Olefinic dioxolane acetal

Hydrogenation of olefin 4, followed by 0-desilylation and 0-tosylation next procured tosylate 3, which cyclised readily when exposed to excess potassium hexamethyldisilazide. Elaboration of the ketophosphonate side chain of 20 was accomplished by condensing ester 2 with the lithiated anion of dimethyl methylphosphonate. After concurrent removal of the 1,3-dioxolane acetal and the MOM ether from 20, the resulting secondary alcohol was oxidised with pyridinium chlorochromate (PCC) to produce methyl ketone 1. 

Me3SiI, CH2CI2, 25°, 15 min, 85-95% yield.Under these cleavage conditions i,3-dithiolanes, alkyl and trimethylsilyl enol ethers, and enol acetates are stable. 1,3-Dioxolanes give complex mixtures. Alcohols, epoxides, trityl, r-butyl, and benzyl ethers and esters are reactive. Most other ethers and esters, amines, amides, ketones, olefins, acetylenes, and halides are expected to be stable. 

The acetal radicals have to be trapped by a scavenger before the C-0 bond cleavage takes place, if the desired product should be the one with the acetal ring (or group) preserved. Terminal olefins have been shown to be efficient scavengers for these radicals. Thus, irradiation of a mixture of dioxolan and terminal olefins in the presence of acetone leads to the desired 1 1 adducts (17) ... 

There are no data available on the rate of formation of dialkyloxonium ions like protonated 1,3-dioxolane in Eq. (16). It is remarkable that the rate constants of formation of secondary onium ions from linear ethers, acetals, sulfides etc. are also utdcnovra. These should however be lower than the rate constants of proton transfer in water (an upper limit) being close to 10 mole 1 s but certainly higher than the rate constantsof protonation of olefins... 

Olefin synthesis. Eastwood el at.1 have reported a new method for conversion of wc-diols into alkenes. For example, rac-l,2-diphenylethane-l,2-diol (1) is heated with N,N-dimethylformamide dimethyl acetal to give 2-dimethylamino-/ram-4,5-diphenyl-1,3-dioxolane (2). When this dioxolane is heated with acetic anhydride at 165-180 mws-stilbene (3) is formed in 80% yield. Similarly, meso-1,2-diphenylethane-l, 2-diol is converted into c/r-stilbene (75%) and a trace of /ram-stilbene if the elimination... 

Thus, coordination of the zinc to one of the diastereotopic oxygens and oriented anti to the adjacent dioxolane substituent places the transfer methylene on the face of the olefin toward the viewer, consistent with the observed absolute configuration. Note that coordination to the other oxygen and orienting anti to the adjacent substituent would place the transfer methylene distal to the double bond. A similar explanation can be offered to rationalize the results of the aldehyde acetal additions, assuming that the olefin adopts the indicated conformation in the transition state. 

Dioxolane, the cyclic ethylene glycol acetal of formaldehyde, is also alkylated by olefins under free-radical conditions. Here, it is the CH2 group between the two oxygen atoms that is the preferred site of hydrogen abstraction. 

The intermediary aldehydes derived from the Ru-catalyzed hydroformylation can be trapped as acetals, as shown by Borner and colleagues (Scheme 1.40) [45]. Only traces of alcohols or aldehydes could be detected. The tandem reaction proceeded exclusively with diols that formed thermodynamically stable 1,3-dioxolane and 1,3-dioxane rings. Methanol as the acetalization reagent failed. As olefins, terminal aliphatic olefins as well as styrene derivatives reacted. The catalyst could be recycled and reused at least twice. 

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