Protonation of lactone enolate

A-Phthaloyl-protected (S)-phenylalanine has been used as a ligand for rhodium in the formation of metallocarbenes from diazo compounds for C-H insertion reactions (Section D.1.2.2.3.2.). Ar-Sulfonyl-protected (S)-alanine and (S)-valine are efficient ligands for chiral Lewis acids used in the Diels-Alder reaction (Section D.1.6.1.1.1.3.). A -Sulfonyl-pro-tected (S)-phenylalanine methyl ester has been used for the enantioselective protonation of lactone enolates (Section D.2.I.). The terf-butyl ester of (S)-valine readily forms imines with carbonyl compounds which are used for the highly efficient alkylations of their azaenolates (Sections D.1.1.1.4.1D.1.5.2.4.). All these derivatives can be obtained by the standard methods described in Houben-Weyl3. 

The enantiomers of 2-cyclohexy(-2-hydroxy acetic add 12 and their esters are conveniently obtained from the corresponding mandelic adds (or derivatives) by catalytic hydrogenation with a platinum catalyst16 17. The methyl ester has been used for the enantioselectivc protonation of lactone enolates (Section D.2.1.) and the chiral diene prepared analogously to the O-methylmandclic acid derivatives11-13 described in the preceding section in diastereoselective Diels-Alder reactions (Section D. 1.6.1.1.1.). 

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