Synthesis of 9-fluorenylmethoxycarbonyl-protected amino aldehydes

Article abstract of DOI:10.1016/S0957-4166(98)00183-9

9-Fluorenylmethoxycarbonyl-protected amino aldehydes could be efficiently prepared in good yields by using two methods: (i) NaBH₄ reduction of Fmoc-protected mixed anhydrides, followed by the Swern oxidation of the alcohols; and (ii) LiAlH

Full text of DOI:10.1016/S0957-4166(98)00183-9

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 1855–1858
Synthesis of 9-fluorenylmethoxycarbonyl-protected amino
aldehydes
James J. Wen and Craig M. Crews ∗
Department of Molecular, Cellular, and Developmental Biology, Yale University, New Haven, CT 06520-8103, USA
Received 22 April 1998; accepted 23 April 1998
Abstract
9-Fluorenylmethoxycarbonyl-protected amino aldehydes could be efficiently prepared in good yields by using
two methods: (i) NaBH4 reduction of Fmoc-protected mixed anhydrides, followed by the Swern oxidation of
the alcohols; and (ii) LiAlH4 reduction of Fmoc-protected amino acid Weinreb amides. Both methods afforded
comparable overall synthetic yields (70–80%). © 1998 Elsevier Science Ltd. All rights reserved.
The reduced peptide bond, ψ[CH2NH], is widely used for the synthesis of pseudopeptide-based
enzymatic inhibitors and antagonists.1 Among the features provided by this amide bond isostere are: (1)
increased flexibility; (2) resistance to enzymatic degradation; and (3) introduction of a protonation site
under physiological conditions.2 Traditionally, the incorporation of the ψ[CH2NH] isostere into a peptide
sequence was achieved by performing a resin bound reductive amination on the resin bound protonated
amine and a tert-butoxycarbonyl-protected amino aldehyde in the presence of NaBH3CN.3 This approach
is convenient and facile, however it is incompatible with solid supports incorporating acidic labile link-
ages, such as Wang4 and Rink resins.5 In order to overcome this incompatiblity between the protecting
group and linkages, a non-acid labile protecting group (Fmoc, allyl, etc.) for the amino aldehydes is
needed. Herein, we report our recent studies on the synthesis of 9-fluorenylmethoxycarbonyl-protected
amino aldehydes.
We envisioned that 9-fluorenylmethoxycarbonyl-protected amino aldehydes could be potentially syn-
thesized by two approaches: (i) reduction of the acids to alcohols, followed by oxidation to the corre-
sponding aldehydes; and/or (ii) transformation of the acids to Weinreb amides, followed by reduction of
the corresponding amides (Scheme 1).
To investigate the feasibility of both approaches, we first focused our attention on the reduction of
Fmoc-protected amino acids. Initially, we used a borane–tetrahydrofuran complex (BH3–THF)6 as the
reducing reagent for this purpose. For all acids tested, the reduction proceeded smoothly at 0°C and
afforded the desired alcohols in 56%–80% yields (Table 1), along with a hydrophobic side product
∗
Corresponding author. E-mail: Craig.Crews@Yale.edu
0957-4166/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00183-9

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Scheme 1. Reagents and conditions: (a) iBuOCOCl, NMM, DME, −10°C; (b) NaBH4, H2O, 0°C; (c) (COCl)2/DMSO/DIEA,
CH2Cl2, −60°C to r.t.; (d) N,O-dimethylhydroxylamine hydrochloride/HBTU/DIEA; (e) LiAlH4
(10%–20%) identified as the Fmoc-cleavage product, 9-fluorenylmethylene.7 In an effort to minimize
cleavage of the Fmoc-group during the reduction, we carried out the reduction at −78°C. We found
that cleavage of the Fmoc-group still occurred under lower temperature conditions, however to a smaller
degree, and at the cost of slowing down overall reduction kinetics as well as decreasing the synthetic
yield. Thus it became clear that in order to minimize the cleavage of the Fmoc-group and improve
the reduction yield significantly, an alternative approach was needed. Subsequently, we found that the
reduction of Fmoc-protected mixed anhydrides with NaBH4, as proposed by Martinez et al.8 provides an
excellent approach for preparing Fmoc-protected alcohols with remarkably high yields (83–98%). The
reduction yields from both reduction approaches are listed in Table 1.
Table 1
Our next endeavor was to investigate the oxidation of Fmoc-protected amino alcohols using
well established oxidants. To identify optimal oxidation conditions for this purpose, we used the
Fmoc–Phe-ol as a model substrate, and oxidized it with various oxidants including pyridinium
chlorochromate (PCC),9 pyridinium dichromate (PDC),10 SO3/pyridine/DMSO,11 and the Swern
oxidation (DMSO/(COCl)2/DIEA).12 It was found that: (i) PCC and PDC provided easy and convenient
approaches for oxidizing Fmoc–Phe-ol, but suffered from slow oxidation kinetics and moderate
synthetic yields (40–50%). This resulted primarily from the incomplete oxidation of the alcohol and
partial cleavage of the Fmoc-group during the oxidation. (ii) Sulfur trioxide oxidation of Fmoc–Phe-ol
afforded Fmoc–Phe-al in a moderate yield (50%). The Fmoc-group was also partially cleaved during the
oxidation. (iii) Swern oxidation worked effectively for the oxidation of Fmoc–Phe-ol to Fmoc–Phe-al,
with a good 79% synthetic yield, even though the Fmoc-group was partially cleaved. Based on
these results, we concluded that Swern oxidation is the best choice for oxidizing Fmoc-protected
amino alcohols to Fmoc-protected amino aldehydes. For most of the alcohols used in this study, the
oxidation yields ranged from 72% to 85% (Table 2).13 Under optimal NaBH4 reduction and Swern

