rhenium - tetrahydropyrans - Prins cyclization
Significance
<P>A Re(VII) complex catalyzed the highly selective condensation
of easily accessible starting materials using a very mild and simple protocol
to give densely functionalized tetrahydropyrans
3 (THP).
Aromatic aldehyde partners
2 give a single
diastereomer where all substituents are equatorially oriented. Whereas
aliphatic aldehydes give diastereomeric mixtures (
4 and
5), alkenyl aldehydes provide
an alternative, as illustrated by complex THP
11.
Achieving high atom-efficiency, the catalytic Prins cyclization
reported improves previous protocols using excess acid (the conjugate
base moiety is generally incorporated into the THP at the end of
the reaction).</P>
Comment
<P>Both ReO
3(OH) and Re
2O
7 were shown
to be effective catalysts to mediate the transformation. The authors
were inspired by seminal work on allylic alcohol isomerization done by
Osborn and co-workers (
Angew. Chem. Int. Ed. 1997,
36, 976),
as well as later investigations done by Grubbs and co-workers (
J. Am. Chem. Soc. 2005,
127, 2842). These early studies invoked
a cationic intermediate suggesting that intermediate
C might
be a viable species in the above process. A potential limitation
of the reaction is seen with aliphatic aldehydes
2,
which are prone to form side-products arising from competing oxonia-Cope
rearrangements under these conditions.</P>
