Carbon reaction with cyclopropenones

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

The reaction with optically active hydrazones provided an access to optically active ketones. The butylzinc aza-enolate generated from the hydrazone 449 (derived from 4-heptanone and (,S )-1 -amino-2-(methoxymethyl)pyrrolidine (SAMP)) reacted with the cyclopropenone ketal 78 and led to 450 after hydrolysis. The reaction proceeded with 100% of 1,2-diastereoselectivity at the newly formed carbon—carbon bond (mutual diastereo-selection) and 78% of substrate-induced diastereoselectivity (with respect to the chiral induction from the SAMP hydrazone). The latter level of diastereoselection was improved to 87% by the use of the ZnCl enolate derived from 449, at the expense of a slight decrease in yield. Finally, the resulting cyclopropanone ketal 450 could be transformed to the polyfunctional open-chain dicarbonyl compound 451 by removal of the hydrazone moiety and oxymercuration of the three-membered ring (equation 192). 

Cydopropenone ketals, of which cyclopropenone 1,3-propanediol ketal (1) is a representative and unusually stable example, have proven to be useful equivalents of the 1,3-dipole (1) 1n a regiospeclfic three-carbon + two-carbon cycloaddition with electron-deficient olefins, (eq 1). Table I shows representative results of a study of this reaction.7... 

The reaction of amines with cyclopropenones appears to be extremely sensitive to the reaction conditions Ammonia reacts with diphenylcyclopropenone at room temperature to yield the isomeric /3-lactams 56 and 57, whereas the vinyl aldehyde 58 is produced at — 33°C. These results can be rationalized in terms of the intermediate 59 which would result from conjugate addition of the nucleophile at carbon (equation 56). 

Cyclopropenyl imminium salts can be prepared by the reaction of cyclopropenones with substituted ammonium saltsor by the reaction of ethoxycyclopropenyl salts with amines The ease of rotation about the carbon-nitrogen double bond shows that the positive charge is not located solely on the nitrogen as in 67 but is shared with the cyclopropene ring as in 67a . 

Condensation reactions of 3,3-dichlorocyclopropenes [176] or of ethoxycyclopropenium salts [177] with metal complexes of 6-diketones have produced triafulvenes with two acyl groups on the exocyclic carbon atom and reactions of cyclopropenones with malononitrile in the presence of acetic anhydride have provided dicyanotriafulvenes CXIV) [119,178-180] ... 

Reaction of cyclopropenones with carbon-nitrogen double bonds... 

The reaction of the ethynylestradiol derivative (50) with difluorocarbene represents an equally fascinating series of reactions. The major product obtained in 48% yield is the difluorocyclopropene (51). The latter is hydrolyzed to the cyclopropenone (53), and is readily alkylated at C-21 to (54), which is also obtained by difluoromethylation of the propynyl derivative (57). The cyclopropenone (53) loses carbon monoxide at elevated temperature to... 

To carry out tethered [3-i-2]-cycloadditions Nakamura et al. developed the new annulating reagents 123 [109, 110]. The reagents carry, in one molecule, two cyclopropenone acetals that are connected with an n-carbon methylene tether. Upon thermolysis in the presence of Cjq, the reagent undergoes [3-i-2]-cycloaddition reaction twice in a regio- and stereoselective manner to give Q- and C2-symmetrical bisadducts with cis-l - and cis-3-addition patterns, respectively. The selectivity varies... 

Bis(oxazoline) ligands have been shown to be useful in the many carbon-carbon bond-forming reactions previously listed. They have also been used in a myriad of other carbon-carbon bond-forming reactions. For example, Nakamura and coworkers used bis(oxazoline) ligands ent-2, ent-22, and ent-39 in ligand-induced enantioselective allylzincation. This reaction consisted of the transformation of the cyclopropenone acetal 198 into allylic cyclopropanone acetal 199 in yields ranging from 73 to 90% with selectivities from >98 2 for the isomer shown to 1 99 (Fig. 9.57). 

Reaction of the pyrones 4 and 5 with the cyclopropenone acetal 6 gave cycloheptatrienes 7 and 8, respectively, by a similar sequence of cycloaddition followed by elimination of carbon dioxide. 

---