COMPLETING A SYNTHESIS

Once you have completed a reaction, separated and purified the product(s), there are still questions to be answered  

The first question is addressed by working out the percentage yield, and Table 3.1 gives an idea of the expectations here. Section 4 addresses the problems of checking purity and Section 5 considers identifying the new compound. Determining the structure of a compound is a very complex process, and we devote all of Part 2 of this book, which is to be found on the Spectroscopy CD-ROM, to answering this question. 

0 to 20 Often called a formation rather than a synthesis , the desired product is formed, but in yield too low to be of any practical use in a commercial synthesis. 

Chemists, particularly when considering a reaction for a commercial purpose, are only interested in yields in the last two categories. In research and development work, smaller yields are acceptable because only small quantities of reactants are usually used. 

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A convenient method to affect the oxidation p- to nitrogen in piperidines is based on the anodic oxidation of N-carboalkoxy piperidines (Scheme 35). The electrochemical oxidation of piperidine (152) in the presence of acetic acid and potassium acetate, for example, afforded a mixture of isomeric 2-hydroxy-3-acetoxypiperidines (153) in a combined yield of 93%, following an aqueous workup [61]. Reduction with sodium boro-hydride severed the C-OH bond. Treatment with acid and then base completed a synthesis of pseudoconhydrine (154). 

When you want to erase completely a synthesis created by the CHAOS program, you must erase these two files which form a part of it. 

Many of you have already described procedures in a lab report. Most likely, you included items such as the equation that you used to calculate percent yield or the step-by-step instructions that you followed to complete a synthesis (e.g., Heat to reflux. or Stir constantly for 10 min. ). Are such items also appropriate in a journal article To answer this question, we analyze several different excerpts. Each excerpt describes a common chemical procedure. Although by no means comprehensive, these few examples should get you started and help you understand what an expert audience expects in this move of the Methods section. 

Since arucadiol and miltirone both have an aromatic "B" ring, enone 64 served as a common intermediate for both of these quinone pigments. The aromatization of 64 was readily achieved using 2,3-dicyano-5,6-dichloro-l,4-quinone (DDQ) (Equation 5.2). With substrate 65 in hand, only demethylation of the ethers was required to complete a synthesis of arucadiol (58). This transformation was accomplished in nearly quantitative yield using boron tribromide. Our synthetic arcudiol was spectrally identical with the natural material. 

A general observation is that the more time that it takes to complete a synthesis, the lower the quality of the final product. The exact basis for this... 

Although the trisubstituted olefinic linkage of the pheromone could be fashioned readily in a less selective manner followed by HPLC purification, we sought a stereoselective route in addition. Still and Mitra completed a synthesis of the... 

Tsuji has completed a synthesis of 0-methyl lasiodiplodin (97), a simple analog of the natural product 90. While 97 itself is not found in nature, the strategy is closely related to those employed for 90 and thus the preparation of 97 is also discussed in detail (see Scheme 1.19). 

Takei has completed a synthesis of A26771B in a manner reminiscent of Ease s work (Scheme 2.3). The synthesis begins with the addition of methyl lithium to 10-undecenal (24) and acetylation of the resulting alcohol to give 25... 

Nicolaou and co-workers have also completed a synthesis of carbomycin B employing a strategy similar to Tatsuta s, as depicted in Scheme 2.22. Assembly of the C-11 to C-15 fragment began with the known optically active (/ )-3-... 

Masamune has also completed a synthesis of tylonide hemiacetal (291) based on the creative use of enantioselective aldol condensations, as shown in Scheme 2.26. The aldol condensation of 328, derived from (/f)-hexahydromandelic acid and prop anal, was found to be >100 1 diastereoselective, affording the 2,3 syn compound 329 in 97% yield. Transformation to the p,7-unsaturated ester 330 occurred via selenoxide elimination and periodate cleavage followed by esterification. Formation of the silyl ether, reduction, and protection of the ester followed by ozonolysis of the terminal olefin gave the diol-protected aldehyde 331. The C-11 to C-15 segment 332 was then completed via chain elongation and a subsequent reduction-oxidation sequence in 34% overall yield from 330. 

Tamm has also completed a synthesis of verrucarin A (116), as described in Scheme 4.21. The muconic acid unit was obtained via peroxide oxidation of catechol to produce diacid 117, which was esterified to provide the ( ,Z)-half-ester 108. 

West and Naidu found that the diazoketone 358, prepared by alkylating the benzyl ester of L-proline with 5-bromo-l-diazopentan-2-one, cyclized to give a transient spirobicyclic ammonium ylide 359 when heated with coppeifll) acetylacetonate in toluene (Scheme 44) (355,356). This unstable ylide underwent a diastereoselective [1,2]-Stevens rearrangement to give the quinolizidinone 360 and its bridgehead epimer in a ratio of 95 5. However, some racemization (possibly through an achiral diradical intermediate) must have occurred, since 360 had an ee of only 75%. Reduction of the ester and defimctionalization of thioketal 361 with the unusual combination of sodium and hydrazine in hot ethylene glycol completed a synthesis of the unnatural (- )-enantiomer of epilupinine (ent-331). 

