Standard precursor synthesis alcohol

In combination with the range of standard transformations of alcohols, alkenes, and vinylsulfides, these silicon-tethered additions of functionalized radicals offer a versatile and stereoselective approach to amino alcohol synthesis. Whereas vinyl and 2-oxoethyl radicals have not yet been demonstrated as competent participants in the various intermolecular additions reported in the literature, the temporary tether approach allows such functionalized fragments to be installed in an efficient and stereoselective manner. Synthesis of the aminosugar daunosamine from achiral precursors shows how this concept, employing hydrazone radical acceptors, can be merged with asymmetric catalysis to achieve practical synthetic advances. 

The corresponding thioether fnnctionalised NHC without a second chiral wingtip gronp bnt an asymmetric carbon atom in the thioether sidearm was also synthesised by Ros et al. [261], However, we will tnrn our attention towards a similar example from the same research gronp [262] where the NHC is pyrido[a] annulated [263], The synthesis is again rather facile and follows standard procedures (see Figure 4.86). The chiral thioether precursor compound is accessible as the alcohol and needs to be treated with tetrabromomethane and PPhj to generate the required primary alkyl bromide. 

Amino acids are the monomers from which nature produces proteins and enzymes. As such, they form an important part of the chiral pool. Histidine contains an imidazole ring in the sidechain and thus is a logical target for the synthesis of functionalised and chiral NHC, Given that a primary amine, as found in natural amino acids, is a prerequisite for standard imidazole ring forming reactions [60], it is surprising that only a few protocols are found in the literature where an amino acid [61-66] or its respective alcohol [64,67] forms part of a reaction that leads to a carbene precursor,... 

A vast array of chiral auxiliaries have been derived from naturally occurring compounds containing the bicyclo[2.2.1]heptane unit (for review articles, see refs 1 -3). In all cases, the ultimate sources of these auxiliaries are the ketones camphor and fenchone, and the alcohols borneol and fenchol, as at least one enantiomer of each compound is provided in enantiomericaUy pure form by nature. Thus. ( + )-camphor [( + )-2], (-)-borneol [(-)- ], and (+)-fenchonc [( + )-5] are enan-tiomerically pure, convenient and inexpensive starting materials for organic synthesis and deriva-tization to give chiral auxiliaries. Most other compounds of this series are also commercially available, but can be prepared by oxidation or reduction of inexpensive precursors by standard methods. The evo-alcohols, such as the enantiomeric isoborneols, are accessible by standard complex hydride reductions of the ketones. The interconnection between these compounds is shown diagrammatically. 

A modified Peterson reaction has been used to generate silenes which have then been converted to diols and lactones. The reaction involves the formation of the sila-Grignard reagents under standard conditions, followed by treatment with isobutyraldehyde to give the silene precursor 76. Silacyclohexene 76 was then produced by reaction with 1,3-pentadiene. High diastereoselectivity was observed and confirmed by 2D NMR experiments on the subsequent reduction and oxidation products. Thus, standard conditions converted silane 78 into diol 79 which was then oxidised to give lactone 80. The protocol can be expanded to achieve further functionalisation, for example in the synthesis of homoallylic alcohol 82. 

The NaB H4 reduction of carbonyl compoimds is a standard procedure for the synthesis of [ 1- H]alcohols, most usefully in the presence of various other functional groups, as for the precursors to [ll- H]ABT-229 (172) , (85)-20-hydroxy[20- H]hepoxilin A3 methyl... 

---