complete deprotection of PhSO2, 4-cyanophenylsulfonyl (Cbs),
4-BrC6H4SO2 and Ts groups of the corresponding tert-butyl
arylsulfonylcarbamates, but Mbs did not cleave completely
even after 24 h. When the same amount of magnesium was used
and the reactions were allowed to proceed under ultrasonic
conditions‡ very fast and clean cleavage took place even with
Mbs-protected and sterically hindered substrates. Reactions
were subsequently optimised with the tert-butyl N-benzyl
derivatives in each case under sonication with the results that (i)
5 equiv. of magnesium powder were sufficient for an efficient
deprotection of the PhSO2, 4-BrC6H4SO2, Cbs and Ts groups
and (ii) 10 equiv. for the 2,4,6-Me3C6H2SO2 and
2,4,6-Pri3C6H2SO2 groups, whereas (iii) at least 15 equiv. were
required for the cleavage of the Mbs-carbamate. These findings
were subsequently applied in the preparative cleavage experi-
ments involving derivatives of alanine, ethylenediamine, hydra-
zine and N-acyl sulfonamides listed in Table 1.
It should be noted that during the experiment involving the
4-NO2C6H4SO2 group (entry 6), a yellow mixture was obtained,
probably resulting from reduction of the nitro group. The Troc
containing sulfonylcarbamate (entry 14), although cleaved
completely, underwent a known alkyl halide reduction mediated
by the reagent,13 giving rise to the 2,2-dichloroethoxycarbonyl
(Doc) derivative. Furthermore, transesterification took place
(entries 10 and 11) as previously observed during the reduction
of an a,b-unsaturated ethyl ester.14 Concerning the N-Ac and
N-Bz sulfonamides (entries 17 and 18), it is worth mentioning
that the cleavage of the tosyl group proceeded more selectively
in the former case.
In summary, since arenesulfonamides derived from primary
amines are easily converted to tert-butyl and similar sulfonyl-
carbamates by DMAP-catalysed acylation,7 this reductive
cleavage completes a two-step procedure from sulfonamides to
carbamates. Furthermore, since the required sulfonylcarba-
mates can also be directly obtained from alcohols by the
Mitsunobu reaction,2 such carbamates can now also be made
more conveniently by the present procedure.
During the tenure of this work, B. N. was a recipient of a
fellowship from the Swedish Institute which is gratefully
acknowledged. This research is part of a programme supported
by the Swedish Natural Science Research Council.
Footnotes
* E-mail: urbki@bmc.uu.se
† Magnesium powder (Merck 105815, particle size 0.06–0.3 mm) was used.
The quality of methanol is critical for these experiments: we found that after
addition of 0.5% of water, no reaction took place. For other applications of
Mg–MeOH, see Encyclopedia of Reagents for Organic Synthesis, ed. L. A.
Paquette, Wiley, Chichester, 1995, vol. 5, pp. 3202–3204.
‡ 35 kHz, 120–240 W (Bandelin, Berlin, type RK106).
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Received in Cambridge, UK, 28th February 1997; Com.
7/01408B
1018
Chem. Commun., 1997