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Abstract: (Chemical Abstract 2001, 134: 71727)
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Different lipase enzymes have been tested in order to perform regioselective
acetylations on the eudesmane tetrol from vulgarin.
High yields (95%) of 1,12-diacetoxy derivative (4)
were achieved in one hour with Candida antarctica lipase (CAL).
However, only the 12-acetyl derivative (6) was obtained in similar yield with
Mucor miehei (MML) or Candida cylindracea (CCL) lipases.
The enzymatic protection at C-1 and C-12 has been used to form eudesmane
cyclic-sulfites between C-6 and C-4 atoms. The R/S-sulfur configuration
has been assigned by means of the experimental and theoretical
13C and 1H NMR chemical shifts.
The theoretical shifts were calculated
using the GIAO method, with a MM+ geometry optimization followed by a
single-point calculation at the B3LYP/6-31G* level (B3LYP/6-31G*//MM+).
Moreover, B3LYP/6-31G* geometry optimizations were carried out to test the
B3LYP/6-31G*//MM+ results, for the deacetylated sulfites (12 and 15).
In addition to the dC and dH shifts, the 3JHH
coupling constants were also
calculated and compared with the experimental values when available.
Finally, different reactivities have been checked in both sulfites by
biotransformation with Rhizopus nigricans.
While the R-sulfite gave 2a- and 11b-hydroxylated metabolites,
the S-sulfite yielded only regioselective deacetylations.
Furthermore, both sulfites showed different reactivities in redox processes.
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