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4.4 Mo(CO)6-Mediated Cyclocarbonylation Reaction for Preparation of α-Alkylidene
Cyclopentenones
Brummond and coworkers had previously shown that the Mo(CO)6 mediated
cyclocarbonylation reaction of 1,3-disubstituted allenes generally proceeds to give α-alkylidene
cyclopentenones as mixtures of E and Z isomers (see Scheme 4.11). 160 A similar trend was
anticipated with the amino-ester derived substrates. Indeed, subjection of allenyne 73a
containing a methyl group on the terminus of the allene to the standard reaction conditions (1.25
equiv. Mo(CO)6, 10 equiv. DMSO, toluene, 90 °C) resulted in a complex mixture of
cyclocarbonylation products without one particular major product according to 1 H NMR analysis
of the crude reaction mixture (presumably diastereomers and E/Z isomers of 274a and
diastereomers of the minor product 270a).
Scheme 4.29 Mo-mediated cyclocarbonylation reactions of 73a and 73b.
Bz N
Mo(CO) 6, DMSO
toluene, 90 °C
Bz N O
MeO2C Bn
•
77%
MeO2C Bn
H
Cbz N
MeO2C 73a
1.7 : 1 mixture of diastereomers
73f
•
Mo(CO) 6, DMSO
toluene, 90 °C
95%
274a
complex mixture of diastereomers
and E/Z isomers
Cbz N O
MeO2C H
274f
Z : E = 7 : 1
Cbz
MeO 2C
Bz
MeO 2C
Since the starting Bz-protected allenyne 73a was submitted to the reaction as a mixture of
diastereomers in 1.7 : 1 ratio, it was reasoned that each diastereomer reacts to give the product
274a as a mixture of diastereomers with respect to the relative stereochemistry of the two sp 3
104
N
N
Bn
270f
270a
O
O