2-hydroxy-2,6,10,10-tetramethyl-1-oxaspiro[4.5]dec-6-en-8-one: Precursor of 8,9-dehydrotheaspirone in white-fleshed nectarines

2-Hydroxy-2,6,10,10-tetramethyl-1-oxaspiro[4.5]dec-6-en-8-one (2) was identified as precursor of 2,6,10,10-tetramethyl-1-oxaspiro[4.5]deca-2,6-dien-8-one (8,9-dehydrotheaspirone, 1) in an Et2O extract from white-fleshed nectarine (Prunus persica Batsch var. nucipersica Schneid) juice. The identification was verified by comparison of chromatographic and spectral data (MS, H-1 and C-13 NMR) of the new natural product with those of an authentic reference compound synthesized from dehydrovomifoliol (3). NMR spectral data revealed that the precursor compound 2 is present in 5:1 mixture of diastereomeric hemiacetals 2a/b. The absolute stereochemistry of the diastereoisomers (major isomer, 2S,5S; minor isomer, 2R,5S) was elucidated by NOE experiments and dehydration to optically pure (S-configured) spiroether 1. In addition to hemiacetals 2a/b, (6S)-dehydrovomifoliol (3) as well as the beta-D-glucoside of (3S,5R,6S)-3-hydroxy-5,6-epoxy-beta-ionone (5) was also identified in nectarine juice.