Sakurai-carbonyl-ene reaction

A stereoselective tandem Sakurai-carbonyl-ene reaction for the synthesis of steroid derivatives has been reported [48]. When LtAlCl2 and la were employed in this cyclization, stereochemical control was different. The cyclization product obtained with la is only 19 (Sch. 17), even though the starting material contained all four geometrical isomers use of LtAlCl2 resulted in a mixture of two different stereoisomers in lower yield. 

One pot domino (cascade) reactions like tandem Knoevenagel-hetero Diels-Alder, Knoevenagel-ene, Pictet-Spengler-ene, Sakurai carbonyl-ene reactions. 

Tandem Sakurai-carbonyl-ene reaction steroid synthesis The key step in a short synthesis of the BDC rings of steroids from the chiral aldehyde 1 involves a tandem Sakurai-carbonyl-ene reaction of 2 mediated by trimethylsilyl triflatc, which provides 3a as the only cyclized product in 52% yield. Amazingly, catalysis with CjHsAlCl leads to two diastereomers, 3b and 3c (3 1) in 40% yield. Hydrogenation of 3a provides 4, which corresponds to the BCD ring of steroids with an added methyl group in ring B. 

Substituted allylsilanes are subject to ene reaction with aldehydes and a,/3-unsaturated carbonyl compounds in the presence of a Lewis acid. The Et2AlCl-promoted reaction of /3-siloxymethyl-substituted allylsilane 27 with aldehydes gives more functionalized allylsilanes (Equation (37)).148 The use of TiCU instead of Et2AlCl leads to the Hosomi-Sakurai allylation. Catalytic enantioselective carbonyl-ene reactions of methallylsilanes have been achieved by using chiral Ti and A1 complexes.149,150... 

The ene reaction is the major side reaction in the Sakurai-Hosomi coupling reaction. Thus, treatment of 221 with carbonyl or with azo compounds in the presence of TiCl2(OPr-/)2 furnishes a mixture of ene-type product 222 and the Sakurai-Hosomi-type product 223 (equation 182)327. Allylsilylation of alkenes328 and alkynes329-330 proceeds regioselectively to give 224 (equation 183) and 225 (equation 184), respectively when aluminum catalysts are used. 

The Patemo-Btichi photocycloaddition reaction - of various carbonyl compounds to furans was initially investigated by Sakurai in 1963 and was found to afford only the head-to-head photoproducts with high exo relative face selectivity. An NMR study by Whipple and Evanega later confirmed the exo mode of cycloaddition. Since the time of the origin report the photoreaction has been systematically studied by several groups and the 2,7-dioxabicyclo[3.2.0]hept-3-ene ring system has been exploited in several facets of synthesis. 

---