A stereoselective synthesis

The stereochemical outcome of the Michael addition reaction with substituted starting materials depends on the geometry of the a ,/3-unsaturated carbonyl compound as well as the enolate geometry a stereoselective synthesis is possible. " Diastereoselectivity can be achieved if both reactants contain a stereogenic center. The relations are similar to the aldol reaction, and for... 

A matched pair combination of the above silylketene acetal and (/ ,if)-2,4-dimethyl-2-heptenal has been applied in a stereoselective synthesis of the C17-C25 fragment of zincophor-... 

Solanesol and other prenyl alcohols are important as metabolites in mulberry and tobacco leaves and in the synthesis of isoprenoid quinones. Hence, Sato and collaborators107 have developed a stereoselective synthesis of all-trans-polyprenol alcohols up to C50. Construction of the requisite skeletons was accomplished by the alkylation of a p-toluenesulphonyl-stabilized carbanion, followed by reductive desulphonylation of the resulting allylic sulphonyl group. This was achieved most efficiently by the use of a large excess of lithium metal in ethylamine (equation (43)), although all reaction conditions led to mixtures. The minor product results from double bond rearrangement. 

Any reaction in which only one of a set of stereoisomers is formed exclusively or predominantly is called a stereoselective synthesis. The same term is used when a mixture of two or more stereoisomers is exclusively or predominantly formed at the expense of other stereoisomers. In a stereospecific reaction, a given isomer leads to one product while another stereoisomer leads to the opposite product. All stereospecific reactions are necessarily stereoselective, but the converse is not true. 

Many chemical reactions and processes yield cationic racemic products, and either a resolution or a stereoselective synthesis must be envisaged to obtain the chiral cations in an enantioenriched or enantiopure form. Resolution has been strongly studied [130] and selected representative examples of such processes mediated by chiral P( VI) anions are presented. 

Ester (1) is a perfumery compound from civet cats. What factor favours a stereoselective synthesis ... 

In ref. [123], 1-triisopropylbenzenesulfonyl- or mesitylenesulfonyl-5-(pyridine-2-yl)-tetrazole (VIII) was successfully used as a coupling agent for the synthesis of protected di- and trinucleotides. In comparison with the sulfonic acid tetrazolides these are able to achieve a stereoselective synthesis of dinucleoside monophosphate aryl esters. 311 1241... 

Chiacchio and co-workers <97T13855> reported a stereoselective synthesis of 133 via an intramolecular 1,3-dipolar cycloaddition. Intermediate 132 was generated in situ by the reaction of tra .v-/V-(2-formylphenyl)-i -methyl-2-phcnylcthenc- l -sulfonamide 131 with N-methylhydroxylamine and afforded a tricyclic benzothiazine 133 in 51% yield (Scheme 36). 

An intramolecular nitrone 1,3-dipolar cycloaddition reaction to give 46 from 45 followed by reductive N-O bond cleavage afforded a stereoselective synthesis of the tetrahydro 177-1-benzazepines 47 the nitrone precursors 44 were prepared in turn by a Claisen rearrangement from an IV-allylamine <06SL2275>. 

The above method has been applied to a stereoselective synthesis of cyclopenta-nones with quaternary carbon centers (Scheme 16.21) [25],... 

A stereoselective synthesis of the enantiomerically enriched allenic hydrocarbons was described in 2001 (Scheme 18.11) [37]. For example, hydrostannylation of the chiral propargylic alcohol 28 (obtained with 82% ee by enantioselective reduction of... 

Tsuboi and coworkers20 reported a stereoselective synthesis of 3,5-alkadienic ester obtained from 2,4-dienoic isomers and their NMR data. 

In Section 5.03.6.2, a stereoselective synthesis of L-homophenylalanine from the racemic AAacetylated amino acid is described. The authors, however, found that substrate solubility limited the utility of this procedure. Having found an L-N-carbamoylase in Bacillus kaustophilus, they introduced the gene for this enzyme together with that for the N-acyl amino acid racemase from D. radiodurans into E. coli for coexpression. These cells, permeabilized with 0.5% toluene, were able to deliver L-homophenylalanine in 99% yield and were able to be used for multiple reaction cycles. 

A -tritylaziridine-2-(5)-carboxaldehyde. The application of a novel, sequential, trans-acetalation oxonium ene cyclization has delivered a stereoselective synthesis of the C-aromatic taxane skeleton, and a combinatorial sequence of the regioselective propiolate-ene, catalytic enantioselective epoxidation and carbonyl-ene cyclization reactions has been used to complete the synthesis of the A-ring of a vitamin D hybrid analogue. 

Metallic tin, Sn(0), is even more effectively employed. For example, in the presence of Sn(0), allyl bromide and a-halocarbonyl compounds afford nucleophilic organometallic species, which add to aldehydes in good yields to give homoallylic alcohols (12) and g-hydroxycarbonyl compounds (13,14) respectively. a-Diketones could be reduced by activated Sn(0), to give tin(II) enediolates which in turn undergo aldol reaction to form a,g-dihydroxyketones (15,16). This reaction was successfully applied to a stereoselective synthesis of methyl D-glucosaminate (17). 

Acid treatment of a 3 1 mixture of murrayafoline A (7) and koenoline (8) led to chrestifoline A (192) in 70% yield. Addition of murrayafoline A (7) to a mixture of 1057 and lithium aluminum hydride in ether and dichloromethane afforded bismurrayafoline-A (197) in 19% yield (662) (Scheme 5.166). In addition to the aforementioned methods, the same group also reported a stereoselective synthesis of axially chiral bis-carbazole alkaloids by application of their "lactone concept" (663) and a reductive biaryl coupling leading to 2,2 -bis-carbazoles (664). 

The crucial element of these syntheses is control of the stereochemistry of the exocyclic double bond on the basis of the stereochemistry of migration of a substituent from boron to carbon. Discuss the requirements for a stereoselective synthesis, and suggest how these requirements might be met. 

A stereoselective synthesis of testosterone (261) was advanced by Fukumoto and co-workers (331), where ring B was joined to the C/D part by an intramolecular nitrile oxide cycloaddition. The key nitrile oxide dipole was generated in situ from oxime 258, which in turn was derived from the optically active tetrahydroin-danone 257. Tetracyclic isoxazoline (259) was obtained as a single stereoisomer... 

Enantiomeric resolution via the acetyl derivative with acylase or chymotryptic hydrolysis of an alkyl ester was not attempted. Conversely, a stereoselective synthesis of the Boc-Phe(4-CH2C02tBu)-0H has recently been reported11291 which, however, lacks selective protection for incorporation to any position of a synthetic peptide. 

Furo[3,4-7]pyridines can be prepared in a stereoselective synthesis involving a ruthenium-catalyzed asymmetric transfer hydrogenation reaction <2001TL1899>. The reaction proceeds with exceptionally high yield and ee (Equation 50). 

It has recently been demonstrated that a stereoselective synthesis of dipeptides by hydrogenation of the corresponding monodehydropeptides (N-protected free acids or methyl esters) is possible. In this reaction, chiral catalysts, for example BPPM (13), in the form of a Wilkinson complex have been used. These are superior to the corresponding DIOP complexes (DIOP = P,P -[2,2-dimethyl-l,3-dioxolane-4,5-bis(methylene)]bis(diphenylphosphane). A d.s. value of 90—99% was generally obtained 49 ... 

The trausmetallation of a-silylated ester lithium euolates by maguesium bromide aud subsequeut Peterson olefination provides a stereoselective synthesis of a,-unsaturated esters (equatiou 94). 

---