Total synthesis procedures

An example is the preparation of 18-trideuterio 5a-steroids bearing a side chain at C-17. Labeling of this position with three deuteriums was accomplished by utilizing the Johnson procedure for steroid total synthesis. This synthesis involves, in part, introduction of the 18-angular methyl group by methylation of the D-homo-17a-keto-17-furfurylidene intermediate (243). By substituting d3-methyl iodide in this step, the C/D cis- and ra/J5-18,18,18-d3 labeled ketones [(244) and (245)] are obtained. Conversion of the C/D tra 5-methylation product (245) into 18,18,18-d3-d /-3)8-hydroxy-5a-androstan-17-one (246) provides an intermediate which can be converted into a wide variety of C-18 labeled compounds of high (98%) isotopic... 

Deuterium labeling of C-18 has also been accomplished by an alternate procedure adapted from the Nagata steroid synthesis. During the course of the total synthesis of pregnanolone, thevC-18 function is introduced in the form of a nitrile group. Reduction of this function in intermediate (247) with lithium aluminum deuteride leads to a deuterated imine (248), which upon Wolff-Kishner reduction and acid-catalyzed hydrolysis... 

When chiral, drugs and other molecules obtained from natural sources or by semisynthesis usually contain one of the possible enantiomeric forms. However, those obtained by total synthesis often consist of mixtures of both enantiomers. In order to develop commercially the isolated enantiomers, two alternative approaches can be considered (i) enantioselective synthesis of the desired enantiomer or (ii) separation of both isomers from a racemic mixture. The separation can be performed on the target molecule or on one of its chemical precursors obtained from conventional synthetic procedures. Both strategies have their advantages and drawbacks. 

The identical synthetic procedures applied to ( + )-inositol or dl-inositol should lead to ( + )-proto-quercitol and DL-proto-quercitol, respectively. Since the total synthesis of DL-inositol had previously been reported (33), the new syntheses of the various forms of proto-quercitol are total, with the possible exception of the step for resolution of DL-inositol, which so far has been accomplished only by a microbiological method (43). 

The aim of this contribution is to examine the most widely used methods of elaborating the oxirane functionality in the synthesis of complex molecules. In view of the extensive use of epoxides in simple transformation procedures in the early stages of total syntheses, only certain selected manipulations of epoxides representing the key steps of a total synthesis and/or their use at a late stage of the reaction sequence are considered. 

This procedure illustrates a broadly applicable method which is essentially that of Roth, Dubs, Gotschi, and Eschenmoser,2 for the synthesis of enolizable /1-dicarbonyl compounds. Although there are various methods for the preparation of /3-dicarbonyl systems,3 the scheme of sulfide contraction widens the spectrum of available methods. The procedure can also be utilized in the synthesis of aza and diaza analogs of /3-dicarbonyl systems. Eschenmoser2 has utilized the method to produce vinylogous amides and amidines in connection with the total synthesis of corrins and vitamin B12.4... 

The Noyori procedure was applied to a total synthesis of baiyunoside, a sweet principle, using 2,3,4-tri-<9-benzyl-D-xylopyranosyl fluoride (18 see Table 1), and a synthesis of glycotriosyl ceramide. A model experiment for the synthesis, using 18, showed a solvent dependence for the a ratio of the products. In this case, the use of acetonitrile, oxolane, or ether gave the a anomer (1,2-a.v), and the use of toluene or hexane gave the P anomer (1,2-trans), preponderantly. 

Reaction of 2-129 with NaCN followed by treatment with NaH and TIPSC1 led to the anthracene 2-130 in 68% yield (Scheme 2.30). The desired lactam 2-131 was then obtained by reduction of the cyano group and ring closure using CoCl2/ NaBH4. Quite recently, this procedure has also been used for a formal total synthesis of tetracenomycin [63]. 

The described procedure has been widely used by Smith III and coworkers [250] in the efficient total synthesis of natural products containing extended 1,3-hydroxylated chains. This architecture is often found as a structural element in polyene macrolide antibiotics [251] such as mycotoxin A and B, dermostatin, and roxaticin. The Smith group used the above-mentioned approach (e. g., as five-component coupling) for the synthesis of the pseudo-C2-symmetric trisacetonide (+)-2-471 [252], which was employed by Schreiber and coworkers [253] within the synthesis of (+)-mycotoxin A (2-470a) (Scheme 2.108). Thus, lithiation of 2.5 equiv. dithiane 2-462b followed by treat-... 

