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metallocycle can occur only from rotamer 73f-A (Scheme 4.20). The relative stereochemistry at
the C4 is then retained in the course of the reaction giving a single diastereomer 270f with the
relative stereochemistry predicted as shown. p Previously, chirality transfer in the Mo-mediated
allenic cyclocarbonylation reaction affording α-alkylidene cyclopentenones has been
observed. 180
Because the new Rh(I) catalytic conditions were optimized exclusively utilizing amino-
ester tethered allenynes, in was intriguing in whether their scope extends to other, less
functionalized substrates. To test this, tosyl-amide tethered allenyne was subjected to the in situ
prepared catalyst. It was found that the reaction of this substrate required heating to 50 °C, and
gave a 70% yield of the 4-alkylidene cyclopentenone 276 selectively (Scheme 4.21). This result
demonstrates that the selectivity observed in the cyclocarbonylation reaction of amino-ester
tethered substrates is not influenced by the quaternary center, and the conditions may be
applicable to a broader set of substrates.
Scheme 4.21 Cyclocarbonylation reaction of N-tosyl tethered allenyne 275.
TsN
•
275
C 6H 13
10 mol% [Rh(CO) 2Cl] 2
30 mol% PPh 3, 22 mol% AgBF 4
CO (1 atm), DCE, 50 o C, 70%
Finally, it should be noted that allenynes with terminally unsubstituted allenes are readily
transformed to 4-alkylidene cyclopentenones by using the standard catalytic conditions (5 mol %
[Rh(CO)2Cl]2, CO) because triene formation is not an option. For example, allenyne 74c gives
cyclopentenone 277 in 72% yield (Scheme 4.22).
p The stereochemistry of the starting allenyne is obtained in the Claisen rearrangement and is assigned in accordance
with the original results reported by Kazmaier (see Chapter 2; Kazmaier, U.; Görbitz, C. H. Synthesis 1996, 1489).
98
TsN
276
O
C 6H 13