Divergent Routes to Chiral Cyclobutane Synthons from (−)-α-Pinene and Their Use in the Stereoselective Synthesis of Dehydro Amino Acids

Several polyfunctionalized cyclobutane derivatives have been synthesized using commercial (−)-α-pinene and (−)-verbenone as chiral precursors. Thus, oxidative cleavage of these compounds by using ruthenium trichloride afforded quantitatively (−)-cis-pinonic and (−)-cis-pinononic acids, respectively, without epimerization. These products were converted into several types of aldehydes, which are the key intermediates in the synthesis of cyclobutane dehydro amino acids via Wittig−Horner condensations with suitable phosphonates. These reactions are highly stereoselective, affording exclusively (Z) isomers, stereochemistry being assessed by NMR experiments. The obtained dehydro amino acids are polyfunctionalized molecules useful for the synthesis of other α-amino acids, with additional chiral centers, whose configuration must be induced by the chirality of the terpene employed as a precursor.