Facile synthesis of 7-dimethylamino-endo-tricyclo[5.2.2.0^1,6]-undec-10-en-9-ones

Pentacarbonyl[(2E)-3-cyclohexenyl-3-dimethylamino-1-ethoxy-2-propen-1- ylidene]chromium (2) yields 7-dimethylamino-endo-tricyclo[5.2.2.0^1,6]undec-10-en-9-ones 5 (15-88%, 12 examples) upon treatment with alkynes 3 in pyridine, most probably by a 6π-electrocyclization followed by a subsequent reductive elimination/intermolecular Diels-Alder reaction with the alkyne and eventual hydrolysis. The direct cyclization of 2 to the cyclohexane-annelated cyclopentadiene 6 is faster than the usual alkyne insertion of α,β-unsaturated Fischer carbene complexes. The [4+2] cycloadditions of 7 and the added alkynes 3 occur with high degrees of regio- and complete diastereoselectivity.