Simple unactivated imines react with hydrocarbon dienes only with the help of a Lewis acid; however, both electron-rich and electron-poor dienes react with unactivated imines when heated. Vinylketenes, for instance, afford dihydropyridones upon [4+2] cycloaddition with imines. Regio- and stereoselectivity are unusually high in reactions of this class of dienes. (10) Vinylallenes react similarly in the presence of a Lewis acid, often with high diastereoselectivity. (11) Synthetic applications The IDA reaction has been applied to the synthesis of a number of alkaloid natural products. In this example, Danishefsky's diene is used to form a six-membered ring en route to phyllanthine. (12) Comparison with other methods Several other methods can access the 1,2,5,6-tetrahydropyridine ring system afforded by IDA reactions. Partial reduction of pyridinium salts has been used, although regioselectivity issues arise when substituted pyridiniums are used. (13) A modified Ireland-Claisen rearrangement leads to tetrahydropyridines via a silyl ketene acetal intermediate. (14) Ring-closing olefin metathesis has also been used to establish the tetrahydropyridine ring system. (15) See also Oxo Diels–Alder reaction Aza-Diels–Alder reaction References Name reactions Cycloadditions