Download (3249Kb) - D-Scholarship@Pitt - University of Pittsburgh

and 288b in 3 : 1 ratio. Notably, both reactions resulted in ~15% of the 4-alkylidene
cyclopenenone (289a, 289b) resulting from reaction with the distal double bond of the allene.
Scheme 4.30 Cyclocarbonylation reaction of phenylalanine derived allenynes.
R
R
R Mo(CO) 6, DMSO
toluene, 80-90 °C
Bz N
Bz N O Bz N O
MeO2C •
MeO2C MeO2C Bn H H
a
a
Bn Bn
yield
Bz
MeO 2C
74a R = Me 79% 287a 57% 288a 10% 289a 13%
74b R = H 94% 287b 55% 288b 20% 289b 15%
(287a + 288a) : 289a = 5.7 : 1, 287a : 288a = 6.4 : 1 a
(287b + 288b) : 289b = 5 : 1, 287b : 288b = 3 : 1 a
a The ratios were determined by integration of the olefin resonances in the 1 H NMR spectrum.
The relative stereochemistry of the major diastereomer 287b was unambiguously assigned by X-
ray crystallography, where the benzyl group and Ha are syn (Scheme 4.31) (for X-ray data see
Appendix D). The same relative stereochemistry was assigned to 287a, based on an X-ray
structure of a later intermediate (296, in Scheme 4.41, vide infra).
Scheme 4.31 X-ray crystal structure of 287b.
Bz N O
MeO2C Bn
Ha 287b
X-ray of 287b
The cyclocarbonylation reaction of allenynes 74c, 74d and 74e, possessing aliphatic side chains
was examined next (Scheme 4.32). The alanine-derived allenynes (R 1 = Me) 74c and 74d
afforded α-alkylidene cyclopentenones with dramatically increased selectivity (14 : 1), and high
diastereoselectivity (~10 : 1). The leucine derived allenyne 74e also reacted selectively with the
106
N
Bn
R
O