Magnesium compounds cycloaddition

The catalyzed cycloaddition of alkoxy or amino aldehydes has been the subject of extensive studies. The influence of different Lewis acids and protective groups on the diastereoselectivity has been investigated for various types of aldehydes and dienes. Induced and simple diastereoselectivities (endoiexo) in the [4 + 2] cycloaddition of aldehydes 1 are highly dependent on the Lewis acid applied as catalyst. Several reactions ofa-alkoxyaldehydes 1 to dienes 2 under Lewis acid catalysis have been performed to give adducts 3 and 435. Among the catalysts tested (see table below) the best results were achieved for the magnesium bromide catalyzed cycloaddition of 1 to several dienes. Adduct 3 was obtained as a single compound. 

Optically active 3-amino-2-hydroxycarboxylic acid derivatives are often key components of medicinally important compounds. The synthesis of isopropyl (2i ,35)-3-amino-4-cyclo-hexyl-2-hydroxybutyrate (126) (Scheme 28) takes advantage of a [2 + 2]-cycloaddition reaction of the chiral imines 123, prepared from 63, to assemble the important diastereomeric azetidinone 124 as the crucial precursor for completion of this novel synthesis. Protection of the hydroxy group of 63 as either the TBS ether 119 or the tert-buty ether 120, followed by a DIBAL reduction at —78 °C, produces smoothly one of the aldehydes 121 or 122. Condensation of these aldehydes with either di-p-anisylmethylamine or benzylamine in the presence of anhydrous magnesium sulfate affords the four possible chiral imines 123a—d (Scheme 26). 

Phenylperhydro-l,3,4-oxadiazin-2-ones 151 react with aliphatic or aromatic aldehydes or ethyl 2-oxoacetate to give the intermediates 152. The azomethine imine ylides 152 yield primary oxazolidines 153 <200081170, 2002S1885, 2004TL3127>. These compounds can be synthesized also by treatment of 151 with ethyl oxoacetate or aldehydes in the presence of magnesium bromide etherate. The tandem cycloreversion-cycloaddition of 153 with various electron-poor dipolarophiles then leads to pyrazolidines 154-156 (Scheme 22). 

The styrylpyrilium salt derivatives (130) undergo (2 + 2)-cycloaddition to afford the corresponding dimers. The magnetic properties of the radical cations formed from these dimers were compared with those formed from (130). Styryl dyes of the type shown as (131) undergo E-Z isomerization on irradiation at 436 nm. The dyes align themselves in the pattern shown in (132, where the filled blob represents the crown ether complex). These undergo dimerization on irradiation to afford compounds (133), from which the magnesium ions can be removed. 

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