ROLE IN SYNTHESIS

The versatility of the Wittig, and related reactions, has resulted in examples being used as key steps in many organic syntheses of complex natural products.2,3 

Wittig-based reactions have been used in both the late stages of multi-step synthetic sequences and also to produce large quantities of key synthetic intermediates (see Ref. 2 for a discussion and some examples). A real advantage of this chemistry is that both the (Z) and (E) isomers of the alkene are generally available by careful choice of conditions (compare Protocols 11 and 12). The products of Protocols 11 and 12 have been conveniently converted into isomeric tetrahydrofurans which bear substituents at all four atoms of the ring. These ring systems form the basis of many complex natural products.25 

Caution All procedures should be carried out in a well-ventilated fume-hood and chemical resistant gloves and safety glasses should be worn at all times. 

To a one-necked round-bottomed flask (10 mL) equipped with a magnetic stirrer, water condenser and under a nitrogen atmosphere, add (2S,3/7)-3-[(triethylsilyl)oxy]-2-methylhexanal (168 mg, 0.678 mmol), I1-(ethoxycarbonyl)ethylidene]-triphenylphosphorane (299 mg, 0.743 mmol) and dry dichloromethane (5 mL). 

Allow the solution to cool then filter through a 2 inch plug of silica gel, washing with dichloromethane. 

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In addition to trialkylboranes, various alkoxyboron compounds have prominent roles in synthesis. Some of these, such as catecholboranes (see. p. 340) can be made by hydroboration. Others are made by organometallic or related substitution reactions. Alkoxyboron compounds are usually named as esters. Compounds with one alkoxy group are esters of borinic acids and are called borinates. Compounds with two alkoxy groups are called boronates. Trialkoxyboron compounds are borates. 

Radical hydrocarbon-stable hydrocarbon reactions also play a role in synthesis the reaction,... 

Protective groups play a passive role in synthesis. Each operation of introduction and removal of a protective group adds steps to the synthetic sequence. It is thus desirable to minimize the number of such operations. Fortunately, the methods for protective group introduction and removal have been highly developed, and the yields are usually excellent. 

The S-elimination reactions are also very frequent and they are typical of the chemistry of fluorinated compounds. They play an important role in synthesis and also... 

E. Brener, Nitrones and Nitronic Acid Derivatives Their Structure and their Roles in Synthesis , in The Chemistry of Amino, Nitroso and Nitro Compounds and their Derivatives, Vol. 1, S. Patai ed., Wiley, Chichester, 1982, pp. 459-538. 

In all these complexes with Au—-O bonds, the oxygen donor is easily displaced by other ligands and this gives them a very useful role in synthesis. Some typical reactions of [(Ph3PAu)30]+ are shown in Scheme 1. 

In some cases the functions of phospholipases in cells are purely degradative and result in the release of the phospholipid components (fatty acids, glycerol, phosphate, and head-groups). But in many cases phospholipases have important roles in synthesis and regulation. For example, we have seen how phospholipase A2 catalyzes the first step in the remodeling of phosphatidylcholine to the surfactant... 

Partial oxidation, although a major source of synthesis gas in the 1940Ts, has gone out of favor in recent years due to a shortage of heavy feedstocks and the lower cost of steam reforming. However, due to its feedstock flexibility it will continue to play a growing although secondary role in synthesis gas production in the future. 

The carbanionic species formed by the a-deprotonation of sulfoxides and sulfones play an important role in synthesis. In contrast, the selenoxides are very... 

It should now be clear that functional group transformations play more than an auxiliary role in synthesis. The importance of these reactions, especially with respect to the chemistry of natural compounds, makes it imperative to have a multitude of diverse and, at times, rather sophisticated methods to effect these often apparently trivial transformations. 

After these more or less isolated reports it was the challenge of natural product synthesis that spurred the most relevant exploration and extensions of the intramolecular magnesium-ene reaction. Its strategic role in synthesis is also illustrated by the following examples. 

These two compounds 1 and 2 cannot always be considered as synthetic equivalents of diazomethane for example, they do not react with carboxylic acids to give esters. In fact, they play a very different role in synthesis, and it is demonstrated in this review that they are powerful synthetic building blocks. Since tin chemistry is often compared to that of silicon, a few results involving bis(trimethylstannyl)diazomethane 3 [4] will also be presented (Scheme 1). Juggling the relative steric bulk of the triisopropylsilyl versus the trimethylsilyl group, and the enhanced reactivity of tin-carbon compared to silicon-carbon bonds, the three diazomethane derivatives 1-3 are complementary synthons. Not only are they attractive precursors for the synthesis of stable compounds hitherto believed to be only transient intermediates, but also of new heterocycles or even of polymer crosslinking agents. 

Fat Metabolism The liver also plays a central role in synthesis, oxidation, storage, and distribution of lipids. It not only aids in the absorption of fats through the action of the bile salts, but also (1) both synthesizes and oxidizes fatty acids, cholesterol, triacylglycerols, and phospholipids (the major components of cell membranes) (2) synthesizes most of the plasma lipoproteins and (3) converts carbohydrates and proteins into fat. 

The epoxide (oxirane) functional group has special features that allow it to play very important roles in synthesis and as an electrophilic species in biologically reactive intermediates. The epoxide group has an estimated strain energy of 27kcalmol-1,19 and its reactions with both acidic and nucleophilic reagents result in epoxide ring... 

The simplicity and efficiency of the click chemistry is attractive to fluorine-18 chemistry, where time plays an important role in synthesis due to the relative short half-life of fluorine-18. This one-pot reaction provides a versatile tool for coupling drug-like fragments in high yield and under mild conditions. The product 1,2,3-triazole formed from cycloaddition is biologically stable with polarity and size similar to an amide group that is a common functional group in many radiopharmaceuticals [77],... 

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