Synthetic equivalent

Reagent A compound which reacts to give an intermediate in the planned synthesis or to give the target molecule itself. The synthetic equivalent of a synthon. 

Synthetic Equivalent A reagent carrying out the function of a synthon which caimot itself be used, often because it is too unstable. 

Synthon A generalised fragment, usually an ion, produced by a disconnection. (Some people also use synthon for a synthetic equivalent). 

More usually, none of the substituents gives a stable anion and so we use the synthetic equivalent of the anion - the Grignard reagent or alkyl lithium. - We refer to "Et " as a SYNTH ON for which EtMgBr is the synthetic equivalent. 

The synthetic equivalents of the synthon H" are the hydride donors sodium borohydride NaBH4, and lithium aluminium hydride LiAIHi. How might you make TM 21 using this disconnection ... 

Wc can t protect the carbonyl group without stopping the reaetion, so we activate one position by adding a COiEt group and using the ester A below, the synthetic equivalent of acetone, instead of acetone itself Here is the reaction draw a mechanism for it. 

Now we can disconnect the ketone using our synthetic equivalent for the acetone anion ... 

The anion B is just the enolate anion of a carbonyl compound, actually the same as A. So there is no need to use a Grignard reagent or any other synthetic equivalent in this reaction anion B itself can be the intermediate and we simply treat the aldehyde with mild base ... 

The synthon will be, in a lower valeney state - if X is C then it will be a carbene or the synthetic equivalent of a carbene. Let s see how this disconnection works out for epoxides. Taking X = O first we have... 

The trisannulation reagent 7-acetoxy-l,l l-dodecadien-3-one (134) is derived from the bisannulation reagent 124 in four steps. This reagent is a synthetic equivalent of l-dodecene-3,7,11-trione, and the two ketone groups of the trione are masked as an acetoxy and a terminal alkene. The synthesis of optically active D-homo-19-norandrosta-4-en-3-one (135) by the trisannulation reaction... 

Step 4 Since a Grignard reagent may be considered as synthetically equivalent to a carbanion this suggests the synthesis shown... 

Naturally occurring colored minerals that contain oxides of iron are known by such names as ochre [1309-37-1], umber [12713-03-0], sienna [1309-37-1], etc. These show greater variation in color and tinting power than the synthetic equivalents, and the nature and amount of impurities in the national products is also variable. Most of the pigments identified in Table 9 are, therefore, manufactured synthetically. They are primarily used in skin-makeup products and in eye-area colorants. 

As with isolated rings, individual heterorings in fused systems which are synthetic equivalents of acyclic subunits, e.g. lactone, ketal, lactam, and hemiketal, can be disconnected. 

For a review of such synthetic equivalents see T. A. Hase, Umpoled Synthons, (J. Wiley, New York, 1987). 

Desoxyleukotriene D4 was synthesized to determine whether the 5-hydroxyl group is necessary for biological activity. It is, since the bioactivity of 5-desoxyleukotriene D4 is less than % that of LTD4 itself. An interesting synthetic equivalent of the 4-formyl- , -l,3-butadienyl anion was utilized in the synthesis. 

The 2,2-bis(tnfluoromethyl)-4-methyl-2f/-5-oxazolone, readily available from 2,2-bis(trifluoromethyl)-l,3-oxazolidin-5-one, is a synthetic equivalent of activated pyruvate [90] (equation 16). 

Often the precursor is not defined completely, but rather its chemical nature is emphasized by writing it as a species to which it is equivalent for synthetic purposes. Thus, a Grignaid reagent or an organolithium reagent might be considered synthetically equivalent to a caibanion ... 

Anions of (3-keto esters are said to be synthetically equivalent to the enolates of ketones. The anion of ethyl acetoacetate is synthetically equivalent to the enolate of acetone, for example. The use of synthetically equivalent groups is a common tactic in synthetic organic chemistry. One of the skills that characterize the most creative practitioners of organic synthesis is an ability to recognize situations in which otherwise difficult transfonnations can be achieved through the use of synthetically equivalent reagents. 

The compound 92 is extremely light-sensitive. Under the action of phosphites or compounds with multiple bonds, it readily eliminates a tellurium atom and thus may be considered as a synthetic equivalent of the heterodiene 93a. 

