Hydride ring-closing reactions

The Bronsted acid (274) catalysed asymmetric tandem 1,5-hydride transfer/ring closing reaction of o-aminobenzo-a-ketoesters (271) with anilines (272), gave cyclic aminals (273) in fairly good diastereo- and enantioselectivities (Scheme 70). ... 

It is thought that the above reactions proceed by abstraction of a benzylic hydride ion by the quinone. Proton loss subsequently occurs from the carbocation (106) leading to the o-quinoneallide which ring-closes to the chromene. 

In the case of synthetic studies on mitomycins (e.g. mitomycin A), both techniques were applied to synthesize model compounds. A retro aldol reaction, followed by a trivial transannular reaction, was performed in the same pot and under the same conditions (sodium hydride) [79] [80], Scheme VIII/25. Thus, compound VIII/132 gave the ring closed intermediate, VIII/133, which by a retro aldol reaction, yielded the eight-membered intermediate, VIII/134. A transannular reaction in VIII/134 gave the acid labile VIII/135, which led finally to the indole derivative, VIII/136. 

Carbopalladation of dienes yields 7r-allylpalladium intennediates capable of capturing nucleophiles. An example of this type of reaction in natural product synthesis is the conversion of the aryl iodide 168 into pentacycle 169. After 6-exo carbopalladation the free phenol is captured by the resultant Tr-allylpaUadium intermediate to yield aUyUc ether 169 en route to (-)-morphine 120. The domino reaction closes a srx-membered ring, forges a quaternary center, and forms a five-membered furan ring. The reaction sequence has also been effectively reversed in a construction of (+)-y-lycorane 172. Treatment of aUylic benzoate 170 with a palladium catalyst in the presence of sodium hydride triggers tt-allylpalladium formation and intramolecular amide capture. rr-Allylpalladium formation occurs with inversion, as does reaction with the pendant amide, resulting in a net retentive 1,3-transposition of stereochemical information. Subsequent addition of base and thermolysis induces 6-exo intramolecular carbopalladation to yield alkene 171. 

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