Ketenes, vinyl cycloaddition

Dihydropyrans have been produced by the 1,3 cycloaddition of methyl vinyl ketone (77) or acrolein (29-J7) with enamines (see Section II.A.2). S-Lactones have been formed as a side product in the reaction of dimethyl ketene with enamines (77), and as the primary products in the reaction of excess ketene with enamines derived from ketones (75) (see Section II.A.4). 

Vinyl ethers undergo many cycloaddition reactions similar to those which take place with enamines. In general, however, these cycloaddition reactions with vinyl ethers take place less readily than those with enamines. These reactions include cycloaddition of vinyl ethers with ketene (200-205), phenyl isocyanate (206), sulfene (207,208), methyl acrylate (209), diethyl acetylenedicarboxylate (210), and diphenylnitrilimine (183). 

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

The dimerization of the parent ketene gives the P-lactone. One molecule of ketene reacts across the C=C bond as a donor and the other molecule reacts across the C=0 bond as an acceptor. This is similar to the concerted [2+2] cycloaddition reaction between bis(trifluoromethyl)ketene and ethyl vinyl ether to afford the oxetane (Scheme 26) [127], A lone pair on the carbonyl oxygen in the ketene molecule as a donor activates the C=C bond as the alkoxy group in vinyl ether. 

The reaction course is shown in Scheme 4. Enyne 12 reacts with 2 to give vinyl carbene complex 17, which is in a state of equilibrium with vinyl ketene complex 21. [2+2] Cycloaddition of the ketene moiety and alkene part in 21 gives cyclob-utanone 22. On the other hand, the vinyl carbene complex 17 reacts with the alkene intramolecularly to produce metalacyclobutane 18. From metalacyclob-utane 18, reductive elimination occurs to give cyclopropane derivative 23. Ret-... 

This Asian species is a major agricultural pest. The pheromone has been proposed to consist of three male-specific compounds, only one of which, (Z)-exo-a-bergamotenal 150, has been reported in the literature [114]. The racemic compound was synthesized starting from farnesoic acid chloride 146 (Scheme 25) [114]. Thus, the vinyl ketene prepared from acid chloride 146 underwent 2+2 cycloaddition to give bicyclic ketone 147. The ketone function was removed by reaction with hydrazine followed by treatment of the resulting hy-... 

Among the most commonly applied chiral moiety for nitrones (2) is the N-a-methylbenzyl substituent (Scheme 12.6) (18-25). The nitrones 8 with this substituent are available from 1 -phenethylamine, and the substituent has the advantage that it can be removed from the resulting isoxazolidine products 9 by hydrogeno-lysis. This type of 1,3-dipole has been applied in numerous 1,3-dipolar cycloadditions with alkenes such as styrenes (21,23), allyl alcohol (24), vinyl acetate (20), crotonates (22,25), and in a recent report with ketene acetals (26) for the synthesis of natural products. Reviewing these reactions shows that the a-methylbenzyl group... 

The other major approach to systems of this type is cycloaddition of cumulene ylides with vinyl isocyanate and vinyl isothiocyanate. Thus, Ph3P=C=C=0 reacts with these two reagents to give 160 and 161 respectively while the corresponding reactions of Ph3P=C=C=NPh afford 162 and 163 <88T543>. By using styryl isothiocyanate with the ketene ylide, 164 was obtained. 

Diels-Alder reactions have featured heavily during the period of review. Ar-Vinyl-2-oxazolidinone has been reported as a dienophile for the first time, including the preparation of various tetrahydro-277,77/-pyrano[4,3-. ]pyrans <2002SL952>. The heterodiene cycloaddition reaction of 3-formylchromone with a series of ketene acetals formed from C2-symmetric l,2-diarylethane-l,2-diols is completely diastereoselective (Scheme 40) <1995J(P1)2293>. 

Ketene reactivity in intramolecular cycloadditions parallel those in intermolecular reactions in which chloro-, vinyl-, aryl- and alkoxyketenes are more reactive than the alkylketenes. In most instances the ketene is generated by amine dehydrohalogenation of an acid chloride. There are, however, a few examples of ketenes prepared along less conventional routes as by the examples for the formation of 11.150 12,151 and 13.151... 

Triazines are generally more reactive in [2 + 4] cycloaddition in comparison with 1,2,3-tria-zines. The wide variety of dienophiles can be employed enamines, enaminones, vinyl silyl ethers, vinyl thioethers, cyclic ketene jV,O-acetals, /V-phenylmaleimide, 6-dimethylaminopentafulvene, 2-alkylidene-imidazolidines (cychc ketene aminals), cyclic vinyl ethers, arynes, benzocyclopropene, acetylenes, and alkenes like ethylene, (Z)-but-2-ene, cyclopentene, cyclooctene and bicyclo[2.2.1]hept-2-ene, hexa-1,5-diene, cycloocta-1,5-diene, diallyl ether, cyclododeca-l,5,9-triene,... 

A number of cycloadditions of imines or imino compounds with a variety of alkenes, including allenes (88HCA1025), vinyl ethers (88TL547), methyl acrylate (86ZOR636), ketene acetals (87JOC365) and electrophilic alkenes (85T1953), afford functionalized azetidines. 

Since stepwise reactions are not subject to the rules of pericyclic reactions, they are often invoked to explain how reactions in which the rules have been subverted take place. However, there is a small group of thermal [2+2] cycloadditions that seem to be disobeying the rules, and yet may well be pericyclic. One is the reaction of ketenes with electron-rich alkenes, illustrated by the reaction of diphenylketene 2.167 with ethyl vinyl ether 2.166 giving the cyclobutanone 2.168. Another is a group of electrophile-... 

A stereocontrolled Staudinger cycloaddition reaction has been reported to be performed on vinylketenes, possessing a y-heteroatom, and imines to produce frans-vinyl-(3-lactams [112]. The vinyl side chain adopted stereoselectively the (Z) configuration in the transition state, stabilizing the vinyl ketene and leading, exclusively, to the frans-3-vinyl-(3-lactam (Scheme 37). 

The palladium-catalyzed carbonylation reaction of allyl diethyl phosphate, in the presence of imines, gives either cis- or rraws-3-vinyl-/J-lactams, in high yields and in a stereoselective fashion (equation 160)574,575. The reaction is a [2 + 2] cycloaddition process which occurs under simple and mild conditions and has significantly more potential than the reaction of imines with ketenes (due to the more forcing conditions that are usually required to form the ketene intermediates). This reaction, however, only proceeds in low yield if the allyl phosphate is replaced with allyl acetate576. 

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