Ketenes with aromatic rings

The previous sections have dealt with stable C=N-I- functionality in aromatic rings as simple salts. Another class of iminium salt reactions can be found where the iminium salt is only an intermediate. The purpose of this section is to point out these reactions even though they do not show any striking differences in their reactivity from stable iminium salts. Such intermediates arise from a-chloroamines (133-135), isomerization of oxazolidines (136), reduction of a-aminoketones by the Clemmensen method (137-139), reductive alkylation by the Leuckart-Wallach (140-141) or Clarke-Eschweiler reaction (142), mercuric acetate oxidation of amines (46,93), and in reactions such as ketene with enamines (143). 

An acidic solution of 2,4-dinitrophenylhydrazine reacts with N-p-chlorophenyl-sulfonyl-3-ethoxy-l,2-thiazetidine 1-oxide to give (80%) the bis-2,4-dinitrophenyl-hydrazone of glyoxal. The adduct of A-sulfinyl-p-chlorophenylsulfonamide with dihydropyran is inert to catalytic hydrogenation and bromination. Treatment of two l,2-thiazetidine-3-one 1-oxides (e.g., 421) with hydriodic acid results in ring-cleavage and loss of sulfur. They are not oxidized to 1,1-dioxides by peracetic acid, ° but m-chloroperbenzoic acid accomplishes this oxidation. The unstable adducts with ketene decompose to amides with loss of hydrogen sulfide and sulfur dioxide or may be trapped by reaction with aromatic amines as shown for thiazetidine 1-oxide 422.An aldol-type condensation has been reported for A -cyclohexyl-1,2 thiazetidine-3-one 1-oxide and p-(A(A"-dimethylamino)benz-aldehyde. " Sulfur monoxide is lost in the flash-vacuum thermolysis of 422a. ... 

The linear ionic intermediate can also be intercepted with either one of the reagents, for example, in the cycloaddition reaction of ketenes with aliphatic imines (Chapter 4, Section 4.1.4.2). Sometimes [2+2+2] cycloadducts resulting from the reaction of the initially formed linear adduct with either one of the reagents are observed. The basicity of the imine plays a role because aliphatic imines react in this manner, while aromatic imines produce the four-membered ring [2+2] cycloadducts... 

In the first step, catalyst 64c attacks ketene 66 to form a zwitterionic enolate 71, followed by Mannich-type reaction with imine 76 (Fig. 40). A subsequent intramolecular acylation expels the catalyst under formation of the four-membered ring. Utilizing 10 mol% of 64c, N-Ts substituted (3-lactams 77 were prepared from symmetrically as well as unsymmetrically substituted ketenes 66, mainly, but not exclusively, with nonenolizable imines 76 as reaction partners [96]. Diastereos-electivities ranged from 8 1 to 15 1, yields from 76 to 97%, and enantioselectivities from 81 to 94% ee in the case of aliphatic ketenes 66 or 89 to 98% ee for ketenes bearing an aromatic substituent. Applying complexes 65 or the more bulky and less electron-rich 64b, ee values below 5% were obtained. 

Azido-l-methylbenzimidazole with diphenylketene gave the imidazobenzimidazole (232 R = Ph), but decomposition of the azide to give a nitrene, followed by cycloaddition to the ketene, was ruled out as a mechanism. Instead, the authors propose that 232 is formed by attack of the endocyclic ring nitrogen on the C=C bond of the ketene followed by nucleophilic displacement of N2 from the azide.168 Preparation of an analog (232, R = H) would constitute a possible route to an aromatic azapentalene 233. 

The photochemical rearrangements of 2,4-cyclohexadienones are of three types (1) ring fission to a cis-diene-ketene ( ) expulsion of a heteroatom from the 6-position with concomitant aromatization of the... 

Cyclopropylidene-l,3-dioxanes or -1,3-dioxolanes are ketene acetals and examples of bis-donor-substituted methylenecyclopropanes which undergo facile cycloaddition with electron-deficient alkenes 12. Thus, spiro[2.3]pentanone acetals 13 were obtained after reaction in an aromatic solvent either at room temperature or 40 C. Upon subsequent aqueous workup, hydrolytic cleavage of a C-C bond of the cyclobutane ring took place rather than the expected acetal hydrolysis (Table 7). This cycloaddition was found to proceed in a stepwise fashion as reaction of the dimethyl ( )-but-2-enedioate gave a cisj tram mixture of cycloadducts on further heating, this ratio changed. 

It can be taken as a measnre of the low intrinsic aromaticity associated with fused pyrone rings, that 3-acyl-chromones undergo hetero-Diels-Alder additions with enol ethers," and ketene acetals, 3-formylchromone reacting the most readily. 

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