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3.2.1 Preparation of Enol-ether Trienes via an Allenic Cycloisomerization Reaction
The success of the Rh(I)-catalyzed allenic cycloisomerization reaction of amino-ester
tethered substrates prompted further examination of the scope of this transformation. In these and
previous examples from the Brummond group, variations of the substitution pattern of the allene
have been limited to introducing different alkyl groups. Consequently, the resulting trienes
possess three cross-conjugated olefins with similar electronic properties, and are expected to
react unselectively with various electrophilic reagents (e.g., epoxidation and dihydroxylation
agents). Being able to perform selective reactions on the three olefins is important in applying
these compounds to complex molecule synthesis. Therefore, we were interested in preparing
trienes in which the olefins are electronically differentiated. Although a number of different
strategies can be undertaken towards this goal, we became particularly interested in preparing
trienes possessing an electron rich enol-ether as the appending olefin. Preparing this class of
trienes was envisioned by cycloisomerization reaction of allenynes possessing a protected
alcohol group at the allylic position of the allene (Scheme 3.14). According to the proposed
mechanism of the reaction, β-hydride elimination in metallocycle 113 would have to occur from
a carbon possessing an oxygen substituent. The feasibility of this process has been demonstrated
by Trost 84 in the Ru(II)-catalyzed and Zhang 85 in the Rh(I)-catalyzed cycloisomerization of 1,5-
enynes to prepare five-membered rings possessing an appending enol ether.
Scheme 3.14 Proposed cycloisomerization to form enol-ether trienes.
PN
•
R
H
1
MeO2C OR2 PN
RhLn H
H
OR2 R
H 1
PN
MeO2C MeO2C R1 112 113 114
41
OR 2