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

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.