Regioselective control elements

In Heading 1.4 we have already seen that three different kinds of control elements [3] may be considered 1) chemoselective control elements (controlling chemical reactivity), 2) regioselective control elements (controlling the orientation of reactants) and 3) stereoselective control elements (controlling the spatial arrangement of atoms within the molecule), which may control either the relative (diastereoselective) or the absolute spatial arrangement (enantioselective control elements). 

Regioselective control elements blocking and activating groups. Bridging elements... 

In the case of benzene derivatives, the discrepancy in reactivity between the C-H bonds is generally less pronounced and regioselective controlling elements are required [Ic]. To solve this problem, DGs have been employed [5] (Figure 4.2). However, this strategy comes at a price, as the DG needs to be installed and/or removed or it should be a part of the target compound structure. In this chapter, we will discuss this particular strategy, as well as the more uncommon. 

Nucleophilic substitutions at C(2) and C(3) positions are not always as regioselective as the Payne rearrangement-opening reactions [10b] and the chemist may then utilise the C(l) hydroxyl group as a control element for inducing a... 

In conclusion, one may finally state that from the examples presented in this chapter the expectation is justified that almost any synthetically useful multiple bond reaction can be directed as far as regioselectivity and diastereoselectivity are concerned if the whole range of control elements is considered. 

Bach, T, The 5-silicon effects as a control element for the regioselective ring opening of oxetanes. Tetrahedron, 50, 12319,1994. 

The major problem remains control of regioselectivity in favor of the branched regioisomer. While aryl alkenes as well as heteroatom-substituted alkenes favor the chiral branched isomer, for aliphatic alkenes such an intrinsic element of regiocontrol is not available. As a matter of fact branched-selective and asymmetric hydroformylation of aliphatic alkenes stands as an unsolved problem. In this respect regio- and enantioselective hydroformy-... 

This hierarchy of substituent effects can be exploited to obtain adducts with predictable and high regioselectivity through the use of dienes and dienophiles containing temporary substituents which control the regiochemisby of the addition and are removed thereafter. Sulfide (diene) and nitro (dienophile) groups can serve in this manner as powerful regiocontrol elements (Sections 4.1.3.3 and 4.1.2.1). 

The synthesis of thiazoles 43 adds an element of regioselectivity as the thioamide has soft (S orbital controlled) andhard (N charge controlled) nucleophilic sites anditis essential to differentiate equally clearly between the two electrophilic centres on the carbonyl part of the molecule. We need to know that a thioamide will react with an a-haloketone 48 X = Cl, Br so that the hard nitrogen atom attacks the ketone and the soft sulfur atom attacks the saturated carbon atom. Thus the simple non-steroidal anti-inflammatory drug Fentiazac 50 is made from thiobenzamide 51 and the simple bromoketone 52, made in turn by bromination of the ketoacid. The synthesis is just that.3... 

By photolyzing a solution of the allyl aryl sulfoxide at slightly elevated temperature at appropriate wavelengths, allyloxy radicals were produced from photolysis of the steady state concentration of the sulfenate. Another study examined the regioselectivity of addition of the alkyl and sulfenyl radicals across olefins and allenes [155], Control of these elements allowed potentially useful synthetic transformations to be designed [156], particularly as the sulfenate may be viewed as an O-H abstraction synthon . 

Abstract Carbohydrates generally possess multiple hydroxyl groups of similar reactivity, and selective monofunctionalizatimi is often difficult. Catalysis provides a versatile and potentially general solution to this problem. This chapter provides an overview of catalyst-controlled methods for the regioselective activation of carbohydrate derivatives. The catalysts discussed include organocatalysts (Lewis bases, Brpnsted acids/bases, and others) as well as those based on main group and transition metal elements. 

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