Regioselective reactions reduction

A reiterative application of a two-carbon elongation reaction of a chiral carbonyl compound (Homer-Emmonds reaction), reduction (DIBAL) of the obtained trans unsaturated ester, asymmetric epoxidation (SAE or MCPBA) of the resulting allylic alcohol, and then C-2 regioselective addition of a cuprate (Me2CuLi) to the corresponding chiral epoxy alcohol has been utilized for the construction of the polypropionate-derived chain ]R-CH(Me)CH(OH)CH(Me)-R ], present as a partial structure in important natural products such as polyether, ansamycin, or macro-lide antibiotics [52]. A seminal application of this procedure is offered by Kishi s synthesis of the C19-C26 polyketide-type aliphatic segment of rifamycin S, starting from aldehyde 105 (Scheme 8.29) [53]. 

The latter reaction was further described by Koizumi et al. [86b] with slightly different results (lower facial selectivity for the exo-approach), in connection with the enantiodivergent synthesis of fused bycyclo [2,2,1] heptane lactones 91 (see Scheme 47). The key step of this transformation was the regioselective DIBAL reduction of only one of the two ester groups in the adducts, followed by... 

California Red Scale is a worldwide citrus pest which can be controlled by means of the pheromone 60. Cohen used reductive lithiation to generate versatile allylic nucleophiles applicable to this type of target.6 The allyl sulfide 61 is lithiated by BuLi and reacts with butenyl bromide a to sulfur to yield 63. Reductive lithiation (Li, DBB) of the product yields allyllithium 64. A regioselective reaction of this nucleophile with formaldehyde at the more substituted terminus is ensured by transmetallation to the allyl titanium 65, which gives 66 after treatment with formaldehyde and bromination. 

Redox equations, balancing, 265 Reduction of aromatic compounds, 200 Reductive amination, 404 Reductive hydroboration, 95 Reformatsky reaction, 321 Regioselective reactions, 97 Reimer-Tiemann reaction, 438 Resonance, 23 in benzene, 192 energy, 24 structures, 29 Resorcinol, 327, 432 Retroaldol condensation, 399 Ring activation, 2(fiff Robinson inaction, 398 Rosenmund reduction, 306... 

A wide variety of N-alkyl hydrazinedicarboxylic esters may be obtained in excellent yields by the hydrohydrazination reaction depicted in Eq. 49.215 Use of cobalt complexes results in more highly regioselective reactions at the cost of lower reaction rates as compared to additions where manganese complexes are employed. Di(fert-butyl) azodicarboxylate is the preferred azo ester reduction of the N=N double bond becomes more prominent when less hindered azo esters are used. Alcoholic solvents are essential the reaction fails when methylene chloride or THF is used. 

Regioselective Reactions. An important attribute of DIBAL is its capacity to react regioselectively. Acetal cleavage is a classical example of this property, which finds its source in electronic, steric, and/or chelation effects. The reduction of aryl-1,2-ethanediol benzylidene acetals is controlled mainly by electronic effects a change in electron donating capacity of the aryl moiety changes the regioselectivity of the reaction (eq 43).4 ... 

Colloidal potassium was also used to desulphonylate cyclic sulphones [106]. This reaction is only synthetically useful under these conditions and is extremely slow in the absence of ultrasound. Furthermore, the reaction is highly regioselective and reductive cleavage occurs preferentially at the more substituted centre giving the product shown (13). This contrasts with the low degree of regioselectivity of literature methods (Scheme 43). 

Factors controlling regioselectivity in the reduction of polynitroaromatics in aqueous solution were examined with the use of SM2/AM1 model. The analysis supported the reliability of aqueous geometries predicted by SM2/AM1. Moreover, successful predictions for the regioselectivity of reduction further supported the validity of the modeling approach. The authors stated that "the ability of the SMk series of solvation models to predict the electronic structure of reagents in solution should open the door for the study of many environmentally important reactions taking place in aqueous or other liquid media" [123]. 

The various 3-keto analogs also can be used to give regioselective modifications. Reduction of 3-ketosucrose with sodium borohydride exclusively gives al-losucrose [110] (reaction 4.105). Reaction of the keto group with hydroxylamine... 

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