitable for the latter. This was thought to be because
of the stearic hindrance offered by benzyl groups of com-
pound 2. Hence, myristoyl chloride was employed instead of
myristic acid for synthesis of compound 3, as acyl chlorides
are more reactive than their corresponding carboxylic acid
analogues. On the other hand, HCl produced as a by-product
of acyl chloride coupling reactions give rise to acidic reaction
conditions. Since such acidic conditions can result into
deprotection of BOC-L-glutamic acid [14], acyl chloride me-
diated coupling was undertaken for synthesis of compound 3
but not for the synthesis of compound 1.
TFA was used to provide mild acidic conditions for
deprotection of amine functional group of compound 1. The
crude product obtained is a TFA salt of BOC-deprotected
compound 1. On reaction completion, excess TFA was
neutralised to avoid interference of acidic conditions with next
coupling reaction. During work up with aqueous sodium bi-
carbonate, TFA salt breaks and forms an unstable amine.
Compound 2 was used immediately in next reaction without
further purification to avoid decomposition of free amine. For
synthesis of compound 3, acyl chloride, amine and base were
taken in the equivalents of 1:1.2:2. Reduction of compound 3
was simplified and rapid since in contrast to traditionally used
H , Pd/C catalyst under pressure, NaBH was used as reduc-
2
4
ing agent under atmospheric pressure. Based on the reaction
condition, reduction of compound 3 resulted into correspond-
ing carboxylic acid (reflux) or alcohol (room temperature)
analogues. Reduction at room temperature resulted into the
former, whereas at reflux, it resulted into the latter.
The reactions catalysed by NaBH offer inexpensive, rapid
FTIR analysis of synthesized compounds demonstrated in
Fig. 2 showed considerable variations in functional group
4
and simple alternative to reduction with H , Pd/C catalyst. A
2
reducing agent, nickel boride, was generated in situ for reduc-
tion of compound 3 to its corresponding carboxylic acid.
Compound 3/NiCl .6H O/NaBH was used in the equivalent
ratio of 1:14:42 since reaction involved cleavage of 4 ester
bonds [20]. Filtration through Celite pad separates the inor-
ganic salts, making reaction work up simple and rapid. For
reduction of compound 3 to its corresponding alcohol, NaBH4
was used as reducing agent. Under harsh reaction conditions
peaks. Significant shift in amine peak from ν ~ 3302 to
max
−1
νmax ~ 3283 cm , respectively, confirmed BOC deprotection
of compound 1 and formation of compound 2. Although both
the aforementioned compounds showed presence of aromatic
2
2
4
−
1
−1
C-H at νmax ~ 2900 cm and νmax ~ 2850 cm , appearance
−
1
of sharp C-H stretching bands at 2918.16 cm and
2849.89 cm confirmed attachment of aliphatic myristoyl
−1
group to compound 2 and formation of compound 3. Due to