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( 77 ) could be isolated in 62% yield by column chromatography. Subsequent reaction of
this compound with lithium aluminum hydride produced the diol 78 (plus bornane-2,10-
sultam), both in essentially quantitative yields. Selective protection of the primary alcohol
and oxidation (pyridinium dichromate, PDC) of the secondary gave access to the tricyclic
ketone 79 (62% overall yield), which by heating in o -dichlorobenzene underwent the
expected retro-Diels-Alder reaction. The 4-substituted cyclopentenone 80 thus obtained
showed an enantiomeric excess greater than 96%.
O
Me
Me
O
SiMe 3
O
H
77
O
N
O 2 S
H
H
Zn, 1:1 AcOH/H 2 O,
rt, 1 h; SiO 2 chromat.
62% (19% of epimer)
N
H
H
O 2 S
Me
Me
75b
Me
LiAlH 4, Et 2 O-THF,
0 °C, 1 h, 97%
HN
O 2 S
Me
(100%)
O
O H
TBDMSCl, imidazole,
DMF, rt, 14 h, 66%;
79
78
OTBDMS
OH
H
H
PDC, DMF, rt,
1 h, 94%
H
H
H
H
o -dichlorobenzene, reflux, 24 h (89%)
O
80
OTBDMS
H
Scheme 5.55
The origin of the essentially complete diastereoselectivity observed in the intermolec-
ular PKRs was subsequently studied by a combination of experimental and theoretical
techniques. 55
Semi-empirical PM3(tm) calculations showed that dicobalt hexacarbonyl complexes of
N -(2-alkynoyl)sultams 74 had only two geometrically very similar stable conformations;
the circular dichroism spectra of these complexes gave support to these calculations. ADFT
study of the intermediates in the PKR of the complex derived from 74b with norbornadiene
showed that an oxygen atom of the sultam auxiliary can selectively chelate one of the cobalt
atoms (although the intermediate pentacarbonyl complex is too reactive to be isolated), and
that coordination of the olefin to the same cobalt from the exo -face and with a syn orientation
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