2- Alkenyl-1,3-dioxolanes

The alkenyl oxonium ion dienophiles generated from dioxolanes can be made diastereoselective by use of chiral diols. For example, acetals derived from anti-pentane-2,4-diol react under the influence of TiCl4/Ti(/-OPr)4 with stereoselectivity ranging from 3 1 to 15 1. 

The stereochemical outcome for addition of r-l,3-dioxolan-4-yl and oxiranyl radicals to phenyl vinyl sulfone has been probed. The results indicated that the symanti selectivity could be altered by changing the group next to the radical in the diox-olanyl case but not in the oxiranyl case (bulky groups had a large xyn-directing effect) (Scheme 39). Several alkenyl-lactones and -lactams have been subjected to hydrosilylation conditions using carbohydrate-derived thiols as homochiral polarity reversal catalysts (yields 25-96% ee 5-95%) " ... 

High-yield amination of aryl-, benzyl- and alkenyl magnesium bromides has been successful using l,3-dioxolan-2-one 0-(benzenesulfonyl)oxime 61 (Scheme 66) . 

The Rli2(OAc)4-catalysed reactions of ethyl diazoacetate with substituted benzalde-hydes yielded 1,3-dioxolanes via an initially formed carbonyl yhde. Catalyst-dependent diastereo-control was observed only when /rnitrobcnzaldchydc was used as catalyst.104 The intramolecular cycloaddition of carbonyl yhde dipoles with tethered alkenyl 71-bonds is greatly enhanced by placing an sp2 centre on the tethered side-chain.105 The thermal reaction of 3-phenyloxirane-2,2-carbonitrile and 2-phenyl-3-thia-l-azaspiro-[4,4]non-l-ene-4-thione yields cis- and /raw.v-cycloadducts via a regioselective 1,3-dipolar cycloaddition of an intermediate carbonyl yhde.106... 

Mercury photosensitization has been used to synthesize 1,3-dioxolane dimers by dehydrodimerization. For 1,3-dioxolane itself or analogues with substitution at C-2 or C-2 and C-4, radical formation occurs at the C-2 position, whereas when substitution is at the C-4 position radical formation occurs at the 4-position. In most cases, a small amount of product from ring opening was also reported <1996TL6853>. For 2,2-dimethyl-l,3-dioxolane, radical formation occurs at the 4-position, and one common way to achieve this is the use of dimethylzinc in air. Addition of various alkenes and related groups can then be performed, such as addition to tosyl imines <2004JOC1531>, and addition to perfluorinated alkenes and alkenyl ethers <1999JFC(94)141>. 

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