Ketenes, cycloaddition reactions benzoquinones

Following the same strategy, further improvements in similar catalytic asymmetric [4 + 2] cycloaddition reaction have been made by Lectka group. The cyclic 1,4-benzoxazinones 3 (Scheme 10.4) that rely on the highly enantioselective [4 + 2] cycloaddition of o-benzoquinone imides with chiral ketene enolates were efficiently constructed, which can be derivatized in situ to provide a-amino acid derivatives in good to excellent yields and with virtual enantiopurity [9]. 

Scheme 15.13 [4 + 2] Cycloaddition reactions between ketene enolates and o-benzoquinone derivatives.

Cycloaddition of diphenyl ketene with benzoquinone diimides (44) gives quinoxalinones of type 45.45 Analogous additions of olefins lead to tetrahydroquinoxalines these reactions are classified as Diels-Alder additions with inverse electronic demand. 

The carbonyl ir-bond has been found to add chemo- and regio-selectively across the alkenic ir-bond of ketenes. Thus diphenylketene readily reacts with benzoquinone to yield a stable [2 + 2] adduct (equation 1). With an excess of diphenylketene the bis-adduct is formed, which decomposes into tetraphe-nylquinodimethane and carbon dioxide (equation 2). With the less stable ketene, thermal [2 + 2] cycloadditions are observed with highly electrophilic carbonyl compounds (equation 3). With unactivated aldehydes and ketones, yields are much lower due to a faster oligomerization of the ketene reagent. However, in the presence of a Lewis acid catalyst, most aldehydes or ketones form P-lactones with ketene (equation 4). Cycloadditions with ketones usually require more active catalysts than with aldehydes. The catalyzed reaction of ketene with methyl vinyl ketone is chemoselective, yielding a 10 1 ratio of [2 + 2] versus [4 + 2] adducts (equation 5). In the absence of catalyst, methyl vinyl ketone reacts with ketene to give exclusively the [4 + 2] adduct. 

The reactions of ketene acetals with Schiff bases derived from (S)-valine esters 1.59 (R = /-Pr) under TiCl4 catalysis are highly selective, but the chiral auxiliary has not been removed [264], Engler and coworkers [1532] performed the asymmetric [2+2]-cycloaddition of 2-methoxy-l,4-benzoquinone with 1-arylpro-penes substituted by electron-donating groups. These reactions occur at -78°C when catalyzed by chiral titanium complexes derived from 2.50 (R = Me, Ar = R = Ph). Cycloadducts are obtained with a good selectivity (Figure 9.6). 

Other cycloadditions. A [3+2]-cycloaddition involving a benzoquinone and an alkene to give 2,3-dihydrobenzofuran derivatives, and an intramolecular [4+3]-cycloaddition to provide functionalized polycyclic compounds, are further demonstrations of the utility of LiClQj-OEtj. The reaction of aromatic or a,p-unsaturated aldehydes with acid chlorides proceeds via ketenes and then 2-oxetanones. ... 

---