2H n.m.r. determination of the stereochemistry of an allylic displacement in the biosynthesis of virescenol B

Abstract

7 (a) For a recent review of the application of 2H n.m.r. spectroscopy to biosynthetic studies see M. J . Garson and J. Staunton,

Biosynthesis and catabolism of monoterpenoids

“…161 Comparison of exchange curves for the fenchocamphorones generated from the stereospecifically labeled precursors with those for the corresponding fenchocamphorones obtained from the racemic (LRS)-[l-3H;2-14C]geranyl pyrophosphate provided convincing evidence for net retention of configuration of Cl in this cyclization, completely analogous with results from the pinane and bornane series, and additionally confirming the nearly Universal preference for anti stereochemistry in mono-, sesqui-, and diterpene cyclizations. 139,[166][167][168][169][170][171][172] The summation of the above studies firmly establishes the overall stereochemistry of the isomerization and cyclization of the achiral precursor geranyl pyrophosphate to bornane, pinane, and fenchane monoterpenes. Specifically, the results permitted determination of the reacting helical conformation of geranyl pyrophosphate and the stereochemistry of the initial allylic rearrangement of geranyl to linalyl pyrophosphate (syn), the stereochemistry, following C2-C3 rotation, of the subsequent allylic displacement with C1-C6 cyclization (anti), and the configuration and conformation (anti,endo) of the cyclizing tertiary allylic, linalyl, intermediate and, thus, the face of the C6-C7 x system involved in bonding.…”

Virescenols: Sources, Structures and Chemistry

“…Another intriguing point in the biosynthesis of the pimarane and isopimarane skeletons is the mechanism of the C ring closure starting from labdadienyl pyrophosphate 30 [24] (Figure 7). group possessing an E configuration, deriving from the16R configuration of the intermediate 30b.…”