Chapter 7: Terpenes: Mono-, Sesqui-, and Higher Terpenes

In this chapter, we first describe the various classes of terpenes along with the classes of compounds that are biogenetically related to terpenes, and provide representative examples of these different classes. A discussion of the basic approach that is used to determine terpene structures with the aid of nuclear magnetic resonance (NMR) spectroscopy follows. The first task is to determine whether an isolated terpene is one of the huge number of known terpenes. This process is often hindered by the lack of complete spectroscopic data for a surprisingly large number of known terpenes. In the case where a compound is either clearly unknown or where the question of novelty is ambiguous, 2D NMR methods for skeletal determination are described, along with an outline of different approaches for determining stereochemistry. In the next section, various problems that could potentially lead to an incorrect structure determination are outlined, and ways of avoiding these pitfalls are discussed. Particular attention is paid to possible ambiguities in spectral interpretation. Finally, specialized techniques that are useful for different classes of terpenes are discussed. The main problem addressed in these last two sections is that of spectral overlap and the problems that this can cause in structure elucidation.