Ketenes, diphenyl cycloaddition reactions

Two papers have appeared describing the cycloaddition reactions of ketens which, ultimately, lead to jS-lactams. In the first diphenyl keten is generated in situ, and by reaction with Schiff s bases affords fair yields of N-alkylazetidinones (69). In the second example,[2 + 2]cycloaddition of iminothiadiazoles (70) with ketenes gave spiro-substituted /3-lactams (71), which were readily desulphurized with Raney nickel or by reflux with DMF. 

When C=N groups are part of a heterocumulene system, as in ketenimines and carbodi-imides, they readily undergo [2+2] cycloaddition reactions with isocyanates. For example, C,C-diphenyl-A -p-tolylketenimine 138 reacts with phenyl isocyanate to give the four-membered ring cycloadduct 139 in 83 % yield. The cycloaddition proceeds exclusively across the C=C bond of the ketene imine. ... 

The product profile thus reveals impressive parallels with the reaction of diphenyl-ketene, the carbon analogue of 9, with (p-methoxybenzal)acetophenone, in which, again under thermal conditions, both cycloadditions and fragmentation of the four-membered ring product25) occur. Overall, the rate or rearrangement 7- 9 appears to be more favorable by the thermal route than by the photochemical pathway. 

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. 

Romo et al. have used Lewis acids to catalyze the formation of a-silyl-/ -lactones in their synthesis of potential inhibitors of yeast 3-hydroxy-3-methyl glutaryl-coenzyme A (HMG-CoA) synthase <1998BMC1255>. In addition to various Lewis acid catalysts, a chiral promoter based on the chiral diol (l/ ,2R)-2-[(diphenyl)hydroxymethyl]cyclo-hexan-l-ol was introduced to the reaction in an attempt to improve the stereoselectivity. A variety of chiral 2-oxetanones were formed, with enantioselectivities ranging from 22% to 85%. Dichlorotitanium-TADDOL catalysts 113 and 114 have also been used in an attempt to encourage the stereoselective [2+2] cycloaddition of silyl ketenes and aldehydes (TADDOL = (—)-/ra r-4,5-bis(diphenyl-hydroxymethyl)-2,2-dimethyl-l,3-dioxolane), although this method only afforded 2-oxetanones in moderate yields and optical purity (Equation 41) <1998TL2877>. 

Photochemical cycloaddition has been reported for the dione (248). Irradiation of this compound in the presence of cyclopentadiene affords the adduct (249). Similar adducts. (250) and (251) respectively, are formed when indene and cyclopentene are used as the alkenes. In contrast to the above when six membered ring alkenes such as cyclohexa-1,3-diene and dihydropyran are used cycloaddition occurs at the ring double bond to yield the adducts (252) and (253) respectively. Several examples of the thermal and/or photochemical addition of diphenyl ketene and p-tolyl diphenylketimine to heterocyclic 1,2-diones has been reported. One example of the reaction is that of the... 

Fig. 2.49. The [3 + 2] cycloaddition product formed from the reaction of [Re(0)3(PhTp)] with diphenyl ketene.

However, the equivalent of 49 is known from the reaction of diphenyl ketene with cyclopentadiene. The adduct 50, formed at low temperature, rearranges through [3,3] sigmatropic shift to form what appears to be the [2 + 2] adduct 51. The reaction of 1-methoxybutadiene with diphenyl ketene to form 52 at low temperature is akin to [2 + 2] reaction involving nc=c of the ketene. However, this species also rearranges further by [3,3] sigmatropic shift to form 53, which is akin to [4 + 2] cycloaddition of the butadiene with 7rc=o °f the ketene, just as in 49 [ 18]. 

The thermal and photochemical [4 + 2] cycloadditions of o-quinones with olefinic and acetylenic dienophiles have been extensively re viewed and include their 4ir heterodiene Diels-Alder reactions with olefins, " vinyl ethers,enamines, selected dienes, " diphenyl Iketenimines, ketenes, fulvenes, " and selected heterocycles including furan, " - benzofuran, - - indoles, azepines, and 1,2-diazepines. The tetrahalo-substituted o-quinones, tetrachloro- and tetrabromo-o-quinone, generally participate in heterodiene [4 + 2] cy-cloadditions at an increased rate over the unsubstituted systems and generally provide higher overall yields of the Diels-Alder products." With simple olefins, the dienophile geometry is maintained in the course of the thermal [4 + 2] cycloadditions [Eq. (52)J, ... 

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