Directing groups pyridine

By contrast the well-defined Ru(02CMes)2(arene) catalyst was efficient to perform ortho mono(hetero)aiylation of arenes containing a nitrogen directing group (pyridine, oxazoline, pyrazole, benzoquinoline...), when the second ortho C-H bond was protected, but also of alkenes [(Eq. 12)] [80]. 

Closely related are the pyridine-A-oxides, which are readily lithiated in the 2-position , even in examples such as 228 where there is a powerful competing directing group (Scheme 111). Lithiation of the bipyridine-Af-oxide 229 was used in a synthesis of the caerulomycins and collismycins. ... 

More powerful directing groups such as those based on amides and sulphonamides are successful with pyridines as with carboxylic rings, and will not be discussed separately. The enhanced acidity of pyridine ring protons makes the simple carboxylate substituent an ideal director of lithiation in pyridine systems . The pyridinecarboxylic acids 232-234 are deprotonated with BuLi and then lithiated with an excess of LiTMP all the substitution patterns are lithiated nicotinic acid 233 is lithiated in the 4-position (Scheme 113). The method provides a valuable way of introducing substituents into the picolinic, nicotinic and isonicotinic acid series. 

Another directing group whose value and effectiveness is greatly enhanced in the pyridine series is the a-aminoalkoxide group derived by addition of a lithioamine to an aldehyde (see Section LB.Lb). Aldehyde 237 was converted to 238, and thence to an intermediate 239 in the synthesis of the alkaloid maxonine, by this method (Scheme 115). ... 

The cyano group has been used as an ortho -directing group for lithiation in the pyridine series.79 Lithiation of 4-cyanopyridine using LiTMP and trapping the lithio intermediate with electrophiles have provided an efficient and straightforward access to ortho -substituted-4-cyanopyridines. 

Silylation of benzyl C-H bonds using hydrosilanes can also be performed with the aid of Ru3(CO)12-catalyst (Table 1) [9]. This silylation occurs only at benzylic CH3 groups. Pyridine, pyrazole, and hydrazones function as good directing groups. Benzylamines, oxime ethers, dimethylanilides, and aryl pyridyl ethers have no activity in this silylation. 

Some other directing groups which involve the sp2 nitrogen can also function as a directing group in place of the pyridine ring. The reaction of aromatic imines with CO and ethylene in the presence of Ru3(CO)12 did not stop at the carbonylation step, but rather indenone derivatives were the final products and were formed via an intramolecular aldol-type reaction of the expected carbonylation products in situ (Eq. 27) [44]. The treatment of the reaction mixture with silica gel selectively afforded indenones in good yields. 

The 1,5-diketone formation by the Michael addition of allylsilane (48) to a, -unsaturated ketones was applied to the synthesis of (+)-nootkatone. Reaction of the keto group of keto aldehyde (58) with allyl Orignard reagent and dehydration gave the diene aldehyde (59). The selective oxidation of the terminal double bond afforded the 1,5-dica nyl compound (60), which is not stable and converted directly to pyridines and phenols (Scheme 18). ... 

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