Communication
Green Chemistry
with retention of the stereochemistry at C11a as demonstrated
by HPLC analysis (see ESI†).
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Compound 28 may be transformed in an additional step
into the corresponding imine;32 consequently, this can be con-
sidered as a new formal synthesis of the natural product DC-81.
Analogously, PBD 31, an essential structure of the bioconju-
gate GWL-78, was prepared from compound 29.33 Reduction of
the nitro group with H2 and Pd/C in MeOH gave arylamino
alcohol 30. Cyclisation through the borrowing hydrogen
process mediated by our Ru based nanoparticle–nanomicelle
combination occurred at 100 °C under MW dielectric heating,
and product 31 was isolated in 47% yield.
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In conclusion, we have developed a protocol that enables the
alkylation of aromatic amines with primary and secondary
lipophilic alcohols in water as the unique solvent. The reaction
occurs under the catalysis of Ru nanoparticles embedded in
the surfactant formed from Ru3(CO)12 and tetraphenylcyclo-
pentadienone (or the genuine Shvo catalyst) in the presence of
the alcohol and the aniline derivative. This borrowing hydro-
gen procedure is very efficient, can be carried out without
interference of several functional groups and allows the purifi-
cation of the final product without chromatography. The
process produces secondary amines with low environmental
impact16b as a minimal amount of surfactant is required, and
organic solvents are employed only for product separation/re-
cycling of the catalyst. Recycling was repeated up to four times
without a sensible reduction of the activity. The protocol was
successfully applied to the synthesis of complex PBD natural
products. This MW assisted Ru nanoparticle–nanomicelle cata-
lysis is currently under investigation for different applications
in other hydrogen transfer reactions.
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Conflicts of interest
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There are no conflicts to declare.
Acknowledgements
This work was financially supported by the project
“Development and application of QM/MM technologies for the
design of light responsive proteins or protein-mimics based on
rhodopsin architecture” within the program “Dipartimenti di
Eccellenza – 2018–2022 financed by MIUR (Rome, Italy).
Notes and references
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