Letter pubs.acs.org/OrgLett
Synthesis and Reactivity of 5‑Substituted Furfuryl Carbamates via Oxanorbornadienes Srinivas Tekkam and M. G. Finn* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States S Supporting Information *
ABSTRACT: Furfuryl carbamates are labile and require care to be accessed by activating furfuryl alcohols. An alternative oxanorbornadiene (OND)-based strategy is presented for the preparation of 5-R-substituted furfuryl carbamates via the reactions of amines with intermediate OND carbonates. The resulting OND carbamates, which are stable for several months, undergo thiol mediated retro-Diels−Alder reaction to deliver the desired furfuryl carbamates in a single flask. Conditions for the selective hydrolysis of furfuryl carbamates in the presence of tert-butyloxycarbonyl (Boc) groups were identified, and it was shown that furfuryl carbamates can be used as a prodrug handle.
C
arbamates are ubiquitous in organic chemistry, especially as amine protecting groups in peptide synthesis,1 drug design and medicinal chemistry,2 and materials and polymer chemistry.3,4 Ritonavir, amprenavir, atazanavir, and irinotecan are antiviral or anticancer carbamate-based drugs in clinical use or trials.5−8 Carbamate moieties also serve as prodrugs to increase water solubility and/or allow for in situ activation by carboxyl esterases.9 Their general methods of synthesis are from isocyanates and alcohols or from activated carbonates/ chloroformates and amines.10,11 Our research on furan-derived oxanorbornadiene (OND) electrophiles and cleavable linkers requires the synthesis of various furfuryl carbamate derivatives.12−16 Although seemingly straightforward, most of our attempts to access furfuryl carbamates from furfuryl alcohols gave poor yields of the desired compounds, presumably due to the highly reactive nature of furan derivatives with leaving groups on the 2methylene position.17 Thus, activated furfuryl alcohol substrates such as carbonates or chloroformates are susceptible to nucleophilic substitution at this “furylic” site in competition with addition at the carbonyl carbon as illustrated in Figure 1. This makes the isolation of activated furfuryl carbonates or chloroformates challenging and can lead to poor results even for their in situ generation and amine capture.18 The problem
appeared to be even worse in our hands for 5-substituted furfuryl carbamates by in situ activation of 5-substituted furfuryl (R = methyl or aryl) alcohols. Accordingly, we investigated the synthesis of furfuryl carbamates from oxanorbornadienes, which are easily made from furan derivatives and standard alkynyl dienophiles such as dimethylacetylenedicarboxylate (DMAD).19 Oxanorbornadienes release furan derivatives in the presence of thiols via sequential Michael addition and retro-Diels−Alder reaction under mild conditions.13 Thus, as in previous examples,20−22 we envisaged oxanorbornadienes as masked furans and proposed to synthesize OND activated carbonates (3) from furfuryl alcohols (1) as depicted in the examples of Figure 2. We hoped that these OND carbonates, unlike the corresponding furfuryl carbonates, would be stable enough to be handled conveniently and reactive enough with amines to provide easy access to furfuryl carbamates (5) after r-DA reaction in a onepot sequential process. The application of this strategy to five furfuryl alcohols varying in substituents at the 5-position (1a−e) is shown in Figure 2.23−25 The corresponding oxanorbornadienes (2a−e) were obtained in >90% yield from DMAD with complete consumption of the furans, except for 2e (41% yield). This reaction was clean (good recovery of starting materials) but incomplete even at long reaction times, presumably reflecting a limited equilibrium due to enhanced furan stabilization or relative electron deficiency caused by resonance interaction with the p-nitrophenyl group. The ONDs were then treated with disuccinimidyl carbonate to give 3a−e in good yields (67−91%). All of these activated OND carbonates were found to be solids and stable for at least six months at 4 °C. Reaction with phenylalanine methyl ester at low temperature provided the desired OND carbamates 4a−e
Figure 1. Problem of furfuryl carbamate instability.
Received: April 1, 2017 Published: May 16, 2017
© 2017 American Chemical Society
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DOI: 10.1021/acs.orglett.7b00990 Org. Lett. 2017, 19, 2833−2836
Letter
Organic Letters
Figure 2. Oxanorbornadiene-mediated synthesis of furfuryl carbamates of phenylalanine methyl ester. The embedded table shows the 5-furyl substituents that define each case and the rates (half-lives of 4a−e) and yields of compounds 5a−e of the two-step cleavage process of each carbamate in the presence of excess β-mercaptoethanol.