Acylation of the acid chloride of 75 with a Grignard reagent catalysed by Fe(III) completes a synthesis of juvabione 76 with good stereochemical control. 

An early example of a natural product synthesis featuring an indole C-H bond alkenylation was report by Trost and co-workers (Scheme 54). Using stoichiometric Pd(II)-salts in combination with silver tetrafluoroborate, they were able to mediate a oxidative Heck process onto the bicyclic amino-alkene. Using a reductive work-up to reduce the Pd-C bond, they were able to complete a synthesis of ibogamine [76]. 

An intriguing and important adaptation of the aldose-nitromethane reaction was also studied in Toronto by Fischer and Jean Grosheintz. By addition of nitromethane to 1,2-0-isopropylidene-n-a 2/fo-pentodialdose and subsequent hydrolysis of the acetone group, they obtained a mixture of 6-deoxy-6-nitro-D-glucose and 6-deoxy-6-nitro-L-idose. A second, intramolecular reaction of these substances led to a mixture of deoxynitro-inositols and, by reduction, to the related aminodeoxyinositols. One of the latter was successfully deaminated a few years later, by T. Posternak, to the B-vitamin, mt/o-inositol (12), thus completing a synthesis of the latter that started from n-glucose. 

The intermolecular variant of the O-H insertion reaction gets stuck at the stage of the initial adduct 8. We envisioned that if R1 = allyl, coordination of the double bond to the metal would initiate a Claisen-type process to form the jr-allylruthenium complex 9, whose reductive elimination would form the allyl ketone starting from terminal alkynes and allyl alcohols (Equation 1.10). Gratifyingly, this prediction was fully realized as shown in Equation 1.11 [13]. A tertiary ester does not undergo elimination under these reaction conditions (Equation 1.12). Dihydroxylationofthe double bond and subsequent acidification effect cyclodehydration to form furans in two overall steps. Subsequent elimination of the elements of acetic acid completes a synthesis of rosefuran 10, one of the most prized fragrances [14]. 

Reactions of alkynes. Hydroarylation, " carbonylation, and the formation of a-substituted acrylamides-- are some of the reactions mediated by PdfOAc),. While an intramolecular version of the former completes a synthesis of coumarins and quin-olinones, the latter two incorporate CO group(s) from carbon monoxide. 

In conclusion, Funk completed a synthesis of ( )-FR901483 in 22 steps and 2.4 % overall yield from the starting dioxanone, demonstrating that the easily accessible trifimctional arrays of 2-amidoacroleins can be exploited in the rapid assembly of tricyclic ring systems. Attention should also be drawn to the />-methoxy benzyl substituent introduced at C(6) by alkylation of a tricyclic lactam in an advanced stage of the synthesis. 

Yet another variation of the Aspidosperma skeleton is that of fendleri dine (aspidoalbidine) (210), in which the C-18 carbon is attached to C-21 via an ether bridge. By a route quite analogous to that used for N-acetylaspidospermidine (252), Ban has completed a synthesis of N-acetyl-aspidoalbidine (445) using deoxylimapodine (446) as the key intermediate (211). 

Vinyl radicals are useful in radical coupling and addition reactions. Stork used the cyclization of vinyl radical 163 (derived from vinyl bromide 162) to complete a synthesis of norseychellanone (164). The cyclization step proceeded in 70% yield. 

Opening of the epoxide 101 with dimethylamine leads to two regioiso-meric dimethylamino alcohols, 104 and 105, in a ratio of 4 1. Catalytic hydrogenation of 104, lactonization by means of acetic anhydride, and DIBAH reduction of the lactone 106 completed a synthesis of racemic form of another antibiotic sugar, forosamine (107, 2,3,4,6-tetradeoxy-4-dimethylamino-DL-er-yf/iro-hexopyranose). When the starting epoxide 101 was resolved by means of its (-I- )-phenylethylammonium salt into enantiomers, the same reaction sequence... 

Chang and Fajer have also recently completed a synthesis of a model isobacteriochlorin (101), although their approach will not be applicable to the natural product itself. The approach rests on Inhoffen s earlier syntheses of the bisgeminiketone (102) by treatment of octaethylporphyrin with hydrogen peroxide in acid. This compound was then treated with methylithium to give the tertiary alcohol (103), which was subsequently reduced with HI/H3PO2/AC2O and afforded the required isobacteriochlorin (101). 

Opening of the cyclopropane ring, followed by capture of the resulting iminium moiety by the tosyl-protected amine, completed the reaction sequence to tetracyclic pyrroloindole 117. It is worth noting that the Qin group was able to employ the same chemistry to make the des-methoxy version of 117 and thereby complete a synthesis of ( )-minfiensine (109) in an additional nine steps. ... 

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