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stereocenters and mixture of E/Z isomers of the alkylidene olefin, further complicating the
characterization. Therefore the Cbz-protected allenyne 73f as single diastereomer was tested next
(Scheme 4.29). The cyclocarbonylation reaction of 73f with Mo(CO)6 provided α-alkylidene
cyclopentenone 274f with ~20 : 1 selectivity over the 4-alkylidene cyclopentenone 270f
(determined by 1 H NMR and HPLC/UV). Cyclopentenone 274f was present as a mixture of
mainly two isomers in ~7 : 1 ratio which were separated by normal phase HPLC. The major
isomer was assigned Z-geometry of the alkylidene olefin based on the 1 H NMR signal appearing
at 5.98 (q, J = 7.3 Hz, 1H) while the minor diastereomer was assigned E-geometry since the
olefin signal appeared further downfield at 6.65 (q, J = 7.2 Hz, 1H). Nevertheless, obtaining the
products as mixture of isomers was disappointing particularly since separation of the products
was possible only by HPLC.
To eliminate the formation of E and Z isomers and focus on determining the
diastereoselectivity of the reaction, our subsequent investigations were focused on the reaction of
monosubstituted allenes. When phenylalanine derived allenynes 74a (R = Me) and 74b (R=H)
were submitted to the standard cyclocarbonylation conditions (1.25 equiv. Mo(CO)6, 10 equiv.
DMSO, toluene, 90 °C), the reaction proceeded to give mixtures of products (Scheme 4.30). The
major diastereomer 287a (57%) could be easily separated by column chromatography from 288a
and 289a. Likewise, 287b (55%) was also chromatographed away from 288b and 289b.
Unfortunately, 288a and 289a could not be separated; likewise, 288b and 289b also coeluted.
The composition of the reaction mixture was determined by integration of the olefinic
resonances in the 1 H NMR spectrum of the crude reaction mixture (Scheme 4.30). s Allenyne 74a
gave the α-alkylidene cyclopentenones 287a and 288a in 6.4 : 1 ratio, whereas 74b gave 287b
s These ratios are averages and showed reasonable variation in different experiments (±10%). The ratios in all
subsequent reactions were determined by integration of the olefinic resonances in the 1 H NMR spectrum.
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