Side introduction, stereospecific

N,O-acetal intermediate 172, y,<5-unsaturated amide 171. It is important to note that there is a correspondence between the stereochemistry at C-41 of the allylic alcohol substrate 173 and at C-37 of the amide product 171. Provided that the configuration of the hydroxyl-bearing carbon in 173 can be established as shown, then the subsequent suprafacial [3,3] sigmatropic rearrangement would ensure the stereospecific introduction of the C-37 side chain during the course of the Eschenmoser-Claisen rearrangement, stereochemistry is transferred from C-41 to C-37. Ketone 174, a potential intermediate for a synthesis of 173, could conceivably be fashioned in short order from epoxide 175. 

Kagan H, Namy JL (1999) Influence of Solvents or Additives on the Organic Chemistry Mediated by Diiodosamarium. 2 155-198 Kakiuchi F, Murai S (1999) Activation of C-H Bonds Catalytic Reactions. 3 47-79 Kakiuchi F, Chatani N (2004) Activation of C-H Inert Bonds. 11 45-79 Kant J (2004) Stereospecific Introduction of Cephalosporin Side Chains Employing Transition Metal Complexes. 6 247-262 Keen SP see Gibson SE (nde Thomas) (1998) / 155-181 Kiessling LL, Strong LE (1998) Bioactive Polymers. 1 199-231 Kim DD see Beak P (2003) 5 139-176... 

The second approach to thiooligosaccharides follows that outlined in O Scheme 2, i. e., an anomeric thiol (or thiolate) is first constructed and then reacted with a saccharide electrophile to give the thiooligosaccharide through an Sn2 displacement reaction. Problems to consider are the stereospecific introduction of the anomeric sulfur group, its (preferably selective) conversion into a thiol, and the nature of the electrophile. Side-reactions encountered in the couplings are elimination of the electrophile and anomerization of the thiol. In a model study of the s)uithesis of methyl 4-thiocellobioside, this approach was found to be superior to the one using thiol acceptors [179]. 

The steps of prenylation and dehydrogenation which follow (94) in the biosynthesis of these neoechinulins is unknown but from knowledge of echinulin biosynthesis (Scheme 6) introduction of the side chain at C-2 may be the next step. Prenylation of the benzene unit seems, by inspection of structures (97) through (101), to depend on C-8—C-9 unsaturation rather than the structure of the dioxo-piperazine ring. The stereochemistry of the desaturation reaction has been explored with L-tryptophan (85) samples stereospecifically labelled with tritium at... 

The chiral auxiliary is cleaved off first and replaced by a benzyloxy-group. Alkylation at the a-position proceeds stereospecifically. However, the projected next step with 9-BBN leads also to side-products involving the ester function. This is a conceptual disadvantage of the overall synthesis, since the ester must first be reduced and then protected (total of four additional steps). The introduction of the hydroxy-group proceeds with high stereoselectivity. The new hydroxy-group is protected as its benzyl ether under acid catalysis, since the basic... 

As we know from experiment, any chemical reaction will result in byproducts and side-reacti(Mis that will limit the overall yield to <100%. Quantitative yields are quite rare for synthetic chemistry that is, until the introduction of click chemistry by Sharpless and coworkers in 2001. " By definition, click chemistry involves reactions that occur by high/quantitative yield, generate few/no byproducts, and are stereospecific - setting an important precedent toward mimicking nature s synthetic efficiency. In particular, nature efficiently links small molecules together via het-eroatomic C—X—C bonding to yield primary metabolites (polypeptides, polynucleotides, and polysaccharides - Figure 5.36), which are essential for life. 

To solve the problem in sialidation (1) introduction of an auxiliary group at C-3 in sialyl donors, (2) specific activation of the anomeric carbon C-2 by use of thioglyco-sides, xanthates or phosphites in acetonitrile medium, and (3) the use of sterically less hindered sugar acceptors have been attempted and found to be highly effective for obtaining a-sialosides in a regio- and stereospecific manner. Combinations of these procedures have resulted in further improvements. 

The third synthesis differs from all earlier ones in that the side-chain is introduced after the biotin ring-system has been constructed. The reaction of the sulphoxide (29) with n-butyl-lithium followed by I(CH2)4COaBu gave cf,/-biotin in 30% yield (Scheme 2). The stereospecific introduction of the alkyl group trans to the... 

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