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temperatures. The reaction conditions were also tolerant of a phenyl-substituted alkyne (entries 3
and 7) giving α-phenyl-cyclopentenones 270d and 270i in ca. 75% yield. The reaction of serine
derived allenyne 73e proceeded to give cyclopentenone 270e in 81% yield. In nearly all cases,
filtration of the reaction mixture over a short column of silica gel and eluting with
EtOAc/hexanes provides the product with satisfactory purity for characterization (the rhodium
catalyst shows high affinity for silica gel and does not elute with these solvents).
Table 4.3 Preparation of 4-alkylidene cyclopentenones via a Rh(I)-catalyzed allenic
cyclocarbonylation reaction.
PN
MeO 2C
R 1
73b-j
•
R 3
R 2
H
10 mol % [Rh(CO) 2Cl] 2
30 mol% PPh 3, 22 mol % AgBF 4
DCE, rt - 40 o C, 1h
P
N
MeO2C R1 entry/allenyne P R 1 R 2 R 3 product yield% entry/allenyne P R 1 R 2 R 3 product yield%
1/73b Bz Bn Me H 270b 32 5/73f Cbz Me Me Me 270f 75
2/73c Bz Bn Me TMS 270c 98 6/ 73h Cbz Me Me TMS 270h 78
3/73d Bz Bn Me Ph 270d 74 7/73i Cbz Me Me Ph 270i 75
4/73e Bz -CH 2OTBS Me Me 270e 81 8/73j Cbz Me i-Pr Me 270j 74
Note: the products in entries 2, 3 and 4 were obtained as mixtures of diastereomers in ~1.7 : 1 ratio.
To test whether the newly developed catalytic reaction conditions are compatible with terminal
alkynes, allenyne 73b was subjected to the reaction conditions. The reaction resulted in a
complex mixture of unidentified products with the desired 4-alkylidene cyclopentenone 270b
isolated in only 32% yield (entry 1). This observation is attributed to competing reaction
pathways resulting from oxidative insertion of the Rh(I)-catalyst into the acetylenic C-H bond,
which is well precedented. 179 Finally, reaction of 73j with an isopropyl group on the allene
terminus afforded 74% yield of the cyclopentenone 270j demonstrating that increase in steric
bulk at this position does not affect the selectivity (entry 8).
96
270b-j
R 3
R 2
O