Cyclopropenecarboxylates

Semiempirical calculations on the Favorskii rearrangement of a-chlorocyclobutan-one to cyclopropenecarboxylic acid suggest that it proceeds via a stepwise semibenzilic acid pathway, both in solution and in vacuo, rather than by a cyclopropanone... 

Functionalized cyclopropenes are viable synthetic intermediates whose applications [99.100] extend to a wide variety of carbocyclic and heterocyclic systems. However, advances in the synthesis of cyclopropenes, particularly through Rh(II) carboxylate—catalyzed decomposition of diazo esters in the presence of alkynes [100-102], has made available an array of stable 3-cyclopropenecarboxylate esters. Previously, copper catalysts provided low to moderate yields of cyclopropenes in reactions of diazo esters with disubstituted acetylenes [103], but the higher temperatures required for these carbenoid reactions often led to thermal or catalytic ring opening and products derived from vinylcarbene intermediates (104-107). 

The addition of alkoxycarbonylcarbene derived by catalysed decomposition of methyl diazoacetate to several simple, and in particular terminal, alkynes leads to low yields S7), but the reaction with 1 -trimethylsilylalkynes proceeds reasonably efficiently subsequent removal of the silyl-group either by base or fluoride ion provides a route to l-alkyl-3-cyclopropenecarboxylic acids. In the same way 1,2-bis-trimethylsilyl-ethyne can be converted to cyclopropene-3-carboxylic acid itself58 . The use of rhodium carboxylates instead of copper catalysts also generally leads to reasonable yields of cyclopropenes, even from terminal alkynes 59). 

The hydrogenation of cyclopropenecarboxylic acids leads to good yields of the saturated acid. Two elegant routes to cis-chrysanthemic acids have been reported based on this process. In the first, the ester (249) is reduced by either diimide or nickel boride and hydrogen 49) ... 

The photochemical rearrangement of 3-vinylcyclopropenes exhibits some regioselec-tivity. Thus, contrary to the selectivity observed with the thermal rearrangement of cyclopropenecarboxylates (see equation 149), the major product obtained from the irradiation of unsymmetrical substituted cyclopropenes was derived from the preferential cleavage of the cyclopropene single C-C bond which is methyl rather than phenyl substituted (equation 154). ... 

Diazoalkanes undergo facile cycloaddition to a range of cyclopropenecarboxylates and car-bonitriles. In the case of cyclopropene-l-carboxylates, the 2,3-diazabicyclo[3.1.0]hex-2-ene produced normally has the ester substituent at Cl, although a bulky 2-substituent on the cyclo-propene can lead to mixtures of regioisomers being produced (Table 18). 

Additionally, i-propyl 3-methyl-2-(trichloromcthyl)cyclopropenecarboxylate (6% bp 104-105 C/7 Torr) was formed. It was separated from the main product using Fisher apparatus (SPALTROHR column). 

Tris(trimethylsilyl)cyclopropenylium hexachloroantimonate (13) was obtained by decarbonyla-tion of the corresponding cyclopropenecarboxylic acid chloride 12. 

After saponification The rearranged product, methyl 2-methoxymethyM-cyclopropenecarboxylate, was obtained. 

Other asymmetric chlorinations are possible by the use of tert-butyl hypochlorite. A hydroxi-molactone is stereoselcctively transformed to an a-chloro nitroso ether by this chlorination reagent in 89 % yield191. Allylic chlorination of ethyl 2,3-dialkyl-2-cyclopropenecarboxylate is also highly stereoselective192. 

A mixture of benzonitrile and 1.5 eqs. Mn02-on-silica gel in cyclohexane stirred gently at reflux for 3 h benzamide. Y 100%. Reaction is considerably faster than with other forms of active Mn02 (Synth. Meth. 21, 166), and no more than 1.5 molar eqs. of oxidant are required. The method may also be used for preparing a,P-ethylenecar-boxylic and cyclopropenecarboxylic acid amides, but phenylacetamide was partially oxidised. F.e.s. K.-T. Liu et al., Synthesis 1988, 715-7. 

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