SYNTHESIS OF THE SPATANE NUCLEUS (CIS-ANTI-CIS-TRICYCLO[5.3.0.0(2,6)]DECANE) USING THE PAUSON-KHAND REACTION WITH A REMARKABLE REVERSAL IN REGIOSELECTIVITY
Ba. Kowalczyk et al., SYNTHESIS OF THE SPATANE NUCLEUS (CIS-ANTI-CIS-TRICYCLO[5.3.0.0(2,6)]DECANE) USING THE PAUSON-KHAND REACTION WITH A REMARKABLE REVERSAL IN REGIOSELECTIVITY, Journal of organic chemistry, 63(5), 1998, pp. 1379-1389
The Pauson-Khand reaction of cyclobutenes 3-5 with a variety of acetyl
enes yielded cis-anti cis-tricyclo[5.3.0.0(2,6)] decanes 9-17. The unw
anted regioisomers 9a and 10a were the sole products using acetylene,
but there was a remarkable reversal in orientation of the cyclobutene
component yielding the desired regioisomer 13b upon using (trimethylsi
lyl)acetylene. The importance of the allylic methyl group in the cyclo
butenes in directing the regiochemical outcome was substantiated by th
e lack of selectivity in Pauson-Khand reactions of desmethylcyclobuten
e 5 with acetylene and (trimethylsilyl)acetylene. The relative unimpor
tance of electronic control of regiochemistry was concluded from the c
onsistent ratio of Pauson-Khand reaction products from norbornenone 22
with various acetylenes. A hypothesis rationalizing the regiochemical
outcome was based on steric interactions of the allylic methyl roup f
rom the cyclobutene component with either the smaller acetylene substi
tuent or the CO ligands on the cobalt. This steric interaction was fur
ther hypothesized to be influenced by the larger acetylene substituent
sterically crowding the CO ligands on the cobalt.