Pyridine, reactions with—continued substitution

The scope and efficiency of [4+2] cycloaddition reactions used for the synthesis of pyridines continue to improve. Recently, the collection of dienes participating in aza-Diels Alder reactions has expanded to include 3-phosphinyl-l-aza-l,3-butadienes, 3-azatrienes, and l,3-bis(trimethylsiloxy)buta-l, 3-dienes (1,3-bis silyl enol ethers), which form phosphorylated, vinyl-substituted, and 2-(arylsulfonyl)-4-hydroxypyridines, respectively <06T1095 06T7661 06S2551>. In addition, efforts to improve the synthetic efficiency have been notable, as illustrated with the use of microwave technology. As shown below, a synthesis of highly functionalized pyridine 14 from 3-siloxy-l-aza-1,3-butadiene 15 (conveniently prepared from p-keto oxime 16) and electron-deficient acetylenes utilizes microwave irradiation to reduce reaction times and improve yields <06T5454>. 

Interest in photochromic systems other than those based on the hexa-fluorocyclopentene moiety continues to grow. The photochemical reactivity of the two photoswitches (35) is similar, and irradiation is efficient with conversions of 85% and quantum yields of around 0.6. The novel photo-chromic systems (36) undergo reversible ring closure in a reaction analogous to that observed in the bisthienyl system. Qin et al. have studied the novel pyridyl substituted cyclopentene system (37). This undergoes photocyclization with an enhanced quantum yield when the reactions are carried out in the presence of a metal. The pyridine units are capable of co-ordinating with the metal. The photochromic dithienylethene unit tethered to 3-cyclodextrin (38) has been used as a photoswitch to control the uptake of porphyrin. A series of new photochromic molecules (39) have been synthesized and studied. These exhibit the usual cyclization on irradiation. " The terthiophene derivatives (40) exhibit reversible photochemical cyclization (at 313 nm) and reversion (at wavelengths >460 nm) reactions. The cycles can be carried out many times... 

The photocycloaddition chemistry of pyridines substituted with electron-donor and electron-acceptor groups at the 2- and 3- positions continues to be exploited. The results of irradiation of such pyridines in the presence of 2-cyanofuran have now been described. The yields of the (47r+47r) cycloadducts (29) and (30), the pyridine dimer (31) and the transposition isomer (32) are dependent on the level of methyl substitution on the heteroarene and are given in Scheme 2. Other photocycloadditions to heteroarenes reported within the year include the reactions of benzodithiophene (33) with butadiyne derivatives and dimethyl acetylene dicarboxylate, giving low yields of (34) and (35) respectively, the latter from photorearrangement of the primary adduct (36). The (271+471) photocycloaddition of indoles (37) to cyclohexa-1,3-dienes (38) is sensitized by the aromatic ketones (39), and yields (14-46%) of the exo and endo isomers of the adduct (40) in ratios which are dependent on the substituents on the addends. 

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