in good yields (77−88%). At higher temperatures, Michael addition of the amine to the electron-deficient OND double bond was observed as a side reaction. The corresponding furfuryl carbamates were then conveniently produced by treatment of the OND carbamates with a 3fold excess of β-mercaptoethanol in the presence of base (trimethylamine) at room temperature. Michael addition of the thiol was found (by TLC and 1H NMR) to be very rapid for all OND carbamates 4a−e, but the retro-Diels−Alder fragmentation of the resulting oxanorbornane ring was not. The progress of this step was monitored by 1H NMR (example shown in Figure 3), following the intensities of the isolated OND alkene (red) and furan (green) resonances. As summarized in Figure 2, the room temperature half-lives of these unimolecular (firstorder) processes varied tremendously, from less than 10 min for all of the aryl-substituted cases to approximately 15 days for the parent (H-substituted) case (4a → 5a). All the furfuryl carbamates (5a−e) were isolated in good yields after standard workup and flash chromatography. A set of primary amines (6− 12) containing a range of other functional groups (alcohol, ester, bromide, thioether, aldehyde) were successfully employed in the same series of steps for both 5-methyl and 5-phenylfuran derivatives as summarized in Table 1. In order to understand the stability of furfuryl carbamates and to assess them as a potential protecting group for amines or as a pro-drug handle for drug delivery, we exposed the representative compounds 5b and 5c to a variety of conditions as shown in Table 2. Overall, the furfuryl carbamate was unstable toward acidic conditions and resistant to base. Thus, clean deprotection of 5-phenyl-furfuryl carbamate/5-methylfurfuryl carbamate was observed with 1% trifluoroacetic acid (TFA) in CH2Cl2 within 10 min, 0.5% TFA in CH2Cl2 in approximately 3 h, and with BF3·OEt2 in 5 min. Partial deprotection of 5b and 5c with the formation of unidentified side products was observed with 0.25% TFA in CH2Cl2 over 12 h; no reaction was observed with pyridinium p-toluenesulfonate (PPTS) in CH2Cl2/MeOH, or with Zn dust (4 h). These furfuryl carbamates were also found to be labile to hydrogenolysis (Pd/C, H2, 1 atm), cleaving completely in 2 h. Compounds 5b and 5c were unreactive with piperidine, sodium hydroxide, and sodium borohydride. These properties may
Figure 3. 1H NMR (CDCl3, 25 °C, showing 4.0−7.5 ppm) of the βmercaptoethanol adduct of 4b, highlighting the vinylic C−H resonances used to determine the rate of the r-DA reaction.
Table 1. Additional Furfuryl Carbamates Formed from 5-Phand 5-Me-Furfuryl Alcohol
warrant the designation of the furfuryloxycarbonyl as the “Furoc” protecting group for amines, in analogy to tertbutyloxycarbonyl (Boc). The relative stabilities of Boc and Furoc carbamates were evaluated by treating a mixture of Boc-Tyr-OMe 13 and 5-Phfurfuryl carbamate 3c with 1% TFA in CH2Cl2 in the presence of triisopropylsilylhydride as a scavenger (Figure 4A). After 15 min, 3c was completely cleaved leaving 13 unchanged (>90% 2834
DOI: 10.1021/acs.orglett.7b00990 Org. Lett. 2017, 19, 2833−2836
Letter
Organic Letters Table 2. Stability Assessment of Compounds 5b and 5c entry
conditions
time
1 2 3
1% TFA in CH2Cl2 0.5% TFA in CH2Cl2 0.25% TFA in CH2Cl2
10 min ∼3 h 12 h
4 5 6
Pd/C, H2, MeOH piperidine in THF PPTS (CH2Cl2/ MeOH) 1 M NaOH (THF/ H2O) NaBH4 (MeOH) Zn dust (AcOH/Et2O) BF3·OEt2 (CH2Cl2)
2h 5h 6h
clean deprotection clean deprotection partial deprotection with impurities clean deprotection no deprotection no deprotection
4h
no deprotectiona
5h 5h 5 min
no deprotectionb no deprotection clean deprotection
7 8 9 10
a Methyl ester hydrolysis observed. observed.
results
b
Figure 5. HPLC chromatogram of the unmasking of doxorubicin from OND carbamate 21. The identities of both thiomaleate E and dox were confirmed by mass spectrometry of the eluted compounds and by comparison of elution time of authentic samples (not shown).
Ester reduction to alcohol
initiate the cascade is unique to this methodology. Its ultimate utility will also depend on a variety of additional factors such as the stability of the OND carbamate toward enzymatic cleavage, which remain to be assessed. In conclusion, a simple series of transformations allow convenient access to a variety of 5-R furfuryl carbamates from process-friendly oxanorbornadiene activated carbonates. The resulting carbamates, accessible in general fashion, can be differentially cleaved in the presence of Boc groups and are hydrolyzed slowly in mild aqueous acid.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b00990. Experimental procedures for new compounds, spectroscopic data (PDF)
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Figure 4. Feasibility of some useful reactions in the presence of furfuryl carbamates.
AUTHOR INFORMATION
Corresponding Author
recovered), thus showing that differential deprotection of Furoc and Boc is feasible. The furfuryl carbamate was also shown to be tolerant to standard solution-phase peptide coupling (Figure 4B), copper-catalyzed azide−alkyne cycloaddition (CuAAC, Figure 4C), and Suzuki coupling (Figure 4D) conditions. To further elaborate the use of furfuryl carbamates as a prodrug handle,26 we synthesized doxorubicin 5-methyl-OND carbamate 21 under standard conditions and subjected it to thiol in a mixture of DMSO and PBS buffer (pH 7.3). The pH of the resulting thiol adduct mixture was then adjusted to 5 and incubated for several hours to assess the cleavage/stability of the corresponding doxorubicin 5-methyl furfuryl carbamate. The entire reaction sequence (thiol addition, r-DA reaction, and carbamate hydrolysis) was monitored by LCMS (see Supporting Information). Free doxorubicin appeared after 16 h, with almost complete release in about 50 h (Figure 5). Doxorubicin release was also observed after extended incubation at pH 7, but with several uncharacterized side products that were not formed at pH 5. These results suggest that furfuryl carbamates may represent a viable amine prodrug for endolysosomal activation via the two-step process of r-DA fragmentation followed by acid-mediated hydrolysis. The need for a thiol component such as intracellular glutathione27 to
*E-mail: mgfi
[email protected]. ORCID
M. G. Finn: 0000-0001-8247-3108 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the NIH (AI119971) and NSF (CHE 1011796). REFERENCES
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