## 1 Introduction  
Terpenes are hydrocarbon natural products constructed from connected isoprene units (each containing five carbons). They are often polycyclic and are derived from rearrangement, involving carbocation intermediates, of acyclic precursors by terpene synthase/cyclase enzymes [1; 2; 3; 4; 5]. For example, monoterenes (10 carbons) are synthesized from geranyl diphosphate (**GPP**), while sesquiterpenes (15 carbons) are produced from farnesyl diphosphate (**FPP**) and di terpenes (20 carbons) from geranylgeranyl diphosphate (**GGPP**; Figure 1). Red and white wines predominantly contain monoterpenes and sesquiterpenes. Terpenes are biosynthesized in the grapevine primarily in flowers and berries. Terpenes produced in grape berries and flowers are thought to play roles in defense against pathogens, attraction of seed dispersing animals, and attraction of pollinators [6; 7; 8]. These terpen also create complex flavor and aroma in finished wines. Two pathways can generate **GPP**, **FPP**, and **GGPP**: (1) the mevalonate (MVA) pathway and (2) the 2-\(C\)-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate (MEP/DXP) pathway (Figure 1) [9, 10, 11, 12]. Each pathway employs a unique means of generating isopentenyl pyrophosphate (**IPP**) and dimethylallyl diphosphate (**DMAPP**), which are subsequently combined to form **GPP** by geranyl diphosphate synthase. **GPP** itself is a substrate for farnesyl diphosphate synthase. In wine grapes, _Vitis vinifera_, **GPP** and **FPP** are biosynthesized via the MEP/DOXP pathway in the mitochondria and the MVA pathway in the cytosol, respectively [13]. Removal of the pyrophosphate group from **GPP** or **FPP** by terpene synthase/cyclase enzymes in _vinifera_ grape vines generates carbocationic intermediates that rearrange within the enzyme active sites to form monoterpenes and sesquiterpenenes [1, 2, 3, 4, 5]. Most compounds derived from terpene biosynthesis in wine grapes are actually terpenoids, which are terpen or terpene alcohols (terpene synthases can produce alkenes via deprotonation or alcohols via water capture) that have undergone chemical transformation(s), typically oxidation (sometimes promoted by enzymes, sometimes not). Genomes for the various _Vitis vinifera_ grape variatals have been mapped, with an emphasis on identifying terpene synthase/cyclase coding regions and sub-families [8, 14, 15]. As described by [PERSON], both red and white wines contain numerous terpen that contribute most of the aroma and flavor in wine [16], although non-terpene compounds, commonly esters and mercaptans,  
Figure 1: Overview of chemical transformations in terpene biosynthesis. also contribute [17]. Herein we will describe typical chemical mechanisms at play in the biosynthesis of terpenes and terpenoids in wine grapes. The representative compounds we will discuss are: (1) **1,8-Cineole**. The characteristic eucalyptus aroma in Australian Syrah and other full-bodied red wines comes from the monoterenoid 1,8-cineole, also known as eucalypt (Figure 1) [18, 19, 20]; (2) **\(\mathbf{a}\)-Ylanguage**. \(\mathbf{a}\)-Ylanguage (Figure 1) is a sesquiterpene found in various red and white grapes, and is likely the precursor to other terpenoid products found in wine [21]; (3) **Botrydial**. _B. cinerea_ is a fungal parasite that can destroy entire vineyards. Botrydial, derived from the sesquiterpene alcohol presilphiperfolanol (Figure 1), is responsible for the phytopathogenice development of _Botrytis cinerea_ and is found on the skins of _Botrytis_ infected grape clusters [22, 23, 24, 25]; (4) **Rotundone**. A ubiquitous terpenoid in red wine varietals, rotundone is a derivative of the sesquiterpene \(\mathbf{a}\)-guaiene (Scheme 1). Rotundone is found in Syrah, Cabernet Sauvignon, Zinfandel, Merlot, and other red _vinifera_ varietals (Figure 1) [26, 27, 28, 29, 30]. Oxidation of \(\mathbf{a}\)-guaiene produces rotundone in red wines that are allowed to oxidize [28].