Another very new radical/radical domino procedure was used in the total synthesis of the alkaloid lennoxamine by Ishibashi and coworkers. Here, a 7-endo cycliza-tion/homolytic aromatic substitution reaction cascade led to the target compound in 41% yield [75]. 

The procedure has been used for the total synthesis of the natural product stypol-dione (5-18) starting from 5-16 to give the 6/6/6-membered tricyclic compound 5-17 which was further manipulated (Scheme 5.4) [7]. 5-17 was obtained as a mixture of two diastereomers in low yield ranging from 20 to 30% [8, 9]. However, three... 

This procedure was used for the asymmetric total synthesis of the steroid (+)-equilenin (7-7) [3]. Cyclopropylidene derivates 7-4 could be converted into the cyclobutanones 7-5 in good yields by applying an asymmetric epoxidation using the chiral (salen)Mnm complex 7-6 (Scheme 7.2) [4]. It is of interest that the demethoxy-lated substrate 7-4b led to 7-5b with a very high enantiomeric excess of 93%, whereas 7-4a gave 7-5a with only 78% ee. 

A convenient procedure for the lactonization of alkenols has been recently revealed by Borhan and coworkers [19]. This methodology was successfully applied in the total synthesis of (+)-tanikolide (7-37), a natural product of marine origin which exhibits antifungal activities. Thus, when alkenol 7-36 is subjected to soluble oxone and a catalytic amount of 0s04, a smooth domino oxidative cleavage/lactonization process takes place which leads, after debenzylation, to the desired product in good overall yield (Scheme 7.12). 

A similar procedure was applied to the synthesis of quinazolidine 189 from precursor 188 in the total synthesis of the natural product known as ( )-quinolizidine 2071 190, an alkaloid isolated from the skin of the Madagascar mantelline frog Mantella baroni, that shows an exceptional axial stereochemistry for the ethyl group at C-l. Quinolizidine 189 was transformed into 190 by oxidation and two consecutive Wittig methylenations (Scheme 34) <1999CC2281>. 

The stereoselective total synthesis of (+)-epiquinamide 301 has been achieved starting from the amino acid L-allysine ethylene acetal, which was converted into piperidine 298 by standard protocols. Allylation of 297 via an. V-acyliminium ion gave 298, which underwent RCM to provide 299 and the quinolizidine 300, with the wrong stereochemistry at the C-l stereocenter. This was corrected by mesylation of the alcohol, followed by Sn2 reaction with sodium azide to give 301, which, upon saponification of the methyl ester and decarboxylation through the Barton procedure followed by reduction and N-acylation, gave the desired natural product (Scheme 66) <20050L4005>. 

L4165> have been used for this cyclization often catalyzed by TsOH or camphorsulfonic acid (CSA). Compound 358, used as an intermediate in the total synthesis of (—)-kaitocephalin, was prepared from 357 following this procedure shown in Equation (62) <20050L4165>. 

Novel synthetic procedures for indolizidine alkaloids were developed via a samarium diiodide-promoted carbon-nitrogen bond cleavage as a key step. Application of the procedure led to the total synthesis of (+)-(8R, 8aR)-perhydro-8-indolizidinol <2006H193>. 

The final steps in a total synthesis of (+ )-gloeosporone (3, a natural germination inhibitor of a fungus) required oxidation of the acetylene group of 1 to a diketone group. The oxidation was carried out in 74% yield by the catalytic Ru02 procedure of Sharpless. On liberation (pyridine-HF) of the C7-hydroxyl group, the hydroxy... 

This strategy [20b] has been applied, for instance, in a total synthesis of pentalenolactone E methyl ester (27) (Scheme 5.17), and more recently Marino and Long [20c] have described the intramolecular version of this annulation procedure to synthesise an octahydronaphthalene derivative (28) which is a key intermediate for a new synthesis of dihydrocompactin (Scheme 5.18). However, the reaction with aldehydes affords only very low yields of a mixture of lactones. 

The construction of carbocydic compounds by ring-annulation procedures frequently plays a prominent role in total synthesis. The tolerance of various functional groups in the zinc reagents employed in copper-catalyzed asymmetric 1,4-additions forms the basis for three novel catalytic enantioselective annulation methods discussed here. 

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