Synthesis of (R)‐ and (R)‐4‐methyl‐6‐2′‐methylprop‐1′‐enyl‐5,6‐dihydro‐2H‐pyran (Nerol oxide) and Natural Occurrence of its Racemate

Abstract
Following a known procedure, a mixture of (−)‐(2S,3R)‐ and (+)‐(2R,3R)‐2,3‐epoxy‐citronellols (5) was prepared from (−)‐(R)‐linalool (3)viaepoxy alcohol4and then reduced to (−)‐(R)‐3‐hydroxy‐citronellol (6). Sensitized photooxygenation of (−)‐(R)‐diol6led in part to (−)‐(R)‐triol8which was cyclodehydrated by dilute acid to a mixture of diastereoisomeric tetrahydropyran‐4‐ols9and10. Dehydration of hydroxy ethers9and10afforded (−)‐(S)‐nerol oxide (11) and (+)‐(R)‐nerol oxide (12), respectively, with an optical purity of 91%. Nerol oxide isolated from Bulgarian rose oil (0.038%) proved to be racemic. These results shed some light on the formation of nerol oxide in plants.