J. J. Wen, C. M. Crews / Tetrahedron: Asymmetry 9 (1998) 1855–1858
1857
Table 2
oxidation conditions, the overall synthetic yields for making Fmoc-protected amino aldehydes from the
Fmoc-protected amino acids could be between 70% to 80%.
The reduction of tert-butoxycarbonyl-protected Weinreb amides to the corresponding aldehydes was
first reported by Castro.14 The reduction was effected using LiAlH4 at 0°C and afforded the desired
aldehydes in high yields. By the same measure, we envisioned that the Fmoc-protected amino aldehydes
could also potentially be prepared by this reduction method. However, since LiAlH4 is a very strong
reducing reagent,15 a low reduction temperature appears to be necessary for minimizing the cleavage of
the Fmoc-group during the reduction process. Subsequently, we synthesized a series of Fmoc-protected
Weinreb amides (>90% yields) and subjected them to LiAlH4 reduction at −78°C under nitrogen. As
expected, the reduction proceeded smoothly and was usually complete within one hour. Under these
reduction conditions, only a small amount of the Fmoc-group was cleaved (5%–10%). The desired Fmoc-
protected amino aldehydes could be synthesized in 73–92% yields (Table 2). The following procedures
are illustrative.
1. Swern oxidation of Fmoc–Phe-ol
A solution of 26 µl oxalyl chloride (0.3 mmol) in 0.5 ml CH2Cl2 was stirred at −60°C under nitrogen.
To the solution was added 43 µl DMSO (0.6 mmol), then the solution was stirred vigorously. After
10 min, 75 mg Fmoc–Phe-ol in 1 ml CH2Cl2 was added dropwise to the solution using a syringe. The
reaction mixture was stirred at −60°C under nitrogen for 15 min, followed by the addition of 209 µl DIEA
(1.2 mmol). The cooling bath was removed after 5 min, and the reaction mixture was allowed to warm
to room temperature. Water (5 ml) was then added to the reaction mixture. The mixture was stirred at rt
for an additional 10 min, and extracted twice with CH2Cl2 (60 ml). The organic layer was washed with
1 N HCl solution, saturated NaCl solution, dried over anhydrous MgSO4, and evaporated in vacuo. The
crude product was purified by flash column chromatography (SiO2, 15%–30% EtOAc:hexane). Yield:
79%; mp: 106–108°C; [α]D=−48.7 (c 0.21, MeOH); 1H-NMR (CDCl3): δ 3.17 (d, 2H, J=6.4 Hz); 4.22
(t, 2H, J=6.8 Hz); 4.42 (m, 1H); 4.52 (m, 1H); 7.13–7.79 (m, 13H); 9.56 (s, 1H). FAB MS: calcd for
[C24H21NO3, M+H+] 372.4, found 372.1.

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J. J. Wen, C. M. Crews / Tetrahedron: Asymmetry 9 (1998) 1855–1858
2. LiAlH4 reduction of Fmoc–Phe–CON(CH3)OCH3
A solution of 53 mg LiAlH4 (1.4 mmol) in 4 ml THF was stirred at −78°C under nitrogen. To the
solution was added dropwise 300 mg Weinreb amide (0.7 mmol) in 2 ml THF. The reaction was quenched
after 60 min by adding cold water to the solution. The solution was evaporated in vacuo and the residue
was extracted twice with ethyl acetate (40 ml) and 1 N HCl solution (5 ml). The organic layer was washed
with brine, dried over anhydrous MgSO4, and evaporated to afford 240 mg of a white solid product. Yield:
92%; mp: 106–108°C; [α]D=−48.7 (c 0.23, MeOH); 1H-NMR and FAB MS data are identical to those
described above.
In conclusion, we have demonstrated that the 9-fluorenylmethoxycarbonyl-protected amino aldehydes
could be successfully synthesized by either reducing the Fmoc-protected mixed anhydrides with NaBH4,
followed by a Swern oxidation of the alcohols, or converting the Fmoc-protected amino acids to the
corresponding Weinreb amides, followed by a reduction of the amides with LiAlH4. Overall, both
approaches afforded comparable synthetic yields.
Acknowledgements
Financial support of this work was provided by the NIH (HG01703-01) and the Association for the
Cure of Prostate Cancer (CaPCURE). C.M.C. is a Burroughs Wellcome Fund New Investigator in the
Pharmacological Sciences.
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