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cyclopentenones 287b, 288b and the isomer 290b. The calculations indicated that the
transformation of 288b to 290b is favorable by ca. 1.9 kcal/mol, whereas 287b and 290b are
nearly isoenergetic (Scheme 4.36). 193 Therefore, these calculations support the observed results.
Scheme 4.36 Ab initio energy calculations of α-methylene cyclopentenones 287b, 288b and
290b.
Bz N O
MeO2C Bn
H
Bz N O
MeO2C Bn
H
Bz N O
MeO2C Bn
287b -26.0 kcal/mol 288b -23.8 kcal/mol 290b -25.9 kcal/mol
To determine whether the diastereoselectivity in the cyclocarbonylation reaction of allenynes
with aromatic side chains could be increased, we tested four other substrates containing a p-
OMe-phenyl (74f), p-F-phenyl (74g), 2-thienyl (74h) and 3-N-Boc-indolyl (74i) moiety. These
were chosen to provide an electron donating, electron withdrawing and two heterocyclic groups
respectively. Interestingly, the cyclocarbonylation reaction of all four substrates proceeded with
nearly the same diastereoselectivity (2-3 : 1) as the phenylalanine case giving the product with
the aromatic group and Ha in syn orientation as the major diastereomers (Scheme 4.37).
Scheme 4.37 Cyclocarbonylation reaction of allenynes 74f-i.
Bz N
MeO2C Ar
•
Mo(CO) 6, DMSO,
toluene, 80-90 °C Bz N O
MeO2C Ar
Ha 74f 287f Ar = p-OMe-Ph 42% dr = 2.3 : 1 a
74g 287g Ar = p-F-Ph 40% dr = 2 : 1 b
74h 287h Ar = 2-thienyl 39% dr = 2 : 1 a
74i 287i Ar = 3-N-Boc-indolyl 45%, dr = 2.7 : 1 c
a yield of mixture 95% b total yield of mixture 76%, yield of major diastereomer
determined by NMR. c total yield of mixture 77%, yield of major diastereomer
determined by NMR.
From these results, it was concluded that opposite diastereomers are favored in the
cyclocarbonylation reaction of allenynes with aliphatic vs. aromatic side chains. It is speculated
110