Chiral formyl cyclopropanes

Scheme 18.15 Extension of the redox opening of epoxyaldehyde, according to Bode and coworkers, to strained chiral formyl cyclopropanes.

Chiral electrophilic cyclopropanes (63) are prepared in high enantiomeric excess starting from butadiene-iron tricarbonyl complexes (60) containing a non-complexed double bond. Reaction with diazomethane and decomposition of the resulting pyrazolines (61) in the presence of Ce" gave the corresponding chiral cyclopropanes (62). Breakdown of the dienic substituent of electrophilic cyclopropane (62) by means of ozonization resulted in the formation of formyl-substituted electrophilic cyclopropane (63) still carrying the asymmetric centre (equation 10) " . ... 

Chiral tricarbonyl(butadienyl)iron complexes are easily accessible by resolution and show excellent diastereofacial selectivities with a variety of reagents55. The tricarbonyl(trienyl)iron complex 2 and methyl diazoacetate (copper bronze catalysis) give only two cis 7/m -isomeric cyclopropanes 5< These can be separated by column chromatography, and each diastereomer transformed into methyl traits- or 7.r-2-formyl-3.3-dimcthylcyclopropanecarboxylate (hemica-ronic aldehyde) by destructive cleavage of the diene complex auxiliary. The enantiomeric excess in these compounds is above 90%. 

The preferred rotational conformations of acetyl and formyl groups can be predicted by temperature-dependent c.d. measurements, and the technique has been applied to some monoterpene aldehydes. The sign of the Cotton effect has been related to the chirality of a series of Ti-molecular complexes of monoterpene (and other) hydrocarbons with tetracyanoethylene. Inconclusive results found with (-h )-sabinene were ascribed to complexation with the cyclopropane ring. °... 

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