Siloxy-3-acetylenes

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>. 

Stereospecific synthesis of 1,3-dienes. Corey et a .3 have described a new synthesis of 1,3-dienes based on the highly stereospecific cis addition of alkylcopper reagents to a, -acetylenic carbonyl compounds (3, 108). Thus the reaction of methyl 4-trimethyl-siloxy-2-nonynoate (l)4 in THF with divinylcopperlithium (1.25 eq) at —90° and then at — 78° affords the pure e(r adduct (2) in > 90% yield. Treatment of (2) with methanolic hydrochloric acid effects cleavage of the trimethylsilyl ether and lactonization to give (3). 

Poly[l-phenyl-2-(/7- -butyldimethylsiloxyphenyl)acetylene] is solution cast to produce a membrane that is subsequently transformed into a durable membrane via desilylation using a mixture of trifluoroacetic acid (TFA)/water. A detailed method of desilylation of membranes is as follows the membrane of polymer is immersed in a mixture of TFA and water (9 1 v/v) at room temperature for 24 h. To neutralize the remaining acid in the polymer matrix, the membrane is then immersed in aqueous sodium carbonate solution at room temperature for 24 h, and then dried in air at room temperature. Finally, the membrane is immersed in hexane for 24 h to remove residual impurities and dried to constant weight at room temperature for 24 h. Complete desilylation is confirmed by IR speetroscopy (no absorptions are seen at 1250 d, Si-CH), 855 (Vas, Si-CHs), and 812 cm (Vas, Si-CHs), which are observed in the siloxy polymer). Desilylation is also confirmed by measuring the weight of the membrane before and after desilylation, thereby verifying loss of the silyl group e.g., mass of siloxy polymer = 53.6 mg, mass of hydroxy polymer = 33.5 mg (measured), 33.5 mg (calculated). ... 

Tetrabutylammonium iodide Siloxy-2-acetylenes from silylacetylenes and oxo compds. 

SCHEME 2.122 Synthesis of a siloxy alkyne starting from a terminal acetylene [191]. 

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