Formation of 7-hetera- and 7,8-diheterabicyclo[2.2.2]octa-2,5-dienes using synthetic equivalents of cyclohexatriene in [4- -2] cycloaddition reactions 97SF1327. 

As mentioned in the introduction, l-heterobut-l-en-3-ynes, RXCH=CHC=CH (X = RN, O, S R = organic radical), are the nearest and most important diacetylene derivatives readily formed by nucleophilic addition of amines, alcohols, and thiols to diacetylene. In many heterocyclization reactions (especially those leading to fundamental heterocycles) l-heterobut-l-en-3-ynes behave as diacetylene synthetic equivalents, but unlike diacetylene, they are nonhazardous. Therefore, the syntheses of heterocycles therefrom are often more attractive in preparative aspect. 

The reaction has wide scope in respect of the dienophUe / -substituent. The representative less reactive dienophiles, crotonoyl- and cinnamoyl-oxazolidinone, react with cyclopentadiene at -15 °C and 25 °C for 20 h and 24 h giving cycloadducts in 99% ee and 96% ee, respectively. The 3-chloropropenoyl derivative also affords the adduct in high optical purity (96% ee) this adduct is transformed to 2-(methoxycar-bonyl)norbornadiene, a useful chiral building block. Thus, the 3-chloropropenoyl derivative can be regarded as a synthetic equivalent of an acetylene dienophile. 

Nitronates derived from primary nitroalkanes can be regarded as a synthetic equivalent of nitrile oxides since the elimination of an alcohol molecule from nitronates adds one higher oxidation level leading to nitrile oxides. This direct / -elimination of nitronates is known to be facilitated in the presence of a Lewis acid or a base catalyst [66, 72, 73]. On the other hand, cycloaddition reactions of nitronates to alkene dipolarophiles produce N-alkoxy-substituted isoxazolidines as cycloadducts. Under acid-catalyzed conditions, these isoxazolidines can be transformed into 2-isoxazolines through a ready / -elimination, and 2-isoxazolines correspond to the cycloadducts of nitrile oxide cycloadditions to alkenes [74]. 

Accordingly, cyclic nitronates can be a useful synthetic equivalent of functionalized nitrile oxides, while reaction examples are quite limited. Thus, 2-isoxazoline N-oxide and 5,6-dihydro-4H-l,2-oxazine N-oxide, as five- and six-membered cyclic nitronates, were generated in-situ by dehydroiodination of 3-iodo-l-nitropropane and 4-iodo-l-nitrobutane with triethylamine and trapped with monosubstituted alkenes to give 5-substituted 3-(2-hydroxyethyl)isoxazolines and 2-phenylperhydro-l,2-oxazino[2,3-fe]isoxazole, respectively (Scheme 7.26) [72b]. Upon treatment with a catalytic amount of trifluoroacetic acid, the perhydro-l,2-oxazino[2,3-fe]isoxazole was quantitatively converted into the corresponding 2-isoxazoline. Since a method for catalyzed enantioselective nitrone cycloadditions was established and cyclic nitronates should behave like cyclic nitrones in reactivity, there would be a good chance to attain catalyzed enantioselective formation of 2-isoxazolines via nitronate cycloadditions. 

The general features of this elegant and efficient synthesis are illustrated, in retrosynthetic format, in Scheme 4. Asteltoxin s structure presents several options for retrosynthetic simplification. Disassembly of asteltoxin in the manner illustrated in Scheme 4 furnishes intermediates 2-4. In the synthetic direction, attack on the aldehyde carbonyl in 2 by anion 3 (or its synthetic equivalent) would be expected to afford a secondary alcohol. After acid-catalyzed skeletal reorganization, the aldehydic function that terminates the doubly unsaturated side chain could then serve as the electrophile for an intermolecular aldol condensation with a-pyrone 4. Subsequent dehydration of the aldol adduct would then afford asteltoxin (1). 

The elaboration of the polyunsaturated side chain of asteltoxin requires a stereoselective coupling of aldehyde 2 with a suitable synthetic equivalent for the anion of 4-formyl-1,3-butadiene (see intermediate 3 in Scheme 4). Acid-induced skeletal reorganization of the aldehyde addition product, followed by an intermolecular... 

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