Formation of tricyclic compounds in domino reactions of alkenal oximes

5‐Alkenal oximes 4, 12 , and 13 were prepared by various methods. These oximes undergo a domino reaction with mercaptoacetaldehyde ( 1‐M ) affording tricyclic compounds 6, 17 and 18 , respectively. At first the mercapto group adds to the oxime function forming a hydroxyamino moiety which then condenses with the aldehyde group giving the cyclic nitrones 10, 15 , and 16 , resp., as intermediates. These nitrones undergo spontaneously an intramolecular cycloaddition yielding the tricyclic compounds ( 6, 17, 18 ). The regioselectivity of the latter reaction step is opposite to that observed for corresponding intermolecular nitrone cycloadditions. The domino reaction proceeds highly selectively affording only one of the diastereomers. Starting from enantiomerically pure oxime 13 , we prepared optically active 18 . The stereogenic center of 13 induces the formation of three further stereogenic centers in the product. The configuration at C‐5 of 18 was determined by a NOESY experiment.