1-aryl-enolethers

An unusual 4-exo-trig cyclization accompanied by a later rearrangement was observed with aryl enolether 13 (Scheme 6) [48], The benzoxetane 14, which is formed in the initial cyclization step, undergoes ring-opening to form the phenoxy radical 15. 5-Endo cyclization of 15 and reduction of the resulting benzylic radical leads to the spirocycle 16. Diverse polycyclic products have also recently been obtained by intramolecular aryl radical cyclizations to tetrahydropyridines [49]. 

In model studies directed towards the synthesis of ( )-gelsemine, 5-exo-trig cyclization of an aryl radical, derived from the vinylogous uretane 58, onto a methoxymethyl enolether resulted in partial fragmentation of the intermediate... 

Cycloadditions involving ketene derivatives as one or both reaction partners are assumed to be rare examples of concerted [tTj - - n cycloadditions [81]. The activation volumes determined for the [2 + 2] cyclodimerization of diphenylketene [82] and the [2 + 2] cycloadditions of diphenylketene to various enolethers [83] gave values of —30 cm mol and —22 to -52 cm mol , respectively, and were highly negative. Thus, the effect of pressure leads to a powerful acceleration of these [2 + 2] cycloadditions comparable to that with Diels-Alder reactions, a characteristic which is usefiil for synthetic purposes. For example, various /1-lactams can be easily synthesized by pressure-induced [2 + 2] cycloaddition of alkyl and aryl isocyanates and enolethers [84]. 

Isoquinolinium salts represent 2-aza-1,3-butadienes activated by A-arylation. Highly stereoselective cycloadditions with chiral enolethers have been reported leading to enantiomerically pure functionalized tetralines in high yields. 

The enantioselective a-enolation of aldehydes reported by MacMillan and co-workers [136] proceeded well with various ir-rich silyl enolethers containing aUtyl, vinyl, or aryl moieties without loss of reaction efficiency and enantiocontrol. The incorporation of bulkier silyl group (TBS) not only increased substrate stability versus hydrolysis, but also led to increased enantioselectivity. Furthermore, inter-molecular enolation rather than intramolecular enolation was observed as preferred. 

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