Magnesium ene reactions

A modern variant is the intramolecular magnesium-ene reaction, e.g. the reaction of the alkene-allylic-Grignard compound 9 to give the five-membered ring product 10. This reaction proceeds regio- and stereoselectively, and is a key step in a synthesis of the sesquiterpenoid 6-protoilludene ... 

Oppolzer and Battig have prepared the marine sesquiterpene via ingenious application of iterative intramolecular magnesium-ene reactions . Aldehyde 672 was converted to the allylic chloride 673, the Grignard of which was heated at 60 °C for 23 hours and subsequently treated with acrolein to furnish alcohol 674 (Scheme LXX). An analogous sequence transformed 675 to 676 and set the stage for final transformations which were patterned after earlier work. 

Oppolzer and coworkers extensively utilized such intramolecular magnesium-ene reaction in the synthesis of complex natural products. For example, A -capnellene has been synthesized by using this intramolecular allylmagnesiations iteratively (Scheme... 

The magnesium-ene reaction provides a unique approach to remote functional groups. ... 

On the other hand, Intramolecular versions of the magnesium-ene reaction commonly proceed in high yield, with good regio- and stereoselectivity, and have furnished elegant syntheses of the carbon skeleton of natural products. Oppolzer [32] has classified intramolecular magnesium-ene reactions as type I, in which the enophile becomes linked to the terminal carbon atom of the magnesium-ene unit, or type II, in... 

Which of the products 6,7 and 8 is formally derived from a magnesium-ene reaction ... 

Zizaene sesquiterpenes were the subject of considerable synthetic interest in the 1970s and several synthetic strategies to construct the tricyclo[6.2.1.0 ]undecane skeleton involved an intramolecular diazoalkane-carbonyl ring expansion, an intramolecular magnesium-ene reaction, and a titanium-promoted reductive coupling. - ... 

Table 7 Intramolecular Zinc- and Magnesium-ene Reactions of Alkynes...

After these more or less isolated reports it was the challenge of natural product synthesis that spurred the most relevant exploration and extensions of the intramolecular magnesium-ene reaction. Its strategic role in synthesis is also illustrated by the following examples. 

Stoichiometric, intramolecular magnesium-ene reactions involving terminal or strained alkene eno-phile units have served extensively as a cornerstone in efficient syntheses of natural products. One of the reasons is the propensity of the cyclized Grignard intermediates to be trapped by a vast array of electrophiles. On the other hand, the use of Mg is incompatible with several functionalities such as certain heteroatoms in the acyclic precursor (zinc-ene cyclizations seem to surmount some of these limitations). 

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